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


Email: scranmer@cfa.harvard.edu