The thermal emission of Centaurs and trans-Neptunian objects at millimeter wavelengths from ALMA observations

¹ LESIA, Observatoire de Paris, PSL Research University, CNRS, Sorbonne Universités, UPMC Univ. Paris 06, Univ. Paris Diderot, Sorbonne Paris Cité, 5 place Jules Janssen, 92195 Meudon, France
e-mail: emmanuel.lellouch@obspm.fr
² Max-Planck-Institut für Extraterrestrische Physik, Giessenbachstraße, 85748 Garching, Germany
³ Instituto de Astrofísica de Andalucía-CSIC, Glorieta de la Astronomía s/n, 18008 Granada, Spain
⁴ National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903, USA
⁵ Harvard-Smithsonian Center for Astrophysics, Cambridge, MA 02138, USA
⁶ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD 21218, USA
⁷ Instituto de Astrofísica, Facultad de Física, Pontificia Universidad Católica de Chile, Av. Vicuña Mackenna 4860, Santiago, Chile
⁸ National Radio Astronomy Observatory, Socorro, NM 87801, USA
⁹ IRAM, Domaine Universitaire, 300 rue de la Piscine, 38400 Saint-Martin-d’Hères, France

Received: 30 July 2017
Accepted: 20 September 2017

Abstract

The sensitivity of ALMA makes it possible to detect thermal mm/submm emission from small and/or distant solar system bodies at the sub-mJy level. While the measured fluxes are primarily sensitive to the objects’ diameters, deriving precise sizes is somewhat hampered by the uncertain effective emissivity at these wavelengths. Following recent work presenting ALMA data for four trans-Neptunian objects (TNOs) with satellites, we report on ALMA 233 GHz (1.29 mm) flux measurements of four Centaurs (2002 GZ₃₂, Bienor, Chiron, Chariklo) and two other TNOs (Huya and Makemake), sampling a range of sizes, albedos, and compositions. These thermal fluxes are combined with previously published fluxes in the mid/far infrared in order to derive their relative emissivity at radio (mm/submm) wavelengths, using the Near Earth Asteroid Standard Model (NEATM) and thermophysical models. We reassess earlier thermal measurements of these and other objects – including Pluto/Charon and Varuna – exploring, in particular, effects due to non-spherical shape and varying apparent pole orientation whenever information is available, and show that these effects can be key for reconciling previous diameter determinations and correctly estimating the spectral emissivities. We also evaluate the possible contribution to thermal fluxes of established (Chariklo) or claimed (Chiron) ring systems. For Chariklo, the rings do not impact the diameter determinations by more than ~5%; for Chiron, invoking a ring system does not help in improving the consistency between the numerous past size measurements. As a general conclusion, all the objects, except Makemake, have radio emissivities significantly lower than unity. Although the emissivity values show diversity, we do not find any significant trend with physical parameters such as diameter, composition, beaming factor, albedo, or color, but we suggest that the emissivity could be correlated with grain size. The mean relative radio emissivity is found to be 0.70 ± 0.13, a value that we recommend for the analysis of further mm/submm data.