Bruno Fernandes
email: b.fernandes@leeds.ac.uk

Michel Khodeir

Céline Perlot

Hélène Carré

Jean-Christophe Mindeguia

Christian La Borderie

The behaviour of concrete made with recycled concrete aggregates (RCA) at room temperature is well-studied. However, some points still need to be addressed, especially in extreme conditions such as durability and high temperature. This paper evaluates the effect of elevated temperatures on the durability of concrete made with RCA. Three concrete mixes were studied: concrete with NA (reference), 100 % direct replacement (DR) mix (RCA-100-DR) and 100 % strength-based replacement (SBR) mix (RCA-100-SBR). The latter was designed to achieve the same performance as concrete made with NA. Mixes were exposed to temperatures of 200 • C, 400 • C and 600 • C. After cooling, durability-loss due to thermal exposure was evaluated through water porosity, capillary water absorption, permeability, chloride diffusion and accelerated carbonation tests. At room temperature, the direct addition of RCA decreased all durability parameters. The SBR mix recovered some of the durability properties. Exposure to high temperatures decreases all the properties, but it varies depending on the property. The concrete made with NA and the SBR mix showed similar performance. The durability was also evaluated using a performance-based approach, both at Funding.

Durability of concrete made with recycled concrete aggregates after exposure to elevated temperatures room and high-temperature. The proposed approaches showed potential to evaluate durability indicators, but they should be considered with precaution. Overall, concrete made with RCA reduces the durability of concrete, with or without heat damage, but this decrease can be reduced with proper mix optimization. These evaluations contribute to the post-heating durability of concrete structures made with RCA, which is fundamental to the post-fire assessment of concrete structures.

Introduction

The use of recycled coarse aggregate (RCA) as a replacement for natural aggregates has proven to be a possible solution to make sustainable concrete. This solution showed potential to minimize ::: the :::::::: potential ::: to :::::::: minimise : the consumption of raw materials and also reduce waste landfilling :::::: landfill : [START_REF] De Brito | Recycled Aggregate in Concrete[END_REF][START_REF] Shi | Performance enhancement of recycled concrete aggregate -A review[END_REF][START_REF] Nedeljković | Use of fine recycled concrete aggregates in concrete: A critical review[END_REF]. The production of recycled aggregate concrete (RAC) has been the aim of study for the last decades. In general, concrete made with RCA shows comparable properties to ordinary concrete, depending on RCA's quality and replacement rate [START_REF] Le | Recycled aggregate concretes -A state-of-theart from the microstructure to the structural performance[END_REF][START_REF] De Larrard | Conclusion[END_REF].

Although its use has been proven to be feasible, RCA will severely affect the properties of the concrete. First, related to the aggregate itself, RCA usually presents higher porosity, higher water absorption and lower strength than NA [START_REF] Shi | Performance enhancement of recycled concrete aggregate -A review[END_REF][START_REF] Behera | Recycled aggregate from C&D waste & its use in concrete -A breakthrough towards sustainability in construction sector: A review[END_REF] , due to the attached (or adhered) mortar. Also, incorporating RCAs on concrete will result in a peculiar microstructure, with three different transition zones [START_REF] Shi | Performance enhancement of recycled concrete aggregate -A review[END_REF][START_REF] Wang | Methods for improving the microstructure of recycled concrete aggregate : A review[END_REF]. These changes affect the behaviour of this sustainable concretes. Extensive :: In :::::: recent :::::: years, ::::::::: extensive research has been done in the last years to characterize the RAC concrete ::

to ::::::::::: characterise ::::: RAC : behaviour. However, some topics still need to be addressed, primarily related to exposure to extreme conditions, such as fire and post-fire durability.

Indeed, these properties have been studied before in literature [START_REF] Guo | Durability of recycled aggregate concrete -A review[END_REF][START_REF] Fernandes | Effect of elevated temperatures on concrete made with recycled concrete aggregates-an overview[END_REF], but limited attention was given to durability-loss due to exposure to elevated temperatures. Kou et al. [START_REF] Kou | Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures[END_REF] evaluated water absorption and porosity of RAC after exposure to elevated temperatures. The authors state that after exposure to 500 • C, water absorption was higher for concrete made with RCA, and this behaviour inverses after 800 • C. Mercury Intrusion Porosimetry (MIP) results demonstrated that the concrete with RCA presented a lesser increase in total porosity and smaller average pore diameter. The authors state that this behaviour is associated with better thermal compatibility between new and old paste.

Xuan et al. [START_REF] Xuan | Thermal and residual mechanical profile of recycled aggregate concrete prepared with carbonated concrete aggregates after exposure to elevated temperatures[END_REF] also evaluated porosity through MIP and found RAC after 600 • C are more porous than concrete made with NA and carbonated RCA. They attributed this behaviour to the decomposition of hydration products from the cement mortar in RAC. Laneyrie et al. [START_REF] Laneyrie | Influence of recycled coarse aggregates on normal and high performance concrete subjected to elevated temperatures[END_REF] evaluated the effect of high temperature on the water accessible porosity. They pointed out that water porosity did not change significantly up to 300 • C. Finally, Ma et al. [START_REF] Ma | Effects of high temperature and cooling pattern on the chloride permeability of concrete[END_REF][START_REF] Ma | Chloride permeability of recycled aggregate concrete under the coupling effect of freezing-thawing, elevated temperature or mechanical damage[END_REF] evaluated the residual chloride permeability of RCA after exposure to high temperatures. They state that substituting NA with RCA increased :::::::: increases : the chloride diffusion coefficient, and this value increases as RCA content increases.

There is a lack of knowledge about the post-heating durability of concrete made with RCA. Several crucial parameters, such as permeability and carbonation ingress, were not evaluated after exposure to elevated temperatures. Moreover, even for ordinary concrete made with NA, the durability of firedamaged concrete is little know and often ignored in standards and guidelines [START_REF] Valente Monteiro | Effect of elevated temperatures on the residual durability-related performance of concrete[END_REF]. It is noteworthy that the post-heating durability should be assessed after extreme events such as fire. Moreover, to be in use again, structures should meet the requirements of serviceability, strength and stability during all the remaining service life. The remaining durability properties should be assessed, and only after this evaluation , the strategies for repair, strengthening, and maintenance can be made.

To contribute to these knowledge gaps, this paper experimentally evaluates the residual durability of concrete made with RCA subjected to elevated temperature. Concretes made with NA and with 100 % of RCA were produced. For 100 % of substitution, a specific mix with comparable strength as concrete with NA was also designed. The mixes were heated to three different temperatures (200 • C, 400 • C and 600 • C). A comprehensive range of durability tests was done before and after exposure to high temperatures: water porosity, capillary water absorption, gas permeability, chloride diffusion and accelerated carbonation. Lastly, the concrete durability was evaluated using : a performance-based approach.

Experimental program

Materials and concrete mixes

All materials were the same for the tested concretes except for coarse aggregate in this study. The cement was CEM II/A-L 42.5R (produced by Eqiom). The limestone filler was Betocarb HP-SC (produced by Omya). The superplasticizer was SIKA ViscoCrete Tempo-483. Alluvial sand was the fine aggregate chosen for the mixes. The two coarse aggregates were natural (NA) and recycled concrete aggregates (RCA). The NA was made from diorite, obtained from a quarry located in Genouillac (France). The RCA :::::: RCAs were crushed and screened near Bordeaux (France) by a recycling company. Table 1 present ::::::: presents : materials properties. Three concrete mixes were studied: concrete with NA (reference), 100 % direct replacement (DR) mix (RCA-100-DR) and 100 % strength-based replacement (SBR) mix (RCA-100-SBR). The concrete with NA was initially designed to meet durability requirements (NF EN 206/CN:2014 [START_REF]Comité Européen de Normalisation: NF EN 206/CN: Béton -Spécification, Performance, Production et Conformité -Complément National à la Norme NF EN 206[END_REF]) for the XD3 exposure class: water/cement (w/c) ratio of 0.5, C35/45 compressive strength class and cement content of 350 kg/m 3 . This was done considering the most used concrete mix in the partner company. In RCA-100-DR, NA :::: NAs : were replaced by RCA :::::

RCAs : in volume, without any changes in other constituents. In RCA-100-SBR, cement and water content were also adjusted to achieve the same strength and slump :::::: target :::::: slump :::::::: (between :::::::: 180 mm :::: and ::::::: 200 mm) : as the reference mix. The SBR mix was designed with BetonLab [START_REF] De Larrard | BetonLab[END_REF] software. Based on compressive strength results of reference and DR mixes, aggregates properties were calibrated. Then, the w/c ratio was adjusted to have the same compressive strength as the reference. Finally, water and cement content were modified (maintaining the same w/c) to achieve the same slump. Table 2 present mixture proportions. Compressive strength results of NA and DR mixes were used to calibrate the software's aggregate properties. Then, the w/c ratio and cement paste content were adjusted to achieve the same strength and workability as reference concrete. Table 2 present ::::::: presents :::: the mixture proportions.

Working with wet aggregates instead of fully-saturated aggregates (soaked) was adoptedthinking in : , :::::::: thinking :: of : a method that can be easily reproduced in the concrete plant from the partner company of the project. The aggregates (sand, NA and RCA) were pre-wetted for one hour using a soaker hose and then covered with a plastic sheet for three hours. The aggregates (sand, NA and RCA) were pre-wetted for one hour using a soaker hose and then covered with a plastic sheet for three hours. Before casting, moisture content was measured, and the mix :::::: mixing : water was adjusted to reach the same slump. Additional water corrections were done due to variations in initial mixes. More details are given in previous works [START_REF] Fernandes | Spalling behaviour of concrete made with recycled concrete aggregates[END_REF][START_REF] Fernandes | Fire behaviour, spalling and residual durability of concrete made with recycled concrete aggregates[END_REF].

