Characterizing neutral Trans-Neptunian Objects

Abstract

Trans-Neptunian Objects (TNOs) are small bodies with orbits beyond Neptune (3050 AU). TNOs are cold and small enough to have remained relatively well-preserved, therefore providing our best observable proxy to the early solar system. Roughly 1=3 of TNOs possess neutral colors indicative of fresh surfaces. Three different mechanisms may explain the neutral colors: (i) collisional resurfacing, which only re-coats a fraction of the surface, producing heterogeneous colors; (ii) compositional differences, producing homogeneous colors; and (iii) cometary outgassing, through which either jets produce a heterogeneous object or a global outgassing event produces a homogeneous object. We conducted two surveys to search for homogeneity on the surfaces of neutral TNOs in order to help discern between these resurfacing theories a brightness variation survey (BVS) in which we sparsely sampled lightcurves of 38 neutral TNOs to select follow-up targets, and a color variation survey (CVS) of the 9 follow-up targets to densely sample their rotational lightcurves. Through the BVS, we found that the amplitude distributions of red and neutral TNOs are similar, suggesting a similar collisional histories and refuting the collisional resurfacing mechanism. We detected no close/contact binaries but placed upper limits to the binary fraction of 12-20% at angular component separations of 0:02+0:03 //0:020, which supports the existence of a turnover in the binary fraction as a function of component separation. From our CVS results, updated spin distributions also reveal similarity between red and neutral TNOs, supporting the BVS findings. We constrained the spin periods of seven objects, one of which (collisional family member 2003 OP32) was found to have the fastest measured rotation period of any outer solar system object at 2:4 or 2.6 hours. We found evidence for companions to four of our CVS targets, giving a binary fraction of > 24% for separations 0:04 0:0200. Combined with the BVS and other surveys, these limits suggest that two binary formation mechanisms not involving collisions were simultaneously at work in the Trans-Neptunian belt, one of which was likely the Goldreich et al. (2002) model of dynamical friction.