Spectropolarimetry of B-type and C-type asteroids: Phase Curves and Wavelength Dependence

Abstract:

We present recent spectropolarimetric observations of eleven B- and C-type asteroids using SPOL (http://james.as.arizona.edu/~psmith/SPOL/) at the 2.3-m Bok and 1.6-m Kuiper telescopes. Our primary goal is to further constrain our understanding of the wavelength dependence of linear polarization. For each object, we produced polarimetric phase curves (linear polarization vs. phase angle) and measure the wavelength dependence in each observation by calculating the slope of each polarization spectrum. Previous analysis by Belskaya et al. (2009) shows that the slope of the polarization spectrum increases as the wavelength increases. No analysis of the wavelength dependence in B-types has occurred previously to our knowledge. For the five C-type (3 Ch- and 2 C-) objects observed, those of the Ch-type have larger minimum polarizations than of the C- subtype. This is consistent with polarimetric phase curves of similar asteroids by Gil-Hutton and Cañada-Assandri (2012). With respect to the wavelength dependence, our observed targets show an increase in polarization slope with wavelength, confirming the Belskaya (2009) result. After dividing the data by group, C- subtype objects have relatively flat polarization spectra, as opposed to the Ch- subtypes that have more rapidly changing slopes as phase angle increases. A mineralogical basis for the variety of wavelength dependences observed is most likely. Additional observations of the other C- subtypes will aid to distinguish how mineralogical variation affects polarization. It may also explain the wide range of polarization slopes measured near a phase angle of ten degrees in the Belskaya (2009) analysis. Six B-type asteroids were observed in a similar manner. As with the C-types, the polarimetric phase curves vary in shape, suggesting differences in albedo. Also, the wavelength dependence increases with wavelength, as shown for the C-types. On the other hand, all of the B-type objects have a similar wavelength dependence trend with phase angle regardless of its polarization curve; there is a steady increase in polarization slope with phase angle for all objects. Results such as these will hopefully provide additional constraints to theoretical work related to regolith scattering processes.