On the polarisation of the Red Rectangle optical emission bands
  The origin of the narrow optical emission bands seen toward the Red Rectangle
is not yet understood. In this paper we investigate further the proposal that
these are due to luminescence of large carbonaceous molecules. Polarised
signals of several percent could be expected from certain asymmetric molecular
rotators. The ESPaDOnS echelle spectrograph mounted at the CFHT was used to
obtain high-resolution optical spectropolarimetric data of the Red Rectangle
nebular emission. The RRBs at 5800, 5850, and 6615 Angstrom are detected in
spectra of the nebular emission 7" and 13" North-East from the central star.
The 5826 and 6635 Angstrom 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 artifact. The lack of a clear polarisation
signal for the five detected RRBs implies that, if the emission is due to
luminescence of complex organics, these gas-phase molecular carriers are likely
to have a high degree of symmetry, as they do not exhibit a Q-branch in their
rotational profile, although this may be modified by statistical effects.
