The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration based on Synthetic He I lines

We derive projected rotational velocities (vsini) for a sample of 156 Galactic OB star members of 35 clusters, HII regions, and associations. The HeI lines at $\lambda\lambda$4026, 4388, and 4471A were analyzed in order to define a calibration of the synthetic HeI full-widths at half maximum versus stellar vsini. A grid of synthetic spectra of HeI line profiles was calculated in non-LTE using an extensive helium model atom and updated atomic data. The vsini's for all stars were derived using the He I FWHM calibrations but also, for those target stars with relatively sharp lines, vsini values were obtained from best fit synthetic spectra of up to 40 lines of CII, NII, OII, AlIII, MgII, SiIII, and SIII. This calibration is a useful and efficient tool for estimating the projected rotational velocities of O9-B5 main-sequence stars. The distribution of vsini for an unbiased sample of early B stars in the unbound association Cep OB2 is consistent with the distribution reported elsewhere for other unbound associations.Comment: Accepted for publication in The Astronomical Journa

First Results from the CHARA Array. I. An Interferometric and Spectroscopic Study of the Fast Rotator alpha Leonis (Regulus)

We report on K-band interferometric observations of the bright, rapidly rotating star Regulus (type B7 V) made with the CHARA Array on Mount Wilson, California. Through a combination of interferometric and spectroscopic measurements, we have determined for Regulus the equatorial and polar diameters and temperatures, the rotational velocity and period, the inclination and position angle of the spin axis, and the gravity darkening coefficient. These first results from the CHARA Array provide the first interferometric measurement of gravity darkening in a rapidly rotating star and represent the first detection of gravity darkening in a star that is not a member of an eclipsing binary system.Comment: Accepted for publication in The Astrophysical Journal, 44 pages w/ 14 figure

A Statistical Study of Threshold Rotation Rates for the Formation of Disks around Be Stars

This paper presents a detailed statistical determination of the equatorial rotation rates of classical Be stars. The rapid rotation of Be stars is likely to be linked to the ejection of gas that forms dense circumstellar disks. The physical origins of these disks are not understood, though it is generally believed that the ability to spin up matter into a Keplerian disk depends on how close the stellar rotation speed is to the critical speed at which the centrifugal force cancels gravity. There has been recent disagreement between the traditional idea that Be stars rotate between 50 and 80 percent of their critical speeds and new ideas (inspired by the tendency for gravity darkening to mask rapid rotation at the equator) that their rotation may be very nearly critical. This paper utilizes Monte Carlo forward modeling to simulate distributions of the projected rotation speed (v sin i), taking into account gravity darkening, limb darkening, and observational uncertainties. A chi-squared minimization procedure was used to find the distribution parameters that best reproduce observed v sin i distributions from R. Yudin's database. Early-type (O7e-B2e) Be stars were found to exhibit a roughly uniform spread of intrinsic rotation speed that extends from 40 to 60 percent up to 100 percent of critical. Late-type (B3e-A0e) Be stars exhibit progressively narrower ranges of rotation speed as the effective temperature decreases; the lower limit rises to reach critical rotation for the coolest Be stars. The derived lower limits on equatorial rotation speed represent conservative threshold rotation rates for the onset of the Be phenomenon. The significantly subcritical speeds found for early-type Be stars represent strong constraints on physical models of angular momentum deposition in Be star disks.Comment: 36 pages (AASTeX), 11 figures, Ap. J., in press (November 20, 2005