Publication Year 2016 Acceptance in OA@INAF 2020-12-21T10:07:06Z Title Spectral characterization of V-type asteroids - II. A statistical analysis Authors IEVA, Simone; DOTTO, Elisabetta; Lazzaro, D.; PERNA, Davide; Fulvio, D.; et al. DOI 10.1093/mnras/stv2510 Handle http://hdl.handle.net/20.500.12386/29033 Journal MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY Number 455 MNRAS 455, 2871–2888 (2016) doi:10.1093/mnras/stv2510 Spectral characterization of V-type asteroids – II. A statistical analysis S. Ieva,1‹ E. Dotto,1 D. Lazzaro,2 D. Perna,3 D. Fulvio4 and M. Fulchignoni3 1INAF–Osservatorio Astronomico di Roma, via Frascati 33, I-00040 Monteporzio Catone (Roma), Italy 2Observatorio Nacional, Rua General Jose´ Cristino, 77 – Sao˜ Cristov´ ao,˜ Rio de Janeiro – RJ-20921-400, Brazil 3LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universites,´ UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cite,´ 5 place Jules Janssen, F-92195 Meudon, France 4Departamento de Fis´ıca, Pontif´ıcia Universidade Catolica´ do Rio de Janeiro, Rua Marques de Sao˜ Vicente 225, Rio de Janeiro 22451-900, Brazil Downloaded from https://academic.oup.com/mnras/article/455/3/2871/2892629 by guest on 06 November 2020 Accepted 2015 October 23. Received 2015 October 22; in original form 2015 August 9 ABSTRACT In recent years, several small basaltic V-type asteroids have been identified all around the main belt. Most of them are members of the Vesta dynamical family, but an increasingly large number appear to have no link with it. The question that arises is whether all these basaltic objects do indeed come from Vesta. To find the answer to the above questioning, we decided to perform a statistical analysis of the spectroscopic and mineralogical properties of a large sample of V-types, with the objective to highlight similarities and differences among them, and shed light on their unique, or not, origin. The analysis was performed using 190 visible and near-infrared spectra from the literature for 117 V-type asteroids. The asteroids were grouped according to their dynamical properties and their computed spectral parameters compared. Comparison was also performed with spectral parameters of a sample of HED meteorites and data of the surface of Vesta taken by the VIR instrument on board of the Dawn spacecraft. Our analysis shows that although most of the V-type asteroids in the inner main belt do have a surface composition compatible with an origin from Vesta, this seem not to be the case for V-types in the middle and outer main belt. Key words: methods: data analysis – methods: statistical – minor planets, asteroids: individ- ual: V-types – minor planets, asteroids: individual: 4 Vesta. Vesta (Marchi et al. 2012), that with all probability are the origin 1 INTRODUCTION of Vesta’s dynamical family. The scenario was then complete: sev- 4 Vesta, an asteroid of 530 km diameter orbiting at 2.36 au, is the eral impacts created a swarm of basaltic fragments, some forming biggest ‘small body’ to show a basaltic crust. Its basaltic nature was the dynamical family while others, due to collisions and dissipative first inferred by McCord, Adams & Johnson (1970), who found, in effects, were injected into strong resonances, who pumped their the spectrum of Vesta, a deep absorption band near 0.9 µm, rep- eccentricities. Some were ejected from the Solar system or fell di- resentative of pyroxene composition. Because of the spectroscopic rectly into the sun (Farinella et al. 1994). Others, extracted from a similarity with basaltic achondrites, Vesta was also considered the close encounter with a terrestrial planet, became NEAs (Cruikshank parent body for Diogenites, Howardites and Eucrites meteorites et al. 1991) or Mars-crossers (Ribeiro et al. 2014). Finally, some (Drake 2001), collectively known as HED meteorites. With the in- of them ended up colliding on Earth and were recovered as HED crease in the number of asteroids discovered in the inner main belt, meteorites. a dynamical family of ten members was first identified by Williams Previous dynamical and observational works confirmed the above (1989). scenario characterized by the diverse steps of the evolution of frag- A major breakthrough was the discovery that smaller bodies in or- ments from Vesta. However, the discovery of a basaltic asteroid in bits between Vesta and the 3:1 mean motion resonance with Jupiter the outer main belt (Lazzaro et al. 2000) can be regarded as the have spectra similar to Vesta (Binzel & Xu 1993), confirming a ge- iceberg tip on the presence and extension of differentiated mate- netic link between them and suggesting a suitable transport mecha- rial in the asteroid belt. Nowadays basaltic asteroids with no dy- nism. Another was the discovery from HST images of a giant crater namical link with Vesta have been discovered all over the main in the south pole of Vesta (Thomas et al. 1997). The Dawn mission belt (Nesvornyetal.