V-type candidates and Vesta family asteroids in the Moving Objects VISTA (MOVIS) Catalogue

V-type candidates and Vesta family asteroids in the Moving Objects VISTA (MOVIS) Catalogue

Key Words.:
minor planets; techniques: photometric, spectroscopic; methods: observations,statistical

Abstract

Context: Basaltic asteroids (spectrally classified as V-types) are believed to be fragments of large differentiated bodies. The majority of them are found in the inner part of the asteroid belt, and are current or past members of the Vesta family. Recently, some V-type asteroids have been discovered far from the Vesta family supporting the hypothesis of the presence of multiple basaltic asteroids in the early solar system. The discovery of basaltic asteroids in the outer belt challenged the models of the radial extent and the variability of the temperature distribution in the early solar system.

Aims:We aim to identify new basaltic V-type asteroids using near-infrared colors of 40000 asteroids observed by the VHS-VISTA survey and compiled in the MOVIS-C catalogue. We also want to study their near-infrared colors and to study the near-infrared color distribution of the Vesta dynamical family.

Methods: We performed a search in the MOVIS-C catalogue of all the asteroids with () and () in the range and , associated with V-type asteroids, and studied their color distribution. We have also analyzed the near-infrared color distribution of 273 asteroid members of the Vesta family and compared them with the albedo and visible colors from WISE and SDSS data. We determined the fraction of V-type asteroids in the family.

Results:We found 477 V-type candidates in MOVIS-C, 244 of them outside the Vesta dynamical family. We identified 19 V-type asteroids beyond the 3:1 mean motion resonance, 6 of them in the outer main belt, and 16 V-types in the inner main belt with proper inclination , well below the inclination of the Vesta family. We computed that 85% of the members of the Vesta dynamical family are V-type asteroids, and only 1-2% are primitive class asteroids and unlikely members of the family.

Conclusions:This work almost doubles the sample of basaltic asteroid candidates in regions outside the Vesta family. Spectroscopic studies in the near-infrared and dynamical studies are needed to confirm their basaltic composition and to determine their origin.

1 Introduction

Basaltic asteroids are believed to be fragments of large bodies whose interiors reached the melting temperature of silicate rocks and subsequently differentiated (Gaffey et al., 2002), forming a core of heavy minerals and a mantle of lighter minerals (olivine and pyroxene). Differentiation is a fundamental process that determines the later geological evolution of the objects, so determining wether specific asteroids are differentiated or not is of scientific interest for understanding the process that sculpted the asteroid belt. Taxonomically, basaltic asteroids are classified as V-types, named after their most representative member (4) Vesta, which was for a long time the only known asteroid presenting a basaltic crust (McCord et al., 1970; Binzel & Xu, 1993). The NASA Dawn mission provided a detailed study of Vesta’s composition (see McCord & Scully 2015 and references therein). The visible to near-infrared (VNIR, 0.4 - 2.5 m) spectra of basaltic asteroids are characterized by two deep absorption bands around 1 and 2 m, associated with the presence of pyroxene on their surfaces. This type of spectra are similar to those of Howardite-Eucrite-Diogenite meteorites (known as HEDs), which are thought to originate from V-type asteroids.

The majority of basaltic asteroids are found in the inner part of the asteroid main belt, between 2.15 and 2.5 AU. Most of them are current or past members of the largest asteroid family in the belt, the Vesta family, dynamically defined as having members with proper semi-major axis (), eccentricity (), and inclination () in the ranges , and , respectively (Nesvorný et al., 2015). The Vesta collisional family originated from a collisional event that excavated a crater in the surface of Vesta (Asphaug, 1997; Thomas et al., 1997) and that occurred more than 1.2 Gyr ago (Carruba et al., 2005). Consistently, spectral studies show that a large fraction of the Vesta family members present a basaltic composition (Binzel & Xu, 1993; Burbine et al., 2001).

Vesta and its dynamical family are not the only asteroids with a basaltic composition. In the last decades, more V-type asteroids were discovered far from Vesta family in the outer main belt (2.8 - 3.3 AU), as is the case of asteroid (1459) Magnya (Lazzaro et al., 2000), or in the middle main belt (2.5 - 2.8 AU) (Moskovitz et al., 2008; Duffard & Roig, 2009). The orbits of these asteroids suggest that they are not scattered Vesta family objects because they cannot be explained by the typical ejection velocities produced during the cratering event. The case of (1459) Magnya is the most intriguing one because it is so far away from the Vesta family that it is almost impossible for it to be a fragment of the crust of Vesta. Michtchenko et al. (2002) proposed that Magnya is a fragment of another large differentiated asteroid that existed in the outer belt region.

