Integral-field spectroscopy of the Galactic cluster [DBS2003]8 ^*,**

Discovery of an ultra-compact HII region and its ionizing star in the bright rimmed cloud SFO49

¹ European Southern Observatory, Karl Schwarzschild-Strasse 2, 85748 Garching bei Munchen, Germany e-mail: mmessine@eso.org; messineo@cis.rit.edu
² Max-Planck-Institut für Radioastronomie, Auf dem Hügel 69, 53 121 Bonn, Germany
³ Leiden Observatory, PO Box 9513, 2300 RA Leiden, The Netherlands

Received: 10 September 2006
Accepted: 23 April 2007

Abstract

Context.We have started a program of infrared (IR) studies of the stellar clusters associated with HII regions in order to understand the structure of the spiral arms and their interaction with the central bar of the Galaxy better.

Aims.This is accomplished by determining the distance to the OB stars embedded in the clusters. We want to use IR stellar spectro-photometric measurements to complement the kinematic distances from the radial velocity of the gas.

Methods.SINFONI, the infrared integral-field spectrograph of the VLT, enabled us to image the clusters and to resolve them into individual stars, to get the spectra of the brightest stars, and to analyze the possible nebular emission. We performed pilot observations of [DBS2003]8, an embedded Galactic stellar cluster in the bright rimmed cloud SFO49, during the SINFONI science verification. The results are presented in this paper.

Results.The center of the cluster is resolved for the first time and four stars are detected, the brightest being a late O type or early B star. A spectro-photometric distance to the cluster of  kpc is derived. We discovered that the O type star is located in the center of a nebula, which is quite circular in shape with an observed radius of ~0.03-0.06 pc, a value typical of an ultra-compact HII region. We measured the nebula's emission, which is generated by photo-ionization of the central star; and from the hydrogen molecular line ratio, we have proved that the ionized nebula is surrounded by a photo-dissociated region (PDR). We also detected [FeII] line emission at 1.664 μm. The ratio of the iron flux to that of the  line is found 15 times higher than in a classical HII region. This suggests that at least part of the [FeII] line emission emanates from shocked stellar wind material.