The Spectral Energy Distributions of Infant Super Star Clusters in Henize 2-10 from 7mm to 6cm
Abstract
We present observations from our continuing studies of the earliest stages of massive star cluster evolution. In this paper, radio observations from the Very Large Array at 0.7cm, 1.3cm, 2cm, 3.6cm, and 6cm are used to map the radio spectral energy distributions and model the physical properties of the ultra-young embedded super star clusters in Henize 2-10. The 0.7cm flux densities indicate that the young embedded star clusters that are powering the radio detected ``ultradense HII regions'' (UDHIIs) have masses greater than \~105 Msun. We model the radio spectral energy distributions as both constant density HII regions and HII regions with power-law electron density gradients. These models suggest the UDHIIs have radii ranging between ~2-4pc and average electron densities of ~103-104 cm-3 (with peak electron densities reaching values of ~105-106 cm-3). The pressures implied by these densities are P/kB~107-1010 cm-3 K, several orders of magnitude higher than typical pressures in the Galactic ISM. The inferred HII masses in the UDHIIs are \~2-8x103 Msun; these values are <5% of the embedded stellar masses, and anonamously low when compared to optically visible young clusters. We suggest that these low HII mass fractions may be a result of the extreme youth of these objects.
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