Spectropolarimetry of Galactic stars with anomalous extinction
  sightlines
  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
($\lambda_{max} \lesssim 0.4 \mu m$). 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, and used archival data for three additional
stars, and run dust extinction and polarization simulations to infer a simple
dust model 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 $\lambda_{max}
\sim 0.53 {\mu m}$, and is consistent within 3$\sigma$ to a larger coherent
sample of Galactic stars from literature. Despite the low $R_V$ values of dust
towards the stars in our sample, the polarization curves do not show any
similarity to the continuum polarization curves observed towards SNe Ia with
low $R_V$ values. There is a correlation between the best-fit Serkowski
parameters $K$ and $\lambda_{max}$, but we did not find any significant
correlation between $R_V$ and $\lambda_{max}$. Our simulations show that the
$K-\lambda_{max}$ relationship is an intrinsic property of polarization.
Furthermore, we have shown that in order to reproduce polarization curves with
normal $\lambda_{max}$ and low $R_V$ values, a population of large (a $\geq 0.1
\mu m$) interstellar silicate grains must be contained in the dust's
composition.