The mixing procedure was the same for all mixes. First, aggregates, cement and filler were mixed for 2 minutes. Then, water and SP were added and mixed for two minutes. The slump was measured following NF EN 12350-2 [START_REF]Comité Européen de Normalisation: NF EN 12350-2: Testing Fresh Concrete -Part 2 : Slump Test[END_REF]. Concrete with NA presented slump of 205 mm, RCA-100-DR was 208 mm and RCA-100-SBR was 174 mm. Cylindrical specimens of 11 cm x 22 cm and prisms of 7 cm x 7 cm x 28 cm were cast. The samples for mechanical characterization at 28 days were kept submerged in water at 20 • C. The remaining experimental samples were kept submerged for seven days and then placed into sealed plastic containers until test age: approximately 6 months for all tests, with the exception of carbonation samples, which aged 12 months. For concrete mechanical characterization, compressive strength (NF EN 12390-3 [START_REF]Comité Européen de Normalisation: NF EN 12390-3: Testing Hardened Concrete -Part 3 : Compressive Strength of Test Specimens[END_REF]), splitting tensile strength (NF EN 12390-6 [START_REF]Comité Européen de Normalisation: NF EN 12390-6: Testing Hardened Concrete -Part 6 : Tensile Splitting Strength of Test Specimens[END_REF]) and elastic modulus(NF EN 12390-13 [START_REF]Comité Européen de Normalisation: NF EN 12390-13: Testing Hardened Concrete -Part 13 : Determination of Secant Modulus of Elasticity in Compression[END_REF]) were determined (Table 2).

Elevated temperature exposure

The heating process was done using an electric furnace of internal dimensions of 55 cm x 50 cm x 31 cm. The time-temperature parameters are controlled by an automated regulator connected to a thermocouple located inside the furnace. The furnace has electric resistance located in the back and a fan at the top to homogenize temperature properly. A low heating rate of 2 °C/min was chosen to avoid significant thermal gradients inside the concrete sample. Three temperatures were considered: 200 temperatures :::: were :::::::: selected ::::: since :::: the ::::: most ::::::: relevant ::::::::::::::: physicochemical :::::::: changes :::::

occur :::::: within :::: this :::::::::::: temperature :::::: range. ::: In :::::::: addition, ::::: after ::::::: 600 • C, :::::: most :: of :::: the ::::::: concrete :::::::: strength :::: has ::::::: already ::::: gone ::: [START_REF] Fernandes | Effect of elevated temperatures on concrete made with recycled concrete aggregates-an overview[END_REF]. ::::: The :::::: target temperature was kept for 180 min at the target temperature, then naturally cooled to room temperature inside the furnace. After cooling, samples were stored inside sealed containers with silica gel until testing.

Determination of durability procedures

Water accessible porosity

The water porosity was measured based on AFREM recommendations [START_REF]et d'essais sur les matériaux et constructions: APC-AFREM: Durabilité des Betons -Méthodes Recommandés Pour la Mesure des Granulats Associées a la durabilité[END_REF]. Three 1 /4 of 11 cm x 5 cm disks, removed from the middle of cylinders, were used for each mix and temperature. For room temperature measurements, samples were oven-dried at 80 • C until constant mass (mass variation less than 0.05 % in one day) before testing. Eq. (1) presents the equation for water porosity (η, in %) calculation.

η = m ssd -m dry m ssd -m hyd • 100 (1)
In these equations, m ssd (kg) is the saturated superficially dried mass, m dry (kg) is the dried mass, and m hyd (kg) is the hydrostatic mass.

Gas permeability

Axial gas permeability was measured using the RILEM-CEMBUREAU method [START_REF] Kollek | The determination of the permeability of concrete to oxygen by the Cembureau method-a recommendation[END_REF]26], based on the Klinkenberg approach [START_REF] Klinkenberg | The permeability of porous media to liquids and gases[END_REF]. The test was carried out at room temperature and after each heating/cooling procedure described above. Three samples ( 11 cm x 5 cm) per mix and temperature were evaluated. These disks were cut from 11 cm x 22 cm cylinders using a diamond saw. The extremities (2 cm) of the 11 cm x 22 cm cylinders were discarded to avoid any possible edge effect on the permeability measurements. Before testing, samples were oven-dried at 80 • C. Then, the curved face of the cylinders was wrapped with aluminium tape to seal the sample, avoid any leakage and induce unidirectional flow.

The sample was placed in the permeameter cell, encased by a polyurethane ring and an inflated rubber tube. Once the sample was set, the rubber tube was inflated to apply lateral pressure on the specimen. The lateral pressure applied was 0.3 MPa, determined in trial tests with different confining pressures. These trial tests were done to verify the minimum value without any gas leakage on the setup. The reduction in lateral pressure was made based on Miah et al. [START_REF] Miah | The effect of compressive loading on the residual gas permeability of concrete[END_REF], which showed that confining pressure strongly affects permeability measurements due to the closure of heat-induced microcracks. In any case, injected pressures were limited to half of the confining pressure.

The test principle consists of applying a :: an inert gas (nitrogen in this work) at constant pressure and, after stabilization, measuring the flow rate at the inlet. Three inlet gas pressure levels were used for each sample. For each pressure, the apparent permeability (K a ) was first determined according to Eq. ( 2) (based on Darcy's Law). The intrinsic permeability (K int ), which is independent of inlet pressure, was determined from the three apparent permeability measurements using the Klinkenberg correction for gas slippage [START_REF] Klinkenberg | The permeability of porous media to liquids and gases[END_REF]. For this, experimental data is fitted to the Eq. ( 3) to determine the intrinsic permeability of the material. Also, apparent permeability at 0.2 MPa, used for the performance-based approach (Section 4), was determined from the fitting.

K a = 2P atm Q i Lµ A(P i 2 -P atm 2 ) ( 2
)
K a = K int 1 + β P m (3)
In these equations, K a (m 2 ) is the apparent permeability, K int (m 2 ) is the intrinsic permeability, P i (Pa) is the inlet pressure , P atm (Pa) is the atmospheric pressure, Q i (m 3 /s) is the gas flow, L (m) is the sample thickness, µ (Pa/s) is dynamic viscosity of the inlet gas at the temperature measured during the test, A (m 2 ) is the cross-section sample area, and β (Pa) is the klinkenberg's coefficient.

Capillary water absorption

The capillary water absorption was measured based on AFREM recommendations [START_REF]et d'essais sur les matériaux et constructions: APC-AFREM: Durabilité des Betons -Méthodes Recommandés Pour la Mesure des Granulats Associées a la durabilité[END_REF]. From the 28 cm x 7 cm x 7 cm prisms, two samples of 14 cm x 7 cm x 7 cm (height x length x width) were obtained and used for each mix and each temperature level. The height of the specimen was chosen based on a previous work [START_REF] Carré | Durability of ordinary concrete after heating at high temperature[END_REF] : , which showed a high capillary rise of samples heated up to 600 • C. For room temperature measurements, samples were oven-dried at 80 • C before testing , until constant mass (mass variation less than 0.05 % in one day). The lateral surface (7 cm x 14 cm) were coated with epoxy resin to ensure one-dimensional water transport. After resin drying, one of the uncoated sides (14 cm x 14 cm) was placed inside a covered recipient with water. Small metallic supports were used to provide 3 mm of immersed depth. The mass of each sample was measured at different time intervals (from 0.25 and up to 72 hours). Capillary water absorption was calculated using Eq. ( 4).

C wa = m t -m 0 A (4)
Where C wa (kg/m 2 ) is the capillary absorption, m t (kg) is mass at time t, m 0 (kg) is initial mass and A (m 2 ) is the cross-section sample area in contact with water.

Chloride diffusion

The effective chloride diffusion coefficient was measured based on the accelerated migration test in steady-state conditions proposed by LMDC [START_REF] Truc | A new way for determining the chloride diffusion coefficient in concrete from steady state migration test[END_REF]. In this test, an electrical field is applied to a saturated concrete specimen placed between two compartments, upstream (a solution rich in chlorides) and downstream ( : a : solution without chlorides). The chloride diffusion coefficient is determined from the chloride ion flux between compartments [START_REF] Truc | A new way for determining the chloride diffusion coefficient in concrete from steady state migration test[END_REF][START_REF] Perlot | Influence of cement type on transport properties and chemical degradation: Application to nuclear waste storage[END_REF][START_REF] Rozière | A performance based approach for durability of concrete exposed to carbonation[END_REF]. For this, concrete disks of 11 cm x 5 cm were sawn from 11 cm x 22 cm cylinders. Two samples per mix and temperature were evaluated. Measurements were limited to 200 • C , since the crack state of samples after 400°C did not allowedproper :::: allow ::::::: proper : determination of the chloride coefficient with the proposed method.

After vacuum saturation with a support solution (demineralised water, 1 g/l of N aOH, and 4.65 g/l of KOH), samples were placed in the migration cell. A constant potential of 30 V was applied between the electrodes to provide an electric field (E = 600 V/m) between downstream and upstream compartments. The upstream compartment contained a solution of N aCl (12 g/l), N aOH (1 g/l), KOH (4.65 g/l) and demineralised water, while the downstream compartment was filled with the the support solution. The concentration of chloride ions in the upstream was determined by AgN O 3 titration. This measurement was done before the start of the test and at regular intervals: two times per day for five days. For the calculation, first, the ionic flux of chlorides (J, in mol/m 3 s) was determined using the slope of the curve of the variation between the cumulative quantity of chlorides leaving the upstream versus time, as shown in Eq. ( 5). Then, the effective chloride diffusion coefficient (D ef f , in m 2 /s) was calculated, following Eq. ( 6).

J = C up,1 -C up,2 t • V S (5)
D ef f = RT J C up F E (6)
Where C up,1 and C up,1 (mol/m 3 ) are the concentration of chlorides at upstream in the start and at the moment of the test, t (s) is test duration, V (m 3 ) is the volume of the upstream solution, S (m 2 ) is :::

the surface area of test specimen exposed to chloride, R (8.314 J/molK) is the perfect gas constant, T (K) is ::: the temperature during chloride test, C up (mol/m 3 ) is chloride concentration of :::

the upstream solution, F (96 487 C/mol) is Faraday constant and E (V/m) is the electric field applied.

