Super Hot Cores in NGC 253: witnessing the formation and early evolution of super star clusters

Rico Villas, F., Martín Pintado, J., González Alfonso, E., Martín, S., Rivilla, V. M. (2020). Super Hot Cores in NGC 253: witnessing the formation and early evolution of super star clusters. Monthly Notices of the Royal Astronomical Society, 491, 3,  4573-4589 DOI: 10.1093/mnras/stz3347

Using 0.2 arcsec (similar to 3 pc) ALMA images of vibrationally excited HC3N emission (HC3N*) we reveal the presence of eight unresolved Super Hot Cores (SHCs) in the inner 160 pc of NGC253. Our LTE and non-LTE modelling of the HC3N* emission indicate that SHCs have dust temperatures of 200-375 K, relatively high H-2 densities of (1-6) x 10(6) cm(-3) and high IR luminosities of (0.1-1) x 10(8) L-circle dot. As expected from their short-lived phase (similar to 10(4) yr), all SHCs are associated with young super star clusters (SSCs). We use the ratio of luminosities from the SHCs (protostar phase) and from the free-free emission (ZAMS star phase), to establish the evolutionary stage of the SSCs. The youngest SSCs, with the larges ratios, have ages of a few 10(4) yr (proto-SSCs) and the more evolved SSCs are likely between 105 and 10(6) yr (ZAMS-SSCs). The different evolutionary stages of the SSCs are also supported by the radiative feedback from the UV radiation as traced by the HNCO/CS ratio, with this ratio being systematically higher in the young proto-SSCs than in the older ZAMS-SSCs. We also estimate the SFR and the SFE of the SSCs. The trend found in the estimated SFE (similar to 40 per cent for proto-SSCs and > 85 per cent for ZAMS-SSCs) and in the gas mass reservoir available for star formation, one order of magnitude higher for proto-SSCs, suggests that star formation is still going on in proto-SSCs. We also find that the most evolved SSCs are located, in projection, closer to the centre of the galaxy than the younger proto-SSCs, indicating an inside-out SSC formation scenario.

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