Rotational mixing in early-type main-sequence stars

We present quantitative observational investigations into the importance of rotationally induced mixing in late-O stars. First, we conduct non-LTE, hydrostatic, plane-parallel H/He model-atmosphere analyses of the optical spectra of three of the most rapidly rotating late-O near-main-sequence stars known: HD 93521 (O9.5 V), HD 149757 (zeta Ophiuchi; O9.5 V), and HD 191423 (ON9 III: n), all of which have equatorial rotation velocities of similar to 430 km s(-1) and omega (e)/omega (e)(crit) similar or equal to 0.9. The analysis allows for the expected (von Zeipel) variation of T-eff and log g with latitude. These three stars are found to share very similar characteristics, including substantially enhanced surface-helium abundances (y similar or equal to 0.2). Secondly, we compare the distribution of projected rotational velocities for ON and morphologically normal dwarf O stars, and demonstrate that the ON stars are drawn from a population with more rapid rotation. The results provide qualitative support for rotationally induced mixing, although there remain discrepancies between atmospheric and evolutionary models (which we show employ inappropriate mass-loss rates for late-O main-sequence stars). We show that the most rapid rotator known, HD 191423, is an ON star, and note the implied disparity between O/ON morphology and surface helium abundance; we discuss consequences for the interpretation of spectral morphology in O-type main-sequence stars. We demonstrate a new, purely spectroscopic, method of distance determination for rapid rotators, and thereby confirm that HD 93521 lies at similar to 2 kpc, and is not, as previously suggested, a low-mass Population II star. Finally, our models contradict earlier claims of strongly differential surface rotation, and are consistent with uniform angular velocity at the surface.