Ultraviolet photometry from the Orbiting Astronomical Observatory. XVI. The stellar Lyman-alpha absorption line.
The Wisconsin far-ultraviolet spectrometer on OAO-2 has observed the stellar La line at 1216 A in 29 lightly reddened stars ranging in spectral type from B2.5 to B9. For these stars, La equivalent widths have been derived by fitting synthetic profiles to the observed spectra. This procedure corrects for a small amount of blending between the stellar La line and a stellar Si "`line at 1206.5 A. The final equivalent widths, which include a small correction for interstellar La absorption, range from 15 A at spectral type B2. 5 to 65 A at spectral type B8. For the late-B stars the La line removes 2 to 3 percent of the total stellar flux. For our sample of stars (24 dwarfs, four giants, and one supergiant) the strength of La correlates quite well with measures of the Balmer discontinuity and Balmer line strengths. With the exception of the one supergiant, luminosity classification does not appear to strongly influence the La line strength. The observed La equivalent widths have been compared with the predictions of Mihalas's grid of non-LTE model atmospheres. With the possible exception of the stars of high luminosity, the calculations and observations are in basic agreement over their range of overlap (T6 = 1 5, ,000 K). We have shown that in a number of cases the stellar La line influences the interstellar column densities reported in the OAO-2 interstellar La survey. Because of this contamination the OAO-2 interstellar column densities toward lightly reddened B2 and B1 .5 stars should be regarded as only upper limits. Subject headings: early-type stars - interstellar matter - line profiles - spectra, ultraviolet