On the polarisation of the Red Rectangle optical emission bands: (Research Note)

Context. The origin of the narrow optical emission bands seen towards the Red Rectangle is not yet understood. We investigate the proposal that these are caused by luminescence of large carbonaceous molecules. Aims. We aim to measure the polarisation of the optical narrow Red Rectangle bands (RRBs). Polarised signals of several percent could be expected from certain asymmetric molecular rotators. Methods. The ESPaDOnS échelle spectrograph mounted at the CFHT was used to obtain high-resolution optical spectropolarimetric data of the Red Rectangle nebular emission. Results. The RRBs at 5800, 5850, and 6615 Ã? are detected in spectra of the nebular emission 7″ and 13″ north-east from the central star. The 5826 Ã? and 6635 Ã? RRB are detected only at the position nearest to the central star. For both positions the Stokes Q and U spectra show no unambiguous polarisation signal in any of the RRBs. We derive an upper limit of 0.02% line polarisation for these RRBs. A tentative feature with peak polarisation of 0.05% is seen for the 5800 RRB at 7″ offset. However, the null spectra suggest that this may be an instrumental artefact. Conclusions. The lack of a clear polarisation signal for the five detected RRBs implies that if the emission is caused by luminescence of complex organics, these gas-phase molecular carriers are likely to have a high degree of symmetry because they do not exhibit a Q-branch in their rotational profile, although this may be modified by statistical effects.