Observations of Members of Very Young Asteroid Families

1
Observations of Members of Very Young Asteroid
Families
Poster 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 (SwRI, Boulder CO USA)
Several asteroid families or clusters have been
found cf. D. Nesvorny et al. 2003, Ap.J.
591486-497 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 lt5 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, 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, 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
APPROACH Our approach is to observe known members
of the Karin cluster (a2.87 AU, e0.044,
i2.1º), the Veritas family (a3.17 AU, e0.065,
i9.3º), and the Iannini cluster (a2.64 AU,
e0.267, i12.2º), 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.
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 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. Merlines programs using HST, Keck,
Gemini, etc.)
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 Dtarget33 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 pV0.2). 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 at http//www.boulder.swri.edu/davidn/p
apers/karin_sph.pdf
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 (V17.5-19.5).
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 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.
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 lt0.32gt 7719 0.33 3.7
14.0 10783 0.23 4.7 13.9 13807 0.41 3.8 13.7
47640 0.33 3.3 14.2 Large Koronis lt0.40gt 13432
0.41 3.4 13.9 14071 0.39 3.7 13.8 23022 0.43 3.4
13.9 23023 0.37 3.6 13.9 Small Karin
lt0.28gt 55852 0.31 1.4 16.1 56285 Still to be
observed 75176 0.18 1.6 16.5 76019 0.36 1.3
16.1 Small Koronis lt0.37gt 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