HST/GHRS Spectroscopy of the Hybrid Star Alpha Aquarii

GHRS observations were made in July 1993 of the luminous hybrid supergiant Alpha Aqr (HD 209750, G2 Ib). This cool supergiant gives evidence for both high temperature emission from N V and C IV (T= 2 times 10(5) K) and a massive wind. In addition, the He I lambda 10830 transition indicates supersonic wind velocities in the chromosphere. To identify the emitting regions, and determine the profile of wind acceleration, we obtained moderate resolution (G160 M) observations and wavelength calibration exposures at four grating positions centered on He II (lambda 1640), C II (lambda 1335), C IV (lambda 1550), and N V (lambda 1240). All four species were detected. The IUE obtained simultaneous ultraviolet spectra including profiles of Mg II (lambda 2800). The C IV and N V lines appear blue-shifted (~ 30 km\ s(-1) ), and broad, with FWHM ranging from 125 to 180 km\ s(-1) , similar to those found previously in alpha TrA, another hybrid star, and interpreted as arising from wind expansion. Hydrostatic and dynamic atmospheric models are calculated with and spherical geometry in order to interpret the observed line profiles.