Rotational velocities for early-type stars in the young large magellanic cloud cluster R136: Further study of the relationship between rotation speed and density in star-forming regions

Abstract

We present the results of a study aimed at assessing the distribution of rotation speeds, N(v sin i) among O- and early B-type stars located in R136, a young (t 1-4 Myr) cluster in the Large Magellanic Cloud (LMC) characterized by a stellar density at least three times that of the densest Galactic clusters in which stellar rotational velocities have been measured. Our goals are (1) to determine whether the distribution of N(v sin i) in R136 shows the same paucity of slowly rotating stars and high mean rotation speed that distinguish early-type stars located in bound clusters in the Milky Way Galaxy (MWG) from their analogs among members of the field and unbound associations and (2) to determine whether the mean rotation speed in the extremely dense R136 cluster is even higher than the values measured for lower-density bound clusters. Our data comprise vsin i estimates for 24 stars obtained by comparing line profile measurements obtained with the Gemini Multi-Object Spectograph on the Gemini South Telescope with a grid of He I and He II line profiles generated from model atmospheres and broadened to emulate the effects of stellar rotation. We find that for R136, 13 stars with masses in the range 6-12 M have an average apparent rotational velocity of 〈vsin i〉 = 233 19 km s-1; by comparison, for LMC stars in this same mass range in the field and in lower-density clusters, 〈vsin i〉 is, respectively, 105 8 km s -1 and 147 14 km s-1. For 11 15-30 M stars in R136, 〈vsin i〉 = 189 23 km s-1; by comparison, the LMC stars in this same mass range but drawn from lower-density regions have 〈vsin i〉 = 129 13 km s-1. Moreover, we find that throughout this entire mass range, R136 lacks the cohort of slow rotators characteristic of early-type field stars, both in the LMC and in the MWG. We provide arguments that these differences in N(v sin i) are unlikely to arise from evolution-driven changes in angular momentum (e.g., angular momentum loss through stellar winds), but rather may reflect differences in the rotation speeds imprinted at the time the stars formed. This result appears most certain for stars with masses in the range 6-12 M; for stars of higher masses, larger samples from regions of differing densities are needed to more firmly establish that the observed differences are imprinted during the stellar assembly phase as opposed to being the result of subsequent evolution. We further argue that the differences in N(v sin i) between R136 and the LMC and MWG field stars likely result from a difference in the initial conditions in protostellar cores that are found in the types of molecular cloud regions that form rich, dense clusters (e.g., higher turbulent speeds) rather than from differences in the environment surrounding the core (e.g., stellar density, UV radiation field). © 2008. The American Astronomical Society. All rights reserved.