Ices, Colors, and Dynamical Properties of Centaurs

1. Ices, Colors, and Dynamical Properties of Centaurs Emily L. Schaller (Lunar and Planetary Laboratory, University of Arizona, els@lpl.arizona.edu) Nader Haghighipour (University of Hawaii) & Michael E. Brown (Caltech) No correlations between water ice and visible color, albedo or diameter Abstract We have obtained near infrared spectra of twenty-two centaurs with the Near Infrared Camera (NIRC) on the Keck I telescope. We parameterize all the spectra with a linear mixture of crystalline water ice and a dark continuum component. Surprisingly, we find no correlation between object visible color and the fraction of water ice found in the infrared spectra. Visible color should therefore not be interpreted as a predictor of the presence or absence of surface ices on these bodies. We also find that the physical properties of centaurs (visible colors and infrared spectra) are not correlated with their current orbital elements nor their integrated dynamical lifetimes. Colors from Tegler et al. 2008 Albedos from Stansberry et al. 2008 Diameters from Stansberry et al. 2008 No correlations between centaur physical properties and dynamics The Sample of Centaurs We integrated 100 clones of each centaur for 100 million years and then calculated the median dynamical lifetime. The fraction of water ice in the infrared spectra does not correlate with the dynamical lifetime. H-K (1.5-2.5 micron) spectra of centaurs (and small KBOs) show featureless to moderate absorptions at 1.5 and 2.0 microns due to water ice Pure Water Ice Continuum? Featureless Spectrum Conclusions Centaur water ice fraction does not correlate with visible color Gray and red centaur populations each have members with water ice and featureless spectra Exposed water ice from collisions or evaporation cannot account for blue/gray visible colors Water ice fraction ranges from 0-40% (wider range than for non-Haumea family and KBOs) Physical properties do not correlate with dynamical lifetimes nor current orbital elements The spectra of three centaurs showing weak/moderate absorptions at 1.5 and 2 microns. All spectra were parameterized by a simple model consisting of a linear mixture of water ice + dark continuum component with a variable slope. Centaurs are objects with perihelia between the giant planets. They are thought to be the dynamical intermediaries between Kuiper Belt Objects (KBOs) and short period comets (Levison & Duncan 1997). Thus, centaurs provide us with the unique opportunity to study what happens to the surfaces of KBOs when they are heated (brought closer to the sun). Twenty-two centaurs were observed with the Near Infrared Camera (NIRC) on the Keck I Telescope. All centaurs had visual magnitudes brighter than 21.5 at the time of observation. The above figure shows the perihelia and semi-major axes of all the centaurs in our survey. Relative Reflectance