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

TL;DR

This study examines how interstellar polarization efficiency varies with wavelength and dust temperature, finding a correlation consistent with radiative torque alignment theory, suggesting grains align better with stronger, bluer radiation.

Contribution

It provides observational evidence linking polarization efficiency to dust temperature and supports the radiative torque alignment mechanism for interstellar grains.

Findings

Polarization efficiency follows an exponential decay with wavelength.

Higher dust temperatures correlate with increased polarization efficiency at shorter wavelengths.

Results support radiative torque alignment as a key mechanism for grain alignment.

Abstract

We investigate the efficiency of interstellar polarization $p_\lambda/A_\lambda$ where $p_\lambda$ is the fractional linear polarization and $A_\lambda$ is extinction, in 16 lines of sight as a function of wavelength $\lambda$. We have used the data obtained with the low-dispersion spectropolarimeter HBS as well as those in literature. It is found that the polarization efficiency $p_\lambda/A_\lambda$ is proportional to $\exp(-\beta/\lambda)$ in wavelength $\lambda \approx 0.4-0.8 \micron$, where $\beta$ is a parameter which varies from 0.5 to 1.2 $\micron$. We find that $\beta$ is negatively correlated with the dust temperature deduced from infrared data by Schlegel et al., suggesting that the polarization efficiency is higher in short wavelength for higher temperature. According to the alignment theory by radiative torques (RATs), if the radiation is stronger and bluer, RATs will make small grains align better, and the polarization efficiency will increase in short wavelength. Our finding of the correlation between $\beta$ and the temperature is consistent with what is expected with the alignment mechanism by RATs.