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