De-Biased Populations of Kuiper Belt Objects from the Deep Ecliptic Survey

De-biased populations of Kuiper Belt objects from the Deep Ecliptic Survey The MIT Faculty has made this article openly available. Please share how this access benefits you. Your story matters. Citation Adams, E. R., A. A. S. Gulbis, J. L. Elliot, S. D. Benecchi, M. W. Buie, D. E. Trilling, and L. H. Wasserman. “De-Biased Populations of Kuiper Belt Objects from the Deep Ecliptic Survey.” The Astronomical Journal 148, no. 3 (August 18, 2014): 55. © 2014 The American Astronomical Society As Published http://dx.doi.org/10.1088/0004-6256/148/3/55 Publisher IOP Publishing Version Final published version Citable link http://hdl.handle.net/1721.1/92725 Terms of Use Article is made available in accordance with the publisher's policy and may be subject to US copyright law. Please refer to the publisher's site for terms of use. The Astronomical Journal, 148:55 (17pp), 2014 September doi:10.1088/0004-6256/148/3/55 C 2014. The American Astronomical Society. All rights reserved. Printed in the U.S.A. DE-BIASED POPULATIONS OF KUIPER BELT OBJECTS FROM THE DEEP ECLIPTIC SURVEY E. R. Adams1,A.A.S.Gulbis2,3, J. L. Elliot3,8, S. D. Benecchi1,4,M.W.Buie5, D. E. Trilling6, and L. H. Wasserman7 1 Planetary Science Institute, 1700 East Fort Lowell, Suite 106, Tucson, AZ 85719, USA 2 The Southern African Large Telescope and South African Astronomical Observatory, Cape Town, 7935, South Africa 3 Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA 4 Carnegie Institution of Washington, Department of Terrestrial Magnetism, 5241 Broad Branch Road NW, Washington, DC 20015-1305, USA 5 Southwest Research Institute, 6220 Culebra Road, San Antonio, TX 78238, USA 6 Department of Physics & Astronomy, Northern Arizona University, S San Francisco Street, Flagstaff, AZ 86011, USA 7 Lowell Observatory, 1400 W. Mars Hill Road, Flagstaff, AZ 86001, USA Received 2013 October 11; accepted 2014 June 3; published 2014 August 18 ABSTRACT The Deep Ecliptic Survey (DES) was a survey project that discovered hundreds of Kuiper Belt objects from 1998 to 2005. Extensive follow-up observations of these bodies has yielded 304 objects with well-determined orbits and dynamical classifications into one of several categories: Classical, Scattered, Centaur, or 16 mean-motion resonances with Neptune. The DES search fields are well documented, enabling us to calculate the probability on each frame of detecting an object with its particular orbital parameters and absolute magnitude at a randomized point in its orbit. The detection probabilities range from a maximum of 0.32 for the 3:2 resonant object 2002 GF32 −7 to a minimum of 1.5 × 10 for the faint Scattered object 2001 FU185. By grouping individual objects together by dynamical classes, we can estimate the distributions of four parameters that define each class: semimajor axis, eccentricity, inclination, and object size. The orbital element distributions (a, e, and i) were fit to the largest three classes (Classical, 3:2, and Scattered) using a maximum likelihood fit. Using the absolute magnitude (H magnitude) as a proxy for the object size, we fit a power law to the number of objects versus H magnitude for eight classes with at least five detected members (246 objects). The Classical objects are best fit with a power-law slope of α = 1.02 ± 0.01 (observed from 5 H 7.2). Six other dynamical classes (Scattered plus five resonances) have consistent magnitude distribution slopes with the Classicals, provided that the absolute number of objects is scaled. Scattered objects are somewhat more numerous than Classical objects, while there are only a quarter as many 3:2 objects as Classicals. The exception to the power law relation is the Centaurs, which are non-resonant objects with perihelia closer than Neptune and therefore brighter and detectable at smaller sizes. Centaurs were observed from 7.5 <H<11, and that population is best fit by a power law with α = 0.42±0.02. This is consistent with a knee in the H-distribution around H = 7.2 as reported elsewhere. Based on the Classical-derived magnitude distribution, the total number of objects (H 7) in each class is: Classical (2100 ± 300 objects), Scattered (2800 ± 400), 3:2 (570 ± 80), 2:1 (400 ± 50), 5:2 (270 ± 40), 7:4 (69 ± 9), 5:3 (60 ± 8). The independent estimate for the number of Centaurs in the same H range is 13 ± 5. If instead all objects are divided by inclination into “Hot” and “Cold” populations, following Fraser et al., we find that αHot = 0.90 ± 0.02, while αCold = 1.32 ± 0.02, in good agreement with that work. Key words: Kuiper belt: general – methods: data analysis – planets and satellites: formation – planets and satellites: general – surveys Online-only material: color figures, machine-readable and VO tables 1. INTRODUCTION the solar