Cellino et al.: Spectroscopic Properties of Asteroids 633 Spectroscopic Properties of Asteroid Families A. Cellino Istituto Nazionale di Astrofisica, Osservatorio Astronomico di Torino S. J. Bus University of Hawai‘i A. Doressoundiram Observatoire de Paris–Meudon D. Lazzaro Observatório Nacional, Rio de Janeiro Asteroid families have been the target of several dedicated campaigns of spectroscopic obser- vations during the last 10 years. Preliminary studies were mainly devoted to obtain a confirma- tion of the cosmochemical reliability of groupings identified by purely statistical analyses of the distributions of objects in the space of the orbital proper elements. These early attempts led to some spectacular confirmations of the common collisional origin of some families, like that associated with 4 Vesta. Subsequently, spectroscopic investigations started to be mostly focused on the issue of characterizing the overall mineralogical compositions of different families, at the same time looking for possible evidence of thermal differentiation of the original parent bodies and for possible evidence of ongoing space-weathering processes. Spectroscopy has also proven to be crucially needed to identify likely interlopers that can seriously affect the derived size distributions of families and the reconstruction of the original fields of ejection velocity of the fragments. At the same time, spectroscopic properties have been recognized as an invaluable tool to assess the real memberships of families mutually overlapping in the space of proper elements. Moreover, spectroscopic surveys have in general been found to be an excellent comple- ment to conventional family searches purely based on the identification of concentrations of objects in the proper-element space. A challenging unsolved problem comes from the fact that several families have been found to exhibit peculiar spectroscopic properties. This opens up new exciting possibilities for future developments in the interpretation of asteroid taxonomic classes. 1. INTRODUCTION some definite answer could come from spectroscopy. Spec- troscopy was able to find, through the observations per- In the early 1990s, the availability of large datasets of formed by Binzel and Xu (1993), the first spectacular confir- asteroid proper elements (Milani and Knezevic, 1990, 1992) mation of the collisional origin of a family, the one associ- and the simultaneous development of new statistical tech- ated with the large asteroid 4 Vesta (see Fig. 1). In that case, niques for the identification of asteroid families (Zappalà et a spectroscopic check was particularly suitable, due to the al., 1990; Bendjoya et al., 1991; Bendjoya, 1993; Zappalà fact that 4 Vesta was a unique (at that time) case of an object et al., 1994, 1995) promised to open new perspectives for belonging to the V taxonomic class, characterized by spec- the physical studies of these groupings, thought to be the troscopic properties similar to those of basaltic achondrites. direct outcomes of collisional events that occurred in the Subsequently, the activities of spectroscopic monitoring of asteroid belt. Previously, extensive physical studies of fami- families have been steadily increasing. From the point of lies had been prevented or seriously slowed down by strong view of spectroscopy, families provide a unique opportunity discrepancies in the family lists proposed by different au- to obtain information on the inner layers of their parent thors [see the review chapter by Valsecchi et al. (1989) in bodies. As a consequence, intense observational activity has the Asteroids II book], and by cosmochemical inconsisten- been devoted to family members in order to characterize cies in some of the proposed family memberships (Chap- their plausible mineralogical composition. At the same time, man et al., 1989). spectroscopy has been used as a very important tool for In that uncertain situation, the first important goal to be identifying random interlopers within families. These ob- pursued was to find some convincing evidence that the most jects share by chance the same orbital properties as “true” recently identified families could really be considered as family members, but they were not produced by the disrup- “true” collisionally originated groups, and not simply sta- tion of the family’s parent body. Interlopers can be identi- tistical flukes. In this respect, it was immediately clear that fied when they are characterized by spectroscopic properties 633 634 Asteroids III Spectroscopic properties can also be used to enlarge the nominal membership of some families, in cases in which 0.14 some peculiar spectral features can be found to characterize their members. In this way, objects sharing the same feature, but located beyond the nominal family borders, can be added 0.12 to the list of plausible members. A good example of this was given, again, by the quoted spectroscopic observations of the Vesta family by Binzel and Xu (1993). In particular, these 0.1 authors discovered a number of genetically related objects (belonging to the V taxonomic class) well beyond the nomi- nal borders of the family, all along the region separating the 0.08 Proper Eccentricity family from the 3:1 mean-motion resonance with Jupiter (Fig. 1). In this way, it was convincingly shown for the first time that large ejection velocities of the fragments, of the 0.06 order of several hundred meters per second, are possible in family-forming events. 