H II Regions in the Vicinity of the Galactic Center

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Session 50 -- The Milky Way
Display presentation, Thursday, June 15, 1995, 9:20am - 4:00pm

[50.05] H II Regions in the Vicinity of the Galactic Center

Sean W. J. Colgan, Janet P. Simpson, Edwin F. Erickson, Michael R. Haas, Robert H. Rubin (NASA/Ames), Mark Morris (UCLA)

We present KAO FIR spectroscopy of four compact, thermal radio sources in the vicinity of the galactic center: G359.98--0.07 (A/B), G359.97--0.08 (C1/C2), G359.98+0.03 (H2), and G0.04+0.02 (H5), where the nomenclature is from Ho {{et~al$.$}} (1985, ApJ, 288, 575) and Yusef-Zadeh \& Morris (1987, ApJ, 320, 545). Our measurements of [SIII] (19 and 33 $\mu$m), [SiII] (35 $\mu$m), [OIII] (52 and 88 $\mu$m), [NIII] (57 $\mu$m), [OI] (63 and 146 $\mu$m), [CII] (158 $\mu$m), and [NII] (205 $\mu$m) emission suggest that these objects are HII regions, in agreement with their radio continuum spectra. The excitation of the gas is consistent with photoionization by $T_{eff} \sim34,000$~K Kurucz LTE stellar atmospheres and the electron densities are $\sim 10^2-10^3$ cm$^{-3}$. The FIR continuum luminosity ranges from $1-5\times10^5$ L$_{\odot}$. This, and the lower excitation [OI] and [CII] line emission are consistent with the formation of a photodissociation region adjacent to the ionized material. More detailed models will be used to estimate the abundances in these sources.

We also discuss FIR observations at four positions in the source Sgr C (G359.43--0.09). From an $\lambda$18 cm VLA map, Liszt (1985, ApJ, 293, L65) describes Sgr C as a shell with a bright spot on its western edge and the eastern part of the ring connected to a linear filamentary structure which runs perpendicular to the galactic plane. From our measurements, the bright spot (position 1) produces FIR emission typical for HII regions, with an electron density of only 100 cm$^{-3}$. The western part of the ring (position 2) shows similar emission, with the fluxes down by a factor of 2. The FIR line and continuum emission at a position on the straight filament, separated from the ring but with a larger radio flux than position 2, is lower by factors of three to seven, implying significantly less photoionization. A nearby radio peak at G359.47--0.17, whose continuum morphology is suggestive of a compact HII region, produced no detectable emission to levels a factor of three or more below position 1, also implying a lack of strong photoionization.

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