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, hy drostatic, plane-parallel H/He model-atmosphere analyses of the optical spe ctra 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 t o 430 km s(-1) and omega (e)/omega (e)(crit) similar or equal to 0.9. The a nalysis allows for the expected (von Zeipel) variation of T-eff and log g w ith latitude. These three stars are found to share very similar characteris tics, including substantially enhanced surface-helium abundances (y similar or equal to 0.2). Secondly, we compare the distribution of projected rotat ional velocities for ON and morphologically normal dwarf O stars, and demon strate that the ON stars are drawn from a population with more rapid rotati on. 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 a n ON star, and note the implied disparity between O/ON morphology and surfa ce helium abundance; we discuss consequences for the interpretation of spec tral morphology in O-type main-sequence stars. We demonstrate a new, purely spectroscopic, method of distance determination for rapid rotators, and th ereby confirm that HD 93521 lies at similar to 2 kpc, and is not, as previo usly suggested, a low-mass Population II star. Finally, our models contradi ct earlier claims of strongly differential surface rotation, and are consis tent with uniform angular velocity at the surface.