Sonia Fornasier
email: sonia.fornasier@obspm.fr

B E Clark

E Dotto

Spectroscopic Survey of X-type Asteroids 1

Keywords: Asteroids, Surfaces, Spectroscopy, Meteorites

. The X complex in the Tholen taxonomy comprises the E, M and P classes which have very different inferred mineralogies but which are spectrally similar to each other, with featureless spectra in visible wavelengths.

, can be used to suggest new Tholen classifications for these objects. We describe 1 A-type (1122), 1 D-type (1328), 1 E-type (possibly, 3447 Burckhalter), 10 M-types (77, 92, 184, 337,

Introduction

The X complex in the Tholen taxonomy comprises the E, M and P classes which have very different inferred mineralogies but which are spectrally similar to each other and featureless in the visible wavelengths [START_REF] Tholen | Asteroid taxonomy from cluster analysis of photometry[END_REF]. By convention X-types with measured geometric albedos are classified into Tholen E-type (high albedo, p v > 0.3), M-type (medium albedo, 0.1 < p v < 0.3), or P-type (low albedo, pv < 0.1) asteroids (Tholen&Barucci, 1989). The X-type asteroid indicates an "EMP" asteroid for which we do not have albedo information.

The X-type asteroids are distributed throughout the main belt [START_REF] Mothé-Diniz | Distribution of taxonomic classes in the main belt of asteroids[END_REF]) but tend to be concentrated around 3.0 AU, and in the Hungaria region [START_REF] Warner | Analysis of the Hungaria asteroid population[END_REF]). The X complex includes bodies with very different mineralogies. M asteroids may be composed of metals such as iron and nickel and may be the progenitors of differentiated iron-nickel meteorites [START_REF] Gaffey | Spectral reflectance characteristics of the meteorite classes[END_REF][START_REF] Cloutis | Metal Silicate Mixtures: Spectral Properties and Applications to Asteroid Taxonomy[END_REF], Gaffey et al. 1993). Enstatite chondrites have also been proposed as M-type asteroid analogs [START_REF] Gaffey | Spectral reflectance characteristics of the meteorite classes[END_REF][START_REF] Vernazza | Plausible parent bodies for enstatite chondrites and mesosiderites: Implications for Lutetia's fly-by[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF][START_REF] Ockert-Bell | The composition of M-type asteroids:synthesis of spectroscopic and radar observations[END_REF]. A metallic iron interpretation requires that M asteroids are from differentiated parent bodies that were heated to at least 2000 • C to produce iron meteorites [START_REF] Taylor | Solar system evolution: a new perspective. an inquiry into the chemical composition, origin, and evolution of the solar system[END_REF]). E-type asteroids have high albedo values and their surface compositions are consistent with iron-free or iron-poor silicates such as enstatite, forsterite or feldspar. E-types are thought to be the parent bodies of the aubrite (enstatite achondrite) meteorites [START_REF] Gaffey | Reflectance spectroscopy and aster oids surface mineralogy[END_REF][START_REF] Gaffey | Relationship of E-type Apollo asteroi ds 3103 (1982 BB) to the enstatite achondrite meteorites and Hungaria asteroids[END_REF][START_REF] Zellner | The E asteroids and the origin of the enstatite achondrites[END_REF], Clark et al. 2004a[START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF]). P-type asteroids have low albedo values, and featureless red spectra. P-types are located mainly in the outer region of the main belt [START_REF] Dahlgren | A study of Hilda asteroids. I. CCD spectroscopy of Hilda asteroids[END_REF][START_REF] Dahlgren | A study of Hilda asteroids. II. Compositional implications from optical spectroscopy[END_REF], Gil-Hutton & Lican-dro 2010) and in the Jupiter Trojan clouds [START_REF] Fornasier | Visible spectroscopic and photometric survey of L5 Trojans : investigation of dynamical families[END_REF](Fornasier et al. , 2007a)). They are probably primitive bodies, however they have no clear meteorite analogues. P-type asteroids are presumed to be similar to carbonaceous chondrites, but with a higher organic content to explain the strong red spectral slopes [START_REF] Gaffey | Reflectance spectroscopy and aster oids surface mineralogy[END_REF][START_REF] Vilas | Iron alteration minerals in the visible and near-infrared spectra of low-albedo asteroids[END_REF]).

In the last 15 years, several near-infrared spectral surveys have been devoted to the study of the X group and in particular the M and E-type asteroids, revealing that these bodies show great diversity in the infrared, even within the same class.

In addition, polarimetric measurements (Lupishko & Belskaya 1989) and radar observations [START_REF] Ostro | Asteroid 1986 DA: Radar evidence for a metallic composition[END_REF][START_REF] Ostro | Radar observations of asteroid 216 Kleopatra[END_REF][START_REF] Margot | A low-density M-type asteroid in the mainbelt[END_REF][START_REF] Magri | A radar survey of main-belt asteroids: Arecibo observations of 55 objects during 1999[END_REF][START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF][START_REF] Shepard | A radar survey of M-and X-class asteroids II. Summary and synthesis[END_REF] of selected M-type asteroids have revealed surface properties that are in some cases inconsistent with metallic iron. Moreover, spectra of M-types are not uniformly featureless as initially believed; several spectral absorption bands have been detected in the visible and near infrared ranges.

Faint absorption bands near 0.9 and 1.9 µm have been identified on the surfaces of some M-types [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF]Clark et al., 2004b;[START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF], and of some E-type asteroids (Clark et al. 2004a;[START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF]) and attributed to orthopyroxene. A very peculiar absorption band, centered around 0.49-0.50 µm, extending from about 0.41 to 0.55 µm, was found on some E-type spectra [START_REF] Burbine | The detection of troilite (FeS) on the surfaces of E-class asteroids[END_REF][START_REF] Fornasier | E-Type asteroids: Spectroscopic investigation on the 0.5µm absorption band[END_REF]Clark et al. 2004a,b;Fornasier et al., 2007b[START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF], and attributed to the presence of sulfides such as troilite or oldhamite [START_REF] Burbine | The detection of troilite (FeS) on the surfaces of E-class asteroids[END_REF](Burbine et al., , 2002a)). [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF] detected weak features tentatively attributed to pyroxenes (at 0.51 µm) and oxidized or aqueously altered mafic silicates (at 0.62 and 0.7 µm) in the spectra of 2 M-type asteroids.

