2012MNRAS.423.2425P

We report a study of the stellar content of the near-infrared (NIR) cluster [DBS2003] 157 embedded in the extended H II region GAL 331.31-00.34, which is associated with the IRAS source 16085-5138. JHK photometry was carried out in order to identify potential ionizing candidates, and the follow-up NIR spectroscopy allowed the spectral classification of some sources, including two O-type stars. A combination of NIR photometry and spectroscopy data was used to obtain the distance of these two stars, with the method of spectroscopic parallax: IRS 298 (O6 V, 3.35±0.61 kpc) and IRS 339 (O9 V, 3.24±0.56 kpc). Adopting the average distance of 3.29±0.58 kpc and comparing the Lyman continuum luminosity of these stars with that required to account for the radio continuum flux of the H II region, we conclude that these two stars are the ionizing sources of GAL 331.31-00.34. Young stellar objects (YSOs) were searched by using our NIR photometry and mid-infrared (MIR) data from the Galactic Legacy Infrared Mid-Plane Survey Extraordinaire (GLIMPSE) survey. The analysis of NIR and MIR colour–colour diagrams resulted in 47 YSO candidates.

The GLIMPSE counterpart of IRAS 16085-5138, which presents IRAS colour indices compatible with an ultracompact H II region, has been identified. The analysis of its spectral energy distribution between 2 and {img}m revealed that this source shows a spectral index α= 3.6 between 2 and {img}m, which is typical of a YSO immersed in a protostellar envelope. Lower limits to the bolometric luminosity and the mass of the embedded protostar have been estimated as L= 7.7 {x} 10³ L_☉ and M= 10 M_☉, respectively, which correspond to a B0–B1 V zero-age main sequence star.

Based on observations carried at the Southern Observatory for Astrophysical Research (SOAR) observatory, a joint project of the Ministério de Ciência, Tecnologia e Inovação (MCTI) of the República Federativa do Brasil, the U.S. National Optical Astronomy Observatory (NOAO), the University of North Carolina at Chapel Hill (UNC) and the Michigan State University (MSU).