Young Populous Clusters in the Magellanic Clouds.

Abstract

This work examines a sample of young populous clusters in the Magellanic Clouds. The population of these clusters offers insight into the course of stellar evolution for high-mass stars in the 8–20 M, mass range. The clusters of the present study provide a statistically significant sample with which to confront the predictions of stellar evolutionary models. We have paid particular attention to the issue of internal mixing, the consequence of which is extension of the convective core boundary over the classical Schwarzschild limit. We present a quantitative determination of the degree of internal mixing within the convective core overshoot paradigm, which parameterizes the degree of extension to the convective core. The convective core overshoot parameter, Lc, represents the degree of extension that is required to match our observations. The basis of our determination of Lc is Hubble Space Telescope WFPC2 photometry of the clusters NGC 330 (SMC) and NGC 1818, NGC 2004, and NGC 2100 (LMC). Our photometry in visual (F555W), ultraviolet (F160BW), and narrowband Ha (F656N) enabled us to construct the highly temperature sensitive F160BW F555W color and to distinguish the Be star population among the upper main-sequence (MS) population. Infrared photometry in the J and K bands was conducted to obtain accurate temperatures and luminosities for the evolved members. Comparison of the cluster population is made against stellar evolutionary models that incorporate a variable level of core overshoot. The luminosity function of MS stars and the number of evolved stars provide joint constraints on Lc. It is found that models with no convective core overshoot are strongly disfavored by the temperature and luminosity of the MS turnoff of the cluster sample. With a binary fraction of 30% the cluster populations indicate a . L p 0.62 0.22 c This study has established a photometric survey technique for Be stars, a technique that spectroscopic follow-up shows is highly complete. Our study has highlighted the large proportion of Be stars on the upper MS of young Magellanic Cloud clusters. Be stars are characterized by rapid rotation. Our study of the Be star fraction within the cluster and field environs of the LMC and SMC has provided evidence for an evolutionary enhancement of the Be fraction toward the end of the MS. We propose that this enhancement is the consequence of an increase in the ratio of the surface angular velocity to the critical angular velocity toward the end of the MS. In order to verify this scenario and to investigate the dependence of average rotation rates on metallicity, we present the rotational velocities for a sample of B0–B2 V–III stars drawn from the clusters and the field of the LMC. Compared with an equivalent sample of Galactic stars, the LMC cluster stars show faster projected rotational velocities. We discuss this result in the light of emergent evolutionary models that incorporate the effects of rotational mixing.