2011ApJ...733...44D

We present Very Large Array observations of ammonia (NH₃) (1,1), (2,2), and dicarbon sulfide (CCS) (2₁-1₀) emission toward the infrared dark cloud (IRDC) G19.30+0.07 at ∼22 GHz. The NH₃ emission closely follows the 8 µm extinction. The NH₃(1,1) and (2,2) lines provide diagnostics of the temperature and density structure within the IRDC, with typical rotation temperatures of ∼10-20 K and NH₃ column densities of ∼10¹⁵/cm2. The estimated total mass of G19.30+0.07 is ∼1130 M_☉. The cloud comprises four compact NH₃ clumps of mass ∼30-160 M_☉. Two coincide with 24 µm emission, indicating heating by protostars, and show evidence of outflow in the NH₃emission. We report a water maser associated with a third clump; the fourth clump is apparently starless. A non-detection of 8.4 GHz emission suggests that the IRDC contains no bright H II regions and places a limit on the spectral type of an embedded zero-age main-sequence star to early-B or later. From the NH₃emission, we find that G19.30+0.07 is composed of three distinct velocity components or "subclouds." One velocity component contains the two 24 µm sources and the starless clump, another contains the clump with the water maser, while the third velocity component is diffuse, with no significant high-density peaks. The spatial distribution of NH₃ and CCS emission from G19.30+0.07 is highly anti-correlated, with the NH₃ predominantly in the high-density clumps and the CCS tracing lower-density envelopes around those clumps. This spatial distribution is consistent with theories of evolution for chemically young low-mass cores, in which CCS has not yet been processed to other species and/or depleted in high-density regions.