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Linear Spectro-Polarimetry: a New Diagnostic Tool for the Classification and Characterisation of Asteroids

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Linear Spectro-Polarimetry: a New Diagnostic Tool for the Classification and Characterisation of Asteroids Mon. Not. R. Astron. Soc. 000, 1–5 (2014) Printed 13 November 2018 (MN LATEX style file v2.2) Linear spectro-polarimetry: a new diagnostic tool for the classification and characterisation of asteroids S. Bagnulo1, A. Cellino2 and M. F. Sterzik3 1Armagh Observatory, College Hill, Armagh BT61 9DG, UK. E-mail: [email protected] 2INAF - Osservatorio Astrofisico di Torino, I-10025 Pino Torinese, Italy. E-mail: [email protected] 3European Southern Observatory, Karl-Schwarzschild-Strasse 2, D-85748 Garching, Germany. E-mail: [email protected] Accepted 2014 September 14. Received 2014 September 08; in original form 2014 July 24 ABSTRACT We explore the use of spectro-polarimetry as a remote sensing tool for asteroids in addition to traditional reflectance measurements. In particular we are interested in possible relationships between the wavelength-dependent variation of linear polariza- tion and the properties of the surfaces, including albedo and composition. We have obtained optical spectro-polarimetric measurements of a dozen asteroids of different albedo and taxonomic classes and of two small regions at the limb of the Moon. We found that objects with marginally different relative reflectance spectra (in the optical) may have totally different polarization spectra. This suggests that spectro- polarimetry may be used to refine the classification of asteroids. We also found that in some cases the Umov law may be violated, that is, in contrast to what is expected from basic physical considerations, the fraction of linear polarization and the reflectance may be positively correlated. In agreement with a few previous studies based on multi- colour broadband polarimetry, we found that the variation of linear polarization with wavelength and with phase-angle is correlated with the albedo and taxonomic class of the objects. Finally, we have serendipitously discovered that spinel-rich asteroid (599) Luisa, located very close to the Watsonia family, is a member of the rare class of Barbarian asteroids. We suggest that future modelling attempts of the surface structure of asteroids should be aimed at explaining both reflectance and polarization spectra. Key words: polarization – minor planets, asteroids: general – Moon. 1 INTRODUCTION Broadband linear polarization (BBLP) measurements have long been used as a remote sensing tool for the char- acterisation of the objects of our solar system. BBLP mea- Light scattered by surfaces is polarized. This may be intu- surements in the standard optical filters are usually plot- itively understood by thinking that an electron sitting in a ted as a function of the phase-angle (the angle between the planar surface and hit by an electromagnetic wave is more arXiv:1409.4620v1 [astro-ph.IM] 16 Sep 2014 sun and the observer as seen from the target object) and free to oscillate in the direction parallel to the surface itself the morphology of the resulting phase-polarization curves rather than perpendicular to it. Accordingly, the radiation may be used for the purposes of albedo determinations (see re-emitted by the electron is partially linearly polarized in Cellino et al. 2012, and references therein), and for aster- the direction parallel to the surface and perpendicular to the oid classification (Penttil¨aet al. 2005). Since main-belt as- scattering plane (i.e., the plane containing the incident and teroids orbit at a significantly longer distance from the Sun the scattered light beams). Since the radiation produced by than Earth, the phase-angles at which they may be observed the oscillations of an electron moving up and down through ◦ are restricted to a small interval, typically ∼ 0 − 30 . In the the surface is more efficiently damped by a darker surface case of near-Earth objects the maximum attainable phase- than by a brighter one, one can expect that the light re- ◦ angle can be higher, well above 40 . Perhaps the most sur- flected by a darker surface is more polarized than the light prising feature of asteroid polarimetric properties is that at reflected by a brighter surface. The state of the polariza- small phase-angles the plane of linear polarization is parallel tion of the scattered radiation depends on the structure and to the scattering plane, in contrast to the simple scattering composition of the reflecting surface and on the scattering mechanism sketched out above. This phenomenon, which is angle, and its measurement may reveal information about traditionally referred to with the somehow confusing term of the physical properties of the