system. Scattering and migration of the giant planets should leave directly observable signatures in the dynamical Studies of the Kuiper Belt have progressed in the last two structure and numbers of small bodies remaining in the asteroid decades from finding individual objects (Jewitt & Luu 1993) and Kuiper Belts and the outer satellites (Levison et al. 2008; to estimating the total number of objects in the outer solar Morbidelli et al. 2009; Bottke et al. 2012). Some models, such system through observations and statistics (e.g., this work; as the Nice Model (Gomes et al. 2005; Morbidelli et al. 2005; Petit et al. 2011; Gladman et al. 2012). The Kuiper Belt itself Tsiganis et al. 2005) and the “Grand Tack” (Walsh et al. 2012), has been subdivided into many distinct dynamical classes of invoke large-scale, abrupt motion by the giant planets. Other objects, including objects in mean-motion resonances with models assume that giant planet migration proceeded more Neptune, high and low inclination populations (Hot versus smoothly, and predict different ratios of objects captured into Cold; e.g., Morbidelli et al. 2008), and various groupings of mean-motion resonances with Neptune (Malhotra 1993; Hahn Scattered objects that have undergone dynamical interactions & Malhotra 2005). with Neptune. The relative numbers of objects in all of these Most models to date can successfully account for some—but populations offer one of the best direct observational constraints not all—features of the Kuiper Belt, such as reproducing the on different dynamical models of solar system formation and Cold but not the Hot Classical populations (smooth migra- evolution. tion), or leaving particular resonances over- or under-populated A recent renaissance in modeling has resulted in a number relative to the apparent populations (most models). Observa- of mostly successful attempts to reproduce the architecture of tional constraints can be used to distinguish between models and resolve fundamental questions, such as how Neptune mi- 8 Deceased. grated outward: smoothly on a nearly circular orbit (e.g., Hahn 1 The Astronomical Journal, 148:55 (17pp), 2014 September Adams et al. & Malhotra 2005), through dynamic scattering with potential field was within ±6◦.5 of the ecliptic and was selected to have at high eccentricity which was later damped down (e.g., Levison least 35 USNO-B astrometric reference stars on each chip. The et al. 2008; Walsh et al. 2012), or some other mechanism. This fields were also chosen to exclude stars brighter than magnitude fundamental debate is evident in recent meetings, where one V = 9.5 (Millis et al. 2002). Typically two 300 s exposures presentation concluded that chaotic capture models are more would be taken on the same night a few hours apart, with a likely based on colors of resonant Kuiper Belt objects, or KBOs third frame on another night during the same observing run. (Sheppard 2012), while another (Noll et al. 2012) claimed that This resulted in orbital arcs of at least 24 hr (required for the large binary fraction in the Cold population favors smooth a designation by the MPC). Most of the fields imaged were migration, since the mechanism of Levison et al. (2008) to push distinct, but sometimes frames of the same field taken several out the Cold population would not preserve as many binary ob- years apart were counted as new search fields, since by that time jects. However, it is also possible to have models with elements a new set of objects would have moved into the field area. In of both: a Nice-like initial migration that proceeds smoothly 2005, the DES stopped surveying new fields, although recovery into a pre-existing Cold population of objects that formed in efforts to improve orbits and classifications of known objects situ would not be ruled out by the binary fraction (Parker & have continued. Kavelaars 2010). In order to obtain as uniform a sample as possible, for this Until recently, comparisons to observations have been disad- work several selection criteria were applied to both the search vantaged because the known sample (over 1600 objects) is an fields and the objects discovered by the DES, as described below. inherently biased population, due to the difficulty of obtaining observations of faint, distant objects. What is needed is a de- 2.1. Search Frame Selection biased population of objects, that is to say, a relatively large sample of objects discovered by a wide-field, all-sky survey that The discovery phase of the DES ended on 2005 May 11. A have had the sources of bias accounted for and removed. Two few KBOs were discovered during subsequent recovery efforts, surveys with large discovery samples have recently begun to but are not included in our sample since they were found under provide such results.

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