“TNOs are Cool”: A survey of the trans-Neptunian region

Abstract

<i>Context. <i/>The goal of the <i>Herschel<i/> open time programme “<i>TNOs are Cool!<i/>” is to derive the physical and thermal properties for a large sample of Centaurs, and trans-Neptunian objects (TNOs), including resonant, classical, detached and scattered disk objects.<i>Aims. <i/>Based on observations of two targets we tried (i) to optimise the SPIRE observing technique for faint (close to the background confusion noise), slowly moving targets; (ii) to test different thermal model techniques; (iii) to determine radiometric diameter and albedo values; (iv) to compare with <i>Spitzer<i/> results whenever possible.<i>Methods. <i/>We obtained SPIRE photometry on two targets and PACS photometry on one of the targets.<i>Results. <i/>We present results for the two targets, (90482) Orcus and (136472) Makemake, observed with SPIRE and for one of those targets, Makemake, observed with PACS. We adopt <i>p<i/><sub><i>V<i/><sub/> = 0.27 and <i>D<i/> = 850 km as our best estimate of the albedo and diameter of Orcus using single terrain models. With two-terrain models for Makemake, the bright terrain is fitted by, 0.78 < <i>p<i/><sub><i>V<i/><sub/> < 0.90, and the dark terrain 0.02 < <i>p<i/><sub><i>V<i/><sub/> < 0.12, giving 1360 < <i>D<i/> < 1480 km.<i>Conclusions. <i/>A single terrain model was derived for Orcus through the SPIRE photometry combined with MIPS data. The Makemake data from MIPS, PACS and SPIRE combined are not compatible with a single terrain model, but can be modelled with a two-terrain fit. These science demonstration observations have shown that the scanning technique, which allows us to judge the influence of background structures, has proved to be a good basis for this key programme.