reflecting surface. c 2014 RAS 2 S. Bagnulo et al. ◦ − ◦ negative polarization, is normally seen in the 0 20 phase- Table 1. BBLP values in the Bessel V RI filters from PQ spectra. angle range (usually referred to as the negative branch of the Photon-noise is negligible, and accuracy is limited by instrumen- phase-polarization curve) and may be explained in terms of tal polarization, which we estimate 6 0.1 %. The double taxon- coherent backscattering (Muinonen et al. 2002). omy classification given in col. 2 are from Tholen (1984) (left) and A widely adopted remote-sensing tool for the physi- Bus & Binzel (2002) (right). Asteroid observations were obtained cal characterization of small solar system bodies is spec- from September 2013 to March 2014. (1) Ceres was observed with troscopy. Similarly to what happens in stellar spectroscopy, ISIS, all the remaining targets with FORS. The Moon was ob- asteroid reflectance spectra are classified into distinct taxo- served with FORS in April and June 2011. nomic classes. Taxonomy based on multi-band optical pho- tometry was first developed by several authors in the ’70s, Object Class α V R I and culminated in the classical work by Tholen (1984). More (%) (%) (%) recently, broadband photometry has evolved in full-fledged spectroscopy using spectrographs equipped with CCDs. A (1) Ceres G/C 22.4◦ 1.17 1.21 1.25 commonly adopted taxonomic classification based on spec- (2) Pallas B/B 27.5◦ 2.25 2.29 2.33 tra at visible wavelengths was published by Bus & Binzel 22.9◦ 0.99 1.00 1.03 (2002), and an extension to the near IR region was more (7) Iris S/S 26.9◦ 0.58 0.52 0.48 recently proposed by DeMeo et al. (2009). ◦ In this paper we want to assess whether spectro- 27.5 0.68 0.62 0.56 28.2◦ 0.75 0.68 0.64 polarimetry may be used to complement and refine the ob- serving techniques of spectroscopy and broadband polarime- (8) Flora S/S 28.4◦ 0.78 0.68 0.60 try, that so far have been only separately considered. For this (21) Lutetia M/Xk 14.6◦ −1.19 −1.23 −1.23 reason, we have started a survey of spectro-polarimetry of ◦ asteroids, to our knowledge the first of its kind. (24) Themis C/B 14.0 −1.23 −1.18 −1.12 The taxonomic classifications of reflectance spectra by (44) Nysa E/Xc 9.1◦ −0.27 −0.30 −0.32 Tholen (1984) and Bus & Binzel (2002) were based on Prin- 24.2◦ 0.23 0.24 0.25 cipal Component Analysis of hundreds of objects. So far, our . ◦ − . − . − . spectro-polarimetric dataset is far too small to allow us any (51) Nemausa CU/Ch 15 7 1 11 1 10 1 06 systematic classification. This paper presents therefore the (208) Lacrimosa S/Sk 13.7◦ −0.46 −0.47 −0.50 results of a pilot project aimed at assessing the usefulness ◦ (236) Honoria S/L 7.1 −1.00 −1.08 −1.17 of further investigations using this technique. (433) Eros S/S 42.0◦ 1.99 1.87 1.86 (599) Luisa S/K 26.9◦ −0.39 −0.30 −0.16 2 OBSERVATIONS Moon E n.a. 81.7◦ 9.86 8.28 7.07 We have obtained spectro-polarimetric measurements of Moon M n.a. 78.3◦ 5.81 4.99 4.36 a sample of asteroids using the FORS2 instrument (Appenzeller et al. 1998) of the ESO Very Large Tele- scope (VLT), and the ISIS instrument of the William Herschel Telescope (WHT) of the Isaac Newton Group tracted then wavelength calibrated using IRAF routines, of Telescopes. During an earlier VLT-FORS visitor mode and then combined with FORTRAN routines. Throughout run dedicated to the observations of the Earthshine this paper we will refer to the reduced Stokes parameter (Sterzik, Bagnulo, & Pall´e2012) we have also observed the PQ(λ) = Q/I representing the flux perpendicular to the sunlit limb of the Moon. plane Sun-Object-Earth (the scattering plane) minus the The instruments employed in our measurements are slit- flux parallel to that plane, divided by the sum of the two fed and are equipped with similar polarimetric optics, con- fluxes. For symmetric reasons, Stokes U is expected to be sisting of a retarder waveplate and a beam-splitter polarizer: zero. From the spectro-polarimetric data we calculated syn- a Wollaston prism in case of FORS2, and a Savart plate in thetic BBLP values (see Table 1). Approximate reflectance case of ISIS. The retarder waveplates may be set at fixed po- spectra r(λ) were obtained by dividing the intensity spec- sition angles, allowing one to exploit the advantages of the tra by the spectrum of solar analogue HD 30246 observed “beam-swapping” technique (Bagnulo et al. 2009). Thanks on 2014-01-30, but without taking into account wavelength to the beam-swapping technique, to the fact that both in- dependent slit losses, and then normalised to λ = 550 nm. struments are slit-fed, and that the light reflected by the Data were rebinned to a spectral bin of ∼ 11 nm. target reaches the polarimetric optics without oblique reflec- Polarization spectra of our targets are shown in Fig.
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