Doppler Ghosts in a Hall of Mirrors: Polarisation Profiles of Scattered Emission Lines

Abstract

The Doppler shifts of optical emission lines which have been scattered by surrounding dust and electrons can provide useful information about the kinematics, geometry and physical conditions of astrophysical flows. In principle, the scatterers can provide views of the line-emitting gas from different directions, allowing the 3-d velocity field of the emitting gas to be determined and revealing sources which are hidden from direct view. Unfortunately, as in a Hall of Mirrors, the resultant multiple images can be confusing and hard to interpret. In general, the scattering geometry will be unknown, which makes it difficult to disentangle the effect of the motion of the scatterers from that of the motion of the emitting source. In this situation, spectropolarimetric observations can be a great help, since the scattered light will be partially polarised to a degree dependent on the angle of scattering and on the details of the scattering process. Analysis of the polarisation profiles of the scattered emission lines can then determine the relative orientation and velocity of the source and the scatterers. Such techniques are applied to the upstream scattered emission lines in the HH 1 jet, where it is shown that spectropolarimetric measurements would allow an independent determination of the bowshock velocity with respect to the upstream matter, which would test ``multiple outburst'' models of the jet.