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

XI. A Herschel-PACS view of 16 Centaurs^⋆

¹ Instituto de Astrofísica de Andalucia – CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
e-mail: duffard@iaa.es
² Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Dr, Knoxville, TN 37996-1410, USA
³ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany
⁴ LESIA-Observatoire de Paris, CNRS, UPMC Univ. Paris 6, Univ. Paris-Diderot, 5 place J. Janssen, 92195 Meudon Cedex, France
⁵ Deutsches Zentrum fur Luft- und Raumfahrt e.V., Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
⁶ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
⁷ SRON, Netherlands Institute for Space Research, Low Energy Astrophysics, 9747 AD Groningen, The Netherlands
⁸ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁹ Laboratoire d’Astrophysique de Marseille, CNRS & Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁰ Unidad de Astronomía, Fac. de Ciencias Básicas, Universidad de Antofagasta, Avda. U. de Antofagasta 02800, Antofagasta, Chile
¹¹ Department of Physics and Astronomy, Northern Arizona University, Flagstaff, AZ 86001, USA

Received: 26 July 2013
Accepted: 19 February 2014

Abstract

Context. Centaurs are the transitional population between trans-Neptunian objects (TNOs) and Jupiter-family comets. Their physical properties provide an insight into TNO properties, but only under restricted conditions since Centaurs are closer to the Sun and Earth. For this reason it is possible to access the smaller ones, which is more difficult to do with the TNO population.

Aims. The goal of this work is to characterize a set of 16 Centaurs in terms of their size, albedo, and thermal properties. We study the correlations, for a more extended sample obtained from the literature, of diameter, albedo, orbital parameters, and spectral slopes.

Methods. We performed three-band photometric observations using Herschel-PACS and used a consistent method for the data reduction and aperture photometry of this sample to obtain monochromatic flux densities at 70, 100, and 160 μm. Additionally, we used Spitzer-MIPS flux densities at 24 and 70 μm when available. We also included in our Centaur sample scattered disk objects, a dynamical family of TNOs, using results previously published by our team, and some Centaurs observed only with the Spitzer-MIPS instrument.

Results. We have determined new radiometric sizes and albedos of 16 Centaurs. The first conclusion is that the albedos of Centaur objects are not correlated with their orbital parameters. Similarly, there is no correlation between diameter and orbital parameters. Most of the objects in our sample are dark (pv< 7%) and most of them are small (D< 120 km). However, there is no correlation between albedo and diameter, in particular for the group of small objects as albedo values are homogeneously distributed between 4 to 16%. The correlation with the color of the objects showed that red objects are all small (mean diameter 65 km), while the gray ones span a wide range of sizes (mean diameter 120 km). Moreover, the gray objects tend to be darker, with a mean albedo of 5.6%, compared with a mean of 8.5% (ranging from 5 to 15%) for the red objects.