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 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 J−H versus H−K diagram does not indicate a nearIR 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).