The presence of multiple differentiated parent bodies is also supported by the meteorite record. Bland et al. (2009) report on the fall of a basaltic meteorite that has orbital properties and an oxygen isotope composition that suggest a parent body distinct from Vesta.

The existence of V-type asteroids outside the Vesta family is currently explained by the presence of multiple basaltic asteroids in the early solar system; this hypothesis is supported by the study of HED meteorites. Huaman et al. (2014) found at least three local sources of V-type asteroids, possibly associated with the parent bodies (349) Dembowska, (221) Eos, and (1459) Magnya. The discovery of basaltic asteroids in the outer main belt challenged the models of the radial extent and the variability of the temperature distribution in the early solar system, which generally did not predict melting temperatures in this region (Hardersen et al., 2014).

Using wide field surveys that cover huge fractions of the sky with several filters in the visible and near-infrared it is possible to search for V-type candidates. Roig & Gil-Hutton (2006) presented a systematic method for identifying possible basaltic (V-type) asteroids using the 3rd release of the Moving Objects Catalog (MOC) of the Sloan Digital Sky Survey (SDSS). The method is based on the Principal Components Analysis of the MOC colors in the visible, combined with some refined criteria of segregation of the taxonomical classes. They found 505 asteroids exhibiting V-type colors, 263 of them outside the Vesta family, almost all of them in the inner asteroid belt, and 8 of them in the middle/outer belt. Moskovitz et al. (2008) also studied the distribution of V-type asteroids in the main belt using SDSS colors and dynamical criteria. They found 50 V-type candidates outside the Vesta family and using visible and near-infrared spectroscopy they confirmed that 10 of the 11 objects they observed are V-types.

The aims of this paper are to identify new V-type candidates using the near-infrared colors of 40000 asteroids observed by the VHS-VISTA survey and compiled in the MOVIS-C catalogue (Popescu et al., 2016) and to study the near-infrared colors of all the Vesta family asteroids identified in the MOVIS-C catalogue.

A general description of MOVIS-C catalogue and the criteria used to select V-type candidates is presented in Section 2. In Section 3 we determine the goodness of the V-type candidate selection by analyzing the candidates with known spectra, SDSS colors, and albedo. The distribution of the V-type candidates in the main belt and their near-infrared colors are analyzed in Section 4. The near-infrared colors of the Vesta family asteroids are studied in Section 5, and the conclusions are presented in Section 6.

2 Identification of V-type asteroid candidates in MOVIS-C

The Moving Objects VISTA Survey (MOVIS) is a compilation of the observations of known minor bodies of the solar system observed during the VISTA-VHS survey (Popescu et al., 2016). The VISTA Hemisphere Survey (VHS) is an all sky survey that uses the 4.1m Visible and Infrared Survey Telescope for Astronomy (VISTA) wide field survey telescope located at ESO’s Cerro Paranal Observatory in Chile, equipped with a near-infrared camera with a 1.65 field of view. VISTA-VHS uses the , , , and broadband filters and targets to image the entire southern hemisphere of the sky, covering about 19.000 square degrees. Popescu et al. (2016) compiled the colors and the magnitudes of the minor planets observed by the VISTA survey in three catalogues: the detections catalogue, MOVIS-D; the magnitudes catalogue, MOVIS-M; and the colors catalogue, MOVIS-C. The catalogues were built using the third data release of the survey (VISTA VHS-DR3). A total of 39.947 objects were detected. The colors found for asteroids with known spectral properties revealed well-defined patterns corresponding to different mineralogies. The distribution of MOVIS-C data in color-color plots shows clusters identified with different taxonomic types. All the diagrams that use in particular the color separate the spectral classes much better than the common and colors used to date: even for quite large color errors () the color-color plots vs. and vs. clearly separate the S- from the C-complex without overlapping between the regions. The end members A-, D-, R-, and V-types also occupy well-defined regions.

Based on their distinctive spectral features asteroids with spectral properties similar to those of V-type asteroids can be easily identified using color-color diagrams obtained from observations with the , , , and filters (Popescu et al., 2016; Mommert et al., 2016). In Popescu et al. (2016) we showed that in the vs. plot V-types appear as a separate group with and (see Fig. 15 in Popescu et al. 2016). In fact, the mean colors of asteroids with know taxonomic type in MOVIS-C show that the mean () and () colors of V-types ( and ) are respectively 0.2 mag larger and more than 0.3 mag smaller than any other spectral class (see Table 1 in Popescu et al. 2016). This is explained well by the deep absorption bands around 1 and 2 m (Band I and Band II) that characterize the near-infrared spectra of V-types (see Fig. 1) and the band-pass of the VISTA filters. We note that the color is determined by the first absorption band and a large (red slope) value is indicative of a deep 1 m band. On the other hand, the is also determined by the second absorption band and a low (blue slope) value is indicative of a deep 2 m band. Based on the template spectrum for V-types from Bus-DeMeo taxonomy (DeMeo et al., 2009) we can determine that a V-type asteroid should have color values around ; ; ; ; ; , which are close to the middle of the region defined by the V-type candidates in the color-color plots. The equivalent colors of the V-types were computed using the taxonomic template spectrum, the response curve of the VISTA filters, and the solar colors, as explained in Popescu et al. (2016).