Of note is the evidence for hydrated minerals on the surfaces of some E and M-type asteroids [START_REF] Jones | The composition and origin of the C, P, and D asteroids: Water as a tracer of thermal evolution in the outer belt[END_REF][START_REF] Rivkin | 3-micron spectrophotometric survey of M-and E-class asteroids[END_REF][START_REF] Rivkin | The nature of M-class asteroids from 3-m observations[END_REF][START_REF] Rivkin | Hydrated minerals on asteroids: The astronomical record[END_REF], inferred from the identification of an absorption feature around 3 µm. This feature is typically attributed to the first overtone of H 2 O and to OH vibrational fundamentals in hydrated silicates. If the origin of the 3 µm band on E and M-type asteroids is actually due to hydrated minerals, then these objects are not all igneous as previously believed and the thermal scenario in the inner main belt would need revision. [START_REF] Gaffey | Mineralogy of Asteroids[END_REF] propose alternative explanations for the 3 µm band: materials normally considered to be anhydrous containing structural OH; troilite, an anhydrous mineral that shows a feature at 3 µm, or xenolithic hydrous meteorite components on asteroid surfaces from impacts and solar wind implanted H. [START_REF] Rivkin | Hydrated minerals on asteroids: The astronomical record[END_REF] provide arguments refuting the Gaffey suggestions, and provide evidence in support of the hydrated mineral interpretation of the observations. The X complex is consistently identified in the two taxonomies recently developed based on CCD spectra acquired in the visible [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic SurveyA Feature-Based Taxonomy[END_REF] or in the visible and near infrared range [START_REF] Demeo | An extension of the Bus asteroid taxonomy into the near-infrared[END_REF], even though there is not a direct one-to-one correspondence between the Bus and Tholen classes. For instance 92 Undina (Tholen M-type), and 65 Cybele (Tholen P-type) are both classified as Xc-type in the Bus taxonomy. A description of the X complex classes (X, Xk, Xe, Xc) in the [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic SurveyA Feature-Based Taxonomy[END_REF] classification is summarized in Clark et al. (2004b), while in section 6 we describe the X complex classes in the Bus-DeMeo taxonomy.

Aiming to constrain and understand the composition of the asteroids belong-ing to the mysterious X group, we have carried out a spectroscopic survey of these bodies in the visible and near infrared range at the 3.5m Telescopio Nazionale Galileo (TNG), in La Palma, Spain, at the 3.5m New Technology Telescope (NTT) of the European Southern Observatory, in La Silla, Chile, and at the Mauna Kea Observatory 3.0 m NASA Infrared Telescope Facility (IRTF) in Hawaii, USA. Results on asteroids classified as E and M-type following the Tholen taxonomy are presented elsewhere [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF]. In this paper we present new VIS-NIR spectra of 24 asteroids belonging to the X type as defined by [START_REF] Tholen | Asteroids taxonomy[END_REF], that is an "E-M-P" type asteroid for which albedo information was not available at the time of their classification. To constrain their compositions, we conduct a search for meteorite analogues using the RELAB database, and we model the asteroid surface composition with geographical mixtures of selected minerals when a meteorite match is not satisfactory. In addition, we present an analysis of X complex spectral slope values and class distributions in the asteroid main belt, where we include previously published observations of M and E-type asteroids obtained during the same survey [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF].

Observations and Data Reduction

[HERE TABLE 1 ]

The data presented in this work were mainly obtained during 2 runs (Febru- 1). We had a total of 11 observing nights. Here we present the results of our observations of 24 X-type asteroids: for 16 objects we obtained new visible and near-infrared wavelength spectra (VIS-NIR), for one object we obtained only a near-infrared spectrum (NIR), and for 7 objects we obtained only visible range spectra (VIS).

The instrument setups are the same as used in [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF].

We refer the reader to those papers for complete descriptions of observations and data reduction techniques. The spectra of the observed asteroids, all normalized at 0.55 µm, are shown in Figs. 1234, and the observational conditions are reported in Table 1. [Here Figs 1,2,3,and 4] To analyze the data, spectral slope values were calculated with linear fits to different wavelength regions: S cont is the spectral slope in the whole range observed for each asteroid, S UV in the 0.49-0.55 µm range, S V IS is the slope in the 0.55-0.80 µm range, S NIR1 is the slope in the 1.1-1.6 µm range, and S NIR2 is the slope in the 1.7-2.4 µm range. Values are reported in Table 2. Band centers and depths were calculated for each asteroid showing an absorption feature, following the [START_REF] Gaffey | Mineralogical variations within the S-type asteroids class[END_REF] method. First, a linear continuum was fitted at the edges of the band, that is at the points on the spectrum outside the absorption band being characterized. Then the asteroid spectrum was divided by the linear continuum and the region of the band was fitted with a polynomial of order 2 or more. The band center was then calculated as the position where the minimum of the spectral reflectance curve occurs, and the band depth as the minimum in the ratio of the spectral reflectance curve to the fitted continuum (see Table 3).

[HERE TABLE 2]

[HERE TABLE 3]

Spectral Analysis and Absorption Features

The observed asteroids show very different spectral behaviors (Figs 1234).

Eleven asteroids show at least one absorption feature (Table 3). 50 Virginia (see Fig. 1 and Table 3) shows spectral features centered at 0.43, 0.69, and 0.87 µm (Table 3) associated with aqueous alteration products [START_REF] Vilas | Iron alteration minerals in the visible and near-infrared spectra of low-albedo asteroids[END_REF][START_REF] Fornasier | Spectroscopic comparison of aqueous altered asteroids with CM2 carbonaceous chondrite meteorites[END_REF], and seems to be a typical 'hydrated' C-type asteroid.

Five asteroids (50 Virginia, 283 Emma, 337 Devosa, 517 Edith, and 1355 Magoeba) show a weak band at 0.43 µm. For 50 Virginia, 283 Emma, and 517 Edith, the band might be associated with an Fe 3+ spin-forbidden transition in the iron sulfate jarosite, as suggested by [START_REF] Vilas | Ferric iron in primitive asteroids -A 0.43-µm absorption feature[END_REF] for low-albedo asteroids. For 337 Devosa and 1355 Magoeba, the band might be associated with chlorites and Mgrich serpentines, as suggested by [START_REF] King | Spectral characteristics of chlorites and Mgserpentines using high resolution reflectance spectroscopy[END_REF] for enstatite chondrites, or to clinopyroxenes such us pigeonite or augite as suggested by [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF] for M-asteroids.

The 1355 Magoeba spectrum also shows a band centered at ∼ 0.49 µm that resembles the absorption seen on some E-type (subclass EII) asteroids [START_REF] Fornasier | E-Type asteroids: Spectroscopic investigation on the 0.5µm absorption band[END_REF]Fornasier et al. 2007b;2008, Clark et al. 2004a)), where it is attributed to sulfides such as oldhamite and/or troilite [START_REF] Burbine | The detection of troilite (FeS) on the surfaces of E-class asteroids[END_REF](Burbine et al. , 2002a(Burbine et al. , 2002b)). Nevertheless, the estimated albedo of 1355 Magoeba has a moderate value (0.267±0.095, [START_REF] Gil-Hutton | Polarimetric observations of Hungaria asteroids[END_REF]) more consistent with a Tholen M-type classification.

92 Undina has a very weak band centered at ∼ 0.51 µm that is similar to the Fe 2+ spin-forbidden crystal field transitions in terrestrial and lunar pyroxenes [START_REF] Burns | Spectral evidence for Cr 3+ , Ti 3+ , and Fe 2+ rather than Cr 2+ , and Fe31 in lunar ferromagnesian silicates[END_REF][START_REF] Adams | Interpretation of visible and near-infrared reflectance spectra of pyroxenes other rock-forming minerals[END_REF][START_REF] Hazen | Effects of compositional variation on absorption spectra of lunar pyroxenes[END_REF]. This band has been previously detected by [START_REF] Busarev | Spectral Features of M-Asteroids: 75 Eurydike and 201 Penelope[END_REF] in the spectra of two M asteroids, 75 Eurydike and 201 Penelope.

