¹ European Organisation for Astronomical Research in the Southern Hemisphere, Alonso de Cordova 3107, Vitacura, Casilla 19001, Santiago 19, Chile
² GEPI, Observatoire de Paris, CNRS, Université Paris Diderot, 5 place Jules Janssen, 92195 Meudon Cedex, France
³ European Organisation for Astronomical Research in the Southern Hemisphere, Karl-Schwarzschild-Str. 2, 85748 Garching b. München, Germany
⁴ Observatorio Astronómico de Valencia, edifici Instituts d'investigació, Poligon la Coma, 46980 Paterna Valencia, Spain

Context.  A fair fraction of all single early-type stars display emission lines well before the supergiant phase. Very rapid rotation is necessary for such stars to form rotationally supported decretion disks. But it is unknown whether and which other parameters may be important.
Aims.  This paper checks on the roles of metallicity and evolutionary age in the appearance of the so-called Be phenomenon.
Methods.  Slitless CCD spectra were obtained covering the bulk (about 3 square degrees) of the Small Magellanic Cloud. For Hα line emission twice as strong as the ambient continuum, the survey is complete to spectral type B2/B3 on the main sequence. About 8,120 spectra of 4,437 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 Hα survey by McSwain & Gies (2005) was used.
Results.  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 small sub-samples (such as single clusters), even if they appear identical, the fraction of emission lines stars can deviate drastically from the mean.
Conclusions.  In all probability, the fractional critical angular rotation rate, Ω/Ω_c, 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 Ω/Ω_c, which not only is dependent on metallicity but differently so for different mass ranges. As the result, even if the Be phenomenon is basically single-parametric (namely Ω/Ω_c), it takes on a complex multi-parametric appearance. The large cluster-to-cluster differences, which seem stronger than all other variations, serve as a caveat that this big picture may undergo significant second-order modulations (pulsations, initial angular momentum, etc).

Accepted by A&A
Preprints from christophe.martayan@eso.org
or on the web at http://cdsads.u-strasbg.fr/abs/2009arXiv0909.2303M