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 ( ζ Ophiuchi; O9.5 V), and HD 191423 (ON9 III: n), all of which have equatorial rotation velocities of ∼ 430 km s −1 and . 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 . 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 ∼ 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.