Figure 1: Template spectrum of a V-type asteroid in the Bus-DeMeo taxonomy. In gray the filter band-pass of the VISTA filters. The and colors are strongly affected by the two deep absorption bands of the spectrum (Band I centered at m and Band II at m)

.

In this work we use the colors obtained from the recently updated VHS-VISTA survey data release (VHSv20160114) to define V-type candidates in the MOVIS-C catalogue: asteroids having and with uncertainties in both colors . These values correspond to the average values of the objects located close to the limit that separates V-types from the rest of the classes in the vs. plot in Fig. 15 from Popescu et al. (2016). This plot shows a distinctive gap between this group and the rest of the objects. We base our selection criteria in these two colors because they clearly separate the V-types from the other classes and because of observational constraints due to the particular observing procedure used in the VHS survey: (1) the number of objects with data is larger than those with data and (2) the colors are more accurate than the ones mainly because of the shorter time interval between the observations in both filters (Popescu et al., 2016).

As discussed above, the existence of V-type asteroids outside the Vesta family can be explained by the presence of multiple basaltic asteroids in the early solar system; for the analysis of the V-type candidates, we will differentiate between those candidates that are members of the Vesta family according to Nesvorný et al. (2015) and those that are non-Vesta family members.

A total of 477 V-type candidates have been identified in the MOVIS-C catalogue using the criteria described above, 233 of them members of the Vesta family (see Table 1) and 244 non-Vesta family members (see Table 2). Only 22 of our V-type candidates, marked with an asterisk in Tables 1 and 2, are also identified as V-type candidates by Roig & Gil-Hutton (2006) based on their SDSS colors.

1959 15781 29334 43388 62406 96698
1979 15881 29979 44097 65068 96890
2011 16234 29994 44805 65504 98086
2508 17139 30097 44877 66111 98167
3153 17162 30329 45792 67498 99381
3613 17431 30358 46281 68759 102813
4311 17562 30818 46698 68782 103042
4444 17769 31132 48644 68801 103132
4993 17976 31517 48734 68879 107709
5051 18508 31575 50048 69742 108250
5307 18581 31778 50082 70248 111976
6014 18754 32276 50084 70277 112087
6085 19025 32541 50086 70940 117819
6096 19573 33049 50241 72246 118295
6506 19680 33100 50248 73109 118305
6877 20252 33477 51368 73174 118532
7005 20302 33491 51487 74010 119085
7012 21633 33512 51628 74898 119136
7810 21692 33590 51687 74924 122101
9076 21883 33875 52792 74936 130756
9204 22080 34081 53000 75636 131609
9616 22155 34534 53580 77122 132010
10056 22654 35222 53734 77324 132347
10614 23522 35284 54084 77590 136439
11189 24085 35414 54152 80798 137512
11326 24115 36021 55315 80800 138192
11522 24255 36761 56169 84328 147141
12340 24261 36834 56599 86520 153392
12591 25220 37113 57104 88781 161029
13054 25708 37149 57818 88958 162518
13164 26238 37192 57930 90548 170306
13191 26611 37234 57943 90639 170309
13287 27539 38335 58271 91343 172447
13530 27939 38732 59215 92646 175841
13569 28397 38876 59336 92686 178112
13855 28543 38879 59569 93339 225334
13994 28737 39949 61521 93394 243137
15032 28902 41558 61741 94355 244232
15756 29186 42644 61986 94413
Table 1: List of MOVIS-C V-type candidates that belong to the Vesta collisional family and have color uncertainties 0.1. Objects marked with an asterisk are also identified as V-type candidates by Roig & Gil-Hutton (2006) based on their visible colors.
2275 12172 26842 44878 70081 113910
2452 12612 27343 44930 71826 114858
2486 12787 27373 44953 71850 122008
2763 12789 27383 45323 71960 122243
2888 13194 27770 45893 73076 122267
3188 13380 28461 45942 73658 123786
3331 13398 29384 47459 74596 126981
3536 13721 29677 47476 74912 130972
3882 13760 29733 47837 75080 133202
3900 15121 30893 48036 75289 135575
3954 15506 30961 49214 75661 136229
4228 15678 31460 50035 77972 145887
4693 15846 31599 50091 79426 147703
5150 16169 31677 50105 80345 152165
5328 16352 31775 50215 80463 152933
5524 16477 32581 51511 82349 159601
5631 16605 33366 52132 82455 164235
5713 16873 33513 52819 86263 165142
5758 17001 33562 52985 86284 166973
5875 17057 33628 52995 86768 177529
5952 17546 35062 53608 89270 179851
6046 17739 36644 54367 89729 180703
6363 18012 37386 55456 89729 183004
6406 18644 37404 55769 90223 188102
6442 19257 37730 56369 94680 189231
6584 19281 38317 56456 96653 197480
6587 19294 38744 57278 96823 203379
6853 19518 39175 57454 98231 207473
7223 19619 39465 59423 98654 218144
7459 19679 39926 59834 99579 235061
7675 19738 40258 61354 100772 240551
7823 19969 40378 61736 101029 245202
7998 19983 40521 61985 102601 249866
8644 20188 40708 63673 102986 284907
8921 21307 41433 63708 103105 313008
9064 22880 41463 65949 103828 322744
9147 22892 41776 67477 109080 326769
9197 24140 41793 67792 111947 354036
9495 24604 42656 67876 112839 364694
9495 25979 43885 68141 112841
9746 26097 44541 69255 113516
Table 2: List of MOVIS-C V-type candidates with color uncertainties 0.1 that do not belong to the Vesta family. The objects marked with an asterisk are also identified as V-type candidates by Roig & Gil-Hutton (2006) based on their visible colors.