Accelerated carbonation

The carbonation was evaluated by accelerated testing. The test protocol was based on French Standard XP P18-458 [START_REF]Essai Pour Béton Durci -Essai de Carbonatation Accélérée -de L'épaisseur de Béton Carbonaté[END_REF], with some differences described below. Concrete disks of 11 cm x 6.5 cm were sawn from 11 cm x 22 cm cylinders using a diamond saw. The pre-conditioning involved different steps. First : , samples were dried in a ventilated oven at 45 • C for 14 days. Then, samples were placed in a climatic chamber at 20 • C and 65 % HR for 14 days. For heated samples, the pre-conditioning involved an additional step. Since they were fully dried after heating, samples were immersed in water for 48 hours. This process was done to re-saturate the sample and recover the necessary humidity (usually between 50 % and 80 % [START_REF] Beushausen | Principles of the performance-based approach for concrete durability[END_REF]) for the carbonation phenomenon. Then, samples underwent the same procedure described before (drying for 14 days followed by 14 days inside the climatic chamber). Just before the test, the curved face of each cylinder was wrapped with aluminium tape to ensure a unidirectional carbonation flow.

The samples were then placed inside the carbonation chamber with 3±0.5 % of CO 2 , relative humidity of 65±5 % and temperature of 20±3 • C. It is noteworthy that the CO 2 content used was much lower than the 50 % specified in the XP P18-458 [START_REF]Essai Pour Béton Durci -Essai de Carbonatation Accélérée -de L'épaisseur de Béton Carbonaté[END_REF] standard. The value was chosen based on previous studies, which showed that 3 % is suitable and representative of long-term carbonation in natural conditions [START_REF] Auroy | Comparison between natural and accelerated carbonation (3% CO2): Impact on mineralogy, microstructure, water retention and cracking[END_REF][START_REF] Castellote | Chemical changes and phase analysis of OPC pastes carbonated at different CO2 concentrations[END_REF]. After 0, 2, 7, 14, 21 and 28 days of exposure, one disk per mix and temperature was removed and split into two parts through splitting tests. Phenolphthalein was sprayed in the fractures ::: on ::: the ::::::::: fractured surfaces, and carbonation depth was measured using a calliper :::::: caliper. Five depth measurements were taken from each side (top/bottom) of each part, and, in total, 20 values were taken per mix/condition. In addition to carbonation depth, the carbonation rate was calculated using Eq. ( 7), based on square root theory and in accord with the second law of Fick [START_REF] Sisomphon | Carbonation rates of concretes containing high volume of pozzolanic materials[END_REF].

X c = k acc √ t (7)
Where X c (mm) is the carbonation depth, k acc (mm/yr 1/2 ) is accelerated carbonation rate and t (yr) is time.

Results

Water accessible porosity

The evolution of water porosity with temperature is presented in absolute (Fig. 1a) and relative values (Fig. 1b). First, at room temperature, concrete with NA showed a porosity of 11.4 %, and the direct substitution of coarse NA by RCA resulted in a porosity increase to 16.2 %. The results and as well the increase ::: the ::: rise : in water porosity are coherent with previous works [START_REF] Rougeau | Durability-related properties[END_REF]. For the SBR mix, the porosity remains close to the DR mix, 15.7 %. This behaviour shows that water porosity, in the case of concrete made with RCA, is mainly driven by RCA porosity. The evolution of water accessible porosity with temperature was similar for all tested concrete (Fig. 1a). First, up to 200 • C, the porosity almost did not change for studied mixes. Similar behaviour has been registered in previous works that studied NA , and RCA concretes [START_REF] Laneyrie | Influence of recycled coarse aggregates on normal and high performance concrete subjected to elevated temperatures[END_REF][START_REF] Mindeguia | On the influence of aggregate nature on concrete behaviour at high temperature[END_REF]. From 200 • C to 400 • C, the porosity starts to increase significantly, and it is mainly related to intense water loss and decomposition of C-S-H [START_REF] Pimienta | Behaviour of high-performance concrete at high temperatures: some highlights[END_REF]. In the last branch, from 400 • C to 600 • C, water porosity increases again at a higher rate and is mainly associated with portlandite decomposition.

In general, concrete with NA presents the lowest water porosity values for all tested temperatures. The difference between NA and RCA is close to 3 % and 4 % at all temperatures. Both RCA mixes (DR and SBR) show similar values, execpt ::::: except : for 400°C , when :::: when :::: the difference reaches 1.8 %. This behaviour highlights again that aggregate plays a crucial role in the water accessible porosity. Reducing w/c and increasing the cement paste did not change the water porosity significantly. In terms of relative porosity evolution, concrete with NA presented higher values than the mixes with RCA. This higher rate is probably related to the higher thermal mismatch in the NA mix that promotes microcracks. The evolution observed in concrete made with RCA is lower, : probably due to the better match between new cement paste and old cement paste.

Gas permeability

Gas permeability results are presented in Fig. 2a and2b. For room temperature (first points on Fig. 2a), the direct addition of coarse RCA appears to increase the gas permeability, as observed in previous works [START_REF] Rougeau | Durability-related properties[END_REF][START_REF] Kwan | Influence of the amount of recycled coarse aggregate in concrete design and durability properties[END_REF]. However, contrary to the phenomenon observed in the water porosity, the strength-based replacement mix shows intrinsic permeability closer to concrete with NA. This result shows :::::::: indicates that the cement matrix plays a key role in permeability. It is noteworthy also that the changes in permeability were observed within the same exponent (around 10 -16 ); hence, care should be taken when interpreting the results. In any case, similar findings were observed in previous studies [START_REF] Rougeau | Durability-related properties[END_REF][START_REF] Gonçalves | Influence of recycled concrete aggregates on concrete durability[END_REF] that verified that the difference between k int in concrete with NA and RCA is lower when the matrix quality is improved. show : differential behaviour depending on the temperature level. At 200 • C, SBR and DR permeability were almost the same. This behaviour can be associated with the cracking and porosity due to water evaporation in cement paste. However, at 400 • C, the DR mix is almost four times more permeable than SBR, even though dispersion is high. Given that RCA porosity is the same, a probable reason is related to the cracking at cement paste and interfaces. The stronger cement paste in the SBR mix possibly reduces general cracking in the paste and reduces thermal mismatch between RCA and paste. At 600 • C, values approach again, with both mixes having similar behaviour. Fig. 2b presents the evolution of permeability relative values (in log scale) with temperature. The same variation observed in absolute is also shown in relative values: concrete with NA presents a steady growth in permeability, while DR and SBR mix varies depending on temperature. The graph reveals the significant increase in DR permeability between 200 • C and 400 • C, highlighting the probable damage of cement paste and interface at this temperature level. Finally, from 400 • C to 600 • C, the growth is higher in the SBR mix, probably related to the degradation in portlandite and sand quartz transformation at this temperature level [START_REF] Pimienta | Behaviour of high-performance concrete at high temperatures: some highlights[END_REF].

Capillary water absorption

The effect of RCA on the capillary water absorption is presented in Fig. 3a. This figure illustrates the evolution of water absorption with the square root of time (each curve is the average of two samples) for samples at room temperature. The RCA-100-DR presents higher absorption than the other two mixes at any given time. This behaviour is the same as reported in previous works and directly linked to the higher absorption and porosity of RCA [START_REF] Kwan | Influence of the amount of recycled coarse aggregate in concrete design and durability properties[END_REF][START_REF] Kou | Enhancing the durability properties of concrete prepared with coarse recycled aggregate[END_REF]. In the SBR mix, the changes in w/c and cement paste decreases the capillary water absorption curves: they get closer to what is observed in concrete made with NA. This improvement isrelated : is ::::::: related : to the improvement in the cement matrix of RCA-100-SBR, which presents higher compacity than RCA-100-DR. The evolution of capillary water absorption with the square root of time varies significantly depending on the temperature level ( :: an : example is given at : in : Fig. 3b). Up to 200 • C, the behaviour is quite linear, as seen in typical absorption curves. However, from 400 • C to 600 • C, the curves present a different evolution, with two distinct phases: a first one, with a higher absorption rate, followed by a flattened phase. The behaviour is similar to what was observed in previous works [START_REF] Ghasemzadeh | Effect of Damage on Moisture Transport in Concrete[END_REF][START_REF] Wang | Investigation on water absorption in concrete after subjected to compressive fatigue loading[END_REF], which evaluated the effect of damage on absorption. The cracks act as a preferential flow and provide access to air-filled pores, usually filled only later in un-damaged concrete. In the second phase, with the lower rate, the air-filled voids had already been filled, explaining the behaviour in this second period. Given this behaviour, to analyze ::::::

analyse : the results with temperature, it was chosen to evaluate the water absorbed at 72 hours as the primary indicator.

Fig. 4a presents the evolution of 72-hour capillary water absorption versus temperature. From room to 200 • C, the absorption was almost constant for all mixes, showing that the crack state is not relevant to induce higher absorption at this temperature. From 200 • C to 600 • C, a linear increase is observed, and it is due to crack development and dehydration. In general, concrete with RCA presented higher absorption than concrete made with NA. The same behaviour was verified in Kou et al. [START_REF] Kou | Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures[END_REF]. This behaviour is mainly attributed to RCA's higher porosity and mortar content, which drive the capillary water rise in these concretes. The SBR mix presented lower values than DR, showing that paste improvement reduced the capillary pores. If the RCA-100-DR presented higher absolute values, this mix presented a lower evolution (Fig. 4) in terms of relative values. NA and RCA-100-SBR showed close growth. The highest evolution of concrete made :::: with : NA could be related to the worst thermal mismatch in this type of concrete. However, this is not the case of ::

for : RCA-100-SBR, which presents an equal mismatch as RCA-100-DR. In this case, the evolution can be related to the higher deterioration of hydration products of cement paste as temperature increases.

