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Observations of Members of Very Young Asteroid FamiliesObservations of Members of Very Young Asteroid FamiliesPoster 15.05, Poster Session 15, 6 p.m., Monday, 5 Sept. 2005, Music Recital Room, 37th 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)Several asteroid families or clusters have been found [cf. D. Nesvorny etal. 2003, Ap.J. 591:486-497] to have very short dynamical ages. TheVeritas family of C-type asteroids, Karin cluster within the S-typeKoronis 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 asteroidalprocesses (e.g. spin evolution, space weathering, devolatilization of near-surface materials, satellite formation and evolution) operate on timescalescomparable to or slower than several Myr, then we may expect toobserve different physical properties for members of these recentlyformed families than for older family members.During the first year of our multifaceted observing program, we haveused numerous facilities (IRTF/MIRSI, IRTF/SPeX, HST, Spitzer, CTIO0.9m, KPNO 0.9m and 2.1m, VLT AO, Gemini AO, and Keck AO), during14 different runs, to obtain about 100 different observational datasets formembers of the 3 young families plus numerous additional observationsof controls (e.g. Themis family and non-Karin members of the Koronisfamily). Techniques employed include lightcurve photometry, visiblecolorimetry, near-IR spectral reflectance, thermal IR, and AO search forsatellites. We discuss representative results from these observations. Theoretical synthesis of the data must await a more complete sampling ofthese family asteroids by the different techniques.This work is being supported primarily by the NASA PlanetaryAstronomy Program and by the observing facilities listed.
PROJECT GOALSThe 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, 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 configurations of asteroid families as a check on hydrocode simulations and other theoretical insights concerning the formation of families.
PROJECT APPROACHOur 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 POSTERWe 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/Instrument Month/Year Type of Observation
VLT July 2004 AO search for sats.
CTIO 0.9m Oct. 2004 Lightcurves/colors
IRTF MIRSI Nov. & Dec. 2004 Radiometry
KPNO 0.9m Nov.-Dec. 2004 Lightcurves/colors
KPNO 0.9m Feb. 2005 Lightcurves/colors
KPNO 2.1m May-June 2005 Lightcurves/colors
IRTF SpeX June & Aug. 2005 Near IR spectra
CTIO 1m Aug.-Sept. 2005 Lightcurves/colors
Spitzer many 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 hypothesized that young asteroids may have a higher propensity for satellites. Such satellites, formed by the collision that created the family, presumably would not have yet had enough time for subsequent loss of the satellite to tidal effects or by subsequent collisions.
Here 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~17.5-19.5). Shown here 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 0Veritas young C-type 18 0Koronis old S-type 9 2Background 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 Dtarget=33 km, diameter Dimpactor= 5.75 km, impact speed Vimpact = 7 km/s and impact angle = 45 deg. The observed and model SFDs are denoted by circles and a line, respectively. We assumed albedo pV = 0.24, which is slightly higher than the average in the Koronis family (average is pV=0.2). For smaller values of pV, the observed SFD would shift slightly to the right and would require a slightly larger Dtarget and Dimpactor. This is 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 athttp://www.boulder.swri.edu/~davidn/papers/karin_sph.pdf
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 <0.32>7719 0.33 3.7 14.010783 0.23 4.7 13.913807 0.41 3.8 13.7 47640 0.33 3.3 14.2
Large Koronis <0.40>13432 0.41 3.4 13.914071 0.39 3.7 13.8 23022 0.43 3.4 13.9 23023 0.37 3.6 13.9
Small Karin <0.28>55852 0.31 1.4 16.156285 Still to be observed75176 0.18 1.6 16.5 76019 0.36 1.3 16.1
Small Koronis <0.37>26782 0.42 1.4 15.8 35241 0.42 1.7 15.4 37695 0.26 1.4 16.3 58400 0.38 1.4 16.0
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)