3 Visible spectra, colors, and albedo of MOVIS V-type candidates

To see how efficiently MOVIS-C can identify V-type candidates, we searched for published spectra of these objects in the two largest spectroscopic databases: the Small Main-Belt Asteroid Spectroscopic Survey (SMASS, Xu et al. 1995, Bus & Binzel 2002) and the Small Solar System Objects Spectroscopic Survey (S3OS2, Lazzaro et al. 2004). Nine of the candidates have spectra in the SMASS database, while there are none in the S3OS2. We also searched for visible and/or near-infrared spectra in the literature and found that there are spectral data of a total 15 of our V-type candidates, 13 of which already spectroscopically classified as V-types (see Table 3). From those that are not, one is classified as Sr, and the other is ambiguously classified as V- or R-type. We note that the spectra of R- or Sr-types are also characterized by deep absorption bands close to 1 and 2 m. On the other hand, eight objects with MOVIS-C data are already classified as V-types based on their visible spectrum obtained by SMASS survey. Six of these objects have well-determined and colors: (2508) Alupka, (3498) Belton, (3536) Schleicher, (3900) Knezevic, (4311) Zguridi, and (4993) Cossard. Except for (3498) Belton, which has a error slightly larger than the 0.1 limit we used, all of them are identified as V-type candidates in this paper. So, 87% of the V-type candidates identified in MOVIS-C with known spectra are V-type, and all the asteroids in MOVIS-C classified as V-type in the SDSS are identified as V-type candidates. This is indicative of the very high success rate of identifying V-types using MOVIS-C and the color criteria presented in this work.

Object Reference Vesta family Class
2011 Veteraniya 4,11 Y V
2486 Metsahovi 1,6 N V
2508 Alupka 2 Y V
2763 Jeans (*) 2,5 N V
3536 Schleiche 2 N V
3613 Kunlun 12 Y V
3900 Knezevic 2 N V
4311 Zguridi 2 Y V
4993 Cossard 2,4 Y V
5051 Ralph 2 Y Sr
6406 Vanavara 1,4 N V
9147 Kourakuen 3,10 N V
10614 1997UH1 3 Y V or R
27343 Deannashea 8 N V
40521 (1999 RL95) 7,9 N V
1
Table 3: MOVIS-C V-type candidates with known spectral class.

We next looked at the SDSS colors and visible albedo () of our V-type candidates, confirming that most of them also have visible colors and albedo compatible with V-type asteroids. The SDSS colors and (as defined by Ivezić et al. 2001), and the calculated from the Wide Infrared Survey Explorer (WISE) data (Masiero et al., 2011) were retrieved using the MPC Minor Planet Physical Properties Catalogue2.

Using SDSS colors from the 3rd release of the MOC, Roig & Gil-Hutton (2006) identified a few hundred V-type candidates. They showed that V-type asteroids appear well segregated in the vs. color-color plot, forming a cluster in the region and . A total of 144 of our V-type candidates presented SDSS colors (Ivezić et al., 2002) in the 4th release of the MOC. Following this criterion (see Fig. 2), 111 out of the 144 V-type candidates presented in this paper (77%) have SDSS colors compatible with V-type asteroids. We note that only 22 of our 144 V-type candidates are identified as V-types in Roig & Gil-Hutton (2006), likely because they used the 3rd release of the MOC that have photometric data for 204,305 moving objects, while the 4th MOV release used in this paper have data for 471,569 moving objects. Considering the error bars of the SDSS colors only 10 asteroids present visible colors far from the V-type cluster ( and ). Only 2 of the 144 present SDSS colors that place them in the region associated with the primitive C-complex (), namely (166973) 2003 OV and (15032) Alexlevin.

