Minor Planet Science with the VISTA Hemisphere Survey

Astronomical Science DOI: doi.org/10.18727/0722-6691/5003

Marcel Popescu1,2 resources for space exploration; and in The survey conducted with the Visible Javier Licandro3,4 order to mitigate the risk of impacts by and Infrared Survey Telescope for Astron- David Morate3,4 near-Earth objects. omy (VISTA), the VISTA Hemisphere Sur- Julia de León3,4 vey (VHS1), provides such a survey for Dan Alin Nedelcu1,2 The physical properties of minor planets minor planets. VHS is the largest survey are known for just a small fraction of being conducted by the VISTA telescope. these objects: spectroscopic studies and VHS images the entire southern hemi- 1 Astronomical Institute of the Romanian light curves exist only for several thou- sphere using four filters in the near- Academy, Bucharest, Romania sand. The results show an unexpected infrared region, namely Y, J, H and Ks 2 IMCCE, Observatoire de Paris, PSL diversity in the composition, density and (McMahon et al., 2013; Cross et al., 2012). Research University, CNRS, Sorbonne shape of these bodies. The Sloan Digital The band centres of these filters are Universités, UPMC Univ Paris 06, Sky Survey (SDSS) and Wide-field Infra- located at 1.02, 1.25, 1.65 and 2.15 μm Université de Lille, France red Survey Explorer (WISE) provide infor- respectively. Figure 1 shows the trans- 3 Instituto de Astrofísica de Canarias mation for about 100 000 minor planets. mission curves of the VISTA filters com- (IAC), La Laguna, Tenerife, Spain The resulting visible colours and albedos pared to the spectra of two of the most 4 Departamento de Astrofísica, Univer show a greater mixing of the bodies as typical asteroid classes, the primitive sidad de La Laguna, Tenerife, Spain a function of their orbital parameters, C-type and the rocky S-type. Notice that which can be explained by the turbulent these four filters allow sampling of some history of the Solar System (DeMeo et al., of the main spectral features we expect We have carried out a serendipitous 2015 and references therein). However, for asteroids: the spectral slope and the search for Solar System objects imaged some of the most important spectral fea- two wide absorption bands at 1 and by the VISTA Hemisphere Survey (VHS) tures used to reveal the compositions of 2 μm, produced by minerals like olivine and have identified 230 375 valid detec- minor planets are in the near-infrared and pyroxene. tions for 39 947 objects. This informa- region. A large survey with observations tion is available in three catalogues, sampling this spectral region allows us to entitled MOVIS. The distributions of the refine and complement the global picture Detection of minor planets in VHS data in colour-colour plots show clusters of these bodies provided by SDSS and identified with the different taxonomic WISE data. With the aim of characterising the minor asteroid types. Diagrams that use (Y–J) planet population in the VHS, we per- colour separate the spectral classes formed a serendipitous search within the more effectively than any other method observational products of the survey. In Figure 1. The normalised throughput profiles of the based on colours. In particular, the end- VISTA filter compared with two asteroid spectral order to detect the minor planets, we class members A-, D-, R-, and V-types types. used the fact that Solar System objects occupy well-defined regions and can be easily identified. About 10 000 aster- 1 oids were classified taxonomically using S-type a probabilistic approach. The distribution of basaltic asteroids across the C-type Main Belt was characterised using the 0.8 MOVIS colours: 477 V-type candidates Filters were found, of which 244 are outside the Vesta dynamical family.

s 0.6

Context unit y The total number of minor planets (small ar Solar System bodies orbiting the Sun) Arbi tr 0.4 known today exceeds 700 000. The vast majority of them are concentrated between the orbits of Mars and Jupiter, in the asteroid Main Belt. These objects are the remnants of planetesimals from which 0.2 YJ HKs the planets formed, and understanding their properties in detail allows the forma- tion and evolution of the Solar System to be constrained. Other arguments for studying minor planets are related to 0.5 1.02 1.25 1.65 2.15 more practical reasons: their use as Wavelength (μm)

16 The Messenger 167 – March 2017 Figure 2. (Left) False- colour image obtained J by combining the stack J H of frames observed with H Ks J, H and Ks filters on Ks 5 November 2010. Owing to the differential motion of 0.24 arcsec- onds per minute, the asteroid (5143) Heracles appears as a different source in each filter.

