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Astron. Astrophys. 344, 943-954 (1999)

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Sh 138: a compact H II region excited by a very young cluster * **

L. Deharveng 1, A. Zavagno 1, D. Nadeau 2, J. Caplan 1 and M. Petit 1

1 Observatoire de Marseille, 2 Place Le Verrier, F-13248 Marseille Cedex 4, France (deharveng@obmara.cnrs-mrs.fr)
2 Observatoire du Mont Mégantic et Département de Physique, Université de Montréal, C.P. 6128, Succ. Centre-ville, Montréal, QC, Canada H3C 3J7 (nadeaud@ere.umontreal.ca)

Received 28 July 1998 / Accepted 14 January 1999


We present a photometric and spectroscopic study of the compact H II region Sh 138 and its associated stellar cluster. The positions and BVRIJHK magnitudes are obtained for more than 400 stars over a field of about 4´ square centred on the H II region. Sh 138 is excited by a cluster of young massive stars. At the cluster's very centre are at least four O-B2 stars separated by less than 4". The brightest of these, both in the visible and the near infrared, exhibits a spectrum similar to those of the more massive Herbig Ae/Be stars. This star, our No. 183, is overluminous by a factor of 2.5 in the visible and four in the near IR with respect to the O9.5 V star required to account for the ionization level of the H II region. However star 183's position in the [FORMULA] versus [FORMULA] diagram does not indicate a near-IR excess. We suggest that this star is a young massive object belonging to a binary or multiple system.

The stellar cluster associated with Sh 138 is very reminiscent of the Orion Trapezium cluster: it is centrally peaked around several massive stars, and is dense - more than 550 stars pc-2 at its centre. The visual extinction in the cluster varies between 5 mag and more than 35 mag; large variations are observed over very small scales (for example, more than 20 mag over less than 4" among the central massive stars).

Key words: stars: pre-main sequence – ISM: H ii regions – ISM: individual objects: Sh 138 – Galaxy: open clusters and associations: individual: Sh 138 – infrared: stars

* Based on observations done at the Observatoire de Haute Provence and the Observatoire du Pic du Midi, France, and at the Canada-France-Hawaii Telescope. The CFHT is operated by the Centre National de la Recherche Scientifique of France, the National Research Council of Canada and the University of Hawaii.
** Table 1 is available in electronic form at the CDS (via anonymous ftp to cdsarc.u-strasbg.fr/pub/cats/J/A+A/344/943 or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/344/943 ) as well as at http://www-obs.cnrs-mrs.fr/matiere/sh138-t1.html

Send offprint requests to: L. Deharveng

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© European Southern Observatory (ESO) 1999

Online publication: March 29, 1999