An absorption feature has been identified in the 0.9 µm region in the spectra of 4 X-type asteroids that have albedos placing them in the Tholen M-class (92 Undina, 337 Devosa, 417 Suevia, and 1124 Stroobantia). The band center ranges from 0.86 µm to 0.92 µm with a band depth (as compared to the continuum) of 3-6 %. Also 522 Helga, having a low albedo (4%) consistent with the Tholen P-type classification, exhibits a faint band centered at 0.94 µm. This band was previously reported by several authors in the spectra of Tholen M, E and X-type asteroids [START_REF] Hardersen | Near-IR spectral evidence for the presence of iron-poor orthopyroxenes on the surfaces of six M-type asteroids[END_REF]Clark et al. 2004a, 2004b[START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF], 2010) and is attributed to low-Fe, low-Ca orthopyroxene minerals.

A peculiar feature centered at 1.6 µm was found on the spectrum of 758 Mancunia. This band resembles one seen on 755 Quintilia [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF], Feiber-Beyer et al. 2006) whose interpretation is still unknown. No bands due to silicates in the 0.9 and 1.9 µm regions have been identified in our spectra, except for a change in slope at around 0.77 µm.

Finally, 1122 Nieth shows a strong 0.96 µm absorption band (depth of 24%) and a steep slope in the NIR region, typical of olivine-rich bodies.

Several asteroids of this survey were observed previously by other authors.

Five X-type asteroids (50 Virginia,283 Emma,337 Devosa,517 Edith,and 909 Ulla) of the Clark et al. (2004b) survey of the X complex were also studied in this work. The spectral behavior for these objects looks quite similar, except for 283 Emma (our spectrum is flat in the NIR, while the Clark et al. spectrum has a concave shape with a higher spectral gradient), and 909 Ulla (our spectrum has a higher spectral gradient as compared to the [START_REF] Birlan | Spectral properties of nine M-type asteroids[END_REF] and the Clark et al. (2004b) spectra). [START_REF] Ockert-Bell | Observations of X/M asteroids across multiple wavelengths[END_REF], 2010) presented near-infrared spectra of 77 Frigga and 758 Mancunia, that look slightly different compared to ours. In particular, they detect faint spectral bands in the 0.9 and 1.9 µm region for 758 Mancunia, and in the 0.9 µm region for 77 Frigga, bands that are not detected in our spectra. 1355 Magoeba and 3447 Burckhalter were observed in the visible range by [START_REF] Carvano | Spectroscopic Survey of the Hungaria and Phocaea Dynamical Groups[END_REF], and their data look very similar to ours.

In sum, comparing our spectra with those in the existing literature, we suggest that asteroids 77 Frigga, 283 Emma, 909 Ulla, and 758 Mancunia may display surface variability as they show different spectral behaviors throughout independent observations. For 758 Mancunia this variability is confirmed also by radar measurements that show the radar cross-section and the polarization ratio to vary considerably with rotation phase [START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF]. For the other 3 asteroids, we cannot exclude that some spectral differences may be linked with unresolved observational differences (such as background stars, or undetected troubles in removing the atmosphere).

Tholen Taxonomic Classification

Our targets were classified as belonging to the Tholen X class on the basis of their spectra from 0.4 to 1 µm and because their albedos were not known at the time of classification. Since the original X-type classification in the Tholen taxonomy, the albedo value has become available for most of the asteroids we observed [START_REF] Tedesco | The supplemental IRAS minor planet survey[END_REF]. Taking into account this important information, together with the visible and near infrared (when available) spectral behavior, we suggest a re-classification of the X-type asteroids in our sample following the Tholen-classification scheme. The suggested new classifications are reported in Table 2.

1122 Nieth (Fig. 4), initially classified as X-type in the Tholen taxonomy, has atypical spectral properties that diverge from all other asteroids. The near infrared spectrum of Nieth clearly shows a strong 0.96 µm absorption band and a steep slope in the NIR region. We infer that this asteroid belongs to the olivine-rich A-types (used in the Tholen, Bus, and Bus-DeMeo taxonomic systems).

1328 Devota has a very steep featureless red spectrum. Considering its low albedo value (4%), Devota falls within the D class in the Tholen taxonomy (Fig. 4).

We suggest that the ten asteroids (77 Frigga, 92 Undina, 184 Dejopeja, 337 Devosa, 417 Suevia, 741 Botolphia, 758 Mancunia, 1124Stroobantia, 1146Biarmia, and 1355 Magoeba) with moderate albedo values (0.1-0.3) are M-type asteroids while those with albedos lower than 10% fall in the C/P classes. To discriminate between C and P-type asteroids, we analyzed spectral slope values in the visible range (S V IS from 0.55 to 0.80 µm) and searched for the UV drop-off below ∼ 0.5 µm that is typical of C-type asteroids. Comparing the S V IS slopes with the S UV slopes (calculated from 0.49 to 0.55 µm) of the low albedo asteroids, we find two distinct distributions correlated with the C and P-classes. The 6 asteroids (50, 220, 223, 283, 536, and 517) having the UV absorption and S vis < 2%/10 3 Å belong to the C-type, while the 4 asteroids (275, 463, 522, and 909) having 3 < S vis < 6%/10 3 Å belong to the P-type. 1902 Shaposhnikov, observed only in the near infrared range, is probably also a P-type, while 3447 Burckhalter, for which the albedo value is 0.336±0.164 [START_REF] Gil-Hutton | Polarimetric observations of Hungaria asteroids[END_REF]) could possibly be an E-type.

Meteorite Spectral Matches and Mixing Models

Meteorite Analogues

To constrain the possible mineralogies of our asteroids, we conducted a search for meteorite and/or mineral spectral matches. We sought matches only for objects observed across the entire VIS-NIR wavelength range. A complete description of the search methodology can be found in [START_REF] Clark | Spectroscopy of B-Type Asteroids: Subgroups and Meteorite Analogs[END_REF] and [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF]. We used the publicly available RELAB spectrum library [START_REF] Pieters | Strength of mineral absorption features in the transmitted component of near-infrared reflected light: First results from RELAB[END_REF], which consisted of nearly 15,000 spectra in November of 2008. RELAB spectra were normalized to 1.0 at 0.55 µm, and then a Chi-squared value was calculated for each RELAB spectrum relative to the normalized input asteroid spectrum. The RELAB spectra were sorted according to Chi-squared values, and then visually examined for dynamic weighting of spectral features by the spectroscopist. Given similar Chi-squared values, a match that mimicked spectral features was preferred over a match that did not. We visually examined the top 200 Chi-squared matches for each asteroid and we constrain the search for analogues to those laboratory spectra with brightness (reflectance at 0.55 µm) roughly comparable to the albedo of the asteroid. It must be noted that the asteroid data are disk integrated, which could mask spectral variations due to differences in particle size, mineralogy, or abundance. However the meteorite and mineral spectra are well characterized powders measured under well defined laboratory conditions. It should also be noted that an asteroid's albedo is defined at phase angle zero, whereas meteorites' reflectance is taken at phase angle > 5 o . For the asteroid albedo value, we used the IRAS albedo published by [START_REF] Tedesco | The supplemental IRAS minor planet survey[END_REF]. For the darker asteroids (below 10% albedo), this is a fairly well-constrained value. The uncertainty for brighter asteroid albedos (above 10% albedo) is relatively larger. To account for this, we filtered out lab spectra if brightness differed by more than ± 3% in absolute albedo for the darker asteroids (albedos less than 10%), and we filtered out lab spectra if brightness differed by more than ± 5% in absolute albedo for the brighter asteroids (albedos greater than 10%). For example, if the asteroid' s albedo was 10%, we searched over all lab spectra with 0.55 µm reflectance between 5% and 15%. Once the brightness filter was applied, all materials were normalized before comparison by least-squares.