Chloride diffusion

Fig. 5 presents the evolution of chloride diffusion coefficient (in log scale) with temperature. At room temperature, the direct addition of RCA increases almost by seven the chloride diffusion coefficient. The lower resistance of RAC to chloride penetration has been verified in previous works [START_REF] Rougeau | Durability-related properties[END_REF][START_REF] Kou | Enhancing the durability properties of concrete prepared with coarse recycled aggregate[END_REF][START_REF] Evangelista | Durability performance of concrete made with fine recycled concrete aggregates[END_REF]. It is noteworthy that RCA-100-DR presented a relevant variation in measurements (relevant error bar), but the mix presented higher diffusion than NA in all samples. The increase in chloride diffusion for SBR was lower than in the DR mix, but the average value did not return to the reference concrete level. Indeed, the chloride evolution behaves similarly to water porosity behaviour: the reduction of w/c and increase in cement paste did not overcome the higher RCA porosity. The influence of RCA porosity on chloride behaviour is also reported in some previous work [START_REF] Evangelista | Durability performance of concrete made with fine recycled concrete aggregates[END_REF][START_REF] Gomes | Structural concrete with incorporation of coarse recycled concrete and ceramic aggregates: Durability performance[END_REF].

For high temperature evolution, as explained in the methodology, the coefficient was not possible to measure after 200 • C , due to the crack state of ::: the : samples. In any case, it is seen that concrete with NA presents the lowest chloride diffusion for the two tested temperatures. However, the difference between concrete with NA and RCA mixes is lower than :: is ::::: lower : at 200 • C. From room temperature to 200 • C, both NA and RCA-100-SBR increased significantly, probably due to the development of microcracks in the material. It is noteworthy that the evolution in RCA-100-DR is much lower than in the other mixes. This is coherent with permeability and porosity evolution, which was lower for RCA-100-DR. The worst behaviour in concrete made with RCA for chloride was also reported in Ma et al. [START_REF] Ma | Effects of high temperature and cooling pattern on the chloride permeability of concrete[END_REF]. They verified that increase in RCA content increased the chloride permeability.

Accelerated carbonation

Table 3 presents an overview of all the results from accelerated carbonation tests. The carbonation rate was calculated using Fick Law's and each graph of carbonation depth versus test duration. The high r 2 of the regression curves shows that this model is appropriate even for the case of high-temperature exposure. For room temperature, the carbonation depth at 28 days of RCA-100-DR was 1.6 times greater than ::: the : reference mix. This is coherent with previous works [START_REF] Silva | Carbonation behaviour of recycled aggregate concrete[END_REF], which verified that relative carbonation depth increases with RCA due to higher porosity and absorption of RCA aggregates. Silva et al. [START_REF] Silva | Carbonation behaviour of recycled aggregate concrete[END_REF], with a statistical approach in a literature compilation, showed that RCA usually presents a carbonation depth 2.5 times higher than ordinary concrete. The adjustment of cement paste in the SBR mix improved the carbonation resistance of RAC mixes, and the depth was even slightly lower than concrete with NA. This behaviour is also verified in previous works, which adjusted the w/c ratio to obtain the same target strength [START_REF] Silva | Carbonation behaviour of recycled aggregate concrete[END_REF][START_REF] Dhir | Suitability of recycled concrete aggregate for use in bs 5328 designated mixes[END_REF]. The same behaviour was identified using the accelerated carbonation rate. The evolution was equal for NA and RCA-100-SBR, highlighting that improving cement paste can result in similar carbonation performance.

To compare the effect of temperature in each mix, carbonation rate versus temperature was plotted in Fig. 6. Fig. 6a presents the evolution of carbonation rate with temperature in absolute values. The behaviour seen in ::

at room temperature keeps similar for all tested temperatures. The direct addition of RCA increased the carbonation rate, while the SBR mix showed similar performance to concrete made with NA. After exposure to 600 • C, the rate in RCA-100-DR was almost 1.5 more elevated than in concrete made with NA. The difference in carbonation depth after 28 days was more than 8 cm higher, as seen in Table 3. The same table and figure show that concrete made with NA and RCA-100-SBR presented minor differences in rate and carbonation depth. This indicates that the paste enhancement for SBR was also helpful for carbonation performance.

For absolute values, the evolution of carbonation rate with temperature is quite similar in all studied mixes. From 23 • C to 400 • C, the evolution rate was relatively low, especially for RCA-100-DR. In this case, the carbonation rate varied from only 0.36 mm/yr 1/2 . At this temperature, the changes are related to the increase of porosity and permeability due to water loss and first C-S-H dehydration. From 400 • C to 600 • C, the increase in carbonation rate was substantial. At 600 • C, concrete suffers from deterioration due to portlandite dehydration and thermal mismatch. When analyzing the relative values (Fig. 6b), a similar behaviour to the other indicators is observed. Concrete with NA and RCA-100-SBR presents higher evolution. And, even though the increase from 400 • C to 600 • C is relevant, the growth in RCA-100-DR is lower than the other two.

Towards a performance-based approach

Different analyses were proposed to evaluate the durability performance of concrete made with RCA. They were divided into two sections concerning temperature exposure: at room temperature and after elevated temperatures. In the first case, the objective was to evaluate the effect of replacement rate on concrete durability. The second case refers to the effect of temperature on concrete durability. Two different performance-based approaches were proposed at room temperature to evaluate the durability of concrete made with RCA. The first is the equivalent performance concept, based on NF EN 206-1/CN:2014 [START_REF]Comité Européen de Normalisation: NF EN 206/CN: Béton -Spécification, Performance, Production et Conformité -Complément National à la Norme NF EN 206[END_REF] prescription. In this method, the new concrete should have an equal or better performance than the concrete designed according to the durability requirements from NF EN 206-1/CN:2014 [START_REF]Comité Européen de Normalisation: NF EN 206/CN: Béton -Spécification, Performance, Production et Conformité -Complément National à la Norme NF EN 206[END_REF]. In this project, both RCA-100-DR and RCA-100-SBR are compared to the concrete made with NA. An index of equivalent performance (EP R ) was calculated for each property. These indexes are the ratio between the property of concrete made with NA (X N A ) and concrete made with RCA (X RCA ). If the ratio is lower than 1, the concrete has deteriorated. If it is higher or equal to 1, concrete has improved or kept the performance of the reference. For permeability and chloride diffusion, the index :::: was calculated using the log values, and the final value was inversed to compatibilize :::::: match with the proposed scale. Eqs. ( 8) and [START_REF] Fernandes | Effect of elevated temperatures on concrete made with recycled concrete aggregates-an overview[END_REF] shows the index calculation. All values were plotted in a spider-plot type of graph (Fig. 7a). Similar approaches were made in previous studies [START_REF] Kaid | Durability of concrete containing a natural pozzolan as defined by a performance-based approach[END_REF][START_REF] San Nicolas | Performance-based approach to durability of concrete containing flash-calcined metakaolin as cement replacement[END_REF][START_REF] Idir | Investigations on the durability of alkaliactivated recycled glass[END_REF][START_REF] Bucher | Performance-based evaluation of flash-metakaolin as cement replacement in marine structures -Case of chloride migration and corrosion[END_REF].

EP R i = X N A X RCA (8)
EP R i = 1 log X RCA log X N A (9)
In these equations, EP R i is the equivalent performance index for ::: the replacement rate. The index i varies from 1 to 5, where 1 is water porosity, 2 is intrinsic permeability, 3 is chloride diffusion coefficient, 4 is accelerated carbonation rate, and 5 is capillary water absorption at 72-hours ::

72 ::::: hours. The second approach for room temperature evaluation was the analysis based on the potential durability established by the AFGC guide for durability [START_REF]Conception des Bétons Pour Une Durée de Vie Donnée des Ouvrages -Maîtrise de la Durabilité Vis-àvis de la Corrosion des Armatures et de L'alcali-réaction[END_REF]. Selected (water porosity, permeability and chloride) were used to classify the concrete according to five classes: very low (VL), low (L), medium (M), high (H) and very high (VH). Table 4 presents the limits associated with each one of the evaluated properties. To :::

For : a comparative analysis, the values were normalized from 0 to 1, using Eq. 10. The 0 was defined as the very low limit (X vl ). It was chosen a value lower than the threshold presented in the table (18 for water porosity; : , : 30 for chloride and 3000 for permeability). The 1 was the very high limit (X vh ) indicated in the table (in the case of water porosity, 9 was the adopted as threshold value). It is noteworthy that gas permeability and chloride diffusion were calculated also ::: also ::::::::: calculated : using base-ten logarithms. Using Eq. [START_REF] Kou | Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures[END_REF] and Table 4, the scale presented in Fig. 7b was proposed. In this figure, all the potential durability zones are indicated. Eq. [START_REF] Kou | Residue strength, water absorption and pore size distributions of recycled aggregate concrete after exposure to elevated temperatures[END_REF] was also used to calculate the potential durability indicator for each of the studied mixes.

DI R i = X i -X vl X vh -X vl (10)
In these equations, DI R i is the potential durability indicators :::::::: indicator for each mix. The index i varies from 1 to 3, where 1 is water porosity, 2 is gas permeability at P i = 0.2 MPa, and 3 is chloride diffusion coefficient. The X i correspond ::::::::::: corresponds to the property to be normalized :::::::::

normalised, X vl is ::: the very low limit and X vh is ::: the : very high limit.

For concrete after exposure to elevated temperatures, similar approaches were applied, but they were adapted considering the objective of the analysis. For the first approach, an equivalent concept was also proposed. But this time, the equivalence of each heated concrete was compared with its respective value at room temperature. In a post-fire scenario, the interest is in how much from ::

of the initial durability remains. Hence, an index of equivalent performance at high temperature (EP T ) was calculated for each property. In this case, these indexes are the ratio between the property of concrete at room temperature and concrete at high temperature :::::::::::: temperatures. The same principle of deterioration and improvement is applied. Since the chloride diffusion coefficient was determined only up to 200 • C, the analysis was done only in the four remaining parameters.

