2012A&A...542A..74G

We have started a campaign to identify massive star clusters inside bright molecular bubbles towards the Galactic center. The CN15/16/17 molecular complex is the first example of our study. The region is characterized by the presence of two young clusters, DB10 and DB11, visible in the near-infrared, an ultra-compact H ii region identified in the radio, several young stellar objects visible in the mid infrared, a bright diffuse nebulosity at 8 µm coming from PAHs and sub-mm continuum emission revealing the presence of cold dust. Given its position on the sky (l=0.58deg, b=-0.85deg) and its kinematic distance of 7.5kpc, the region was thought to be a very massive site of star formation in proximity of the central molecular zone. One of the two identified clusters, DB11, was estimated to be as massive as ∼10⁴M_☉. However the region's properties were known only through photometry and its kinematic distance was very uncertain given its location at the tangential point. We aimed at better characterizing the region and assess whether it could be a site of massive star formation located close to the Galactic center. We have obtained NTT/SofI deep JHK_S photometry and long slit K band spectroscopy of the brightest members. We have additionally collected data in the radio, sub-mm and mid infrared, resulting in a quite different picture of the region. We have confirmed the presence of massive early B type stars and have derived a spectro-photometric distance of ∼1.2kpc, much smaller than the estimated kinematic distance. Adopting this distance we obtain clusters masses of M_DB10≃170M_☉ and M_DB11≃275M_☉. This is consistent with the absence of any O star, confirmed by the excitation/ionization status of the nebula. No He i diffuse emission is detected in our spectroscopic observations at 2.113µm, which would be expected if the region was hosting more massive stars. Radio continuum measurements are also consistent with the region hosting at most early B stars.