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Session 105 - Molecular Cores and Protostars.
Display session, Thursday, January 16
Radiative transfer models of molecular clouds and the atmospheres of evolved stars predict linear polarization in emission lines of a few percent under favorable conditions. Previous attempts to detect this polarization in molecular clouds and the AGB star IRC+10216 have failed, with upper limits ranging from several percent to 0.5%.
We have searched for emission line polarization toward three protostellar outflows and two evolved stars using the new facility polarimeter at the NRAO 12-meter telescope. The outflow velocity gradients and excitation temperature gradients in these objects could lead to polarization. However, polarization was not detected toward the protostars with 3\sigma upper limits of 0.4% (DR 21 and Mon R2, HCO^+ J=1--0) and 1.2% (IRAS 16293-2422, CS J=2--1). A tentative detection of 1.0% (\sim5\sigma) polarization was made toward IRC+10216 (CS J=2--1). The 3\sigma upper limit on polarization toward CRL 2688 is 0.9% (HCN J=1--0).
Our observations of IRC+10216 appear to be the first detection of millimeter emission line polarization that does not arise from a maser. The lack of polarization in the molecular outflows may be because the line-emitting gas is clumpy and turbulent on scales smaller than our 70\char'175\ beam. We hypothesize that if polarization is indeed present in IRC+10216, it is because the stellar wind is smoother than the protostellar outflows. The presence of polarization toward IRC+10216 would then indicate an asymmetry in the optical depth of the radiating gas, perhaps due to a bipolar structure.
Program listing for Thursday