AN OPTICAL COLOR SURVEY OF HILDA GROUP ASTEROIDS: TESTING GIANT PLANET MIGRATION MODELS Erin Lee Ryan1,2, Keith S. Noll2, Charles (Chick) Woodward3 1NASA Postdoctoral Program, 2Planetary Systems Lab, NASA GSFC, 3Minnesota InsItute for Astrophysics Abstract and Introduc@on: The Hilda group asteroids reside in a 3:2 resonance with Jupiter, and in a top down view of the solar system, they trace a triangle with overdensies near L3, L4, L5 (leT figure). The orbits of these bodies are consistent with an inward migraon of Jupiter by ~0.45 AU, but that does not preclude migraons near or at the Ime of giant planet formaon. The Hilda asteroids are a unique populaon for studies of composiIon because while the populaon is dynamically stable now, the populaon would have been completely depleted if Saturn and Jupiter migrate enough to cross their mutual 2:1 resonance. We have begun a survey of Hilda group asteroids in the opIcal with the 2.3-m Bok and 4-m Mayall Telescope at Ki\ Peak and Sloan g,r,i & z filters and find their composiIons consistent with X, C, D and T taxonomic types as illustrated in the figure on the right. Are the color variaons size dependent? Are colors consistent with simple inward What about that Nice Model? migraon of Jupiter? SuggesIons from the literature on Hilda asteroids are that the In the Nice Model, Neptune plows through a proto-Kuiper Belt of relave abundances of D & P taxonomic types are a funcIon of size In the case where the observed gradient in main belt asteroid small bodies resulIng in the scaered disk. It is possible that some in the Hilda populaon [1,2]. There have also been some studies in composiIons are a funcIon of formaon mechanism (inner main of these objects are also thrown into stable orbits in the inner solar the literature that suggest that Hilda asteroid albedo is a funcIon of belt silicate rich -> outer main belt carbon rich) and the orbits of system at semimajor axes > 2.6 AU [6]. In opIcal studies of trans- size but that appears to have been refuted by a larger study [3,4] Hildas only trace an inward migraon of ~0.45 AU by Jupiter, we Neptunian objects, many researchers have noted bimodal color (Figure 1). would expect to see a gradient in spectral type as a funcIon of distribuIons in the various dynamical groups [7-9]. eccentricity (Figure 3). Objects at high eccentricity within the Hilda group would have been preferenIally captured at larger semimajor axes than the low eccentricity objects[5]. FiGure: The Spitzer and IRAS results (red squares) suggested a trend of FiGure: The Sloan colors of TNOs as derived from BVRI photometry show a increasing geometric albedo with decreasing asteroid diameter [3], however bimodal color distribuIon for at least the Centaurs & SDOs consistent with B- in the much larger WISE sample set this relaonship is not found[4]. FiGure: Expected trend of color with proper eccentricity, if Hilda group R color studies. asteroids were captured via inward migraon of Jupiter through a disk of Ques&on: Will any tests of migra&on models need to remove asteroids which are more carbon rich (and thus red) at larger heliocentric Ques&on: Are the colors of the Hildas consistent with the colors of diameter dependent color varia&ons? distance. Colors in plot are the same as below for taxonomic types. any trans-Neptunian dynamical groups? Ques&on: Do we see a gradient of taxonomic type with eccentricity? Answer: Our preliminary results suggest that the C, D and T types have a similar distribuIon as a funcIon of size, however the X types may not be equally distributed as a funcIon of size. We have only observed half our Answer: Yes. The colors of the Hildas are consistent with the blue/neutral desired sample however, so this may change. Answer: Not with taxonomic type, but we need to invesIgate spectral slope populaon of scaered disk objects and Centaurs. While not directly as there may be a small spectral slope correlaon. diagnosIc of a common source region, this result is highly suggesIve. Conclusions: There is no clear trend of a gradient in taxonomic type as a funcIon of eccentricity in the Hildas with our current sample, suggesIng that the Hildas may not be resonance captured objects. The colors of Hildas are however consistent with the colors of the blue/neutral populaon of Scaered Disk Objects and Centaurs. Within the next year we hope to expand our total sample to 500 objects (published photometry & 300+ new photometric measurements) and will be able to compare object colors to TNO colors obtained by the OSSO Survey currently underway at the Canada-France-Hawai’i Telescope (PI: Bre\ Gladman). AcknowledGements: This research was supported by an appointment to the NASA Postdoctoral Program at Goddard Space Flight Center, administered by Oak Ridge Associated UniversiIes through a contract with NASA. This work also funded by NASA Planetary Astronomy Grant NNX13AJ11G. References: [1] Dahlgren & Lagerkvist, 1995, Astronomy &Astrophysics, 302, 907; [2] Dalhgren et al., 1997, Astronomy &Astrophysics, 323, 606; [3] Ryan & Woodward, 2011, Astronomical Journal 141,186, [4] Grav et al., 2012, Astrophysical Journal, 744, 197, [5] Franklin et al., 2004, Astronomical Journal, 128, 1391; [6] Levison et al., 2009, Nature, 460, 364, [7] Tegler & Romanishin, 2003, Icarus, 161, 181, [8] Peixinho et al. , 2003, Astronomy & Astrophysics, 410, L29, [9] Peixinho et al. , 2012, Astronomy & Astrophysics, 546, A86 .