Observations of Members of Very Young Asteroid Families Poster 15.05, Poster Session 15, 6 p.m., Monday, 5 Sept. 2005, Music Recital Room, 37 th AAS/DPS Meeting, Cambridge, England C.R. Chapman, W.J. Merline, D. Nesvorny, P.M. Tamblyn, & E.F. Young C.R. Chapman, W.J. Merline, D. Nesvorny, P.M. Tamblyn, & E.F. Young (SwRI, Boulder CO USA) Abstract. Several asteroid families or clusters have been found [cf. D. Nesvorny et al., 2003, Ap.J. 591, 486] to have very short dynamical ages. The Veritas family of C-type asteroids, Karin cluster within the S- type Koronis family, and the Iannini cluster (apparently S-type) formed about 8.3, 5.8, and <5 Myr ago, respectively. If one or more kinds of asteroidal processes (e.g. spin evolution, space weathering, devolatilization of near-surface materials, satellite formation and evolution) operate on timescales comparable to or slower than several Myr, then we may expect to observe different physical properties for members of these recently formed families than for older family members. During the first year of our multifaceted observing program, we have used numerous facilities (IRTF/MIRSI, IRTF/SPeX, HST, Spitzer, CTIO 0.9m & 1m, KPNO 0.9m and 2.1m, VLT AO, Gemini AO, and Keck AO), during ~14 different runs, to obtain about 100 different observational datasets for members of the 3 young families plus numerous additional observations of controls (e.g. Themis family and non-Karin members of the Koronis family). Techniques employed include lightcurve photometry, visible colorimetry, near-IR spectral reflectance, thermal IR, and AO search for satellites. We discuss representative results from these observations. Theoretical synthesis of the data must await a more complete sampling of these family asteroids by the different techniques. This work is being supported primarily by the NASA Planetary Astronomy Program and by the observing facilities listed. PROJECT GOALS The goals of our project are to study, with various telescopic techniques, the physical properties of members of very young dynamical families. These families, identified by Co-I Nesvorny and his colleagues (see below right), were formed at discrete and well-determined times less than 10 million years ago. Most asteroid families are believed to be hundreds or even billions of years old. Our expectation is that young families, as compared with typical families, may provide vital clues to processes that presumably take place on relatively short timescales. Processes that we are interested in include space weathering, temporary preservation of near-surface volatiles, satellite formation and evolution, the Yarkovsky Effect (which we can help to calibrate), and understanding the initial/early configura- tions of asteroid families as a check on hydrocode simulations and other theoretical insights concerning the formation of families. PROJECT APPROACH Our approach is to observe known members of the Karin cluster (a=2.87 AU, e=0.044, i=2.1º), the Veritas family (a=3.17 AU, e=0.065, i=9.3º), and the Iannini cluster (a=2.64 AU, e=0.267, i=12.2º), plus controls (e.g. non-Karin members of the Koronis family, also small members of the C-type Themis family), using a variety of telescopes, instruments, and techniques. Our approach is to make coordinated space-based and ground-based observations of relevant family members, obtaining low- and medium-resolution spectrophotometry, radiometry, adaptive optics imaging, and lightcurve photometry. PURPOSE OF THIS POSTER We have completed the first year of this three-year project. We have obtained innumerable observations of various types. But it is premature to analyze the systematics of what we have observed, only part-way through the observing phase of this program. Nevertheless, we want others to be aware of the project, its scope, and the degree of progress we have made. The convergence of angles at 5.8 My ago means that the Karin cluster was created by a parent asteroid breakup at that time. The plot shows past orbital histories of Karin members: (above) nodal longitudes and (below) perihelion arguments. Values relative to 832 Karin are shown. At 5.8 My ago (broken vertical line), the nodal longitudes and perihelion arguments converge. Our Observing Runs: A Summary Facility/InstrumentMonth/Year Type of Observation VLTJuly 2004 AO search for sats. CTIO 0.9mOct. 2004Lightcurves/colors IRTF MIRSINov. & Dec. 2004Radiometry KPNO 0.9mNov.-Dec. 2004Lightcurves/colors KPNO 0.9mFeb. 2005Lightcurves/colors KPNO 2.1mMay-June 2005Lightcurves/colors IRTF SpeXJune & Aug. 2005Near IR spectra CTIO 1mAug.