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

This paper presents a detailed statistical determination of the equatorialrotation rates of classical Be stars. The rapid rotation of Be stars is likelyto be linked to the ejection of gas that forms dense circumstellar disks. Thephysical origins of these disks are not understood, though it is generallybelieved that the ability to spin up matter into a Keplerian disk depends onhow close the stellar rotation speed is to the critical speed at which thecentrifugal force cancels gravity. There has been recent disagreement betweenthe traditional idea that Be stars rotate between 50 and 80 percent of theircritical speeds and new ideas (inspired by the tendency for gravity darkeningto mask rapid rotation at the equator) that their rotation may be very nearlycritical. This paper utilizes Monte Carlo forward modeling to simulatedistributions of the projected rotation speed (v sin i), taking into accountgravity darkening, limb darkening, and observational uncertainties. Achi-squared minimization procedure was used to find the distribution parametersthat 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 ofintrinsic rotation speed that extends from 40 to 60 percent up to 100 percentof critical. Late-type (B3e-A0e) Be stars exhibit progressively narrower rangesof rotation speed as the effective temperature decreases; the lower limit risesto reach critical rotation for the coolest Be stars. The derived lower limitson equatorial rotation speed represent conservative threshold rotation ratesfor the onset of the Be phenomenon. The significantly subcritical speeds foundfor early-type Be stars represent strong constraints on physical models ofangular momentum deposition in Be star disks.Comment: 36 pages (AASTeX), 11 figures, Ap. J., in press (November 20, 2005