A total of 240 of our V-type candidates have visible albedo () obtained from WISE observations (Masiero et al., 2011). The albedo distribution (see Fig. 3) is typical of that of rocky asteroids, with , with 232 objects (87%) with and only 3 with (compatible with C-class objects within the uncertainties): (166973) 2003 OV, (5524) Lecacheux, and (112839) 1998 SA. Low-albedo asteroids cannot be basaltic, so the selection criteria includes 1.5% of asteroids that are clearly not V-type likely owing to errors in the color determination.

We note that (166973) 2003 OV has both SDSS colors and compatible with a primitive class asteroid. On the other hand, the albedo of (15032) Alexlevin is too high () and (5524) Lecacheux and (112839) 1998 SA have 0 ( and , respectively).

Finally, of the 477 candidates, 76 have both SDSS colors and WISE albedo, and 56 of these 76 (74%) have the albedo and SDSS colors of the V-types (, and ). We note that the success rate of detecting objects that are V-types according to MOVIS, SDSS, and WISE data depends on the success rate (and uncertainties) of all the surveys. The success rate of MOVIS alone of detecting V-type candidates among those that have been already classified because of their visible or near-infrared spectrum (confirmed V-types) is higher, (13 of 15 objects).

Figure 2: SDSS colors of the V-type candidates presented in Tables 1 and 2. The great majority of them are in the region of the V-types according to Roig & Gil-Hutton (2006).
Figure 3: Visible albedo () distribution of the V-type candidates in Tables 1 and 2. The majority of the objects have large albedos (), compatible with V-type asteroids.

4 V-type candidates outside the Vesta family

4.1 Distribution of V-type candidates in the main belt

As discussed above, the existence of V-type asteroids outside the Vesta family, and in particular in orbits that are not likely to be attained by any asteroid scattered from the Vesta family, is challenging, and suggests that there were multiple basaltic asteroids in the early solar system. For this reason we first analyzed the orbital distribution in the space of proper elements and separated members and nonmembers of the Vesta dynamical family according to Nesvorný et al. (2015).

Figure 4 shows the distribution in proper orbital elements of the V-type candidates presented in Tables 1 and 2. It is clearly seen that the great majority of the non-Vesta candidates are in the inner main belt (objects with semi-major axis AU, hereafter IMB), most of them close to the region occupied by the Vesta family asteroids. This suggests that the large majority of those IMB V-type asteroids outside the Vesta family are probably fragments of (4) Vesta; some of them are probably not included in the family because of the distance criterium used by Nesvorný et al. (2015) or because some of them have been scattered from the family (Nesvorný et al., 2008).

On the other hand, V-type candidates that are in the outer main belt (objects with semi-major axis between 2.8 - 3.3 AU, hereafter OMB) and in the middle main belt (objects with semi-major axis between 2.5 - 2.8 AU, hereafter MMB) are very interesting because they are unlikely scattered Vesta family asteroids.

Asteroids in the MMB are objects that are beyond the 3:1 mean motion resonance with Jupiter, the outer edge of the Vesta family. According to Roig & Gil-Hutton (2006), only fragments of the Vesta family ejected at very high velocities ( m/s) are able to be injected in the MMB. We identified 13 V-type candidates in the MMB (see Table 4), Moskovitz et al. (2008) identified 12 more, and Roig & Gil-Hutton (2006) another 2; asteroid (40521) was identified in all three samples. A total of 27 V-type candidates are identified in the MMB either by their near-infrared (this work) or SDSS colors (Moskovitz et al., 2008; Roig & Gil-Hutton, 2006).

According to Nesvorný et al. (2015) two of the V-type asteroids in the MMB identified in this work, (197480) and (180703), belong to the (173) Ino collisional family; among those identified in Moskovitz et al. (2008), asteroid (208899) belongs to the (2732) Witt family, (84021) belongs to the (170) Maria family, and (55550) belongs to the (15) Eunomia family. The case of (55550) is very interesting because the Eunomia family has been suggested as a possible source of basaltic-type asteroids (Carruba et al., 2007; Roig et al., 2008). Twelve of the V-type candidates identified in the MMB are in the vicinity of the Eunomia family in the space of proper orbital elements.

