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

XIV. Size/albedo characterization of the Haumea family observed with Herschel and Spitzer^★

¹ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
e-mail: vilenius@mps.mpg.de
² Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany
³ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁴ SRON Netherlands Institute for Space Research, Postbus 800, 9700 AV Groningen, The Netherlands
⁵ Rijksuniversiteit Groningen, Kapteyn Astronomical Institute, Postbus 800, 9700 AV Groningen, The Netherlands
⁶ Konkoly Observatory, Research Centre for Astronomy and Earth Sciences, Konkoly Thege 15-17, 1121 Budapest, Hungary
⁷ Instituto de Astrofísica de Andalucía (CSIC), Glorieta de la Astronomía s/n, 18008 - Granada, Spain
⁸ Deutsches Zentrum für Luft- und Raumfahrt e.V., Institute of Planetary Research, Rutherfordstr. 2, 12489 Berlin, Germany
⁹ Northern Arizona University, Department of Physics and Astronomy, PO Box 6010, Flagstaff, AZ 86011, USA
¹⁰ LESIA, Observatoire de Paris, Université PSL, CNRS, Univ. Paris Diderot, Sorbonne Paris Cité, Sorbonne Université, 5 Place J. Janssen, 92195 Meudon Pricipal Cedex, France
¹¹ CITEUC – Centre for Earth and Space Science Research of the University of Coimbra, Observatório Astronómico da Universidade de Coimbra, 3030-004 Coimbra, Portugal
¹² Lowell Observatory, 1400 W Mars Hill Rd, Flagstaff, AZ 86001, USA
¹³ School of Interdisciplinary Social and Human Sciences, Kindai University, Shinkamikosaka 228-3, Higashiosaka-shi, Osaka 577-0813, Japan
¹⁴ Centre for Astrophysics, University of Southern Queensland, Toowoomba, Queensland 4350, Australia
¹⁵ Aix-Marseille Université, CNRS, LAM (Laboratoire d’Astrophysique de Marseille) UMR 7326, 13388 Marseille, France

Received: 29 December 2017
Accepted: 3 July 2018

Abstract

Context. A group of trans-Neptunian objects (TNOs) are dynamically related to the dwarf planet 136108 Haumea. Ten of them show strong indications of water ice on their surfaces, are assumed to have resulted from a collision, and are accepted as the only known TNO collisional family. Nineteen other dynamically similar objects lack water ice absorptions and are hypothesized to be dynamical interlopers.

Aims. We have made observations to determine sizes and geometric albedos of six of the accepted Haumea family members and one dynamical interloper. Ten other dynamical interlopers have been measured by previous works. We compare the individual and statistical properties of the family members and interlopers, examining the size and albedo distributions of both groups. We also examine implications for the total mass of the family and their ejection velocities.

Methods. We use far-infrared space-based telescopes to observe the target TNOs near their thermal peak and combine these data with optical magnitudes to derive sizes and albedos using radiometric techniques. Using measured and inferred sizes together with ejection velocities, we determine the power-law slope of ejection velocity as a function of effective diameter.

Results. The detected Haumea family members have a diversity of geometric albedos ~0.3–0.8, which are higher than geometric albedos of dynamically similar objects without water ice. The median geometric albedo for accepted family members is p_V = 0.48_−0.18^+0.28, compared to 0.08_−0.05^+0.07 for the dynamical interlopers. In the size range D = 175−300 km, the slope of the cumulative size distribution is q = 3.2_−0.4^+0.7 for accepted family members, steeper than the q = 2.0 ± 0.6 slope for the dynamical interlopers with D < 500 km. The total mass of Haumea’s moons and family members is 2.4% of Haumea’s mass. The ejection velocities required to emplace them on their current orbits show a dependence on diameter, with a power-law slope of 0.21–0.50.