New cases of unusual polarimetric behavior in asteroids

Abstract

Aims. Results of different polarimetric campaigns at Complejo Astronómico El Leoncito (Casleo), San Juan, Argentina are presented. The aim of these campaigns was to search for objects exhibiting anomalous polarimetric properties, similar to those shown by the Ld-class asteroid (234) Barbara, among members of the same or similar taxonomic classes. Methods. The data have been obtained with Torino and CASPROF polarimeters at the 2.15 m telescope. The Torino polarimeter is an instrument that allows simultaneous measurement of polarization in five different bands, and CASPROF polarimeter is a two-hole aperture polarimeter with rapid modulation. Results. The campaigns began in 2005, and we found four new asteroids with Barbara-like polarimetric properties: the L-class objects (172) Baucis, (236) Honoria and (980) Anacostia, and the K-class asteroid (679) Pax. The polarimetric properties of the phase-polarization curves of these objects may be produced by a mixture of high-and low-albedo particles in their regolith as a result of the fragmentation of a substrate that is spectrally analog to the CO3/CV3 chondrites.

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“…6). This group of moderate-albedo asteroids of spectral type L, K or Ld have anomalous polarization properties which may be related to their specific surface composition (Cellino et al 2006;Gil-Hutton et al 2008;Masiero & Cellino 2009). We note, that the polarization minimum value of these asteroids deviates considerably from the well-known correlation "P min -albedo" and cannot be used for albedo estimation.…”

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“…A full description of the Torino photopolarimeter can be found in Piirola (1988) and Scaltriti et al (1989), while a description of CASPROF is given in Gil-Hutton et al (2008). From the analysis of several standard stars, we found the instrumental polarization to be fairly constant and stable, always below 0.1% for both instruments and for each filter.…”

“…Observations of the asteroids (172) Baucis, (234) Barbara, and (980) Anacostia, are included in that figure although they were previously published, are included in that figure. These objects, commonly known as Barbarians, from their prototype (234) Barbara, were found by Cellino et al (2006) and Gil-Hutton et al (2008) to show an unusual polarimetry behavior characterized by an inversion angle α 0 30 • . Although (234) Barbara itself is classified as Ld-class (which might suggest that this class might be a good candidate to include all Barbarians), there are other known Ld asteroids, such as (269) Justitia, which are not Barbarians, as shown in Fig.…”

“…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).…”

Polarimetric survey of main-belt asteroids

“…3). This last group is mainly composed of observations of asteroids that are known to show unusual polarimetric behavior and follow a phase-polarization curve with a large inversion angle: (172) Baucis, (234) Barbara, (236) Honoria, (387) Aquitania, (679) Pax, and (980) Anacostia (Cellino et al 2006;Gil-Hutton et al 2008;Masiero & Cellino 2008). From these observations there are three interesting points that must be indicated.…”