* Astronomy

Title: Multiple episodes of star formation in the CN15/16/17 molecular complex
Authors: M. Gennaro (1), A. Bik (1), W. Brandner (1), A. Stolte (2), B. Rochau (1), H. Beuther (1), D. Gouliermis (1,3), J. Tackenberg (1), N. Kudryavtseva (1), B. Hussmann (2), F. Schuller (4), Th. Henning (1) ((1) MPIA, Heidelberg, (2) AIfA, Bonn, (3) ITA-ZAH, Heidelberg, (4) ESO, Santiago)

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 characterised by the presence of two young clusters, DB10 and DB11, visible in the NIR, an ultra-compact HII region identified in the radio, several young stellar objects visible in the MIR, 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=0.58, b=-0.85) and its kinematic distance of ~7.5 kpc, the region was thought to be a very massive site of star formation in proximity of the CMZ. The cluster DB11 was estimated to be as massive as 10^4 solar masses. 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 characterising 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 JHKs 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 ~1.2 kpc, much smaller than the kinematic distance. Adopting this distance we obtain clusters masses of M(DB10) ~ 170 solar masses and M(DB11) ~ 275 solar masses. This is consistent with the absence of any O star, confirmed by the excitation/ionisation status of the nebula. No HeI diffuse emission is detected in our spectroscopic observations at 2.113 µm, 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.

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