Our search techniques effectively emphasized the spectral characteristics of brightness and shape, and de-emphasized minor absorption bands and other parametric characteristics. As such, we suggest that our methods are complementary to band parameter studies.

[HERE TABLE 4] [HERE FIGURES 5 and 6]

The best matches between the observed X-type asteroids and meteorites from the RELAB database are reported in Table 4. We matched only the 15 asteroids observed both in the visible and near infrared range that have published albedo values. Some meteorite matches are quite good, while for some asteroids the best meteorite match we found does not satisfactorily reproduce both the visual and near infrared spectral behavior. In particular our attempt to find a meteorite or mineral match failed for the 3 asteroids 1122 Nieth (A-type), 1328 Devota (Dtype), and 1355 Magoeba (M-type).

The best matches we found for 7 moderate albedo M-type asteroids are presented in Figure 5. Two asteroids, 741 Botolphia and 1146 Biarmia were not compared with RELAB spectra due to the lack of near-infrared spectral data. We find that iron or pallasite meteorites are the best matches both for the featureless M-type asteroids and for those having minor absorption bands. This result supports the link between M-type asteroids and iron or pallasite meteorites suggested in the literature by several authors. In this small sample, no enstatite chondrites, which have also been suggested to be linked with M-type asteroids, have been found as the best meteorite analogue.

The strongest analogy we found is between 758 Mancunia and a sample of the iron meteorite Landes which has a large number of silicate inclusions in a nickel-iron matrix. It must be noted that no specific grain-size sampling filter was applied, and of concern is the fact that Mancunia's best match is with a spectrum obtained of the cut slab surface of the Landes iron meteorite.

758 Mancunia has a radar albedo of 0.55±0.14 [START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF]) one of the highest values measured for the asteroids, suggesting a very high metal content. [START_REF] Shepard | A radar survey of M-and X-class asteroids[END_REF] also found variations in the radar cross-section and polarization ratio with Mancunia's rotational phase, variations that are not related to shape but to the regolith depth, porosity, or near-surface roughness. The [START_REF] Ockert-Bell | The composition of M-type asteroids:synthesis of spectroscopic and radar observations[END_REF] Mancunia spectrum shows absorption bands in the 0.9 and 1.9 µm region, and is found to be similar to the ordinary chondrite Paragould.

Our spectrum does not show these features, but a change of slope at 0.77 µm and a faint (1.3% depth) feature centered at 1.6 µm, similar to that observed on 755 Quintilia [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF][START_REF] Fieber-Beyer | Near-Infrared Spectroscopic Analysis of Mainbelt M-Asteroid 755 Quintilla[END_REF]), whose origin is not yet understood. Considering both radar and spectral observations, 758 Mancunia could present an heterogeneous surface composition, with a high metal content but also the presence of some silicates that are not uniformly distributed on the surface.

The Landes iron meteorite sample was found to be the best spectral match also for 337 Devosa, even if the meteorite does not fully reproduced the 0.88 µm absorption band of the asteroid. A similar problem occurred for 417 Suevia and 1124 Stroobantia, where different grain sized samples of the iron meteorite DRP78007 are found to be the best spectral matches, but they did not reproduce the asteroids absorption bands in the 0.9 µm region. We note that grain size effects can play an important role in the spectral behavior of iron-nickel meteorites.

In fact [START_REF] Britt | Bidirectional reflectance properties of ironnickel meteorites[END_REF] found that the spectra of M-type asteroids show good agreement with those of iron meteorites with surface features in the range of 10 µm to 1 mm, that is larger that the wavelength of incident light. Meteorites with these roughness values are diffuse reflectors and show the classic red slope continuum of iron, with practically no geometric dependence on reflection. On the other hand, a decreasing of the meteorite surface roughness changes the reflectance characteristics: complex scattering behavior is seen for roughness in the 0.7-10 µm range, while for roughness values < 0.7 µm the reflectance is characterized by two distinct components, the specular one which is bright and red sloped, and the nonspecular one which is dark and flat.

For 77 Frigga we were not able to find any meteoritic/mineral analog within the medium albedo range that could reproduce Frigga's red visible slope and flat near-infrared slope. The best spectral match is shown in Fig. 5. The iron meteorite Chulafinnee models the spectral behavior below 0.55 µm very well, and the albedo values are similar, however the change in spectral slope around 0.9 µm is not reproduced.

For 92 Undina we propose two different meteorites: the best spectral match is with a metal-rich powder sample of the pallasite meteorite Esquel, whose albedo of 0.14 is lower than the 0.25 value of the asteroid. The iron meteorite Babb's Mill was a better numerical fit (minimum chi squared fit) and had a more comparable albedo, but was not favored because Babb's Mill has a near infrared spectrum that is much flatter than that of Undina.

In Figure 6 we present the best matches proposed for six low albedo asteroids. It has long been noticed that reflectance spectra of carbonaceous chondrites are similar to those of the low-albedo asteroids [START_REF] Gaffey | Asteroid surface materials -Mineralogical characterizations from reflectance spectra[END_REF][START_REF] Hiroi | Thermal metamorphism of the C, G, B, and F asteroids seen from the 0.7 micron, 3 micron and UV absorption strengths in comparison with carbonaceous chondrites[END_REF]. In particular, CM meteorites show features due to aqueous alteration processes and are linked to low albedo aqueous altered asteroids like the C and G-types (see [START_REF] Vilas | Iron alteration minerals in the visible and near-infrared spectra of low-albedo asteroids[END_REF][START_REF] Burbine | Could G-class asteroids be the parent bodies of the CM chondrites?[END_REF][START_REF] Fornasier | Spectroscopic comparison of aqueous altered asteroids with CM2 carbonaceous chondrite meteorites[END_REF]). Indeed, all the observed low albedo asteroids are best fit with CM meteorites, either unaltered (50 Virginia and 283 Emma) or altered by heating episodes (275 Sapientia and 517 Edith) or laser irradiation (522 Helga and 909 Ulla) (see Fig. 6).

The CM2 MET00639 meteorite matches very well the visible range spectrum of 50 Virginia and in particular the 0.7 µm band attributed to Fe 2+ → Fe 3+ charge transfer absorptions in phyllosilicate minerals. Nevertheless, the meteorite does not well reproduce the asteroid's near-infrared spectrum. The same meteorite was found as a best match for the near infrared region of 283 Emma, but it does not well reproduce the spectrum in the visible range, where the asteroid is featureless.

The absence of the 0.7 µm band in the asteroid spectrum led us to discard this as a spectral match.

