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

We present a photometric and spectroscopic study of the compact H II region Sh 138 and its associated stellar cluster. The positions and BVRIJHK magni tudes 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 require d to account for the ionization level of the H II region. However star 183' s position in the J-H versus H-K 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, an d is dense - more than 550 stars pc(-2) at its centre. The visual extinctio n in the cluster varies between 5 mag and more than 35 mag; large variation s are observed over very small scales (for example, more than 20 mag over l ess than 4 " among the central massive stars).