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Download This Article in PDF Format A&A 626, A42 (2019) Astronomy https://doi.org/10.1051/0004-6361/201935246 & © ESO 2019 Astrophysics Polarimetric survey of main-belt asteroids VII. New results for 82 main-belt objects?,?? C. López-Sisterna, E. García-Migani, and R. Gil-Hutton Grupo de Ciencias Planetarias, Departamento de Geofísica y Astronomía, Facultad de Ciencias Exactas, Físicas y Naturales, Universidad Nacional de San Juan - CONICET, Av. Ignacio de la Roza 590 (O), J5402DCS Rivadavia, San Juan, Argentina e-mail: [email protected] Received 11 February 2019 / Accepted 6 May 2019 ABSTRACT Aims. We present the 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 and to estimate the diversity in the polarimetric properties of asteroids. Methods. The survey began in 1995 and a second period began in 2013 using the CASPOL polarimeter with a more sensitive detector to study small asteroids, families, and special taxonomic groups. The data were obtained using this instrument at the 2:15 m telescope of CASLEO. Results. We present 128 observations for 82 asteroids of different taxonomic types. These results revealed phase-polarization curves and polarimetric parameters for 20 asteroids, amounting to a total of 135 objects with sufficient good data in the Catalogue of Asteroid Polarization Curves. Using the values obtained for the objects with a taxonomic classification, we obtained the mean polarimetric parameters for 19 taxonomic types and the Barbarians. The asteroids with large mean scatter separation distances have a minimum of the phase-polarization curve greater than 1%, slopes at the inversion angle of less than 0:12 0:15% per degree, and perihelion distances 1:8 < q < 2:5 au; these measurements− indicate the asteroids could have high or moderate albedos− and that they are objects with perihelia in the inner asteroid belt. These large mean scatter separation distance values could be the result of an electrostatic mechanism acting on the small grains of the regolith, a manifestation of a coherent backscattering mechanism, or the result of a surface formed by a mixture of dark and bright particles. Key words. minor planets, asteroids: general – techniques: polarimetric 1. Introduction provide information about the properties of the surface (Dollfus et al. 1989; Muinonen et al. 2002a; Kaasalainen et al. 2003). For The light that we receive from any asteroid at visible wavelengths phase angles <20◦, Pr turns out to be negative, reaching a mini- consists of partially polarized light produced by the scattering of mum of polarization,∼ P , at phase angles α 6 12 . Beyond min min ≈ − ◦ the sunlight on the solid surface of the body. The polarization 20◦ of phase, the polarization changes sign at the inversion is usually found to be linear with its azimuth either normal or ≈ angle, α0, and becomes positive, increasing linearly for larger parallel to the scattering plane, which in the solar system is the phase angles with a slope h. plane containing the asteroid, the Sun, and the Earth at the epoch Although polarimetry provides useful information about the of observation. In polarimetry, the results of observations are physical properties of the asteroid surface, polarimetric observa- usually expressed using the parameter Pr = (I I )=(I + I ), tions of these objects are not easy to obtain because an asteroid where I and I are the intensities of the scattered? − k light? polar-k ? k must be followed at several phase angles to study its polariza- ized along the planes perpendicular and parallel to the scattering tion curve; this kind of coverage is difficult owing to constraints plane, respectively. such as object faintness, limited visibility, and weather prob- The parameter Pr is affected by the composition, roughness, lems. As a consequence, the database of asteroid polarimetric and other physical properties of the target and also depends on measurements was very small until about 1990 and very few the illumination conditions. The variation of the degree of linear objects had their polarimetric parameters well determined. Since polarization as a function of the phase angle α, which is the angle 1995 we have made an extensive effort to conduct polarimetric between the directions to the Sun and to the observer as seen observations at Complejo Astronómico El Leoncito (CASLEO), from the object, produces the so-called phase-polarization curve. Argentina; the main objective has been to increase the avail- This curve is described by some polarimetric parameters that able polarimetric database of main-belt asteroids. The first epoch ? of this survey ended in 2012 and a second epoch started in Table 1 is only available at the CDS via anonymous ftp to 2013 using new and more sensitive equipment. This survey pro- cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsarc. u-strasbg.fr/viz-bin/qcat?J/A+A/626/A42 vided a large number of polarimetric measurements of main-belt ?? Based on observations carried out at the Complejo Astronómico El asteroids, including objects not observed before, and has the Leoncito, operated under agreement between the Consejo Nacional de advantage of being a homogeneous dataset that has always been Investigaciones Científicas y Técnicas de la República Argentina and observed and reduced following the same procedures. These the National Universities of La Plata, Córdoba, and San Juan. data, combined with results previously published, were used Article published by EDP Sciences A42, page 1 of5 A&A 626, A42 (2019) to obtain phase-polarization curves and polarimetric parameters 3. Results of good quality for 120 main-belt asteroids1 (Gil-Hutton & García-Migani 2017).