Rotational mixing in early-type main-sequence stars

Details Rotational mixing in early-type main-sequence stars Rotational mixing in early-type main-sequence stars

Oct 2001

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2001mnras.327..353h&link_type=abstract

Monthly Notices of the Royal Astronomical Society, Volume 327, Issue 2, pp. 353-368.

Astronomy and Astrophysics

Astronomy

39

Stars: Early-Type, Stars: Evolution, Stars: Individual: Hd 93521, Stars: Individual: &Zeta, Ophiuchi, Stars: Individual: Hd 191423, Stars: Rotation

Scientific paper

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 ~430kms-1 and [formmu8]ωe/ωe(crit)~=0.9. The analysis allows for the expected (von Zeipel) variation of Teff and logg with latitude. These three stars are found to share very similar characteristics, including substantially enhanced surface-helium abundances [formmu9](y~=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 ~2kpc, 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.

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