Sagittarius B2 Main: A Cluster of Ultra-Compact HII Regions and Massive Protostellar Cores

Abstract

The ionized core in the Sgr B2 Main star-forming region was imaged using the Submillimeter Array archival data observed for the H26$\alpha$ line and continuum emission at 0.86 millimeter with an angular resolution 0.3\arcsec. Eight hyper-compact H26$\alpha$ emission sources were detected with a typical size in the range of 1.6--20$\times 10^2$ AU and electron density of 0.3--3$\times 10^7$ cm${}^{-3}$, corresponding to the emission measure 0.4--8.4$\times 10^{10}$ cm${}^{-6}$ pc. The H26$\alpha$ line fluxes from the eight hyper-compact HII sources imply that the ionization for each of the sources must be powered by a Lyman continuum flux from an O star or a cluster of B stars. The most luminous H26$\alpha$ source among the eight detected requires an O6 star that appears to be embedded in the ultra-compact HII region F3. In addition, $\sim$ 23 compact continuum emission sources were also detected within the central 5\arcsec$\times$3\arcsec\,($\sim 0.2$ pc) region. In the assumption of a power-law distribution for the dust temperature, with the observed brightness temperature of the dust emission we determined the physical properties of the submillimeter emission sources showing that the molecular densities are in the range of 1--10$\times 10^8$ cm${}^{-3}$, surface densities between 13 to 150 $g$ cm${}^{-2}$, and total gas masses in the range from 5 to $\gtrsim$ 200 $M_{\odot }$ which are 1 or 2 orders of magnitude greater than the corresponding values of the Bonnor-Ebert mass. With a mean free-fall time scale of 2$\times 10^3$ y, each of the massive protostellar cores are undergoing gravitational collapse to form new massive stars in the Sgr B2 Main core.

19 pages, 4 figures, ApJ accepted

References in corpus (7)

• Theory of Star Formation
• Star formation through gravitational collapse and competitive accretion
• A Minimum Column Density of 1 g cm^-2 for Massive Star Formation
• The High-Density Ionized Gas in the Central Parsec of the Galaxy
• Infall and Outflow of Molecular Gas in Sgr B2
• Reversal of infall in SgrB2(M) revealed by Herschel/HIFI observations of HCN lines at THz frequencies
• A bipolar outflow from the massive protostellar core W51e2-E

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• The Chemistry and Kinematics of Two Molecular Clouds near Sagittarius A*
• Clustered Star Formation in the center of NGC 253 Contributes to Driving the Ionized Nuclear Wind