Millimeter-Wave Spectropolarimetry of Evolved Stars: Evidence for Polarized Molecular Line Emission

We present spectropolarimetry of CS J = 2-1 emission from the evolved stars IRC +10216 and CRL 2688 and spectropolarimetry of the HCN J = 1-0 transition from CRL 2688. We detected a line-averaged linear polarization of ~1% toward IRC +10216 on three separate occasions. We found no evidence for velocity dependence of the polarization within the emission line. A deviation from spherical symmetry in the radiative transfer of the envelope is implied because polarization was detected toward the central star. The polarization position angle of about -70° is orthogonal to the envelope elongation apparent in previous observations. Polarizations of 5.1% ± 1.5% (1 σ) and less than 0.9% (3 σ), respectively, were derived for CS and HCN from CRL 2688.