1999ApJS..125..143W

We present continuum and spectral line observations toward three ultracompact (UC) H II regions (G34.26+0.15, G11.94-0.62, and G33.92+0.11) and one candidate precursor to a UC H II region (IRAS 18511+0146). The free-free emission, estimated from the spectral index of the centimeter data, is subtracted from the λ=2.7 mm data to place limits on the dust emission. We do not conclusively detect thermal dust emission from compact cores in these regions. Mass limits on warm dust cores (T_dust∼100 K) in or near the UC H II regions are 137, 163, and 236 M_☉.beam^–1 for G34.26+0.15, G11.94-0.62, and G33.92+0.11, respectively. Limits on cool dust cores (T_dust∼30 K) anywhere within the primary beam are 355, 350, 1315, and 194 M_☉.beam^–1 for G34.26+0.15, G11.94-0.62, G33.92+0.11, and IRAS 18511+0146, respectively.

Emission in the C¹⁸O J=1⟶0 and ¹³CO J=1⟶0 lines was detected from G34.26+0.15, G33.92+0.11, and IRAS 18511+0146. In G33.92+0.11 and IRAS 18511+0146, the emission is associated with extended, moderate density (n∼10⁴-10⁵ cm^–3), cool (T∼20 K) cores; in G34.26+0.15, the C¹⁸O traces a dense (n∼10⁷ cm^–3), warm (T∼50 K), compact core. Column densities estimated for C¹⁸O regions range from 2x10²³ to 2x10²⁴ cm^–2, and their total masses are 200-5000 M_☉.

The CH₃CN J=6⟶5 line was detected strongly in emission toward G34.26+0.15 and weakly toward G33.92+0.11. The CH₃CN emission arises primarily from compact regions, characteristic of hot core emission.

For G34.26+0.15, fits to the K-component lines derive a gas temperature in the range 80-175 K and CH₃CN column densities from 3x10¹⁵ to 1.5x10¹⁶ cm^–2. For G34.26+0.15 there are systematic spatial offsets between the distributions of the C¹⁸O, CH₃CN, NH₃, and free-free emission. We propose that the hot molecular gas in G34.26+0.15 is the outer layer of a cool massive core that is being externally heated by interaction with the cometary UC H II region C. The amount of heated gas could be modest, 2-20 M_☉, if the molecular abundances are as high as found in some chemical models. In this scenario, the CH₃CN emission traces layers closer to the UC H II, which have been heated for some time; the NH₃ emission arises from a deeper layer, which has been more recently heated; and the C¹⁸O emission traces the bulk of the core, which is only mildly heated. We argue that the velocity gradients in the molecular and ionized gases are due to the kinematics of the H II-molecular interaction and the geometry of the environment rather than gravitationally bound rotational motion.

IRAS 18511+0146 is unlikely to be an immediate precursor to UC H II regions. The weak free-free emission seen at λ=3.6 cm is consistent with an optically thin H II region surrounding a B0 star. The large extent of the molecular gas, the modest density, and the lack of compact millimeter dust continuum emission suggest that additional massive-star formation is not occurring in this region at the present time.