Physical properties of the extreme Centaur and super-comet candidate 2013 AZ₆₀

¹ Konkoly Observatory, MTA Research Centre for Astronomy and Earth Sciences, Konkoly-Thege Miklós út 15–17, 1121 Budapest, Hungary
e-mail: apal@szofi.net
² Department of Astronomy, Loránd Eötvös University, Pázmány Péter sétány 1/A, 1117 Budapest, Hungary
³ Computational Engineering and Science Research Centre, University of Southern Queensland, Toowoomba, 4350 Queensland, Australia
⁴ Australian Centre for Astrobiology, UNSW Australia, Sydney, 2052 New South Wales, Australia
⁵ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, Giessenbachstr., 85741 Garching, Germany
⁶ Max-Planck-Institut für Sonnensystemforschung, Justus-von-Liebig-Weg 3, 37077 Göttingen, Germany
⁷ Instituto de Astrofísica de Canarias, 38205, La Laguna, Tenerife, Spain
⁸ Departamento de Astrosfísica, Universidad de La Laguna, 38206, La Laguna, Tenerife, Spain
⁹ Gothard Astrophysical Observatory, Loránd Eötvös University, 9700 Szombathely, Hungary
¹⁰ Lowell Observatory, 1400 W Mars Hill Rd, Arizona, AZ 86001, USA
¹¹ Centre for Astrophysics Research, University of Hertfordshire, College Lane, Hatfield AL10 9AB, UK
¹² Instituto de Astrofísica de Andalucía – CSIC, Apt 3004, 18008 Granada, Spain

Received: 2 April 2015
Accepted: 7 July 2015

Abstract

We present estimates of the basic physical properties including size and albedo of the extreme Centaur 2013 AZ₆₀. These properties have been derived from optical and thermal infrared measurements. Our optical measurements revealed a probable full period of ≈9.4 h with a shallow amplitude of 4.5%. By combining optical brightness information and thermal emission data, we are able to derive a diameter of 62.3 ± 5.3 km and a geometric albedo of 2.9%, which corresponds to an extremely dark surface. Additionally, our finding of ≳50 Jm^-2 K^-1 s^{− 1/2} for the thermal inertia is also remarkable for objects in such a distance. The results of dynamical simulations yield an unstable orbit, with a 50% probability that the target will be ejected from the solar system within 700 000 yr. The current orbit of this object and its instability could imply a pristine cometary surface. This possibility agrees with the observed low geometric albedo and red photometric colour indices for the object, which match the surface of a dormant comet well, as would be expected for a long-period cometary body approaching perihelion. Although it was approaching ever closer to the Sun, however, the object exhibited star-like profiles in each of our observations, lacking any sign of cometary activity. According to the albedo, 2013 AZ₆₀ is a candidate for the darkest body among the known trans-Neptunian objects.