THE AXISYMMETRICAL STELLAR WIND OF AG CARINAE

We present optical linear spectropolarimetry of the Luminous Blue Vari able AG Carinae obtained after a recent visual brightness increase. Th e absence of He II lambda4686 emission, together with the weakening of the He I spectrum and the appearance of Fe lines in the region around 5300 angstrom, confirm that AG Car has started a new excursion across the HR diagram. The Halpha line profile exhibits very extended line w ings that are polarized differently in both amount and position angle from either the continuum or the line core. The polarization changes a cross Halpha, together with variable continuum polarization, indicate the presence of intrinsic polarization. Coexistence of the line-wing p olarization with extended flux-line wings evidences that both are form ed by electron scattering in a dense wind. The position angle rotates across the line profiles, in a way that presently available models sug gest is due to rotation and expansion of the scattering material. AG C ar displays very large variations of its linear polarization with time , DELTAP approximately 1.2%, indicating significant variations in enve lope opacity. We find that the polarization varies along a preferred p osition angle of approximately 145-degrees (with a scatter of +/- 10-d egrees) which we interpret as a symmetry axis of the stellar wind (wit h an ambiguity of 90-degrees). This position angle is co-aligned with the major axis of the AG Car ring nebula and perpendicular to the AG C ar jet. Our observations thus suggest that the axisymmetric geometry s een in the resolved circumstellar environment at various distances alr eady exists within a few stellar radii of AG Car. From the Halpha pola rization profile we deduce an interstellar polarization of Q = 0.31%, U = - 1.15 % at Halpha. The inferred interstellar polarization implies that the intrinsic polarization is not always of the same sign. This indicates either significant temporal changes in the envelope geometry , or it may arise from effects of multiple scattering in conjunction w ith density variations.