The young stellar cluster [DBS2003]157 associated with the Hii region GAL331.31-00.34

We report a study of the stellar content of the near-infrared (NIR) cluster [DBS2003]157 embedded in the extended Hii region GAL331.31-00.34, 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: IRS298 (O6V, 3.35 plus or minus 0.61kpc) and IRS339 (O9V, 3.24 plus or minus 0.56kpc). Adopting the average distance of 3.29 plus or minus 0.58kpc and comparing the Lyman continuum luminosity of these stars with that required to account for the radio continuum flux of the Hii region, we conclude that these two stars are the ionizing sources of GAL331.31-00.34. 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 IRAS16085-5138, which presents IRAS colour indices compatible with an ultracompact Hii region, has been identified. The analysis of its spectral energy distribution between 2 and m revealed that this source shows a spectral index alpha = 3.6 between 2 and 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 103L[odot] and M= 10M[odot], respectively, which correspond to a B0-B1V zero-age main sequence star.