The P-type asteroids 275 Sapientia and the C-type 517 Edith have as best match two different samples of the Murchison carbonaceous chondrite heated at 600 o C (grain size < 63 µm) and 700 o C (63 µm < grain size < 125 µm), respectively. It must be noted that at these temperatures the phyllosilicates are dehydrated and transformed into olivine and pyroxene and that the 3 µm hydratation band vanishes [START_REF] Hiroi | Thermal metamorphism of the C, G, B, and F asteroids seen from the 0.7 micron, 3 micron and UV absorption strengths in comparison with carbonaceous chondrites[END_REF]. Our spectral match may indicate that Sapientia and Edith have no hydrated silicates on their surfaces. If hydrated silicates were originally present, the asteroids may have experienced important thermal episodes that dehydrated them.

The P-type asteroids 522 Helga and 909 Ulla are reproduced by a sample of the CM meteorite Migei after laser irradiation. This meteorite is composed of a black matrix with olivine-rich chondrules, olivine aggregates and individual grains, carbonates, and sulfides [START_REF] Moroz | Reflectance Spectra of CM2 Chondrite Mighei Irradiated with Pulsed Laser and Implications for Low-Albedo Asteroids and Martian Moons[END_REF]. Its unaltered spectrum is dark and shows absorption features related to hydrated silicates such as the 2.7-3 µm band, the UV-falloff and a 0.75 µm band. Once irradiated, the meteorite is dehydrated, the absorption bands are significantly weakened [START_REF] Moroz | Reflectance Spectra of CM2 Chondrite Mighei Irradiated with Pulsed Laser and Implications for Low-Albedo Asteroids and Martian Moons[END_REF]), and the material remains dark -most probably due to abundant submicron inclusions of Fe-rich phases finely dispersed in the glassy mesostasis [START_REF] Shingareva | Mineralogy and Petrology of Laser Irradiated Carbonaceous Chondrite Mighei[END_REF].

Because laser irradiation is a laboratory technique for simulating micrometeorite bombardment, the spectral match between Helga and Ulla and a laser irradiated sample of CM Migei may indicate that the surfaces of these outer belt asteroids, if composed of CM carbonaceous-like materials, may be spectrally reddish due to micrometeoritic bombardment.

Mixing Models

[HERE FIGURE 7] [HERE TABLE 5]

To constrain the surface compositions of the investigated X-type asteroids and the materials needed to reproduce the weak 0.9 µm band seen on some spectra we considered geographical (spatially segregated) mixtures of several terrestrial and meteoritic materials in different grain sizes: in particular we took into account all the samples included in the US Geological Digital Spectral Library (http : //speclab.cr.usgs.gov/spectrallib.html), in the RELAB database, and in the ASTER spectral library (http : //speclib. jpl.nasa.gov), together with organics solids (e.g. kerogens by Clark et al. 1993 and[START_REF] Khare | Optical Constants of Kerogen from 0.15 to 40 micron: Comparison with Meteoritic Organics[END_REF]; Titan tholins from [START_REF] Khare | Optical constants of organic tholins produced in a simulated Titanian atmosphere -From soft X-ray to microwave frequencies[END_REF]and Triton tholins from McDonald et al. 1994) and amorphous carbon (by [START_REF] Zubko | Optical constants of cosmic carbon analogue grains -I. Simulation of clustering by a modified continuous distribution of ellipsoids[END_REF]. The synthetic spectra were compared with the asteroid spectra, using the known IRAS albedo, and the VIS-NIR spectral behavior and continuum slopes. We present these matches as non-unique examples of how geographical mixtures can be used to explain some of the variety found in X-type asteroid spectra. We do not intend to indicate that these are unique derivations of the composition of these asteroids. Such an inversion requires much more information than we currently possess (grain sizes, optical constants, endmembers present, etc.). Nevertheless, the models we present can provide a first order understanding of possible surface composition.

In Figure 7 and Table 5 we present mixing models for the 4 X-type asteroids showing the 0.9 µm band (this band is attributed to low-Fe, low-Ca orthopyroxene and is not reproduced by the meteorite analogues proposed in Table 4). We also present the low albedo asteroids 517 Edith (for which the spectral match with a heated CM meteorite is poor in the 1-2 µm region), and 1328 Devota (for which we cannot find any satisfactory meteorite analogue).

As shown in Fig. 5 and discussed in Section 5.2 we did not find a meteorite with a similar albedo value that could match the whole VIS-NIR spectral behavior of 92 Undina. However, we were able to produce a model of the surface of this M-type asteroid with a geographical mixture of 99% of pallasite Esquel and 1% orthopyroxene. This mixture model reproduces the spectral behavior of Undina, however its albedo is much lower (0.14) than that of the asteroid (0.25).

For 337 Devosa the spectral behavior has been reproduced by two different mixtures: one composed of 99% pallasite Esquel and 1% orthopyroxene (for an albedo of 0.14) (red line in Fig. 7), and one composed of 98% pallasite Esquel and 2% goethite (for an albedo of 0.14). 337 Devosa has spectral properties in the VIS-NIR spectral range that are very similar to the M-type asteroid 22 Kalliope (see [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF]. For these asteroids, the 0.9 µm band is consistent with a small amount of anhydrous silicate (such as orthopyroxene), or with a small amount of goethite (an aqueous alteration product).

For 417 Suevia the proposed meteoritic analogue (the iron meteorite DRP78007, Fig. 5) does not fit the observed 0.9 µm band. We found that a geographical mixture of 97% IM DRP78007 (RELAB file cdmb47) and 3% orthopyroxene models the asteroid spectral behaviour. The albedo of the mixture is 0.16, a bit lower than the value estimated for Suevia (0.20). For 1124 Stroobantia the proposed meteorite analogue (metallic meteorite DRP78007) also does not fit the faint 0.9 µm band. For this asteroid, we propose two mixing models. One is a geographical mixture of 98% IM MET101A Odessa and 2% orthopyroxene, with an albedo of 0.13 (blu line in Fig. 7). This model reproduces the asteroid spectrum below 1.4 µm but not the longer wavelengths. Alternatively, a mixture of 96% iron meteorite DRP78007 and 4% olivine (albedo 0.16, red line in Fig. 7) fits Stroobantia beyond 0.6 µm, but does not match the spectral region below 0.6 µm. Although none of the modeled mixtures reproduces all spectral features (albedo, spectral slopes, band depth, and band center) of the asteroid, our best inferrence from the modeling is to suggest that the surface composition of Stroobantia is probably consistent with metallic meteorites enriched in silicates.

These results show that a few percent (less than 3%) of orthopyroxenes or goethite added to iron or pallasite meteorites can reproduce the weak spectral features around 0.9 µm seen on some asteroids belonging to the X-complex.

517 Edith shows a flat and featureless spectrum. The observed spectral behavior appears similar to the spectrum of pure amorphous carbon (from [START_REF] Zubko | Optical constants of cosmic carbon analogue grains -I. Simulation of clustering by a modified continuous distribution of ellipsoids[END_REF], although we cannot exclude the possibility that a small amount of silicates may be present and masked by the dark and opaque materials.

