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