Showing papers by "Brian Babler published in 1997"

Constraints on the Geometry of Circumstellar Envelopes: Optical Interferometric and Spectropolarimetric Observations of Seven Be Stars

Abstract: We have obtained contemporaneous optical interferometry and spectropolarimetry of seven Be stars. The interferometry was done using both continuum and narrow Ha line -lters to observe the circumstel- lar envelope emission. The envelopes of all seven stars were resolved interferometrically in Ha. Of these seven stars, the envelopes of four were not circularly symmetric and showed clear evidence of elongation. The position angles of the major axis of the elongation were in good agreement with the disk orientation inferred from the intrinsic polarization data, which samples material within a few stellar radii, and also agreed with previous results from the radio, which samples material out as far as 100 stellar radii but at lower resolution. This -nding indicates that the envelope alignment persists over 2 orders of magnitude in radius. Minimum inclination angle estimates from the interferometry are presented for six of the seven stars. Under the assumption that the envelopes are fairly thin circularly symmetric disks, there is a straightfor- ward explanation of the interferometric results. The stars with the greatest elongations, f Tau, / Per, and t Per, are seen nearly equator-on; c Cas is seen at a more intermediate latitude, consistent with previous results; and 48 Per and g Tau, which are only slightly asymmetric, are nearly pole-on. b CMi had insuf- -cient coverage to determine whether its envelope is asymmetric. These results are consistent with inde- pendent indications of the inclinations based on the polarimetry. Spectral diagnostics have been used in the past to classify Be stars as pole-on or as shell stars (usually interpreted to indicate an equator-on orientation). Our inclination angle for 48 Per is consistent with its pole-on classi-cation by Slettebak, and the three equator-on stars have all been previously classi-ed as shell stars. However, both c Cas and g Tau have also been previously classi-ed as shell stars, but we -nd that they have intermediate and pole-on orientations, respectively. We interpret this -nding as evidence that, while equator-on stars may preferentially be shell stars, not all shell stars are actually equator-on. Our results show that the Ha emission region extends up to about 12 stellar radii, possibly depending on spectral type. The size of this region correlates with the equivalent width of the Ha emission. Our results also provide strong constraints for allowed models of Be star envelopes. In particular, for f Tau, we derive an upper limit to the disk opening angle of 20i, which limits the vertical extent of the envelope. We also -nd that the polarization position angle is perpendicular to the interferometric major axis in all cases. This rules out envelope models that are both optically and geometrically thick, since these models produce polarization parallel to the plane of the disk. We conclude that results from the combined interferometry and polarimetry strongly favor the disk paradigm for Be stars over mildly ellip- soidal models. Subject headings: circumstellar matter E stars: emission-line, Be E techniques: interferometric E techniques: polarimetric

Solar System Observations by the Wisconsin Ultraviolet Photopolarimeter Experiment.I.The First Ultraviolet Linear Spectropolarimetry of Mars

Abstract: A B STR ACT Portions of the Moon were observed by the Wisconsin Ultraviolet Photopolarimeter Experiment (WUPPE) on 1995 March 12, 14 and 17, and represent the first ultraviolet (UV) spectropolarimetric observations of the Moon. The polarimetric observations confirm that a change in the dominant scattering process occurs in the UV, changing from volume scattering in the near-UV to surface scattering in the far-UV. The data are investigated empirically. It is found that Umov’s relationship holds when the polarization is perpendicular to the scattering plane. It is also found that the degree of polarization can be modelled by a phase-angle-dependent polarization modified by a wavelength-dependent depolarization factor. The scattering function for each observation is determined.