2018A&A...615A..42C

Highly reddened type Ia supernovae (SNe Ia) with low total-to-selective visual extinction ratio values, R_V, also show peculiar linear polarization wavelength dependencies with peak polarizations at short wavelengths (λ_max≤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 similar R_V values. We investigate polarization profiles of a sample of Galactic stars with low R_V values, 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 low R_V values and anomalous extinction sightlines displays normal polarization profiles with an average λ_max∼0.53µm, and is consistent within 3σ to a larger coherent sample of Galactic stars from the literature. Despite the low R_V values 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 low R_V values. There is a correlation between the best-fit Serkowski parameters K and λ_max, but we did not find any significant correlation between R_V and λ_max. Our simulations show that the K-λ_max relationship is an intrinsic property of polarization. Furthermore, we have shown that in order to reproduce polarization curves with normal λ_max and low R_V values, a population of large (a≥0.1µm) interstellar silicate grains must be contained in the dust composition.