Considering the low albedo value and the featureless and reddish spectral behavior, we classify 1328 Devota as a D-type. Since the Tagish Lake meteorite is usually considered the best meteorite analog for D-type asteroids [START_REF] Hiroi | The Tagish Lake Meteorite: A Possible Sample from a D-Type Asteroid[END_REF], we compared the spectrum of Devota with laboratory spectra of several samples of this meteorite. The spectrum of Devota appears to be redder than that of Tagish Lake. This could be due to the different ages of the asteroids and meteorites. The surface of Devota could be older than the meteorite, and the reddening of the asteroid spectrum could be the result of space weathering processes.

Alternatively, the spectral behavior of Devota can be reproduced with a geographical mixture of 94% Tagish Lake meteorite (RELAB file c1mt11) and 6% Triton tholins (from McDonald et al. 1994), resulting in an albedo of 0.02. Our observations of Devota do not cover the wavelengths necessary for detection of the tholin spectral feature at about 3 µm, so although we cannot constrain the presence of tholins with the data in hand, we suggest that some tholin or tholin-like component could be the reddening agent producing the observed spectral slope between 0.5 and 2.5 µm.

The X complex: overview and discussion

[HERE TABLE 6 and7] [HERE FIGURES 8 and 9] Our survey devoted to the X-complex asteroids as defined by the [START_REF] Tholen | Asteroid taxonomy from cluster analysis of photometry[END_REF] taxonomy is composed of 78 objects, including the E and M-type asteroids already published in [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF][START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF]. On the basis of their spectral behavior and albedo values, we classified 22 E-types and distributed them into 3 subclasses I, II, and III, (see Fornasier et al. 2008 and references therein). One of the objects (3447) was originally classified as X-type and was tentatively attributed to the E(I) class in this work, due to its moderately high albedo value but with a high uncertainty. Our survey includes 38 M-types, ten of which (77,92,184,337,417,741,758,1124,1146) were originally classified as X-types, 7 C-types (498 Tokio was originally classified as an M-type, and all the others as X-type), 1 D (originally classified as X-type), 5 P-type (originally classified as X-type), 3 A-types (2577, 7579 and 1122 which were originally classified as E-types, and an X-type, respectively), and 2 S-types (5806 and 516, which were originally classified as an E and M-type, respectively). In Fig. 8 we show the spectral slope value S V IS calculated in the visible wavelength range versus the semimajor axis for all the E, M and C/P-types observed. As expected, high albedo E-type asteroids populate mainly the Hungaria region and the inner part of the main belt, while low albedo C and P-types are located mainly in the outer part of the main belt or beyond it, and M-type are located between 2.4 and 3.2 AU. The mean visible spectral slope for the different E, M and P classes are very similar, as expected (Table 6). In the NIR range, M-type asteroids having a band in the 0.9 µm region have mean spectral slope S NIR1 and S NIR2 values similar to P-type asteroids. These values are higher than those of the M-type asteroids without the 0.9 µm band. For the Etype asteroids, observations in the infrared range are available only for a very few objects. We give the NIR mean slope values only for the subtype III (4 asteroids observed in the NIR range: 44 Nysa, 214 Aschera, 317 Roxane, and 437 Rhodia), and the subtype II (64 Angelina, 2867 Steins, and 4660 Nereus), that show the lower S NIR1 mean value within the X complex (no data are available in the NIR range for the subtype I).

One M-type asteroid, 77 Frigga, show a very peculiar spectrum, with a near infrared spectrum similar to that of other M-type bodies, but with a very high S V IS value comparable with that of the D-type Devosa. Nevertheless its moderate albedo value allows us to exclude any possible link with dark P or D-types. Also A or S type asteroids have S V IS value similar to that of Frigga, but the absence of any absorption band due to olivine and pyroxene lets us exclude any link with these asteroid classes. As discussed in section 3, Frigga may show surface variability, and the 3 µm band, usually associated with hydrated silicates, has been detected on its surface [START_REF] Rivkin | The nature of M-class asteroids from 3-m observations[END_REF]. Polarimetric data reveals that Frigga has a large inversion angle [START_REF] Gil-Hutton | Polarimetry of M-type asteroids[END_REF], implying that the surface is composed of small particles (comparable to the wavelength), and/or of a mixture of particles with high contrast in albedo, like refractory inclusions seen on some carbonaceous chondrites (see Belskaya et al. 2010, and references therein). Our attempt to find a meteorite analogue was unsuccessful as the best match found (the iron meteorite Chulafinnee) does not satisfactory reproduce the asteroid spectrum. No radar observations are available for this body. Additional observations of 77 Frigga will be very important to confirm its surface heterogeneities and possibly to constrain its surface composition.

In our sample 43 objects were observed in the complete V+NIR spectral range.

Among them 37 have spectral features and albedo values compatible with the Xcomplex (5 E-types, 29 M-types, and 3 P-types). For all objects we investigated the spectral matches with meteorites/minerals or geographical mixture models.

For the 5 E-type asteroids (44, 64, 214, 317 and 437) we found good spectral matches with the enstatite achondrite meteorites, in several cases enriched with troilite, oldhamite or orthopyroxenes [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF]). The 3 P-type asteroids presented in this paper (275, 522, and 909) exhibit spectral behaviors and albedo values compatible with carbonaceous chondrite meteorites. Most of the Mtype asteroids have spectral features and albedo values well represented by iron meteorites, pallasites, and enstatite chondrites -in several cases enriched with orthopyroxenes, olivines, or goethite.

Our new spectral observations enhance the available physical information for the observed asteroids and allow us to apply the Bus-DeMeo classification recently published [START_REF] Demeo | An extension of the Bus asteroid taxonomy into the near-infrared[END_REF]. The Bus-DeMeo system is based on the asteroids' spectral characteristics over the wavelength range 0.45 to 2.45 µm without taking the albedo into consideration. In the Bus-DeMeo taxonomy, the X complex comprises 4 types (X, Xk, Xe, Xc) as in the [START_REF] Bus | Phase II of the Small Main-Belt Asteroid Spectroscopic SurveyA Feature-Based Taxonomy[END_REF] system: an asteroid belongs to the Xe-type if the 0.49 µm feature is present, to the Xk-type if a feature is present in the 0.8-1.0 µm range, to the Xc-type if the spectrum is red and featureless with slight concave-down curvature, and to the X-type if the spectrum is straight and featureless. The Tholen X-type asteroids we observed show different spectral behaviors in the near infrared range. We therefore re-classified 16 of the asteroids presented in this paper and observed both in the VIS and NIR range according to the Bus-DeMeo taxonomy. The corresponding classes are summarized in Table 2. All 5 asteroids showing the faint 0.9 µm band fall in the Xk-type (337 Devosa also has the 0.43 µm band), while 1122 Nieth, which shows a broad and deep 0.96 µm band and a steep infrared spectrum, is classified as an A-type. The 5 asteroids having the 0.43 µm absorption band (note that this region is outside the wavelength limits where the Bus-DeMeo taxonomy is defined) belong to 5 different classes: 337 Devosa is classified as Xk; 50 Virginia, showing bands due to hydrated materials, is a Ch; 283 Emma is a C-type; 517 Edith is an Xc; and 1355 Magoeba, having also the peculiar band at 0.49 µm, is classified as Xe-type in the Bus-DeMeo taxonomy. The featureless objects belong to the X-type (77, 758, and 909), D-type (1328, and 1902), and to the C-type (275).

