Differences between the Pallas collisional family and similarly sized B-type asteroids^⋆

¹ Laboratoire Lagrange, UMR 7293, Université de la Côte d’Azur, CNRS, Observatoire de la Côte d’Azur, 06304 Nice Cedex 4, France
e-mail: valilagoa@oca.eu
² Instituto de Astrofísica de Canarias (IAC), c/ vía Láctea s/n, 38205 La Laguna, Tenerife, Spain
³ Departamento de Astrofísica, Universidad de La Laguna, 38071 La Laguna, Tenerife, Spain
⁴ Complejo Astronómico El Leoncito (CONICET), Av. España 1512 sur, J5402DSP San Juan, Argentina
⁵ Universidad Nacional de San Juan, J. I. de la Roza 590 oeste, 5400 Rivadavia, San Juan, Argentina
⁶ Physics Department, University of Central Florida, PO Box 162385, Orlando, FL 32816.2385, USA
⁷ Department of Earth and Planetary Sciences, University of Tennessee, 1412 Circle Dr, Knoxville, TN 37996-1410, USA
⁸ Florida Space Institute/University of Central Florida, Orlando, FL 32826-0650, USA
⁹ Department of Astronomy, University of Maryland, College Park, MD 20472-2421, USA

Received: 29 October 2015
Accepted: 14 March 2016

Abstract

Context. B-type asteroids constitute a peculiar spectral class within the C-complex. Previous work has pointed out the difference between the visible geometric albedos of B-types and those of the Pallas collisional family (PCF), whose few members with observed spectra are B-types (one exception out of eight objects). This has been interpreted as being due to compositional differences. However, the PCF members are typically smaller than the spectroscopically classified B-types, and the following possibilities have not been ruled out: the albedo differences might be related to a size-albedo dependence and/or to the generally larger errors of the WISE data and best-fitting values of the derived parameters expected for smaller objects.

Aims. We compare albedos and beaming parameters of PCF members and B-types of similar sizes and re-examine our conclusion on the different composition of the PCF.

Methods. By modelling their WISE/NEOWISE data, we derived sizes and albedos of all objects whose Sloan Digital Sky Survey reflectances are similar to the typical B-type reflectance spectra. In particular, we derived the so-called infrared beaming parameters (η), effective diameters (D), and corresponding visible geometric albedos (p_V), and studied their value distributions.

Results. We obtained the effective diameter and geometric visible albedo for ~600 B-type asteroids whose sizes range between 2 and 100 km, approximately half of which have fitted η-values that are inversely correlated to size. We found that the albedo distributions of the PCF is significantly different from that of other B-types in the same size range (2−20 km), and we rule out any size-albedo dependency or biases related to the lower quality of the p_V-values of smaller objects. In addition, we also found differences between the η-value distribution of the PCF and the other similarly-sized B-types.

Conclusions. The differences in the visible albedos of PCF members and the other B-types of similar sizes is a strong indication that their compositions are different. The trend between beaming parameter and size of the B-types (excluding the PCF) is consistent with the expectation that the surfaces of smaller asteroids have coarser rocks than do those of larger objects (D> 40 km), which are thought to be covered by a finer, more insulating layer of dust.