DISTINCT CHEMICAL REGIONS IN THE “PRESTELLAR” INFRARED DARK CLOUD G028.23–00.19

We have observed the Infrared Dark Cloud (IRDC) G028.23-00.19 at 3.3 mm using the Combined Array for Research in Millimeter-wave Astronomy. In its center, the IRDC hosts one of the most massive (~1520 M sub([middot in circle])) quiescent, cold (12 K) clumps known (MM1). The low temperature, high NH sub(2)D abundance, narrow molecular line widths, and absence of embedded infrared sources (from 3.6 to 70 mu m) indicate that the clump is likely prestellar. Strong SiO emission with broad line widths (6-9 km s super(-1)) and high abundances ((0.8-4) x 10 super(-9)) is detected in the northern and southern regions of the IRDC, unassociated with MM1. We suggest that SiO is released to the gas phase from the dust grains through shocks produced by outflows from undetected intermediate-mass stars or clusters of low-mass stars deeply embedded in the IRDC. A weaker SiO component with narrow line widths (~2 km s super(-1)) and low abundances (4.3 x 10 super(-11)) is detected in the center-west region, consistent with either a "subcloud-subcloud" collision or an unresolved population of a few low-mass stars. We report widespread CH sub(3)OH emission throughout the whole IRDC and the first detection of extended narrow methanol emission (~2 km s super(-1)) in a cold, massive prestellar clump (MM1). We suggest that the most likely mechanism releasing methanol into the gas phase in such a cold region is the exothermicity of grain-surface reactions. HN super(13)C reveals that the IRDC is actually composed of two distinct substructures ("subclouds") separated in velocity space by ~1.4 km s super(-1). The narrow SiO component arises where the subclouds overlap. The spatial distribution of C sub(2)H resembles that of NH sub(2)D, which suggests that C sub(2)H also traces cold gas in this IRDC.