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

Abstract

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