The first ultraviolet and optical spectropolarimetry of the B[e] star HD 50138

We report the first ultraviolet spectropolarimetry of the B[e] star HD 50138, obtained with the Wisconsin Ultraviolet Photo-Polarimeter Experiment aboard the Astro-2 Space Shuttle mission in 1995 March. The data cover the spectral range 1500-3200 Angstrom. Spectropolarimetry obtained contemporaneously in the range of 3800-10,000 Angstrom in the visual-to-near-IR region are presented as well. The presence of intrinsic polarization is detected from the UV to the near-IR. Strong evidence of a thin gaseous disk around the star is found. An almost flat wavelength dependence of the intrinsic polarization in the optical spectral region indicates that electron scattering, rather than dust scattering, is the dominant polarizing mechanism, although a small contribution due to dust scattering cannot be completely ruled out. A small inclination of the disk away from edge-on with respect to the line of sight is suggested. Comparison with similar polarimetric observations obtained for different objects that have the same circumstellar geometry shows that the envelope of HD 50138 probably has a rather large density contrast between the equatorial and polar regions that does not produce a position angle flip like that observed in other objects, notably HD 45677.