TNOs are cool: A survey of the trans-Neptunian region

V. Physical characterization of 18 Plutinos using Herschel-PACS observations^⋆

¹ Deutsches Zentrum für Luft- und Raumfahrt (DLR), Institut für Planetenforschung, Rutherfordstraße 2, 12489 Berlin, Germany
e-mail: michael.mommert@dlr.de
² Konkoly Observatory of the Hungarian Academy of Sciences, 1525 Budapest, PO Box 67, Hungary
³ LESIA-Observatoire de Paris, CNRS, UPMC Univ Paris 06, Univ. Paris-Diderot5 place J. Janssen, 92195 Meudon Principal Cedex, France
⁴ Max-Planck-Institut für Sonnensystemforschung (MPS), Max-Planck-Straße 2, 37191 Katlenburg-Lindau, Germany
⁵ The University of Arizona, Tucson, AZ 85721, USA
⁶ Max-Planck-Institut für extraterrestrische Physik (MPE), Postfach 1312, Giessenbachstr., 85741 Garching, Germany
⁷ Center for Geophysics of the University of Coimbra, Av. Dr. Dias da Silva, 3000-134 Coimbra, Portugal
⁸ Astronomical Observatory of the University of Coimbra, Almas de Freire, 3040-04 Coimbra, Portugal
⁹ Instituto de Astrofísica de Andalucía (CSIC) C/ Bajo de Huétor 50, 18008 Granada, Spain
¹⁰ SRON Netherlands Institute for Space Research, Postbus 800, 9700 AV Groningen, The Netherlands
¹¹ UNS-CNRS-Observatoire de la Côte d’Azur, Laboratoire Cassiopée, BP 4229, 06304 Nice Cedex 04, France
¹² Univ. Paris Diderot, Sorbonne Paris Cité, 4 rue Elsa Morante, 75205 Paris, France
¹³ Laboratoire d’Astrophysique de Marseille, CNRS & Université de Provence, 38 rue Frédéric Joliot-Curie, 13388 Marseille Cedex 13, France
¹⁴ Department of Astronomy, University of Maryland, College Park, MD 20742, USA

Received: 1 December 2011
Accepted: 31 January 2012

Abstract

Context. The Herschel open time key programme TNOs are Cool: A survey of the trans-Neptunian region aims to derive physical and thermal properties for a set of  ~140 Centaurs and trans-Neptunian objects (TNOs), including resonant, classical, detached and scattered disk objects. One goal of the project is to determine albedo and size distributions for specific classes and the overall population of TNOs.

Aims. We present Herschel-PACS photometry of 18 Plutinos and determine sizes and albedos for these objects using thermal modeling. We analyze our results for correlations, draw conclusions on the Plutino size distribution, and compare to earlier results.

Methods. Flux densities are derived from PACS mini scan-maps using specialized data reduction and photometry methods. In order to improve the quality of our results, we combine our PACS data with existing Spitzer MIPS data where possible, and refine existing absolute magnitudes for the targets. The physical characterization of our sample is done using a thermal model. Uncertainties of the physical parameters are derived using customized Monte Carlo methods. The correlation analysis is performed using a bootstrap Spearman rank analysis.

Results. We find the sizes of our Plutinos to range from 150 to 730 km and geometric albedos to vary between 0.04 and 0.28. The average albedo of the sample is 0.08    ±    0.03, which is comparable to the mean albedo of Centaurs, Jupiter family comets and other TNOs. We were able to calibrate the Plutino size scale for the first time and find the cumulative Plutino size distribution to be best fit using a cumulative power law with q = 2 at sizes ranging from 120–400 km and q = 3 at larger sizes. We revise the bulk density of 1999 TC36 and find ρ = 0.64_-0.11^+0.15 g cm^-3. On the basis of a modified Spearman rank analysis technique our Plutino sample appears to be biased with respect to object size but unbiased with respect to albedo. Furthermore, we find biases based on geometrical aspects and color in our sample. There is qualitative evidence that icy Plutinos have higher albedos than the average of the sample.