Figure 3. (Right) False- colour image obtained by combining the stack Heracles of frames observed with Comet 279P J, H and Ks filters for the comet 279P. appear as moving sources compared MOVIS-D contains the parameters corre- information to be inferred (Popescu et with the background stars. This is exem- sponding to all detections; MOVIS-M al., 2016a). plified in Figure 2, using a false-colour contains the magnitudes obtained with image obtained by combining the stack different filters for each object and The VHS-DR3 release covers ~ 40 % of of frames observed with J-, Ks- and selected by taking into account the timing the planned survey sky area; thus by the H-band filters on 5 November 2011 at constraints; and MOVIS-C lists the colours end of the survey the total number of 02:32, 02:42 and 02:52 UT, respectively. which are useful to infer the mineralogy. Solar System objects observed will be at In this case, the near-Earth asteroid least double these numbers. (5143) Heracles moved about 2.5 arcsec- The MOVIS catalogues are available onds between the three consecutive online via the Centre de Données observations and hence appears as three astronomiques de Strasbourg (CDS2) Near-infrared colours of minor planets separate images compared with the portal. The information provided includes background stars. observational details, and photometric In order to derive compositional informa- and astrometric measurements. The tion for minor planets, it is necessary to Identifying the Solar System objects astrometric positions, corresponding to correlate the spectral behaviour with the observed in a given field requires cross- 230 375 valid detections, were submitted colours. This can be accomplished using matching of the detected coordinates to the International Astronomical Union the taxonomic classes of asteroid spec- with those computed at the moment of Minor Planet Center3, and all of them were tral data (since the large majority of the observation. A dedicated pipeline — validated. The observatory site received objects are asteroids). The main goal of called MOVIS (Moving Objects VISTA the code W91-VHS-VISTA. taxonomies is to identify groups of aster- Survey) was designed for this task. The oids that have similar surface composi- first step consists of predicting the posi- The first published catalogues (Popescu tions. The fact that spectra similar to the tion of Solar System objects potentially et al., 2016a) used the VHS-DR3 data templates proposed by the taxonomic imaged in each field and retrieving the release, which contains the observations systems were systematically recovered corresponding detections from the sur- performed between 4 November 2009 by independent authors using diverse vey products. Secondly, MOVIS removes and 20 October 2013. A total of 39 947 data sets and different methodologies the mis-identifications based on an algo- objects were detected, including 52 provides confidence in these systems. rithm that takes into account the differ- Near Earth Asteroids (NEAs), 325 Mars ence between the observed and the Crossers, 515 Hungaria asteroids, The first approach consisted of analysing computed (O–C) positions and also by 38 428 Main Belt asteroids, 146 Cybele the observed MOVIS-C colours for the comparing with the PPXML star cata- asteroids, 147 Hilda asteroids, 270 objects already classified taxonomically. logue (of positions, proper motions, Two Trojans, 13 comets (see example in Fig- There are about 185 objects with spectra Micron All Sky Survey [2MASS] near- ure 3), 12 Kuiper Belt objects and obtained by the Small Main-belt Asteroid infrared and optical photometry). Finally, Neptune with its four satellites. About Spectroscopic Survey (SMASS) and the the information is provided in three cata- 10 000 objects have accurate spectro- S3OS2 visible spectroscopic survey of logues for the purpose of organising the photometric data (i.e., magnitude errors 820 asteroids. Within an error of ~ 5 %, the data for different types of analysis: less than 0.1) allowing compositional main compositional groups are completely

The Messenger 167 – March 2017 17 Astronomical Science Popescu M. et al., Minor Planet Science with the VISTA Hemisphere Survey

Figure 4. Left panel: the ACDSVX ACDSVX 0.9 colours of asteroids with visible spectra, having an assigned taxonomic type. Right panel: the 0.7 colours computed for the template spectra of the taxonomic classes J 0.5 from DeMeo et al. Y– (2009) compared with the MOVIS-C data with colour errors less than 0.3 0.033 mag. From Popescu et al. (2016a). N = 185 N = 1335 0.1