The case of V-type asteroids in the OMB and the possible link with (1459) Magnya is crucial in order to validate the hypothesis of the presence of multiple basaltic asteroids in the early solar system. We identified six V-type candidates in the OMB (see Table 4), one of which – (126981) – belongs to the (9506) Telramund family. Moskovitz et al. (2008) identified four V-types in the OMB, and Roig & Gil-Hutton (2006) identified another six V-types.

Finally, it is interesting to study the V-type candidates in the IMB that are far from the space of proper orbital elements of the Vesta dynamical family. This can help explain what dynamical path was taken by the Vesta asteroids to reach these orbits, and can test whether there are basaltic asteroids in the IMB that are not chunks of the crust of Vesta. In particular there are two groups: (1) those with AU that are well inside the 7:2 mean motion resonance (between 2.23 and 2.27 AU), which is the inner edge of the Vesta family, and (2) those with proper inclination well below the proper inclination of the Vesta family. Nesvorný et al. (2008) showed that scattered Vesta family asteroids populate most of the semi-major axis extent of the IMB, but do not extend to proper inclinations much smaller than that of the family (e.g., ). There are 19 V-type candidates with AU and 16 with in our sample. From our list of V-type candidates in the IMB 13 of them dynamically belong to the (8) Flora family, 5 belong to the (44) Nysa family, 2 belong to the (434) Hungaria, and 1 belongs to the (27) Euterpe family.

Spectroscopic observations of V-type candidates in the MMB, the OMB, and other non-Vesta family V-types are extremely important in order to confirm their taxonomical classification as V-types, and to allow mineralogical studies to compare their surface properties to those of Vesta family asteroids (e.g., Duffard & Roig 2009; Fulvio et al. 2016; Ieva et al. 2016). These asteroids, together with dynamical studies to determine their origin (e.g., Carruba et al. 2014; Nesvorný et al. 2008; Huaman et al. 2014; Roig et al. 2008), are crucial to understanding the population and origin of basaltic asteroids in this region of the belt.

Figure 4: Distribution in proper orbital elements of our V-type candidates. The upper panel shows the proper semi-major axis () vs. the proper eccentricity (). The lower panel corresponds to the proper semi-major axis vs. the sine of the proper inclination (). Red circles are Vesta family members, while blue circles indicate the objects not in the Vesta family. The vertical dashed lines correspond to the most relevant mean motion resonances.
Number Name
2452 Lyot 0.548 0.006 0.216 0.017 3.157603 0.157434 0.203073
26842 Hefele 0.543 0.019 0.200 0.054 3.070801 0.161099 0.11862
47837 2000 EB118 0.513 0.052 0.188 0.099 3.047409 0.116868 0.198066
112839 2002 QJ18 0.847 0.057 0.297 0.090 3.161972 0.090493 0.263182
123786 2001 BS17 0.560 0.081 0.143 0.096 3.094369 0.199381 0.045462
126981 2002 FW26 0.615 0.042 0.231 0.087 2.985409 0.064602 0.152313
6442 Salzburg 0.545 0.032 0.023 0.078 2.686807 0.058888 0.047547
7459 Gilbertofranco 0.788 0.010 0.125 0.019 2.597774 0.135546 0.088471
13760 Rodriguez 0.558 0.029 0.142 0.072 2.556707 0.106708 0.040244
20188 1997 AC18 0.693 0.024 0.067 0.058 2.593472 0.12641 0.133965
40521 1999 RL95 0.668 0.034 0.073 0.075 2.531127 0.05667 0.218938
52132 5034 P-L 0.602 0.073 0.216 0.046 2.590746 0.247551 0.188448
59423 1999 GE4 0.520 0.027 0.270 0.075 2.648131 0.120942 0.228428
61354 2000 PY10 0.690 0.027 0.029 0.084 2.516033 0.105526 0.206352
61985 2000 RW30 0.679 0.024 0.101 0.045 2.564561 0.090761 0.153342
100772 1998 FN34 0.614 0.043 0.289 0.092 2.748188 0.246466 0.076894
180703 2004 HW46 0.502 0.031 0.141 0.093 2.721743 0.1747 0.237132
197480 2004 BE8 0.510 0.020 0.221 0.052 2.724477 0.171389 0.231051
249866 2001 QM165 0.929 0.077 0.273 0.097 2.666192 0.196148 0.172099
Table 4: Near-infrared colors and proper orbital elements of the V-type candidates identified in the outer and middle main belt.

4.2 Near-infrared color distribution of the V-type candidates

The vs. plot of the V-type candidates is presented in Figure 5. A first look at this figure suggests that the colors of the Vesta and non-Vesta V-types are slightly different. To better analyze this possible difference the normalized histograms showing the and color distribution of both groups are presented in Figure 6, and the mean value and corresponding standard deviation () of the colors as well as the number of objects used to compute them (N), separated into Vesta and non-Vesta family asteroids, are presented in Table 5.