≈ During this period of the survey we obtained 128 observa- In this paper we report polarimetric observations obtained tions for 82 main-belt asteroids. Each asteroid’s name, date, during 2017 and 2018, which have not yet been published, and total integration time in seconds (Tint), phase angle (α), position angle of the scattering plane (θ ), observed polarization (P) we use these in combination with previously published data to find polarimetric parameters for several asteroids. In Sect.2 we and its error (σP), position angle in the equatorial reference describe the observations, in Sect.3 we present and discuss our frame (θ) and its error (σθ), Pr, and Bus taxonomic classifica- results, and in Sect.4 we draw our conclusions. tion (Bus 1999) taken from Bus & Binzel(2002) or Lazzaro et al.(2004) are listed in Table 1. For 6 asteroids the Bus type is not determined, therefore we used the taxonomic classification of 2. Observations Tholen(1989) and these asteroids are indicated with bold type in Table 1. Our observations were carried out during various observing runs The data obtained for the asteroid (152) Atala confirm the between April 2017 and November 2018 at the 2:15 m telescope overall shape of a phase-polarization curve with an inversion of CASLEO using the CASPOL polarimeter. The CASPOL angle of 15◦ and a small Pmin, which suggest a misclassification instrument is a polarization unit inserted in front of a CCD as a S-type≈ object (Fornasier et al. 2006). The polarization val- camera that allows high precision imaging polarimetry. This ues obtained for (678) Fredegundis in combination with previous polarimeter was built following the design of Magalhaes et al. published observations suggest that this object has significa- (1996), and uses an achromatic half-wave retarder and a Savart tive variations in Pr, which could be a result produced by the plate as an analyzer. The reduction and analysis of the images change of the surface properties for different rotational phases. are done in the usual way using the Image Reduction and Analy- The asteroids (107) Camilla, (307) Nike, and (346) Hermentaria sis Facility (IRAF) tasks and scripts specially designed to reduce 2 are observed for the first time. observations made with CASPOL . The polarimetric parame- With the data presented in this work it is possible to find ters and errors are obtained from a least-squares solution to polarimetric parameters for 20 asteroids not included previously the measurements made at different half-wave plate positions. in our Catalogue of Asteroid Polarization Curves3, increasing All the polarimetric measurements were made using a V-band to 135 the number of objects with phase-polarization curves of filter. sufficient quality to find their polarimetric parameters. As usual, From the analysis of several standard stars, we found the the phase-polarization curve was obtained by fitting the obser- instrumental polarization to be fairly constant and stable, always vations available for these objects to the function proposed by below 0:05%. Whenever possible, we observed the targets dur- Kaasalainen et al.(2003) and Muinonen et al.(2009), i.e. ing runs several weeks apart to obtain measurements during the " ! # same apparition at different phase angles. Observing nights were α generally assigned around the new Moon to minimize the con- Pr(α) = A0 exp 1 + A2α, (1) − A1 − tamination of sky polarization by moonlight. Each night, we observed at least two zero-polarization standard stars and two where A0, A1, and A2 are constant coefficients. The phase- high-polarization stars to determine instrumental polarization. polarization curve was then used to find αmin, α0, Pmin, and h. The standard star data were obtained from Turnshek et al.(1990), These polarimetric parameters can be used to find the index of Gil-Hutton & Benavidez(2003), and Fossati et al.(2007). refraction n, and the product of the wave number, k, and the We observed the targets consecutively several times each mean scatter separation distance, d, applying the procedure used night with individual exposure times long enough to reach by Gil-Hutton & García-Migani(2017) in the expressions for signal-to-noise ratios (S/N) 40 in flux.
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