The potential durability approach following AFGC criteria was also applied for high-temperature evaluation. The method, including limits and normalization :::::::::::: normalisation : procedure, was the same as that used at room temperature. Since the chloride diffusion coefficient was determined only up to 200 • C, the analysis was done this time considering room temperature and 200 • C.

Room temperature

Fig. 8 presents the equivalent performance for the three studied mixes. When analyzing :::::::: analysing : the DR mix, it can be seen that the substitution of NA with RCA worsened all durability indicators. An improvement in the durability was achieved when optimizing ::::::::: optimising : the w/c and the cement paste content to obtain the same strength as reference concrete. Even though the water porosity remains high, due to the high porosity of the RCA, the RCA-100-SBR showed reduced deterioration in some indicators. This is especially highlighted for the accelerated carbonation rate. These results also highlight that water accessible porosity may not be the most appropriate indicator to evaluate durability. indicate : that the mix has high (H) durability, while permeability indicates that concrete has medium (M) durability. As the case of the equivalent comparison, the DR mix worsened all the performances. Water porosity and permeability were classified as very low (VL), and chloride showed medium potential durability. For the SBR mix, even though it has improved from the DR mix, this mix did not recover the same potential as the concrete made with NA. Gas permeability and chloride diffusion presented the same potential (medium). Still, water porosity was classified as low (L). It's noteworthy that, in this document, carbonation was not defined as a parameter for potential durability indicator ::::::::

indicators. Hence, if only this approach were used, the improvement in carbonation resistance would not be verified. This highlights the necessity of precaution when evaluating durability through a similar approach.

Elevated temperatures exposure

Fig. 10 presents the equivalent performance analysis for exposure to elevated temperatures. A different mix is evaluated in each plot, and a curve for each temperature level is presented. For all mixes, it is seen that up to 200 • C, the performance is close to the room temperature. This shows that the concrete can retain some initial durability if heated only up to this temperature level. After 400 • C, the deterioration gets high, and the increase is more prominent in Fig. 9: Potential durability for mixes at room temperature concrete with NA and RCA-100-SBR. This indicates that these mixes undergo higher damage up to this temperature, mainly associated with cracks due to water loss and paste hydrates deterioration. The lower evolution of RCA-100-DR can also be associated with the fact that the initial durability of this concrete is lower. Hence, the transformations at this level do not deteriorate indicators as much as the other mixes.

Fig. 11 presents the potential durability evaluation for the mixes after exposure to 200 • C. A similar trend observed in equivalent durability is also verified here: the RCA-100-DR degrades less than the other mixes. However, concrete with NA presents a better performance, even after exposure to high temperatures. The porosity keeps in the high zone potential, but permeability and chloride degrade one class: permeability enter ::::: enters : to low durability zone, and chloride enters the medium durability zone. This behaviour reinforces that this concrete can retain some of its durability properties and may still be functional after exposure to this level. For RCA-100-DR, the values kept almost the same after exposure to 200 • C. However, it's noteworthy that the durability of this mix is already low at room temperature. Lastly, for RCA-100-SBR, porosity and chloride kept similar, with both staying in the same potential after exposure to 200 • C (low and medium, respectively). However, the permeability moved from medium to low potential, indicating a reduction in the performance of this concrete. In general, this methodology seems a good approach for evaluating post-fire durability.

Conclusions

This paper presented an experimental investigation into the post-heating durability of concrete made with recycled concrete aggregates. Based on the results, the following conclusions have been drawn:

• The direct addition of RCA in DR mixes reduced the durability of ordinary strength concrete. The reduction was prominent on all durability indicators. The reduction of w/c and increase in cement paste in ::: the : SBR mix (c) RCA-100-SBR Fig. 10: Equivalent performance for mixes at after exposure to high temperature contributed to the recovery of some durability parameters. Even though water porosity kept ::: was : high for SBR mix, permeability and accelerated carbonation showed the same performance as reference concrete. The chloride diffusion coefficient and capillary water absorption were better than DR but not at the same level as concrete with NA. This behaviour highlights that the same compressive strength is not enough to guarantee the exact same durability, and these parameters should be evaluated. • Exposure to high temperatures deteriorated the durability of all studied mixes, and the extent is greater as temperature increases. In general, the relative increase is lower for the DR mix and similar in the case of the NA and SBR mix. Capillary water absorption and water porosity evolved similarly, with almost constant values up to 200 • C, followed by a significant increase at 400 exposure :: to 600 • C. Permeability showed a different behaviour depending on the temperature level -with slight variations between all mixes. For chloride, NA presented lower values than RCA mixes, but the increase was higher.

• The proposed performance-based approaches showed potential to evaluate durability indicators. The equivalent performance concept showed potential, but some indicators (e.g. water porosity) should be considered with precaution. The potential durability concept was an excellent method to evaluate the durability, including the high-temperature scenario. However, this method should be updated and extended for different indicators (accelerated carbonation, for example). • This paper shows that the addition of RCA reduces the ambient and postfire durability of concrete made with RCA. However, with proper mix optimization ::::::::::: optimisation, the durability can be recovered to some extent.

Fig. 1 :

 1 Fig. 1: Evolution of water porosity with temperature

Fig. 2 :

 2 Fig. 2: Evolution of permeability with temperature

Fig. 3 :

 3 Fig. 3: Evolution of capillary water absorption with time

Fig. 4 :

 4 Fig. 4: Evolution of capillary water absorption with temperature

Fig. 5 :

 5 Fig. 5: Evolution of chloride diffusion coefficient with temperatures

Fig. 6 :

 6 Fig. 6: Evolution of carbonation rate with temperature

Fig. 7 :

 7 Fig. 7: Scale used in performance approaches

Fig. 8 :

 8 Fig. 8: Equivalent performance for mixes at room temperature

Fig. 11 :

 11 Fig. 11: Potential durability for mixes after exposure to elevated temperatures

Table 1 :

 1 Materials properties

	Property	Cement Filler Sand	NA 4/10	NA 10/20	RCA 4/10	RCA 10/20
	Density (kg/m 3 )	3010	2700	2650	2820	2840	2570	2590
	Specific surface (cm 2 /g)	4700	4330	-	-	-	-	-
	Water absorption (%)	-	-	0.35	0.92	0.81	5.6	4.52
	LA coefficient (%)	-	-	-	16	16	30	36
	Fineness modulus	-	-	3.1	-	-	-	-

Table 2 :

 2 • C, 400 • C and 600 • C. The ::::: These Mix proportions and mechanical properties of studied mixes

	Material/Property		Mix
		NA	RCA-100-DR RCA-100-SBR
	Cement (kg/m 3 )	350	350	420
	Filler (kg/m 3 )	60	60	60
	Sand (kg/m 3 )	804.3	804.3	799.8
	NA 4/10 (kg/m 3 )	331.7	-	-
	NA 10/20 (kg/m 3 )	711.1	-	-
	RCA 4/10 (kg/m 3 )	-	302.3	291.5
	RCA 10/20 (kg/m 3 )	-	648.5	630.3
	Water (kg/m 3 )	175	175	168
	SP (% of cement)	0.9	0.9	0.9
	f c28 (MPa)	42.2 (± 0.6)	32.6 (± 0.3)	44.4 (± 0.8)
	f t28 (MPa)	4.0 (± 0.1)	3.8 (± 0.1)	4.0 (± 0.1)
	E c28 (GPa)	32.9 (± 0.5)	27.1 (± 0.2)	30.1 (± 0.3)

::::::::::::

Table 3 :

 3 Carbonation results after exposure to elevated temperature (average values and standard deviation)

	Mix	Parameters		Temperature
			23°C	200°C	400°C	600°C
		Depth at 28 days (mm)	6.42 (1.02)	8.04 (1.08)	9.84 (0.89)	12.20 (1.21)
	NA	Carbonation rate (mm/yr 1/2 )	1.14	1.49	1.79	2.51
		r 2	0.93	0.97	0.97	0.97
		Depth at 28 days (mm)	10.37 (1.85) 11.49 (0.85) 11.62 (0.81) 20.49 (1.66)
	RCA-100-DR	Carbonation rate (mm/yr 1/2 )	1.93	2.15	2.29	3.72
		r 2	0.99	0.98	0.97	0.98
		Depth at 28 days (mm)	6.24 (0.93)	6.76 (0.81)	8.90 (0.87)	13.87 (2.16)
	RCA-100-SBR	Carbonation rate (mm/yr 1/2 )	1.14	1.32	1.77	2.69
		r 2	0.96	0.97	0.95	0.97

Table 4 :

 4 Durability indicators (adapted from[START_REF]Conception des Bétons Pour Une Durée de Vie Donnée des Ouvrages -Maîtrise de la Durabilité Vis-àvis de la Corrosion des Armatures et de L'alcali-réaction[END_REF])

	Property		Classes and limit values
		Very Low	Low	Medium	High	Very High
	Water Porosity η (%)	>16	14 to 16	12 to 14	9 to 12	9 to 6
	Effective chloride coefficient D ef f (10 -12 m 2 /s)	>8	2 to 8	1 to 2	0.1 to 1	<0.1
	Gas permeability at (Pi = 0.2 MPa) kgaz (10 -18 m 2 )	>1000	300 to 10000	100 to 300	10 to 100	<10
	Compressive strength class (indicative)	-	C25 to C40	C30 to C60 C55 to C80	>C80

Acknowledgments. The authors would like to thank the support of Groupe Cassous (Guyenne Environnement and AQIO) and Groupe Garandeau.

This study was funded by Région Nouvelle-Aquitaine (project RECYFEU).

data and materials. All experimental data that support the findings of this study can be obtained from the corresponding author.


