HD-93521, ZETA-OPHIUCHI, AND THE EFFECTS OF RAPID ROTATION ON THE ATMOSPHERES AND WINDS OF 09.5-V STARS

Both low- and high-resolution IUE spectra of the rapidly rotating O9.5 V stars HD 93521 and zeta Oph are used to develop a coherent picture of the effects of rapid rotation on the atmospheres and winds of late, main-sequence O stars. The observational consequences are by far the strongest on HD 93521, most likely because it is being viewed nearly equator-on. In particular, it is shown that HD 93521 has (1) a much smaller UV to optical flux ratio than expected, (2) UV photospheric lines indicative of a B0 supergiant, (3) an abnormally strong N v wind doublet, and (4) wind profiles suggesting that its wind has latitudinally dependent properties. Because HD 93521 has a larger observed upsilon sin i than zeta Oph and yet its Halpha emission is no stronger than in zeta Oph, it is speculated that zeta Oph actually rotates as fast or faster than HD 93521, but has a smaller sin i. Because zeta Oph is significantly reddened, nothing can be determined about its intrinsic UV energy distribution. However, it is shown that its UV photospheric lines are a bit peculiar and that its C iv and N v wind doublets are abnormally strong and have unusual profiles. The C iv profile agrees with models of a rotationally distorted wind similar to the one in HD 93521, except viewed at an angle i approximately 60-degrees-80-degrees. The spectral peculiarities of both stars are attributed to the combined effects of gravity darkening of their atmospheres and rotational distortion of their winds. The differences between their spectra are interpreted as the result of being viewed at different inclination angles. Because of the gravity darkening, atmospheric analyses of either star based on single temperature and surface gravity model atmospheres are probably unreliable. Finally, I describe how different effects conspire to make the spectroscopic signatures of gravity darkening so pronounced at O9.5 V.