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Distinct Chemical Regions in the "Prestellar" Infrared Dark Cloud (IRDC) G028.23-00.19

We have observed the IRDC G028.23-00.19 at 3.3 mm using CARMA. In its center, the IRDC hosts one of the most massive (~1520 Msun) quiescent, cold (12 K) clumps known (MM1). The low temperature, high NH_2D abundance, narrow molecular line widths, and absence of embedded IR sources (from 3.6 to 70 mu) indicate that the clump is likely prestellar. Strong SiO emission with broad line widths (6-9 km s^-1) and high abundances (0.8-4 x 10^-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^-1) and low abundances (4.3 x 10^-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_3OH emission throughout the whole IRDC and the first detection of extended narrow methanol emission (~2 km s^-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^13C reveals that the IRDC is actually composed of two distinct substructures ("subclouds") separated in velocity space by ~1.4 km s^-1. The narrow SiO component arises where the subclouds overlap. The spatial distribution of C_2H resembles that of NH_2D, which suggests that C_2H also traces cold gas in this IRDC.