–0.1 0.30.7 1.1 –0.1 0.30.7 1.1 J–Ks J–Ks separated (Figure 4) using the (Y–J) ver- as the end taxonomic type members A-, in the early age of the Solar System. This sus (J–Ks) colour-colour plot (Popescu D-, R-, and V-types (Popescu et al., hypothesis challenges the models of the et al., 2016a). This is the case for S-, C-, 2016a). This survey thus provides an radial extent and the variability of the A-, D-, V- and C-type asteroids. S-type important tool for investigating the faint early Solar System temperature distribu- are objects with spectra similar to ordi- asteroids which are hard to observe tion, which generally do not predict melt- nary chondrite meteorites; C-type spec- spectroscopically. ing temperatures in this region. V-type tra are similar to carbonaceous chon- asteroids with orbits in the middle and drites; A-type are olivine-rich asteroids; Based on the available data we were able outer part of the Main Belt are unlikely to D-type are objects with low albedo and a to classify about 10 000 objects using be scattered Vesta family objects. featureless red spectrum; V-type are a probabilistic approach which takes into asteroids with a basaltic composition, the account the errors in the object pho Near-infrared colours allow the easy iden- most representative one being the aster- tometry and the position of the class in tification of the V-type candidates: asteroid (4) Vesta. The X-types present fea- colour space (Popescu et al., 2017). oids with (Y–J) > 0.5 and (J–Ks) > 0.3 mag tureless spectra with a moderate slope A further step is to integrate into this are likely to be members of this class. and are representative of objects of dif- schema the photometric data obtained Using the more accurate MOVIS-C data ferent compositions: primitive, like carbo- by SDSS and WISE. (with uncertainties < 0.1 mag), and the naceous chondrites; or metallic, like colour criteria described above, we have enstatite chondrites. Knowledge of the identified 477 objects, of which 244 of the albedo is necessary to derive composi- A particular case: the basaltic asteroids V-type candidates are outside the Vesta tional information of an X-type asteroid. dynamical family (Figure 5). This sample Basaltic asteroids are considered to be almost doubles the number of known The second approach is to compare the fragments of large bodies whose interiors V-types that are not members of the distribution of MOVIS-C data in colour- reached the melting temperature of sili- Vesta family (Licandro et al., 2016). In colour space with the position of colours cate rocks and subsequently differenti- particular we identified 19 V-type aster- computed for the template spectra of the ated (a core of heavy minerals is formed oids beyond the 3:1 mean motion reso- different taxonomic classes spanning the with a mantle of lighter minerals, such nance, 13 of them in the middle part of visible to the near-infrared region (DeMeo as olivine and pyroxene). These asteroids the Main Belt and six in the outer part. et al., 2009). are classified as V-types and are identified by their spectrum in the 0.5–2.5 μm In Popescu et al. (2016a) we show that region which shows two deep and sharp Acknowledgements the (Y–J) colour is a key parameter for a absorption bands at 0.9 and 2 μm. Most The observations were obtained as part of the VISTA taxonomic classification: those with the of the objects with these characteristics Hemisphere Survey, ESO Programme 179.A-2010 1 μm band have a large value of (Y–J) orbit in the inner part of the Main Belt (PI: R. McMahon). J. de León acknowledges support colour. The colour-colour plots which use and are members of the Vesta collisional from the IAC. D. Morate acknowledges the Spanish MINECO for financial support in the form of a the Y filter allow separation of the taxo- family, or are likely scattered members of Severo-Ochoa Ph.D. fellowship. J. Licandro, D. nomic classes much better than has pre- this family. These objects are chunks of Morate, J. de León, and M. Popescu acknowledge viously been possible using other col- the crust of the asteroid (4) Vesta, ejected support from the project ESP2013-47816-C4-2-P ours. Even for large photometric errors from it due to a collision. The presence of (MINECO, Spanish Ministry of Economy and Com- petitiveness). The work of D. A. Nedelcu, and part (up to 0.15 mag), the diagrams (Y–J) vs basaltic asteroids, unlikely to be scat- of the work of M. Popescu, were supported by a (Y–Ks) and (Y–J) vs (J–Ks) clearly sepa- tered members of the Vesta family, clearly grant from the Romanian National Authority for rate the asteroids belonging to the main shows that there were other big differen- Scientific Research — UEFISCDI, project number spectroscopic S- and C-complex, as well tiated basaltic asteroids in the Main Belt PN-II-RU-TE-2014-4-2199.

18 The Messenger 167 – March 2017 Figure 5. Distribution in References Vesta proper orbital elements 0.4 non-Vesta of the V-type candidates Cross, N. J. G. et al. 2012, A&A, 548, A119 from the VHS survey. DeMeo, F. E. et al. 2009, Icarus, 202, 160 3:15:2 2:1 The figure shows the DeMeo, F. E. et al. 2015, Asteroids IV, University of proper semi-major axis Arizona Press, 13 0.3 (ap) vs the sine of the Licandro, J. et al. 2016, A&A, submitted proper inclination (ip). McMahon, R. G. et al. 2013, The Messenger, 154, 35 Red circles are Vesta Popescu, M. et al. 2016a, A&A, 591, A115 P

i family members, while Popescu, M. et al. 2017, in preparation

n blue circles indicate si 0.2 objects out of the Vesta family. The vertical Links dashed lines corre- spond to the most rele- 1 VHS: http://www.vista-vhs.org/ 0.1 vant mean motion reso- 2 CDS catalogues: nances (Licandro et al., http://cdsarc.u-strasbg.fr/viz-bin/Cat 2016). 3 IAU Minor Planet Center: http://www.minorplanetcenter.net/iau/mpc.html 4 0 http://vizier.u-strasbg.fr/viz-bin/VizieR?-source=J/ 1.8 2.0 2.2 2.4 2.6 2.8 3.0 3.23.4 A%2BA/591/A115

aP R. Wesson/ESOR.

Composite image of sunset and star trails over the La Silla Observatory photographed from the 3.6-metre telescope with a series of long exposures. See Picture of the Week potw1647 for information.

The Messenger 167 – March 2017 19