Angle-dependent radiative grain alignment; Confirmation of a magnetic field - radiation anisotropy angle dependence on the efficiency of interstellar grain alignment

TL;DR

This study confirms that the efficiency of interstellar grain alignment depends on the angle between magnetic fields and radiation anisotropy, supporting the Radiative Alignment Torques theory with improved observational accuracy.

Contribution

The paper provides a more precise analysis of grain alignment efficiency dependence on angle using spectral classification and photometric data, confirming previous findings with higher accuracy.

Findings

Grain alignment efficiency varies with the magnetic field-radiation angle.

Grain temperature also depends on the angle, affecting alignment.

Estimated fraction of aligned grains in the cloud.

Abstract

Interstellar grain alignment studies are currently experiencing a renaissance due to the development of a new quantitative theory based on Radiative Alignment Torques (RAT). One of the distinguishing predictions of this theory is a dependence of the grain alignment efficiency on the relative angle ($\Psi$) between the magnetic field and the anisotropy direction of the radiation field. In an earlier study we found observational evidence for such an effect from observations of the polarization around the star HD 97300 in the Chamaeleon I cloud. However, due to the large uncertainties in the measured visual extinctions, the result was uncertain. By acquiring explicit spectral classification of the polarization targets, we have sought to perform a more precise reanalysis of the existing polarimetry data. We have obtained new spectral types for the stars in our for our polarization sample, which we combine with photometric data from the literature to derive accurate visual extinctions for our sample of background field stars. This allows a high accuracy test of the grain alignment efficiency as a function of $\Psi$. We confirm and improve the measured accuracy of the variability of the grain alignment efficiency with $\Psi$, seen in the earlier study. We note that the grain temperature (heating) also shows a dependence on $\Psi$ which we interpret as a natural effect of the projection of the grain surface to the illuminating radiation source. This dependence also allows us to derive an estimate of the fraction of aligned grains in the cloud.