In Table 7 we report Bus-DeMeo taxonomic classifications together with the Tholen classifications (existing or proposed by us) for all the asteroids observed during our survey devoted to the X-complex asteroids. In In particular the asteroid 849 Ara has a steep spectral slope and falls in the D class according to the Bus-DeMeo taxonomy [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF], but its albedo is very high (0.27), excluding a surface composition of organic-rich silicates, carbon, and anhydrous silicates as commonly expected on low-albedo Tholen D-type asteroids.

From this comparison, it is evident that the Bus-DeMeo taxonomy is very helpful in constraining the asteroids' surface compostion (for example Xk-type means presence of orthopyroxene), in particular when some absorption bands are present on the spectra. But it is clear that the asteroid albedo is also a very important parameter for constraining the surface properties and also their evolution in time (space weathering effects). We strongly encourage the development of a next-stage Bus-DeMeo taxonomy that includes the important albedo parameter for asteroid composition/classification.

Summary

We present new visible and near infrared spectra of 24 asteroids belonging to the X-type as defined by [START_REF] Tholen | Asteroids taxonomy[END_REF], that is an "E-M-P" type asteroid for which albedo information was not available at the time of their classification. The X complex in the Tholen taxonomy is comprised of the E, M and P classes which have very different mineralogies but which are spectrally similar with featureless spectra in broadband visible wavelengths. Our observations reveal a large variety of spectral behaviors within the X class, and we identify weak absorption bands on 11 asteroids. We combine our spectra with the albedo values available since 2002 for the observed bodies to suggest new Tholen-like classifications. We find: 1 A-type ( 1122), 1 D-type ( 1328), 1 E-type (possibly, 3447 Burckhalter), 10 M-types (77, 92, 184, 337, 417, 741, 758, 1124, 1146, and 1355), 5 P-types (275, 463, 522, 909, 1902), and 6 C-types (50, 220, 223, 283, 517, and 536). Four new M-type asteroids (92 Undina, 337 Devosa, 417 Suevia, and 1124 Stroobantia) show a faint band in the 0.9 µm region, attributed to low calcium, low iron orthopyroxene. Indeed, several works based on spectral and radar observations show that not all the M-type asteroids have a pure metallic composition (Fornasier et al. 2010 and reference therein).

Three low albedo asteroids (50 Virginia, 283 Emma, and 517 Edith) show a weak band centered at 0.43 µm that we interpreted as due to Fe 3+ spin-forbidden transition in hydrated minerals (hematite, goethite). Also the medium albedo bodies 337 Devosa and 1355 Magoeba have the same absorption. In this case the band may be associated with chlorites and Mg-rich serpentines or pyroxene minerals such us pigeonite or augite. 50 Virginia shows also two absorptions centered at ∼ 0.69 and 0.87 µm which are typical of hydrated silicates.

We performed a search for meteorite and/or mineral spectral matches between the asteroids observed in the visible and near infrared range (with published albedo values) and the RELAB database.

The best matches found for all the M-types of our sample are iron or pallasite meteorites as suggested in the literature, however we note that these meteorites often do not reproduce the faint absorption band features (such as the possible orthopyroxene absorption bands at 0.9 µm) detected on the asteroids. We tried to constraint the asteroids' surface compositions using geographical mixing models for new M-types having the 0.9 µm feature. We found good spectral matches by enriching the iron or pallasite meteorites with small amounts (< 3%) of orthopyroxene or goethite. For the low albedo asteroids we found as the best match CM carbonaceous chondrites, either unaltered or altered (submitted to heating episodes or laser irradiation). Our sample includes also two objects whose spectra diverge completely from other X complex asteroids: 1328 Devota, a low albedo body with a red featureless spectrum, suggested to belong to the D-type, and 1122 Nieth, a medium albedo asteroid with a broad and deep 0.9 µm band and a steep infrared spectrum, suggested to belong to the A-type. A synthetic model made with the Tagish Lake meteorite, considered as the best meteorite analogue of Dtype asteroids [START_REF] Hiroi | The Tagish Lake Meteorite: A Possible Sample from a D-Type Asteroid[END_REF], and a reddening agent (Triton tholin) reproduces the spectral behavior of 1328 Devota.

The whole sample of asteroids included in our work is 72 X-type objects (we exclude the A, D and S/Sq-type asteroids), partly published here and partly already published in [START_REF] Fornasier | Visible and near infrared spectroscopic investigation of E-type asteroids, including 2867 Steins, a target of the Rosetta mission[END_REF] and [START_REF] Fornasier | Spectroscopic survey of M-type asteroids[END_REF]. The analysis of this complete sample has clearly shown that, although the mean visible spectral slopes of M-, E-and P-type asteroids are very similar to each other, the differences in albedo indicate major differences in mineralogy and composition.

Table 1. Observational circumstances for the observed X type asteroids. Solar analog stars named "hip" come from the Hipparcos catalogue, "la" from the Landolt photometric standard stars catalogue, and "HD" from the Henry Draper catalogue Table 2: Physical and orbital parameters of the X-type asteroids observed and selected on the basis of the Tholen taxonomy [START_REF] Tholen | Asteroid taxonomy from cluster analysis of photometry[END_REF].

The Bus and Bus-DeMeo classifications are also reported, together with the new Tholen-like classification proposed here, considering the available albedo values [START_REF] Tedesco | The supplemental IRAS minor planet survey[END_REF]

Figure captions

Fig. 1 -Visible and near infrared spectra of X-type asteroids.

Fig. 2 -Visible and near infrared spectra of X-type asteroids.

Fig. 3 -Visible and near infrared spectra of X-type asteroids.

Fig. 4 -Visible and near infrared spectra of X-type asteroids.

Fig. 5 -Best spectral matches between the observed medium albedo asteroids and meteorites from the RELAB database (see Table 4 for details of the meteorite samples). All these asteroids are re-classified as M-type following the Tholen classification scheme. For 92 Undina 2 meteorites are proposed: in red the IM Babb's Mill and in blue the IM Landes.

Fig. 6 -Best spectral matches between the observed low-albedo asteroids and meteorites from the RELAB database (see Table 4 for details of the meteorite samples). 5). > We present new spectral observations in the VIS+NIR range of 24 asteroids belonging to the X-type (Tholen classification). > New absorption features are detected on 11 objects, reclassified using the new spectra and the albedo values. > We found that iron or pallasite meteorites are the best meteorite analogue of the observed reclassified M-type objects. > C/P-type asteroids are best fit with CM meteorites, either unaltered or altered. > Our mixing models shows that few percent of silicates may reproduce the 0.9 micron band seen on 4 M-type asteroids.

  ary and November 2004) at the Italian Telescopio Nazionale Galileo TNG of the European Northern Observatory (ENO) in La Palma, Spain, and 2 runs (August 2005, and January 2007) at the New Technology Telescope (NTT) of the European Southern Observatory (ESO), in Chile. Two asteroids (92 Undina, 275 Sapientia), investigated in the visible range at the NTT and TNG telescopes, were separately observed in the near infrared at the Mauna Kea Observatory 3.0 m NASA Infrared Telescope Facility (IRTF) in Hawaii during 2 runs in July 2004 and September 2005 (Table

