Hα emission line spectroscopy in NGC 330 ^*

On the hybrid model for global oscillations in Be star circumstellar disks

¹ Institut für Astronomie und Astrophysik und Universitäts-Sternwarte München, Scheinerstr. 1, 81679 München, Germany
² European Southern Observatory, Karl-Schwarzschildstr. 2, 85748 Garching, Germany
³ National Astronomical Observatory of Japan, Saburo Telescope, 650 North A'ohoku Place, Hilo, Hawaii, HI 96720 USA
⁴ Universitäts-Sternwarte Göttingen, Geismarlandstr. 11, 37083 Göttingen, Germany
⁵ CNR -Istituto Fisica Cosmica, Via Bassini 15, 20133 Milano, Italy
⁶ Mount Stromlo Observatory, Private Bag, Weston Creek, Weston, ACT 2611, Australia
⁷ Landessternwarte Heidelberg, Königstuhl 12, 69117 Heidelberg, Germany
⁸ Max-Planck-Institut für extraterrestrische Physik, Postfach 1312, 85741 Garching, Germany

Corresponding author: W. Hummel, whummel@eso.org

Received: 4 October 2000
Accepted: 1 March 2001

Abstract

We perform an observational test on global oscillations in Be star circumstellar disks in the metal deficient environment of the SMC. According to the hybrid model of disk oscillations early-type Be stars require an optically thin line force to establish a density wave. The low metallicity in the SMC should therefore diminish or prevent the formation of disk oscillations in early-type Be stars. We present short wavelength range spectra around Hα of 48 Be stars in the young open cluster NGC 330 in the SMC. We find that the fraction of early-type Be stars in NGC 330 which host a global disk oscillation does not differ from the known fraction of Galactic field Be stars. This observational result is in contradiction to the theoretical prediction. We discuss several interpretations and propose a further observational test.