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Astron. Astrophys. 326, L17-L20

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Letter to the Editor

Detection of WR stars in the metal-poor starburst galaxy IZw 18

F. Legrand1, D. Kunth1, J.-R. Roy2, J.M. Mas-Hesse3, and J.R. Walsh4,*

1Institut d'Astrophysique de Paris, CNRS, 98bis boulevard Arago, F-75014 Paris, France
2Département de physique and Observatoire du mont Mégantic, Université Laval, Québec Qc G1K 7P4
3LAEFF, Apdo 50727, E-28080 Madrid, Spain
4European Southern Observatory, Karl-Schwarzschild-Str. 2, D-85748 Garching, Germany

Received 26 June 1997 / Accepted 10 July 1997


Wolf-Rayet stars (WR) have been detected in the NW region of the metal-poor starburst galaxy IZw 18. The integrated luminosity and FWHM of the bumps at 4650 Å and 5808 Å are consistent with the presence of a few individual stars of WC4 or WC5 type. Evolutionary synthesis models predict few WRs in this galaxy, but only of WN type. The presence of WC stars at such low metallicity could however be explained by high mass loss rates, which would constrain the IMF upper mass cut-off in IZw 18 to be higher than 80 M FORMULA or alternatively favor a binary channel for WR formation. WC stars could also explain the strong and narrow HeII 4686Å emission line which peaks co-spatially with the WR bump emission, as suggested by Schaerer (1996). This detection shows that WR stars, even of WC type, are formed at metallicities below 1/40th solar.

Key words: Galaxies - Galaxies: IZw 18 - Galaxies: WRs galaxies - Galaxies: star formation - Galaxies: enrichment of ISM - Stars: WR -

*DK and JRR Visiting astronomers at Canada-France-Hawaii Telescope, which is operated by the National Research Council of Canada, the Centre National de la Recherche Scientifique de France, and the University of Hawaii

Send offprint requests to: F. Legrand

© European Southern Observatory (ESO) 1997

Online publication: October 1, 1997
Last change: April 8, 1998