Anne-Marie Lagrange
email: anne-marie.lagrange@obs.ujf-grenoble.fr

Markus Kasper

Anthony Boccaletti

Gaël Chauvin

Damien Gratadour

Thierry Fusco

David Ehrenreich

Daniel Apai

David Mouillet

Daniel Rouan

A.-M Lagrange

Constraining the orbit of the possible companion to Beta Pictoris: New deep imaging observations

Keywords: Instrumentation, adaptive optics -stars, early-type -stars, planetary systems -stars, individual (β Pic)

ou non, émanant des établissements d'enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

Introduction

The β Pic disk of dust and gas has been regarded as the prototype of young (12 +8 -4 Myr; Zuckerman et al. 2001) planetary systems since the 1980's and has revealed over the years an impressive amount of indirect signs pointing toward the presence of at least one giant planet.

Using L ′ -band high-angular resolution imaging data obtained with the NAOS-CONICA adaptive optic system (NaCo) on the Very Large Telescope UT4, we discovered a point-like source with an apparent magnitude L ′ = 11.2 ± 0.3 mag at 0. ′′ 411 ± 0. ′′ 008 (∼ 8 au) north-east of β Pic, well aligned with the dust disk. Eventhough no second epoch data were available with a sensitivity enough to detect the companion candidate (cc), we were confident that this signal was not due to a background object as the associated probability was shown to be very low. We therefore attributed the source to a probable bound object. Given Send offprint requests to: A.-M. Lagrange ⋆ Based on observations collected at the European Southern Observatory, Chile, ESO; runs 282.C5037(A), 282.C5037(B) and 282.C5037(D). the star proper motion, a background object would lie now angularly too close to the star to be detected again and a few more years (see below) are necessary to rule out this possibility from an observational point of view.

Very interestingly, if this cc is indeed at a physical separation of 8 au, it would explain most of the β Pic system's morphological and dynamical peculiarities: the disk inner warp, its brightness asymetries, as well as the observed falling evaporating bodies (FEBs; Lagrange et al. 2009). Recently, Lecavelier des Etangs & Vidal-Madjar (2009) investigated whether the observed cc could also be responsible for the photometric variability observed in November 1981 (Lecavelier des Etangs et al. 1995) and analysed in details in a subsequent paper (Lecavelier des Etangs et al. 1997). In the latter paper, the complex photometric curve observed was explained either by a planet located in a dust-free region of the disk, or a cloud of dust passing in front of the star. In the planet scenario, an object with a radius larger than 2 Jupiter radii, located at less than 8 au and surrounded by a void of material within its Hill radius could explain both the eclipse signal and the observed higher brightness a few days around the eclipse. From their dy-namical analysis, Lecavelier des Etangs & Vidal-Madjar (2009) conclude that the cc observed in 2003 could be responsible for the 1981 eclipse provided the semi-major axis of its orbit (circular case or assuming a low eccentricity) is in the range 7.6-8.7 au, corresponding to periods in the range 15.9-19.5 years. Another longer-period orbit, dynamically compatible with the existence of an eclipse and the November 2003 data was excluded on the basis that the cc would have been detected earlier.

We note that the corresponding orbital radius, 17 au is also much larger than the one predicted by the modeling of the photometric curve. As noted by these authors and Lagrange et al. (2009), a cc on an orbit with a radius of ∼ 8 au would not be detectable in fall 2008 nor in early 2009. Lecavelier des Etangs & Vidal-Madjar (2009) furthermore predict that if the cc is responsible for the 1981 eclipse, then it should reach its maximum elongation between 2011 and 2015. Finally, we note that the data available to Lecavelier des Etangs & Vidal-Madjar (2009) did not allow them to disentangle two possible cases: a first case where the cc would have been located before quadrature in 2003, and a second case where the cc would have been located after quadrature in 2003.

Obviously, new deep imaging observations are needed to further constrain the possible orbits of the cc. A major question is the true (unprojected) separation of the cc from the star. Four Quadrant Phase Mask (4QPM) coronagraph observations performed in 2004 did not reveal any point-like source with an absolute magnitude1 in the K s band of M K s ≃ 11.5 at a projected separation > 8 au [START_REF] Boccaletti | [END_REF]. Using the COND and DUSTY models (Baraffe et al. 2003;Chabrier et al. 2000), we obtained a K s -L ′ color of 1.2 (COND) to 1.4 (DUSTY); hence a cc with L ′ = 11.2 ± 0.3 (i.e., an absolute magnitude M L ′ = 9.8 ± 0.3) would have M K s = 11.0 ± 0.3 to 11.2 ± 0.3, respectively. If located beyond 8 au, the cc seen in 2003 would then have been detected on these 4QPM data. This allows to conclude that its projected separation had decreased between 2003 and 2004.

In order to confirm the companionship and/or to further constrain the cc orbit, we obtained discretionary time to perform in January and February 2009 new high-contrast and highspatial-resolution observations of the β Pic system with NaCo (Rousset et al. 2003;Lenzen et al. 2003) at L ′ band, allowing thus a direct comparison between 2003 and 2009 data, as well as with the 2004 K s-band 4QPM data. 2 The observations were designed to detect the faint companion as close as possible to the star. A positive detection would both confirm the companionship and provide crucial constraints on the companion orbital parameters. A non-detection would provide valuable constraints on the orbit of the cc. We present these observations in Section 2, and we use these new results to constrain the possible location of the cc in Section 3.

Observations and data reduction procedures

2.1. L ′ -band observations (February 2009) 2.1.1. Observing strategies L ′ -band images of β Pic (V = 3.8, L ′ = 3.5) were obtained in February 2009 with NaCo. Two different observing strategies were adopted. The first one (run A) followed as closely as possible the observing procedure adopted in November 2003, which consisted in recording β Pic non-saturated and saturated images, followed by similar data on a comparison star, HR 2435 (also used in November 2003), whose saturated images are used to remove the stellar halo on the β Pic saturated images.

For the second strategy (runs B1 and B2), β Pic and HR 2435 images were recorded at two different de-rotator positions, to be able to use either β Pic or HR 2435 to remove the star halo (see examples of the use of the star itself observed at different rotator positions to remove the PSF halo in Kasper et al. 2007). The offset between the two rotator positions was chosen to be 30 • (run B1) or 180 • (run B2). The latter was chosen such that the VLT aperture spiders remain in the same orientations.

For all runs, the time elapsed between β Pic and HR 2435 observations was precisely calculated so that the images for both stars were recorded at similar parallactic angles, within 0.5 • , so as to remove as accurately as possible the PSF wings.

Instrumental set-up

The visible wavefront sensor was used with the 14 × 14 lenslet array, together with the visible dichroic. We used a set-up similar to the one used for the November 2003 observations: CONICA L27 camera, which provides a pixel scale of ∼ 27 mas; note, however, that the CONICA detector was changed between both observing runs. Saturated images of β Pic were recorded, with detector integration times (DITs) of 0.2 s and number of detector integrations (NDIT) of 150. For the saturated images, we used a four-position dithering pattern every two DIT×NDIT exposures. This allows accurate sky and instrumental background removal.

Non-saturated images were also recorded to get images of the stellar point spread function (PSF) as well as a photometric calibration. In such case, we added the Long Neutral Density filter (transmission ∼ 0.018) in the CONICA optical path, and recorded images with DITs of 0.2s. Finally, twilight flat fields were recorded as well.

The log of the observations is reported in Table 1, as well as the observing conditions. Noticeably the observing conditions were not as good as during the November 2003 run, with coherent energies (estimated in the K band) of 40-50% for run A, and 25-30% for runs B1 and B2, instead of 50-70% in 2003, and coherent times τ 0 between 2.8 and 6.2 ms, instead of 20 ms in the best data sets in November 2003. The conditions during run B1 and even more during run B2 were rather poor, resulting in an unstable and mediocre adaptive optics (AO) correction.

Data processing

The data were reduced using different methods. The first two methods are described in Lagrange et al. (2009). A third methods, which takes profit of the different rotator positions in order to remove the β Pic PSF halo consists in the following steps. The initial steps of the image processing are standard (flat fielding, bad-pixel cleaning, sky subtraction). As in the previous methods, the sky for each exposure was determined using the exposure of the next adjacent dithering position. Hence, the sky measure- 1 Range of parallactic angles at the start/end of the observation. 2 The average coherent energy (K-band) as estimated on-line by AO during saturated exposures. 3 The average coherence time as estimated on-line by AO during saturated exposures.

ment for each exposure is never older than about half a minute. Then, the images were superimposed with sub-pixel accuracy by a cross-correlation technique and added up, flux normalized to unit peak intensity using the flux in the unsaturated Airy rings and high-pass filtered. The high-pass filtering was done by subtracting from the image its median filtered (≈ 3 full-width-athalf-maximum box-width) version. This procedure efficiently eliminates the large-scale structures while leaving point sources (such as substellar companions) relatively unaffected. The highpass filtering reduces the PSF peak intensity by only about 15%. Finally, images taken at different roll angles of the instrument were superimposed and subtracted from each other in order to calibrate residual speckle noise.

K s -band observations (January 2009)

Observing strategy and instrumental set-up

As the brightness ratio of the cc in K s is expected to be larger than in L ′ , a dedicated observing procedure was considered. Coronagraphic images of β Pic were recorded in the K s band with the 4QPM installed in 2007 in the NaCo focal wheel [START_REF] Boccaletti | the Spirit of Bernard Lyot: The Direct Detection of Planets and Circumstellar Disks in the 21st Century[END_REF]) in combination with the Full Undersized pupil stop (10% undersizing of the UT pupil). Additionally, a reference star was observed and used to calibrate the residual speckle halo (see below). Two series of exposures with DIT = 0.3 s and NDIT = 200 was obtained at 4 different rotator positions: 0, 15, 30 and 45 • clockwise, totalizing 480 s. Photometric references obtained on out-of-mask unsaturated images (DIT = 0.8 s, NDIT = 20) are corrected for the Neutral Density transmission (1/90) and the difference of pupil stop (Boccaletti et al. 2008). The same observing procedure was repeated on the reference star HR 2435. However, the parallactic angles differs by 0.5, 1.4, 3.5 and 3.3 • at each rotator angle. Differences of more than 0.5 • are not well adapted to our needs.

