A ridge of recent massive star formation between Sgr B2M and Sgr B2N

We present single dish and interferometric maps of several rotational transitions of HC3N vibrationally excited levels towards Sgr B2. HC3N is a very suitable molecule to probe hot and dense regions (hot cores) affected by high extinction since its vibrational levels are mainly excited by mid-IR radiation. The single dish maps show, for the first time, that the HC3N vibrationally excited emission (HC3N*) is not restricted to Sgr B2M and N but extended over an area 40 " x 20 " in extent. We distinguish four bright clumps (Sgr B2R1 to B2R4) in the ridge connecting the main cores Sgr B2M and Sgr B2N, and a low brightness extended region to the west of the ridge (Sgr B2W). The physical properties and the kinematics of all hot cores have been derived from the HC3N* lines. Our high angular resolution images show that the Sgr B2N hot core breaks in two different hot cores, Sgr B2N1 and N2, with different radial velocities and separated by similar to 2 " in declination. We find that the excitation of the HC3N* emission in all hot cores can be represented by a single temperature and that the linewidth of the HC3N* rotational lines arising from different vibrational levels systematically decreases as the energy of the vibrational level increases. The systematic trend in the linewidth is likely related to the increase of the velocity as the distance to the exciting source increases. We have developed a simple model to study the excitation of the HC3N vibrational levels by IR radiation. We find that the single excitation temperature can be explained by high luminosities of embedded stars (similar to 10(7) L-circle dot) and small source sizes (similar to 2 - 3 "). The estimated clump masses are 500 M-circle dot for Sgr B2M, 800 M-circle dot for Sgr B2N and 10-30 M-circle dot for Sgr B2R1 to B2R4. Luminosities are 1 - 2 10(6) L-circle dot for Sgr B2R1-B2R4 and Sgr B2M and 10(7) L-circle dot for Sgr B2N. We estimate HC3N abundances of 5 10(-9) for Sgr B2M and Sgr B2N2 and 10(-7) for the rest of the hot cores. The different HC3N abundances in the hot cores reflect different stages of evolution due to time dependent chemistry and/or photo-dissociation by UV radiation from nearby HII regions. According to the mass and the luminosity of the different hot cores, we propose that Sgr B2M and B2N contain a cluster of 20-30 hot cores, each like that in Orion A, a number similar to the UC HII regions already detected in the region. The Sgr B2R1-B2R4 hot cores represent isolated formation of massive stars.