The colors of the Vesta family candidates seem to have a narrower distribution than the colors of the non-Vesta family asteroids. The colors of non-Vesta candidates spread over a wider range of values and peak at a larger , but the mean values of the two distributions are similar within a 1- deviation. To check if the color distribution of the two groups are statistically different, we use a Kolmogorov-Smirnov (K-S) test. This test assumes that both distributions are compatible and compute the probability that the two distributions are equal. The probability value obtained with the K-S test for the distributions is , and for the is . To reject the null hypothesis the K-S rejection criteria range from the less strict to the more strict (or even ), so the test cannot reject the null hypothesis in the case of the distributions, but shows that the distributions of the Vesta and non-Vesta candidates are significantly different.

The different color distribution of both groups can be interpreted in terms of (1) a different composition of the non-Vesta asteroids with respect to Vesta, supporting an origin in a different parent body than Vesta with a slightly different basaltic composition; (2) a different degree of space-weathering that either affects the spectral slope or the depth of the bands (see, e.g., Pieters et al. 1993; Brunetto et al. 2006; and references therein); and (3) a different degree of contamination of the two samples by the incorrect identification of V-types using MOVIS. In this context the observed narrower distribution of the color of the V-type candidates in the Vesta family is expected as they are chunks of the same asteroid (Vesta), ejected at the same time (thus the affected by weathering during the same amount of time in a similar region of the solar system), and the sample is likely less affected by objects that are incorrectly identified as V-type as it is biased in favor of Vestoids.

Differences in the composition of V-types outside the Vesta family have been suggested by Ieva et al. (2016) for V-types in the MMB and OMB from the analysis of four V-types outside the 3:1 resonance. As discussed above, is related to Band I of the V-type spectrum and is related to Band II (see Fig. 1). The position and the depth of the bands is used to study the mineralogical composition of the V-type asteroids (see, e.g., Duffard et al. 2005). In particular, the Band I center vs. Band II / Band I area ratio (BAR) is a diagnostic of different surface composition. For V-type asteroids with similar , a larger value can be due to either a deeper Band I (than a smaller BAR value), or a lager value of Band I center. In any case, the interpretation in terms of mineralogical composition with only these two broadband colors is not straightforward and needs to be studied further, for example by using the spectra of basaltic meteorites, but this is beyond of the scope of this paper.

On the other hand, when considering only the OMB and MMB V-types (the non-Vesta asteroids that are more likely chunks of other basaltic parent bodies), we obtain a mean value of , almost the same value obtained for the Vesta family candidates (see Table 5). Unfortunately, there are not enough V-types in the OMB and MMB to perform a statistically significant distribution to compare with the Vesta asteroids, so we cannot claim any compositional difference between the two populations. Visible and near-infrared spectra of a significant number of these non-Vesta V-types, in particular in the OMB and MMB, is needed in order to perform a detailed mineralogical analysis and to compare their composition with that of Vesta family asteroids.

Figure 5: VISTA colors of our V-type candidates. In red the objects belonging to the Vesta family according to Nesvorný et al. (2015); in blue the asteroids that are not members of the Vesta family.
Color Mean N
Vesta family
0.62 0.07 233
0.75 0.12 90
0.68 0.12 232
0.12 0.13 94
0.05 0.13 233
-0.07 0.12 233
Non-Vesta family
0.68 0.11 244
0.78 0.14 117
0.73 0.14 244
0.10 0.14 120
0.05 0.13 244
-0.05 0.13 121
Table 5: Mean values of the near-infrared colors of our V-type candidates, separated into members and nonmembers of the Vesta collisional family.
Figure 6: Normalized histograms showing the and color distributions of our V-type candidates. In red the objects belonging to the Vesta family according to Nesvorný et al. (2015); in blue the asteroids that are not members of the Vesta family.

5 The Vesta family

We also used MOVIS-C to study the near-infrared colors of the Vesta dynamical family and to combine them with the analysis of visible colors from the SDSS and visible albedos from the WISE data.

There are 273 Vesta family asteroids according to Nesvorný et al. (2015) observed in the MOVIS-C catalogue with and colors with uncertainties magnitudes. The vs. color-color plot of the asteroids belonging to the Vesta dynamical family is shown in Fig. 7. The regions between the V-, S-, and C-type classes as determined in Popescu et al. (2016) are also shown. The Vesta family asteroids are concentrated in two clearly differentiated groups, one in the upper left (in the region of the V-type asteroids) and the other in the lower right (in the region of the S- and C-type asteroids). A total of 233 Vesta family asteroids ( 85%) are V-types, 39 ( 11%) are located in the region of S-types, and 11 ( 4%) in the region of C-type asteroids.

