A slitless spectroscopic survey for Halpha emission-line objects in SMC clusters

TL;DR

This study investigates the occurrence of Be stars in the Small Magellanic Cloud, revealing higher frequencies than in the Milky Way and suggesting that stellar rotation rates and metallicity influence the Be phenomenon.

Contribution

It provides the first large-scale slitless spectroscopic survey of Halpha emission-line stars in SMC clusters, comparing their properties with Galactic counterparts to understand Be star formation.

Findings

Be stars are 3-5 times more frequent in SMC than in the Galaxy.

The distribution of Be stars by spectral type is similar in both galaxies.

The Be phenomenon mainly occurs towards the end of the main sequence, influenced by stellar rotation.

Abstract

This paper checks on the roles of metallicity and evolutionary age in the appearance of the so-called Be phenomenon. Slitless CCD spectra were obtained covering the bulk of the Small Magellanic Cloud. For Halpha line emission twice as strong as the ambient continuum, the survey is complete to spectral type B2/B3 on the main sequence. About 8120 spectra of 4437 stars were searched for emission lines in 84 open clusters. 370 emission-line stars were found, among them at least 231 near the main sequence. For 176 of them, photometry could be found in the OGLE database. For comparison with a higher-metallicity environment, the Galactic sample of the photometric Halpha survey by McSwain & Gies (2005) was used. Among early spectral sub-types, Be stars are more frequent by a factor 3-5 in the SMC than in the Galaxy. The distribution with spectral type is similar in both galaxies, i.e. not strongly dependent on metallicity. The fraction of Be stars does not seem to vary with local star density. The Be phenomenon mainly sets in towards the end of the main-sequence evolution (this trend may be more pronounced in the SMC); but some Be stars already form with Be-star characteristics. In all probability, the fractional critical angular rotation rate, \omc, is one of the main parameters governing the occurrence of the Be phenomenon. If the Be character is only acquired during the course of evolution, the key circumstance is the evolution of \omc, which not only is dependent on metallicity but differently so for different mass ranges.