Spectropolarimetry of Galactic stars with anomalous extinction sightlines

Abstract

Highly reddened type Ia supernovae (SNe Ia) with low total-to-selective visual extinction ratio values,RV, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths (λmax≲ 0.4μm). It is not clear why sightlines to SNe Ia display such different continuum polarization profiles from interstellar sightlines in the Milky Way with similarRVvalues. We investigate polarization profiles of a sample of Galactic stars with lowRVvalues, along anomalous extinction sightlines, with the aim to find similarities to the polarization profiles that we observe in SN Ia sightlines. We undertook spectropolarimetry of 14 stars, used archival data for 3 additional stars, and ran dust extinction and polarization simulations (by adopting the picket-fence alignment model) to infer a simple dust model (size distribution, alignment) that can reproduce the observed extinction and polarization curves. Our sample of Galactic stars with lowRVvalues and anomalous extinction sightlines displays normal polarization profiles with an averageλmax~ 0.53μm, and is consistent within 3σto a larger coherent sample of Galactic stars from the literature. Despite the lowRVvalues of dust toward the stars in our sample, the polarization curves do not show any similarity to the continuum polarization curves observed toward SNe Ia with lowRVvalues. There is a correlation between the best-fit Serkowski parametersKandλmax, but we did not find any significant correlation betweenRVandλmax. Our simulations show that theK–λmaxrelationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normalλmaxand lowRVvalues, a population of large (a ≥ 0.1μm) interstellar silicate grains must be contained in the dust composition.