For these K s -band observations, the visible wavefront sensor again was used with the 14 × 14 lenslet array, together with the visible dichroic. We used the CONICA S13 camera, which provides a pixel scale of 13.25 mas. The DCS detector mode and readout modes were respectively set to 'HighDynamic' and 'Double'.

Data processing

After standard flat-field correction and bad pixels removal, the 400 images (DIT = 0.3 s) obtained at each rotator angle were co-added resulting in 8 images of 120 s for the target star and similarly for the reference star. The stability of the stellar image behind the 4QPM was sufficient and did not cause significant variations of the coronagraphic attenuation, so image selection was not necessary. In addition to twilight flat field calibration, we acquired background observations with the 4QPM in the beam that we used to evaluate the 4QPM flat field alone. Correction of this flat field was made on the 120 s exposures as the subtraction of the IR background evaluated on the histogram of pixels. Images were recentered at the sub-pixel scale using the maximum of the cross-correlation map (we checked that it provides satisfying results on coronagraphic images). Then, several methods have been considered to combine this set of images.

-Method 1: Images are de-rotated and co-added, resulting in 2 images one for the target star and one for the reference star. The speckle background is averaged by a factor equal to the number of rotator angles (N rot ). An optimal subtraction is performed between the star and the reference. -Method 2: Image at rotator position i + 1 is subtracted to image at position i. The resulting subtraction is de-rotated and co-added to the previous one. The same procedure is repeated on the reference star images. Finally, the reference image is subtracted from the target image. The speckle background is here averaged by a factor N rot -1. -Method 3: The target star image is subtracted with the reference star image for each rotator angle. As in method 2, this image at position i + 1 is subtracted to image at position i.

The final image is obtained after de-rotating and co-adding each of these subtractions. Again, the speckle are averaged by a factor N rot -1. -Method 4: The last method is closer to that of Marois et al. (2006). A median map is estimated on each pixels for the target set of images as for the reference star. This median map is subtracted to each rotator position. Then, the images are derotated and co-added. As for method 1, the background is averaged by a factor of N rot .

Every time a subtraction is made between 2 images, we searched for the intensity ratio for an optimal subtraction by scanning a large range of possible ratios. This intensity ratio is then estimated in different ways (in the whole image, in a disk or in a ring) and the average is taken as the result. In principle, the methods 2 and 3 are more efficient than traditional averaging of derotated images as it is shown in the NaCo user manual. However, in the present case, N rot = 4 is too small and so the gain of these methods is negligible. The results presented in this paper are obtained with the method 4 although the performance of all the methods are not much different at an angular distance of 0. ′′ 4.

Results

Detection limits

We show in Fig. 1 the subtracted image corresponding to run A data, as well as the corresponding 6σ detection limit as obtained when using HR 2435 to remove the PSF halo. The detection limits were computed by measuring for each pixel the noise within a 5 × 5-pixel box centered on the given pixel, and determining the corresponding 6σ limits, using the non-saturated images to get a photometric scaling factor, taking into account the instrumental sets up used to record the non-saturated and saturated images.

No companion with M L ′ = 9.8, corresponding to the absolute magnitude of the cc detected in November 2003 (if bound) is detected on the data down to a separation r ≃ 11-13 pixels, i.e., about 6.5 au. Comparatively, 6σ detection limits of M L ′ = 9.8 were achieved down to r = 10-11 pixels (i.e., about 5.5 au) in the best set (set A) of the better quality November 2003 data. Taking into account the error bars associated to the measured magnitude (0.3 mag) does not significantly change these results (actually, the slope dr/dM ≃ 4 pixels mag -1 at r ≃ 12).

Much less homogeneous results were obtained with run B1 data. Again using HR 2435 to remove the PSF halo, a 6σ detection limits of M L ′ = 9.8 is achieved down to r = 12-17 pixels, depending on the position angle. This is coherent with the lower quality of the data compared to run A data. Using β Pic as a comparison did not improve the results. Finally, run B2 data did not allow us to remove properly the PSF halo, as the image quality had changed signficantly between the recording of β Pic and HR 2435 images. So, no attempt was made to derive detection limits.

Similarly, we show in Fig. 2 the results obtained in the K s band. No companion is detected in the present data which reach a typical limit of M K s = 9 at a separation of 0. ′′ 4 (8 au). An absolute magnitude M K s of 11.3, corresponding to the one expected from the cc if bound (see above) is reached further than 40-44 pixels, i.e., at about 10 au. Hence, the present K s data do not allow to test the presence of the cc at 0. ′′ 4 (or less). For comparison, the computation of the 6σ detection limit in the November 2004 4QPM data show that a cc with M K s = 11.3 would have been detected down to about 6.5-7 au.

Constraints on the physical position of the companion candidate

Constraints from available deep images

First, we remind that as expected, the non-detection of the cc in February 2009 does not bring new constraints on the background object scenario. This can be seen in Fig. 3 where we have plotted the position of the cc between 2003 and 2013, taking into account the stellar proper motion. Note that we assume, as usually done, that the cc has no proper motion). Should the object be a physical companion, our new non-detection would place strong constraints on its possible orbits; in the following we discuss this scenario.

We will assume that the M L ′ = 9.8 cc orbits within the plane of the debris disk itself; the orbit is then seen edge-on as well; we furthermore assume that its orbit is prograde relative to the disk (see, e.g., Olofsson et al. 2001), and circular. Assuming a circular orbit is justified by the fact that the models proposed so far to explain the β Pic disk peculiarities (disk assymetries, FEBs) require planets on circular or low eccentric orbits (i.e., ≤ 0.1; see a review in Lecavelier des Etangs & Vidal-Madjar 2009), and as shown by the same authors, assuming such a low eccentricity does not change significantly the dynamical results. Besides, the relevant parameters adopted for the star itself are its mass, 1.75 ±0.05 M ⊙ , and its distance 19.3 ±0.2 pc (Crifo et al. 1997).

In Fig. 4, we show the various regions of the β Pic disk that have been explored in November 2003 and/or in 2009. With only a single epoch of observation, an important part of the star immediate surroundings is not explored. The new observations, in contrast, allow to significantly increase the explored surroundings. We see, in particular, that the full 8-10 au annulus around the star has now been fully explored, i.e., a companion with a We now constrain the physical position of the cc knowing that its projected separation was 8 au in November 2003 and is less than 6.5 au in February 2009. In Fig. 5, we plot its projected separation in February 2009, assuming its projected separation was 8 au on November 2003, as a function of its orbital radius. We note that the impact of the error associated to the star mass is negligible (≤ 3%). We can see that:

-Initial configurations where the cc orbited in 2003 before quadrature (i.e., before the maximum elongation, see Fig. 7 for an illustration) are ruled out except for separations between 8 and 9.75 au. Larger separations are excluded. We note that to rule out a cc before quadrature and with a separation between 8 and 9.5 au with the L ′ data, we will need to wait until 2012-2013. -Initial configurations where the cc orbited in 2003 after quadrature lead to current positions too close to the star to November 2003) data only, the whole region "in front of" and 'behind' the star at separations smaller than 6.5 au (resp. 5.5 au) were totally un-explored. Observing at two different epochs has allowed to explore most of this region as well, thanks to the keplerian rotation.

be detectable in February 2009. However, we note that for orbits with radii between 8 and 10 au, the projected separation is about 4 au ( ≃ 8 pixels), so not far from the detection limits in case of very good atmospheric conditions.

If we now take into account the non-detection of a cc with M K s = 11.3 down to 6.5-7.0 au in the 4QPM November 2004 data, we see (Fig. 5) that (1) any cc orbiting before quadrature in 2003 would have been easily detected in 2004, (2) a cc orbiting after quadrature in 2003 with an orbital radius larger than 100 au would also have had in 2004 a separation large enough to be easily detected (even assuming the largest possible magnitude for the cc given the error bars, i.e., M K s = 11.5), (3) a cc orbiting after quadrature in 2003 with an orbital radius in the range 8.5 to 30 au would definitely not have been detected in the 4QPM 2004 data, and (4) a cc orbiting after quadrature in 2003 with an orbital radius in the range 30-100 au or smaller than about 8.5 au would have had a separation in the range 6.5-7.1 au in November 2004; given the uncertainties on both the K s photometry and the uncertainty associated to the detection limits themselves, we cannot claim that the cc would have been detected in 2004.

We conclude then that the 2004 data definitely rule out any cc which would have been before quadrature in 2003, as well as cc after quadrature in 2003, with large orbital radius larger than 100 au. The data do not allow to derive conclusions for cc orbital radius in the range 8 to 30-100 au. We remind that the conclusions derived from the 4QPM data rely on K s -L ′ color estimations. Knowing the actual K s magnitude of the cc would therefore be very important.

Can the companion candidate still be responsible for the 1981 photometric eclipse?

Lecavelier des Etangs & Vidal-Madjar (2009) proposed that in order to be responsible for the 1981 eclipse, the cc observed in 2003 should have a physical separation in the range 7.6-8.7 au (see Sect. 1). As underlined, they could not at that time disentangle a cc which was before or after quadrature in 2003. There was thus a degeneracy in the solutions. We now test whether the constraints brought by the 2004 K s and 2009 L ′ images still allow the cc to be the transiting planet of 1981. The 2004 data excludes now cc which would have been before quadrature in 2003. This removes the degeneracy for the possible locations of the cc in November 2003. Assuming that the cc was at a projected separation of 8 au in November 2003, and was after quadrature, we plot in Fig. 6 the positions of the cc back in 1981. Three possible orbital radii are compatible with a projected separation of 0 au in 1981: 8.1, 10.5 and 17 au. The radii at 10.5 au corresponds to an anti-transit solution (where the cc is aligned with the star and the observer, but is located exactly behind the star), so is not acceptable. The 8.1 and 17 au orbital radii correspond to transit positions in 1981 and are therefore dynamically acceptable.

