Correlation between Interstellar Polarization and Dust Temperature: Is the Alignment of Grains by Radiative Torques Ubiquitous?

Abstract

We have investigated the efficiency of interstellar polarization, $p_\lambda/A_\lambda$, where $p_\lambda$ is the fractional linear polarization and $A_\lambda$ is the extinction, in 14 lines of sight as a function of the wavelength, $\lambda$. We used the data of lines of sight to the Pleiades cluster obtained with the low-dispersion spectropolarimeter HBS as well as those in literature. It has been found that the polarization efficiency, $p_\lambda/A_\lambda$, is proportional to exp ($-\beta/\lambda$) in wavelength, $\lambda$$\approx$ 0.4–0.8$\ \mu$m, where $\beta$ is a parameter that varies from 0.5 to 1.2$\ \mu$m. We find that $\beta$ is negatively correlated with the dust temperature deduced from infrared data by Schlegel et al. (1998, ApJ, 500, 525), suggesting that the polarization efficiency is higher in short wavelengths for higher temperature. According to the alignment theory by radiative torques (RATs), if the radiation is stronger, RATs will make small grains align better, and the polarization efficiency will increase at short wavelengths. Our finding of the correlation between $\beta$ and the temperature is consistent with what is expected with the alignment mechanism by RATs.