2.3 2.35 2.4 2.45 2.5 Since then, several families have been extensively ob- Proper Semimajor Axis served, including the most important groupings identified by statistical investigations, like Koronis (Binzel et al., 1993), Eos (Doressoundiram et al., 1998a; Zappalà et al., 2000), 0.14 Eunomia (Lazzaro et al., 1999), Veritas (Di Martino et al., 1997), Hoffmeister (Migliorini et al., 1996), Flora (Florczac et al., 1998), Maria (Zappalà et al., 1997), Nysa/Polana 0.12 (Doressoundiram et al., 1998b; Cellino et al., 2001), and Hygiea (Mothé-Diniz et al., 2001). In addition to the above specific campaigns, a major observational contribution has 0.1 also been produced by the SMASS (Xu et al., 1995) and SMASSII (Bus, 1999) surveys. The infrared JHK spectro- photometry performed by Veeder et al. (1995) for members of the Eos, Koronis, and Maria families also deserves a men- Proper Inclination 0.08 tion in this context. The general results of these investigations, as well as a number of problems they were expected to solve, are briefly 0.06 summarized in the following sections. 2.3 2.35 2.4 2.45 2.5 2. STATE OF THE ART Proper Semimajor Axis A first point deserves mention here. Spectroscopy of as- Fig. 1. Plot of the asteroids located around the family of Vesta teroids has profited in recent years from improvements in in the space of the proper elements. Crosses indicate family mem- instrumental performance because of the availability of bers. Open circles identify the basaltic objects found by Binzel larger telescopes and increasingly better CCDs. Wide spec- and Xu (1993). Filled squares indicate the basaltic objects not tral windows have been sampled, with an extension of the belonging to the nominal family identified by the same authors traditional wavelength range of the classical UBVRI pho- in the region between the family and the border of the 3:1 mean- tometry toward longer wavelengths in the near-IR (see Bus motion resonance with Jupiter. et al., 2002). However, this does not mean that, on average, the wavelength coverage in single observing campaigns has been steadily increasing. In most cases, spectra have been incompatible with those of their family. For instance, an S- obtained for asteroid families in the interval from 5000 Å type object belonging to a family of C-type members should up to 9,500–10,000 Å. This means that, with respect to the be considered as a very likely interloper. The presence of older spectrophotometric investigations based on UBV fil- interlopers and their plausible numbers can be predicted in ters, something has been generally lost at the ultraviolet end purely statistical terms (Migliorini et al., 1995), but it is of the obtained spectra, whereas the longest wavelengths clear that direct observations can quantify not only how covered have been rarely beyond 1 µm. This is all because many nominal members are actual interlopers, but they can of the limitations of the instruments. Although this did not also identify which ones. In turn, this is very useful for in- prevent the observers from obtaining important results, as we vestigations concerning the physical properties of the fami- shall see, it is important to stress that in several cases family lies (size distributions, ejection velocity fields, etc.) de- investigations suffered from a less than ideal coverage of the scribed in Zappalà et al. (2002). spectra, mainly around the region of the silicate absorption Cellino et al.: Spectroscopic Properties of Asteroids 635 bands around 1 µm and beyond, or shortward of 5000 Å, an ing from a differentiated parent body has been found. A important region for objects belonging to the “primitive” possible exception might be a tentative association of the taxonomic classes (C, B, F, and G). Bellona and Phaeo groupings, whose members belong to In spite of the above limitations, spectroscopy of families the S and X taxonomic classes respectively. This possible has produced a wealth of useful data in recent years. A sum- association has been proposed by Bus (1999), but it is not mary of the current situation is given in Table 1, listing for firmly established, being at most a tentative conjecture ac- each family (ordered according to the proper semimajor cording to the same author. Another tentative candidate axis of the lowest-numbered member) the number of mem- might be the Lydia family, but in this case the evidence is bers observed spectroscopically, the identified interlopers also quite weak. (by purely spectroscopic means; this does not include inter- As a general rule, spectroscopic properties turn out to be lopers already known on the basis of taxonomic classifica- fairly homogeneous among members of the same family.