An alternative (but equivalent) approach, as adopted by Lecavelier des Etangs & Vidal-Madjar ( 2009) is to consider the projected separation of the cc in 2003 and 2009, assuming that it was transiting in 1981. In that case, we compute the projected separation assuming that at a time t = 0, the cc is transiting, where as in the previous approach, we assumed that at t = 0, the cc had a projected separation of 8 au. The results are shown in Fig. 7. We see that the projected separation of the cc in 2003 is compatible with the detected (projected) position at 8 au for semi-major axis of 8.1, 8.5, and 17.1 au. The orbits with semimajor axis of 8.1 and 8.5 au correspond to positions before and after quadrature, respectively. The before-quadrature scenario is excluded by the 2004 4QPM images (see Fig. 4, right panel). We furthermore see that with the 8.1, and 17.1 au solutions, the projected separation of the cc in February 2009 is much smaller than 6.5 au (unhatched region in top panel of Fig. 7). We conclude then that both possible semi-major axis of 8.5 and 17.1 au are compatible with the cc being responsible for the 1981 eclipse, We note that a cc with an orbital radius of 8.1 au was at the limit of detectable zone in 2004 in the 4QPM images, but as seen earlier, it is not possible from these data to definitively draw any firm conclusions on the detectability of a cc with a ≤ 30 au in 2004. We therefore regard the 8.1 au solution as a possible one. A cc with an orbital radius of 17.1 au was much too close to the star to be detected in 2004 or 2009. Lecavelier des Etangs & Vidal-Madjar (2009) proposed that a planet orbiting at 17.1 au could have been detected when it was near quadrature between 1993 and 1998, and thus excluded this solution. This is in fact not so clear, as the visible magnitude of the cc is estimated to V = 21, below the detection limits of the 1997 HST/STIS observations, that probed the surroundings of β Pic as close as 15 au for companions with V < 17 (Heap et al. 2000;Lagrange et al. 2009). Circular orbits are assumed and the cc is assumed to rotate prograde with respect to the circumstellar disk.

Expected separations of the cc in forthcoming years

Given the constraints brought by the L ′ and 4QPM images, we plot in Fig. 8 the projected separation of the cc in the forthcoming years, assuming a circular orbit. We see that if its actual orbital radius is low (between 8 and 12-15 au), the cc should be detectable again in fall 2009, under good atmospheric conditions. If we take into account only the constraints provided by L ′ -band data, we cannot exclude a cc before quadrature with an orbital radius smaller than 9.5 au; in that case (see also Fig. 8), the cc would be detectable again after 2012 only.

Summary and future prospects

New L ′ -band data were obtained in February 2009 and did not reveal the presence of the cc detected in 2003 down to 6.5 au. This non-detection does not allow to rule out a background companion. The L ′ -band data allow to exclude initial positions for the cc before quadrature and with radii larger than 9.75 au (assuming circular orbits). They, however, do not constraint the pos- sible orbits of a cc located after quadrature in 2003. 4QPM data obtained in January 2009 did not have the sensitivity to detect the cc at K s band.

Assuming a K s -L ′ color of 1.2-1.5 mag, a similar analysis made on the 2004 4QPM data allowed to exclude all initial configurations where the cc was located before quadrature. They, moreover, restrict the possible orbital radius of the cc to less than a maximum radius of ≃ 30-100 au from the star. We remind, nevertheless, that the conclusions derived from the 4QPM data rely on K s -L ′ color estimations. Finally, we have shown that if its actual orbital radius is small (between 8 and 12-15 au), the cc could be detectable at L ′ as soon as fall 2009 under very good atmospheric conditions in case it was seen after quadrature in 2003, and after 2012 in case it was seen before quadrature.

Fig. 1 .

 1 Fig. 1. Top: Residual L ′ image after subtracting β Pic by the comparison star HR 2435 (run A data). Bottom: Map of the corresponding 6σ detection limit. The contour levels represent isoabsolute magnitudes and are separated by 1 mag. The M L ′ = 12 contour level is indicated for reference. The cc, with an absolute magnitude M L ′ ≃ 9.8 is not detected in these data. The radius of the white circle corresponds to 8 au, i.e., the projected separation of the cc in November 2003.

Fig. 2 .

 2 Fig. 2. Top: Residual K s image after subtracting β Pic by the comparison star HR 2435. Bottom: Map of the corresponding 6σ detection limit. The contour levels represent iso-absolute magnitudes and are separated by 1 mag. The M K s = 10 contour level is indicated for reference. The cc with an expected absolute magnitude M K s ≃ 11.5 is not detected in these data. North is up and east is to the left. The radius of the white circle corresponds to 8 au, i.e., the projected separation of the cc in November 2003.

Fig. 3 .Fig. 4 .

 34 Fig. 3. Expected position of the source detected in November 2003 (black dot), assuming it was a background (BKG) object, taking the parallactic and stellar proper motions into account, for the 10 following years (thin curly line). Note that we assume, as usually done, that the cc has no proper motion. The β Pic debris disk midplane is indicated by the thick plain line. The thick dashed lines represent the uncertainty on the midplane angle.

Fig. 5 .

 5 Fig. 5.Projected separations of the cc in 2009 (dashed curve) and 2004 (solid curve), as a function of its orbital radius, assuming its projected separation was 8au in 2003. Top: orbital radii up to 1000 au are considered. Bottom: zoom of the same plot, with orbital radii up to 100 au. The horizontal dotted lines indicate separations corresponding to the computed 6 sigma detection limits of an M L ′ = 9.8 source in November 2003 (5.5 au ), and to an M L ′ = 9.8 source in February 2009 (6.5 au ) or a M K s =11.3 source in November 2004 (6.5 au ), and the cc projected separation in November 2003 (8 au). Circular orbits are assumed and the cc is assumed to rotate prograde with respect to the circumstellar disk.

Fig. 6 .

 6 Fig. 6. Top: Projected separation of the cc in 2004 (dashed curve) and 1981 (solid line curve), as a function of its orbital radius, assuming its projected separation was 8 au in 2003. Bottom: Projected separation of the cc in 2009 (dashed curve) and in 1981 (solid line curve), as a function of its radius, assuming its projected separation was 8 au in 2003. In both cases, we took into account the fact that the cc was located after quadrature in November 2003. Horizontal lines: same conventions as in Fig. 5. Circular orbits are assumed and the cc is assumed to rotate prograde with respect to the circumstellar disk.

Fig. 7 .

 7 Fig. 7. Top: Projected separation of the cc in 2003 (dash-dotted curve) and 2009 (solid curve) as a function of its orbital radius, assuming it was transiting β Pic in 1981. The horizontal dashdotted line represent the projected separation of the cc in 2003. Circular orbits are assumed, and the cc is assumed to rotate prograde with respect to the circumstellar disk. Bottom: Sketch of the possible locations in November 2003 and February 2009 (as seen from above) of the cc assuming it transited β Pic in 1981. The two cases a= 8.5 and a=17.1 au are represented (see text). The values in parenthesis give the fraction of the orbit that has been covered in 2003 (or 2009).

Fig. 8 .

 8 Fig. 8. Projected separation of the cc in forthcoming years as a function of its orbital radius, assuming its projected separation was 8au in 2003. Left: we assume, as suggested by the Ks-band images, that the cc was located after quadrature in November 2003. Right: projected separation if the cc was observed before quadrature in November 2003. The vertical dashed lines at 8.5 and 17.1 au indicate the possible (circular) separations if we further assume that the cc transited in 1981.

Table 1 .

 1 Observing log of the β Pic saturated images, and corresponding atmospheric conditions.

	Set Star	Date	Filter DIT	NDIT N exp π 1	EC 2	τ 0	3
					(s)		(s)	(o)	(%)	(ms)
	A	β Pic	2009-02-11 L ′	0.2	150	48	-24.2/-9.1	43.4	5.0
		HR 2435 2009-02-11 L ′	0.2/0.22 150	48	-24.7/-10.4 49.0	4.9
	B1 β Pic	2009-02-16 L ′	0.2	150	44	-22.8/-8.4	35.7	3.2
		HR 2435 2009-02-16 L ′	0.2	150	54	-22.9/-6.7	26.9	2.8
	B2 β Pic	2009-02-17 L ′	0.2	150	42	-16.8/-2.4	26.8	4.4
		HR 2435 2009-02-17 L ′	0.2	150	42	-16.9/-2.6	28.0	6.2
	D	β Pic	2009-01-20 K s	0.3	200	8	-30.9/-21.5 51.2	9.6
		HR 2435 2009-01-20 K s	0.3	200	10	-31.3/-18.2 44.2	8.7

In the following, absolute magnitudes in a photometric band i are noted M i and calculated assuming a distance of 19.3 pc. The notation i will simply refer to the apparent magnitude in this band.

Note that we additionally acquired images of β Pic using the new Sparse Aperture Masking mode offered on NaCo in November 2008. However, the experimental observing template we tested as well as the SAM capabilites do not provide constraints on the cc. They are consequently not considered here.

Acknowledgements. We would like to thank ESO staff, in particular Lowell Tacconi for his very efficient support during phase II preparation, Paranal staff for the SM observations, and Christophe Dumas for his help. We thank Z. Haiman for pointing out precious information on galaxies/QSO contamination. We acknowledge financial support from the French Programme National de Planétologie (PNP, INSU), as well as from the French Agence Nationale pour la Recherche (ANR; project NT 05 -4 44463). These results have made use of the SIMBAD database, operated at CDS, Strasbourg, France.

List of Objects

'β Pictoris' on page 1