G76.188+0.098: a newly born massive binary star

¹ European Southern Observatory, Karl-Schwarzschild-Strasse 2, 85748 Garching, Germany e-mail: fcomeron@eso.org
² Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany e-mail: pasquali@mpia-hd.mpg.de
³ Departament d'Astronomia i Meteorologia, Universitat de Barcelona, 08028 Barcelona, Spain e-mail: jordi@am.ub.es

Received: 12 February 2006
Accepted: 4 July 2006

Abstract

Context.We present observations of G76.188+0.098, an object classified in the literature as an ultracompact HII region (UCHIIR) based on its far-infrared and radio continuum characteristics.

Aims.To investigate the nature of this object on the basis of the combination of radio observations existing in the literature and the new observations in the near-infrared presented here, with particular attention to the direct detection of the central ionizing source.

Methods.Near-infrared narrow-band imaging and low resolution spectroscopy in the 2 μm region.

Results.The new observations clearly reveal the central source, which turns out to be a resolved binary system obscured by mag. A general agreement is found between the morphology of the region as outlined by narrow-band, near-infrared imaging, and existing VLA radio continuum maps. Indirect evidence based on the low HeI (2.058 μm) / Brγ intensity ratio of the compact nebula suggests that the binary system is composed of two stars with spectral types O9 and B0 at a distance of ~2.4 kpc, somewhat closer to the Sun than commonly assumed in the literature.

Conclusions.The size of the central cavity of the compact nebula, the low electron densities derived from radio continuum measurements, and the moderate extinction towards the central source by UCHIIR standards suggest that G76.188+0.098 is a very young massive binary observed near the end of the embedded, ultracompact phase, adding one example to the rare cases in which the central source of an ultracompact HII region is detected.