45th Lunar and Planetary Science Conference (2014)
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AN OPTICAL COLOR SURVEY OF HILDA GROUP ASTEROIDS: TESTING GIANT PLANET
MIGRATION MODELS. Erin L. Ryan1, K.S. Noll2, C.E. Woodward 3 1NASA Postdoctoral Program Fellow
(NASA Goddard Space Flight Center, Code 693, 8800 Greenbelt Rd, Greenbelt, MD 20771, [email protected]),
2
NASA Goddard Space Flight Center, 3 Minnesota Institute for Astrophysics, University of Minnesota
Introduction: The bulk of the ~ 400 planetary systems that have been reliably detected via radial velocity measurements [1-2] consist of "hot Jupiters". These
planets have radii equal to or greater than that of Jupiter and are found < 1 AU from their host star. Likely,
these planets did not form in their current locations and
the highly excited nature of these systems, points to the
importance of giant planet migration in the early history of planetary systems as a general process [3].
Within our own solar system, the dynamical evidence
for giant planet migration is vast. The dynamical structure of the Kuiper Belt shows clear evidence of outward migration by Neptune. In addition, the Kuiper
Belt population appears extremely diverse in terms of
color and albedo [4-5] with further evidence that these
are primordial properties rather than environmental
factors [6-7]. More recently it has become evident that
migration may also have reshaped the populations in
the inner Solar System, particularly the outer Main
Belt, the Hildas and the Jovian Trojans [8-9]. Some,
and perhaps all, of the objects found in these dynamical groups may have originated in
the protoplanetary disk located between and beyond
the giant planets and been scattered by giant planet
migration to their current locations. We therefore have
designed a program to obtain optical colors of ~300
Hilda group asteroids that will allow us to identify
correlations between color, albedo, size, and dynamical
properties that can be used to constrain specific models
of origins.
Why Hildas?: The Hildas are a unique population of
small bodies that are locked in a 3:2 mean motion resonance with Jupiter. Unlike other resonances in the
asteroid belt, the 3:2 is a stable resonance a 3.95 AU
with objects stable there for at least 2 GYr and more
likely for the age of the Solar System [10-11]. The
Outer Main Belt (OMB) also has many objects that
may have originated in the outer protoplanetary disk
(OPD). However, the OMB appears to be more mixed
with objects from the elsewhere in Main Belt and enjoys only a small advantage in terms of brightness for a
given diameter and albedo. The intrinsic collisional
probability for objects in the Hilda population is also a
factor of 3 to 5 less than the collisional probabilities
for Trojan and OMB populations [12-13], suggesting a
significant population of objects that may be minimally
altered by collisional processing. Thus, we have cho-
sen to concentrate on the Hildas as possibly the nearest
example of a possibly nearly pure sample of relatively
pristine OPD planetesimals in the Solar System.
Observations: We have observed a preliminary
sample of ~150 Hilda group asteroids with the 2.3-m
Bok Telescope and the 4-m Mayall telescope located at
Kitt Peak National Observatory using the Sloan g,r,i,
and z filters. Targets were selected from the catalog of
WISE detected Hildas [14] such that correlations with
optical colors and taxonomy as a function of albedo
and diameter can be investigated.
Preliminary results: Though bimodal color distributions have been noted within the Kuiper Belt and
Trojan populations [7, 15] and in ~100 object sample
of Hilda group asteroids observed by the Sloan Digital
Sky Survey [16], we find no evidence for a bimodal
color distribution within the Hilda group from our data.
We find that the colors of the Hilda group asteroids in
our preliminary catalog show a smooth transition from
a C taxonomic type to a D taxonomic type as presented
in Figure 1.
We also find no correlations between size, albedo
and dynamical properties, however we caution that this
is a preliminary sample which is being actively expanded with continuing observations in our observing
program.
Figure 1: Sloan r-i vs i-z colors for Hildas in our
preliminary sample. Black asterisks denote D-type
asteroids, red circles denote T-type asteroids, purple
triangles denote X-type asteroids and blue squares denote C-type asteroids using SMASS taxonomic definitions.
45th Lunar and Planetary Science Conference (2014)
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Acknowledgements: This research was supported by
an appointment to the NASA Postdoctoral Program at
the Goddard Space Flight Center, administered by Oak
Ridge Associated Universities through a contract with
NASA. This research also supported by NASA Planetary Astronomy grant NNX13AJ11G.
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