In this sample of 273 Vesta family asteroids 139 of them have visible albedo () based on WISE data (Masiero et al., 2011). Only two of them ( %) have an albedo compatible with primitive asteroids (). The same result is obtained when considering all the Vesta family asteroids observed by WISE with determined (see Fig. 8). Only 31 objects have of a total of 1933, equivalent to 1.5% of the sample. We note that the region occupied by the C-types in the vs. plot also includes the X-types and the K-types. Some X-types (M- and E- types) and the K-type asteroids present , so the Vesta family contains a very small fraction of primitive interlopers.

Figure 7: Colors of asteroids belonging to the Vesta family in the MOVIS-C catalogue with uncertainties mag. The great majority of them are in the region of the V-type asteroids as expected.
Figure 8: Visible albedo distribution of all the Vesta family asteroids observed by WISE.

Using the SDSS data (Ivezić et al., 2002) and only considering objects with uncertainties in the spectral slope and uncertainties in the color 0.13, there are data for 910 Vesta family asteroids (see Fig. 9). A total of 659 of these objects ( %) fulfill the criteria used by Roig & Gil-Hutton (2006) to determine a V-type, i.e., and , and only 23 (2.5%) are in the region of C-type primitive class asteroids. We note that a large fraction of the asteroids in the region of the S-types are concentrated very close to the region of the V-types, 179 of them having color in the range -0.2 ).

Figure 9: SDSS colors of the sample of Vesta family asteroids having SDSS observations. The great majority of them are in the region of the V-types according to Roig & Gil-Hutton (2006).

6 Conclusions

In this paper we used the Moving Objects VISTA Survey Color Catalog (MOVIS-C, Popescu et al. 2016), a compilation of the near-infrared colors of 40000 known minor bodies of the solar system observed during the VISTA-VHS survey, to search for V-type asteroids and to study the color distribution of the Vesta dynamical family.

A total of 477 V-type candidates were identified in MOVIS-C based on their near-infrared and colors, 455 of them not previously recognized by any other spectroscopic or spectrophotometric survey. This sample almost doubles the number of known V-types outside the Vesta family. The MOVIS success rate of detecting V-types is , 15 objects in our list have a previous taxonomical classification based on spectroscopic observations, 13 of them were already classified as V-types. We found that 244 of the V-type candidates are not Vesta family members according to Nesvorný et al. (2015). We also enlarged the sample of V-types with an unlikely origin in the Vesta family: we identified 19 V-type asteroids beyond the 3:1 mean motion resonance, 13 of which in the mean main belt and 6 in the outer main belt, and we found other 16 in the inner main belt with proper inclination well below that of the Vesta family () and well bellow the value that scattered Vesta family objects could attain (Nesvorný et al., 2008).

We also found that the colors of the Vesta family candidates seem to have a narrower distribution compared to the colors of the non-Vesta family asteroids, and peaks at a smaller value. Ieva et al. (2016) found that V-types in the MMB and OMB seems to present a different surface composition with respect to Vesta family asteroids, but when considering only the OMB and MMB V-types in our sample we found that their color distribution is almost equal to that of the Vesta family. Therefore, we conclude that a careful visible and near-infrared spectroscopic study of the V-types in this region of the main belt is needed in order to perform a detailed mineralogical analysis and compare their composition with that of Vesta family asteroids.

Finally the near-infrared colors of the all the Vesta family asteroids in MOVIS-C (a total of 273 asteroids) are presented and analyzed together with their SDSS colors and visible albedo . We conclude that the great majority of the asteroids identified as Vesta family members by pure dynamical considerations are V-types ( 85%), and only a few % are primitive asteroids and so unlikely members of the collisional family.

Acknowledgements.
This article is based on observations acquired with the Visible and Infrared Survey Telescope for Astronomy (VISTA). The observations were obtained as part of the VISTA Hemisphere Survey, ESO Program, 179.A-2010 (PI: McMahon). We thank Daniela Lazzaro for her useful comments that helped to improve this manuscript. J. Licandro, D. Morate, and J. de León acknowledge support from the AYA2015-67772-R (MINECO, Spain). The work of M. Popescu was supported by a grant of the Romanian National Authority for Scientific Research – UEFISCDI, project number PN-II-RU-TE-2014-4-2199.

Footnotes

  1. (1) Alvarez-Candal et al. (2006); (2) Bus & Binzel (2002); (3) de Sanctis et al. (2011b); (4) de Sanctis et al. (2011a) ; (5) Duffard et al. (2004); (6) Fulvio et al. (2016); (7) Moskovitz et al. (2008); (8) Moskovitz et al. (2010); (9) Roig et al. (2008); (10) Popescu et al. (2012); (11) Xu et al. (1995); (12) http://smass.mit.edu/catalog.php .
  2. http://mp3c.oca.eu/

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