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The Spectral Energy Distributions of Infant Super-Star Clusters in Henize 2-10 from 7 Millimeters to 6 Centimeters

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 0.7, 1.3, 2, 3.6, and 6 cm are used to map the radio spectral energy distributions and model the physical properties of the ultrayoung embedded super-star clusters in Henize 2-10. The 0.7 cm flux densities indicate that the young embedded star clusters that are powering the radio-detected "ultradense (UD) H II regions" have masses greater than ~105 M☉. We model the radio spectral energy distributions as both constant-density H II regions and H II regions with power-law electron density gradients. These models suggest that the UD H II regions have radii ranging between ~2 and 4 pc 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 interstellar medium. The inferred H II masses in the UD H II regions are ~(2-8) × 103 M☉; these values are less than 5% of the embedded stellar masses and anomalously low when compared with optically visible young clusters. We suggest that these low H II mass fractions may be a result of the extreme youth of these objects.