Linear spectropolarimetry across the optical spectrum of Herbig Ae/Be stars

Abstract

We present the results of spectropolarimetric observations of 12 Herbig Ae/Be objects. Our data have the largest spectropolarimetric wavelength coverage, 4560 {\AA} to 9480 {\AA}, published to date. A change in linear polarisation across the H{\alpha} line, is detected in all objects. Such a line effect reveals the fact that stellar photons are scattered off free electrons that are not distributed in a spherically symmetric volume, suggesting the presence of small disks around these accreting objects. Thanks to the large wavelength coverage, we can report that H{\alpha} is the spectral line in the optical wavelength range that is most sensitive to revealing deviations from spherical symmetry, and the one most likely to show a line effect across the polarisation in such cases. Few other spectral lines display changes in polarisation across the line. In addition, H{\alpha} is the only line which shows an effect across its absorption component in some sources. We present a scenario explaining this finding and demonstrate that the detection of the line effect strongly relies on the number of photons scattered into our line of sight. We highlight the special case of R Mon, which is the only object in our sample to show many lines with a polarisation effect, which is much stronger than in all other objects. Given that the object and its nebulosity is spatially resolved, we argue that this is due to scattering of the stellar and emission spectrum off circumstellar dust.