´ 2008): some of them can be traced back recently identified not just one, but two craters in the south pole of to Vesta trough dynamical pathways that involve non-linear sec- ular resonances (Carruba et al. 2005); others could derive from a distinct differentiated body than Vesta. Moreover, laboratory E-mail: [email protected] studies on the oxygen isotopic composition of HED meteorites C 2015 The Authors Published by Oxford University Press on behalf of the Royal Astronomical Society 2872 S. Ieva et al. suggest that not all can be related to a unique parent body (Scott other, with those of a sample of HED meteorites and with spectral et al. 2009). parameters of the surface of Vesta as taken by VIR instrument on Asteroids have been classified as V-type if showing a spectrum board the Dawn spacecraft. In the next section, we describe the similar to that of Vesta(Tholen & Barucci 1989; Bus & Binzel 2002; selected sample, while the statistical analysis of the data is given DeMeo et al. 2009) or ‘putative’ V-type if presenting compatible in Section 3. The study of the mineralogy for the subsample with photometric colours (Roig & Gil-Hutton 2006; Carvano et al. 2010; infrared data is given in Section 4. Finally, we conclude discussing DeMeo & Carry, 2013). Extensive numerical simulations of the the results we obtained and their implications on the study of V-type dynamical evolution of Vesta’s ejected fragments over time-scales asteroids in the main belt. comparable to the family age have shown that a relatively large fraction of the original Vesta family members may have evolved 2 THE V-TYPE SAMPLE out the family borders, and be considered as ‘fugitives’ (Nesvorny´ et al. 2008). However, from a dynamical point of view it is quite The selected sample is composed of 190 spectra in the visible and/or difficult to explain large V-type objects in the middle and outer NIR for 117 asteroids classified as V-type according to the most re- Downloaded from https://academic.oup.com/mnras/article/455/3/2871/2892629 by guest on 06 November 2020 main belt. According to Roig et al. (2008) the probability of an cent taxonomy (DeMeo et al. 2009). In order to better investigate asteroid with a mean diameter larger than 5 km to evolve from the similarities between different populations, we divided our sample Vesta family, cross the 3:1 mean motion resonance with Jupiter, into six groups according to their dynamical properties. The highest and reach a stable orbit in the middle belt is almost 1 per cent. It concentration of V-type asteroids is found in the inner main belt (at is noteworthy that the estimated diameters (assuming an albedo of semimajor axis a < 2.5 au), where we identified four dynamical 0.4 typical for basaltic surfaces) for the middle belt asteroids 10537 groups: vestoids or Vesta family objects, fugitives, low inclina- 1991 RY16 and 21238 Panarea are about 7 and 5 km, respectively, tion (low-i) and inner other (IOs). Outside the inner main belt two while for the outer belt asteroid 1459 Magnya, thermal observation other V-type groups were defined: NEAs and Middle/Outer V-types have determined a diameter of 17 km (Delbo et al. 2006). Moreover, (MOVs). Objects outside the limits of the Vesta dynamical fam- these are the three unique V-type asteroids in the middle and outer ily (fugitives, low-i, IOs and MOVs) are often grouped together in belt whose basaltic nature has been confirmed through visible and literature as non-vestoids. Therefore, we considered as follows. near-infrared spectra. In the last decade, several works have been devoted to the ob- (i) A vestoid is a V-type member of the Vesta dynamical family, 1 servation of V-type asteroids, in order to answer the question of as defined by Nesvorny´ using the hierarchical clustering method their origin (Duffard et al. 2004; Moskovitz et al. 2008a,b; Burbine (HCM). et al. 2009; Moskovitz et al. 2010; De Sanctis et al. 2011a,b). In (ii) A fugitive, following the definition of Nesvorny´ et al. (2008), < most of these works, the focus has been to identify mineralogical is a V-type asteroid with a 2.3 au and comparable e and i with differences and/or similarities between V-type asteroids belonging the Vesta family. and not belonging to the Vesta dynamical family. No definitive (iii) Also according to Nesvorny´ et al. (2008) a low-i is a V-type < ◦ < < conclusion has been reached by the diverse authors, with the only asteroid having i 6 and 2.3 a 2.5 au. exception of 1459 Magnya, which mineralogy has been shown to (iv) The remaining V-type asteroids in the inner main belt were be distinct from that of Vesta (Hardersen, Gaffey & Abell 2004).