-Sept. 2005Lightcurves/colors Spitzermany scheduled obs Radiometry (also: numerous searches for satellites of members of these families as part of Dr. Merline’s programs using HST, Keck, Gemini, etc.) Our Adaptive Optics Program: Assessing Satellites of Members of Young Asteroid Families We are searching for satellites of asteroids that are members of young families, using ground-based adaptive optics (AO) observations. We hypothesize that young asteroids may have a higher propensity for satellites. Such satellites, formed by the collision that created the family, might not yet have had enough time for subsequent loss of the satellite to tidal effects or by subsequent collisions. Below we show images of (832) Karin and (490) Veritas taken for this purpose. Any distortions or artifacts adjacent to the objects here are characteristic of AO images. A few background stars can be seen. So far, we have imaged about a half-dozen of the brighter members of the Karin, Veritas, and Iannini families using AO. We have also made observations of representative samples of control populations, such as Koronis (S-type) or Themis (C-type). As part of this overall program, we have completed a large HST Snapshot program (9747, Merline PI) using the Advanced Camera for Surveys. There, we searched for satellites among the smaller-sized members of these populations (V~ ). Shown to the right is one of the Koronis binaries we discovered with the HST program, and is of the type we expected to find among the Karin members. Results of the satellite survey are: Population Characteristics Observed # satellites Karin young S-type 17 0 Veritas young C-type 18 0 Koronis old S-type 9 2 Background old? ? 9 0 Our results indicate that other factors may be more important than age in the production and/or retention of satellites. These factors may include the size of the progenitor asteroid, circumstances (e.g. energy) of the collision, the formation mechanism (type of collision), the location within the size distribution of ejecta fragments being sampled, absolute separation of the binary, or scaled separation (a/R, i.e. Hill separation) of the binary. Example of a good model fit to the observed size-frequency distribution (SFD) of the Karin cluster that we obtained via SPH/N- body modeling with diameter D target =33 km, diameter D impactor = 5.75 km, impact speed V impact = 7 km/s and impact angle = 45 deg. The observed and model SFDs are denoted by circles and a line, respectively. We assumed albedo p V = 0.24, which is slightly higher than the average in the Koronis family (average is p V =0.2). For smaller values of p V, the observed SFD would shift slightly to the right and would require a slightly larger D target and D impactor. This exemplifies one of several scientific motivations for the portion of our observing program dedicated to radiometric determination of albedos of members of young families. The full text of the paper (submitted to Icarus) is available at Our Spitzer Program to Study Albedos of Karin/Koronis What are the albedos of young and older members of the S-type Koronis family? The Karin cluster is presumably the result of a very recent collisional break-up of a typical Koronis member. Very preliminary reductions of Spitzer observations of small as well as very small members of both the Karin cluster and typical Koronis members are shown below; known systematics have yet to be removed from the data. Possibly there is a hint that Karin members are unexpectedly darker than normal Koronis members (or have other thermal differences). ID pv D(km) H Large Karin Large Koronis Small Karin Still to be observed Small Koronis Members of our team, observing two weeks ago with the IRTF SpeX remotely from the Boulder, Colorado, offices of SwRI, from back-left to front- right: Bill Merline, Clark Chapman, Eliot Young, and Peter Tamblyn (our theoretician David Nesvorny was busy preparing his Urey Prize talk for the DPS meeting) Our IRTF SpeX Infrared Spectra of Young Family Members and Controls Earlier this year, we began using the IRTF’s SpeX instrument (J.T. Rayner et al., 2003, PASP 115, 362) in low-resolution prism mode to measure spectra of asteroids in the 0.8 – 2.5 micron region. We are concentrating on members of the Iannini, Karin, and Veritas groups, using occasional Koronis and Themis family members as controls. Preliminary reductions for 1086 Nata (a Veritas member, which was V=14.4 at the time) and 4863 Yasutani (a Koronis member, which was V=15.7 at the time), from our June run, are shown above; they are ratioed to our G2V standard HD156802, normalized to unity at 1.21 microns and smoothed with a running average. The Koronis member shows prominent absorption bands characteristic of the Koronis family, while the slightly “bluish” trending slope for Nata is consistent with the flat, featureless spectrum observed for this asteroid in the visible (e.g. M. DiMartino et al., 1997, Icarus 127, 112) Nata 4863 Yasutani