Polarimetric survey of asteroids with the Asiago telescope

Abstract

Aims. We present the first results of an asteroid photo-polarimetry program started at Asiago-Cima Ekar Observatory. The aim of our survey is to estimate diversity in polarimetric properties of asteroids belonging to different taxonomic and dynamical classes. Methods. The data were obtained with the polarization analyser placed inside the Faint Object Spectrographic Camera (AFOSC) of the 1.8 m telescope. This instrument allows simultaneous measurements of the two first Stokes parameters without any λ/2 retarding plate. Results. Our survey began in 2002, and up to now we have obtained data on a sample of 36 asteroids; most of them are being investigated with the polarimetric technique for the first time. Combining our data with those already available in literature, we present an estimate of the inversion angle for 7 asteroids in this paper. Furthermore, we present the polarimetric measurements of the rare asteroid classes belonging to the A and D types and a detailed VRI observations at extremely small phase angles of the low albedo asteroid 1021 Flammario

Polarimetry of M-type asteroids

“…In the second case, the observation is from Fornasier et al (2006), who also suggest that this object could have a behavior similar to that shown by 21 Lutetia with a possible large inversion angle. In all the cases the observations were also fitted with the phase-polarization curve model proposed by Lumme & Muinonen (1993) and a third order polynomial (Goidet et al 1992), but the polarimetric parameters obtained are always coincident within the nominal error bars.…”

“…Although most targets could be observed at a single phase angle, comparing the data obtained with the available polarimetric measurements of asteroids of the same taxonomic type allows us to draw interesting conclusions on their surface properties. On the other hand, to enlarge the polarimetric database of M-type asteroids, observations of 13 objects classified as M-type were obtained from the Small Bodies Node at NASA's Planetary Data System (http://pdssbn.astro.umd.edu/sbnhtml/asteroids) and from Zellner & Gradie (1976), Belskaya et al (1985Belskaya et al ( , 1987aBelskaya et al ( ,b, 1991, Broglia & Manara (1992), Cellino et al (1999Cellino et al ( , 2005, and Fornasier et al (2006). Since data obtained with an R filter is normally of good quality and generally compatible with V filter data, observations in both filters from these sources were taken into account.…”

First albedo determination of 2867 Steins, target of the Rosetta mission

“…3). Its polarimetric properties are similar to those of the A-type asteroid 863 Benkoela (Cellino et al 2005;Fornasier et al 2006), which has an IRAS albedo of 0.6. However, such high values of IRAS albedos could be an overestimation connected with using non-simultaneous photometric and radiometric data.…”

Puzzling asteroid 21 Lutetia: our knowledge prior to the Rosetta fly-by

“…Other observations of Lutetia were carried out in the framework of a coordinate program at three observatories: the Crimean Astrophysical Observatory (Ukraine), the Asiago Observatory (Italy) and Complejo Astronómico El Leoncito (Casleo, Argentine), to cover phase angles that had not been previously observed. A part of these data was published among results of observations at each telescope (Fornasier et al 2006;Gil-Hutton 2007;Belskaya et al 2009). Here we report complementary observations of Lutetia that have not yet been published.…”

“…Table 2 presents the mean time of observations in UT, the phase angle α, the polarization degree P and position angle Θ in the equatorial coordinate system, together with the root-mean-square errors σ P and σ Θ , the calculated values of the corresponding P r and position angle Θ r in the coordinate system referring to the scattering plane as defined by Zellner & Gradie (1976), and the telescope. Methods of observations and data processing were identical to those described by Fornasier et al (2006) for Asiago, Belskaya et al (2009) for Crimea, and Gil-Hutton (2007) for Casleo. The polarization-phase function of Lutetia obtained using both new and published data is shown in Fig.…”