Physical And Thermal Properties Of The Centaurs 2060 Chiron And 10199 Chariklo: Results From Far-infrared Observations With The Herschel Space Observatory

Abstract

Centaurs are trans-Neptunian Objects (TNOs) on unstable orbits between Jupiter and Neptune. Centaurs may show activity and some of them are classified also as comets. In order to better investigate the albedo, size distribution and thermal properties of TNOs and Centaurs an Open Time Key Program entitled "TNOs are Cool: A survey of the Transneptunian Region" was submitted to the Herschel Space telescope (Muller et al. 2009, EM&P, 105, 209). This proposal was awarded 372.7 hours to perform radiometric measurements of a large sample of 140 TNOs/Centaurs. Eighteen Centaurs were observed within this program with the PACS instrument in 3 bands (70, 100, and 160 micron), and two of them (Chiron and Chariklo), were observed also at longer wavelengths (250, 350, and 500 micron) with the SPIRE instrument. In this work we present the results of the combined SPIRE and PACS instruments observations of 2060 Chiron and 10199 Chariklo. The Herschel data, coupled with those obtained by Spitzer-MIPS at 24 and 70 micron, have been modeled with both NEATM (Harris 1998, Icarus 131) and thermophysical models in order to derive their albedo, diameter and thermal properties, including thermal inertia and emissivity. Particular care was taken in estimating the absolute magnitude from new and literature data of the two Centaurs during the Herschel observations, as both bodies show brightness variation with the heliocentric distance caused by their cometary activity. Chiron and Chariklo have very similar sizes, and both show a decrease of their emissivity with wavelength, decrease which is particularly strong on Chiron. No coma was detected in the far-infrared images for these two Centaurs. We will present the derived physical and thermal properties for both targets and discuss the possible cause for the emissivity drop at submm-wavelengths.