Polarimetric survey of main-belt asteroids

Abstract

Aims. We present results of a polarimetric survey of main-belt asteroids at Complejo Astronómico El Leoncito (Casleo), San Juan, Argentina. The aims of this survey are to increase the database of asteroid polarimetry, to estimate diversity in polarimetric properties of asteroids that belong to different taxonomic classes, and to search for objects that exhibit anomalous polarimetric properties. Methods. The data were obtained with the Torino and CASPROF polarimeters at the 2.15 m telescope. The Torino polarimeter is an instrument that allows the simultaneous measurement of polarization in five different bands, and the CASPROF polarimeter is a two-hole aperture polarimeter with rapid modulation. Results. The survey began in 2003, and up to 2009 data of a sample of more than 170 asteroids were obtained. In this paper the results for 33 X-type objects are presented, several of them are being polarimetrically observed for the first time. Using these data we found polarization curves and polarimetric parameters for different groups among this taxonomic class and that there are objects with very different albedo in the sub-classes of the X taxonomic complex.

Polarimetric survey of main-belt asteroids

“…3. All the observations follow the phase -polarization curve proposed by Cañada-Assandri et al (2012) with the exception of those of the asteroid (402) Chloe, which suggests a large inversion angle, indicating that this object must be added to the list of known Barbarians. Chloe belongs to the K-class but we know that there is at least another asteroid classified as K, (679) Pax, which is also a Barbarian (Gil-Hutton et al 2008).…”

“…6. As mentioned above and as also noted by Cañada-Assandri et al (2012), the high dispersion of the data is a result of merging objects with quite heterogeneous surface properties, but all included in the Bus & Binzel X-complex. Some of these objects were previously assigned to different classes (E-, M-, P-types) by Tholen (1989), and were known to be strongly different in terms of albedo (high, moderate, and low for E, M, and P, respectively).…”

“…Polarimetry is strongly sensitive to albedo differences. As a reference, the phase -polarization curves for moderate and low albedo found by Cañada-Assandri et al (2012) are also included in Fig. 6.…”

“…Since 1995 we have made an extensive effort of polarimetric observations at Complejo Astronómico El Leoncito (CASLEO), Argentina, with the main objective of increasing the available polarimetric database of main-belt asteroids. At the same time, we also developed several observing programs focused on more specific objectives, including studies of the size dependence of the albedo distribution exhibited by the IRAS data (Cellino et al 1999(Cellino et al , 2005a, analyses of the polarimetric properties of different taxonomic classes and asteroid groups (Gil-Hutton 2007;Gil-Hutton et al 2007;Gil-Hutton & Cañada-Assandri 2011Cañada-Assandri et al 2012) and of some individual targets, like the near-Earth A&A 569, A122 (2014) asteroid (25453) Itokawa, the target of the Japanese Hayabusa space mission (Cellino et al 2005b). An unexpected result of this overall observational effort was the discovery of the unusual polarimetric behavior of some objects, the so-called Barbarians (Cellino et al 2006;Gil-Hutton et al 2008).…”

Significantly high polarization degree of the very low-albedo asteroid (152679) 1998 KU2

“…Because this is the first time that polarimetric measurements of a dark asteroid have been performed at large phase angles, there is no precedent that can be directly compared with our results. Certain major polarimetric parameters (i.e., the polarization slope and inverse angle), the majority of which were acquired at phase angles smaller than 30 • , have been known to exhibit typical values of the corresponding taxonomic types according to previous statistical studies (Belskaya et al 2005(Belskaya et al , 2017Gil-Hutton & Cañada-Assandri 2011, 2012Cañada-Assandri et al 2012;Cellino et al 2012).…”