Size and albedo distributions of asteroids in cometary orbits using WISE data

¹ Instituto de Astrofísica de Canarias (IAC), C/vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
e-mail: jlicandr@iac.es
² Departamento de Astrofísica, Universidad de La Laguna (ULL), 38205 La Laguna, Tenerife, Spain
³ Departamento de Astronomía, Facultad de Ciencias, Iguá 4225, 11400 Montevideo, Uruguay
⁴ Physics Department, University of Central Florida, PO Box 162385, Orlando, FL 32816.2385, USA

Received: 30 June 2015
Accepted: 15 September 2015

Abstract

Context. Determining whether asteroids in cometary orbits (ACOs) are dormant or extinct comets is relevant for understanding the end-states of comets and the sizes of the comet population.

Aims. We intend to study the value distributions of effective diameter (D), beaming parameter (η), and visible geometric albedo (p_V) of ACO populations, which can be derived from NASA’s Wide-field Infrared Explorer (WISE) observations, and we aim to compare these with the same, independently determined properties of the comets.

Methods. The near-Earth asteroid thermal model (NEATM) is used with WISE data and the absolute magnitude (H) of the ACOs to compute the D, p_V and η.

Results. We obtained D and p_V for 49 ACOs in Jupiter family cometary orbits (JF-ACOs) and 16 ACOs in Halley-type cometary orbits (Damocloids). We also obtained the infrared beaming parameter η for 45 of them. All but three JF-ACOs (95% of the sample) present a low albedo compatible with a cometary origin. The p_V and η distributions of both ACO populations are very similar. For the entire sample of ACOs, the mean geometric albedo is p̅_V = 0.05±0.02, (p̅_V = 0.05±0.01 and p̅_V = 0.05±0.02 for JF-ACOs and for Damocloids, respectively) compatible with a narrow albedo distribution similar to that of the Jupiter family comets (JFCs), with a p̅_V ~ 0.04. The mean beaming parameter is η̅ = 1.0±0.2. We find no correlations between D, p_V, or η. We also compare the cumulative size distribution (CSD) of ACOs, Centaurs, and JFCs. Although the Centaur sample contains larger objects, the linear parts in their log-log plot of the CSDs presents a similar cumulative exponent (β = 1.85 ± 0.30 and 1.76 ± 0.35, respectively). The CSD for Damocloids presents a much shallower exponent β = 0.89 ± 0.17.

Conclusions. The p_V- and η-value distributions of ACOs are very similar to those of JF comet (JFCs) nuclei. The ACOs in Tancredi’s list are the best possible candidates to be dormant/inactive comets. The CSD for JF-ACOs is shallower and shifted towards larger diameters with respect to the CSD of active JFCs, which suggests that the mantling process has a size dependency whereby large comets tend to reach an inactive stage faster than small comets. Finally, the population of JF-ACOs is comparable in number with the population of JFCs, although there are more tens-km JF-ACOs than JFCs.