  Fig 9 we show the albedo value versus the semimajor axis for the observed asteroids classified with the Bus-DeMeo taxonomy. We aim to investigate if possible correlations between the classes of this taxonomy and the albedo may exist, although the albedo is not a parameter taken into account in the Bus-DeMeo classification system. We dispose of the albedo value for 7 out of the 10 Xe asteroids, that show the 0.49µm band typical of this class and attributed to sulfides. Most of them correspond to the high albedo subgroup II of the Tholen E-class, with only two asteroids (132 Aertha and 1355 Magoeba) being classified as M-type in the Tholen taxonomy. So this peculiar band, attributed to the presence of sulfides like oldhamite, seems not to be exclusively associated with high albedo E-type asteroids. The Bus-DeMeo Xk-class asteroids show two distinct albedo distributions: a high albedo group (albedo > 0.4), corresponding to the subgroup III of the Tholen Eclass, and a medium albedo group (0.1 < albedo < 0.3) which includes all the Tholen M-types showing a faint feature in the 0.9 µm region. The Xk-class includes also a low albedo object, 522 Helga, that we classified here as a P-type in the Tholen taxonomy. Iron bearing pyroxenes such as orthopyroxene are suggested to cause the feature around 0.9 µm, characteristic of the Xk class, and seem to be present on asteroids with very different albedo values. Most of the Bus-DeMeo X-class asteroids have 0.1 < albedo < 0.2, with the exception of 504 Cora (albedo = 0.34, therefore classified as E[I] asteroid) and 909 Ulla (albedo=0.034, therefore classified as Tholen P-type). All the C-types have low albedo values, but the Xc and D-types span low and moderate albedo values.

Fig. 7 -

 7 Fig. 7 -Geographical mixing model for the asteroids 92 Undina, 337 Devosa, 417 Suevia, 517 Edith, 1124 Stroobantia, and 1328 Devota (see details of each model in Table5).

Fig. 8 -

 8 Fig. 8 -Spectral slope value (S V IS ) versus the semimajor axis for the different asteroids observed, classified following the Tholen taxonomy. The size of the symbols is proportional to the asteroids' diameter.

Fig. 9 -

 9 Fig. 9 -Albedo versus semimajor axis for the different asteroids observed, classified following the Bus-DeMeo taxonomy. The size of the symbols is proportional to the asteroids' diameter.

  Figure 2:

  Figure 7:

Table 3 :

 3 ; the albedos marked by a Band center, depth and width for the features detected in the asteroid spectra.

	come from Gil-Hutton et al. 2007) and the spectral behaviors. The

Table 4 :

 4 RELAB Matches (Exp means experimental).

	ASTEROID	ALBEDO	BEST FIT	MET	MET	NAME	GRAIN SIZE
				CLASS	REFL
	50 Virginia	0.04	PH-D2M-032	CM	0.03	MET00639	<75µm
	77 Frigga	0.14	MR-MJG-082	IM	0.18	Chulafinnee
	92 Undina	0.25	MR-MJG-083	IM	0.23	Babb's Mill
	92 Undina	0.25	MB-TXH-043-H	Pall	0.14	Esquel	<63 µm
	184 Dejopeja	0.19	MB-TXH-043-H	Pall	0.14	Esquel	<63 µm
	275 Sapientia	0.04	MB-TXH-064-C	CM Exp.	0.03	Murchison heated to 600 • C	<63 µm
	283 Emma	0.03	PH-D2M-032	CM	0.03	MET00639	<75µm
	337 Devosa	0.16	MB-TXH-046	IM	0.16	Landes	slab
	417 Suevia	0.20	MB-TXH-047-D	IM	0.15	DRP78007	75-125µm
	517 Edith	0.04	MB-TXH-064-D	CM-Exp	0.03	CM Murchison heated to 700 • C 63-125µm
	522 Helga	0.04	MA-ATB-068	CM Exp	0.05	CM Migei Laser Irrad.	<45µm
	758 Mancunia	0.13	MB-TXH-046	IM	0.16	Landes	slab
	909 Ulla	0.03	MA-ATB-068	CM Exp	0.05	CM Migei Laser Irrad.	<45µm
	1124 Stroobantia	0.16	MB-TXH-047-A	IM	0.12	DRP78007	<25

Table 5 :

 5 Geographical mixing models. 1 blue line in Fig.7, 2 red line in Fig.7

	ASTEROID	ALBEDO GEOGRAPHICAL MODEL	FILE SOURCE	MODEL ALBEDO
	92 Undina	0.25	99% pallasite Esquel	RELAB ckmb43	0.14
			1% orthopyroxene	RELAB cbpe40
	337 Devosa 1	0.16	98% pallasite Esquel	RELAB ckmb43	0.14
			2% goethite	ASTER
	337 Devosa 2	0.16	98% pallasite Esquel	RELAB ckmb43	0.14
			2% goethite	ASTER
	417 Suevia	0.20	97% iron met. DRP78007	RELAB cdmb47	0.16
			3% orthopyroxene	RELAB cbpe40
	517 Edith	0.04	100% amorphous carbon	Zubko et al., 1996
	1124 Stroobantia 1	0.16	98% iron met. MET101A	RELAB c1sc99	0.13
			2% orthopyroxene	RELAB cbpe40
	1124 Stroobantia 2	0.16	96% iron met. DRP78007	RELAB cdmb47	0.16
			4% olivine	USGS
	1328 Devota	0.04	94% carb. chond. Tagish Lake	RELAB c8mt11	0.03
			6% Trit. Tholin	McDonald et al. 1994

Table 6 :

 6 Mean spectral slopes, for the different classes of asteroids with the associated standard deviation value. We discard from the following analysis a few asteroids initially classified as E/M/X in the Tholen taxonomy but whose spectral properties diverge completely from these classes and

	who have been since reclassified as A, S or D-type (A: 1122, 2577, and 7579; S/Sq: 516, and
	5806; D: 1328).
	Asteroid Class	Mean S V IS Mean S NIR1 Mean S NIR2
		(%/(10 3 Å) (%/(10 3 Å) (%/(10 3 Å)
	E (all)	3.92±1.73 -	-
	E[I]	4.60±1.18 -	-
	E[II]	5.15±1.31 0.85±0.30 0.32±0.15
	E[III]	2.43±1.12 1.07±0.81 1.09±0.87
	M with the 0.9 µm band	3.54±0.90 3.14±1.51 1.82±1.42
	M without the 0.9 µm band 4.00±1.97 1.73±2.07 0.81±1.21
	P	3.34±1.69 2.66±1.65 1.83±1.25
	C	1.77±1.94 0.48±0.68 0.17±0.35

Acknowledgment

The authors thank Dr. M. Ockert-Bell, Dr. M. Shepard, and Dr. A. Migliorini for their help with observations, and A. W. Rivkin and an anonymous referee for their comments and suggestions. BEC thanks Jonathan Joseph for computer programming assistance. SF thanks Dr. F. DeMeo for her help in the classification of the observed asteroids using her taxonomy. Taxonomic type results presented in this work were determined, in whole or in part, using a Bus-DeMeo Taxonomy Classification Web tool by Stephen M. Slivan, developed at MIT with the support of National Science Foundation Grant 0506716 and NASA Grant NAG5-12355. This research utilizes spectra acquired with the NASA RELAB facility at Brown University.