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

**Steven R. Cranmer**

Harvard-Smithsonian Center for Astrophysics

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.

**Reference: **ApJ, in press (November 20, 2005), astro-ph/0507718

Status: Manuscript has been accepted

**Weblink: **http://arXiv.org/abs/astro-ph/0507718

**Comments: **

**Email: **scranmer@cfa.harvard.edu