Astrophys. J., 558, 693-701 (2001/September-2)
Small-scale abundance variations in TMC-1: dynamics and hydrocarbon chemistry.
DICKENS J.E., LANGER W.D. and VELUSAMY T.
Abstract (from CDS):
We present high spectral resolution observations of eighteen molecules, including high-quality maps of CCS and HC7N in TMC-1, using NASA's Deep Space Network 70 m antenna to study the interaction between cloud dynamics and chemistry. Other molecules shown in our study are C18O, NH3, CS, C3S, C3H2, H2C3, H2C4, H2C6, HC3N, HC5N, DC5N, HC9N, C4H, C5H, C6H, and a successful detection of the rare C8H molecule. In addition, we have searched for and set meaningful abundance limits on several carbon chain and ring molecules such as C7H, H2C5, c-H2C5, and biogenic molecules such as pyrrole and glycine. All the species observed in TMC-1 show large spectral-line variations in both intensity and shape over extremely small scales (∼0.03 pc). Maps of CCS and HC7N display abundance ratio variations of 3-5 along individual lines of sight. The high degree of clumpiness, transient nature of clumps, and gas-phase enrichment adequately explain the ``early-time'' chemistry and the molecular complexity in TMC-1. This enrichment has interesting implications for hydrocarbon chemistry in TMC-1, and presumably other clumpy, dark clouds. Specifically, the large number of clumps at various stages of early-time chemical evolution increases the chances for detection of complex hydrocarbons, since the probability of observing a clump at the time of peak abundance for a given molecular species is increased. We suggest two mechanisms for explaining the small-scale variations: (1) the passage of MHD waves in a clumpy medium and (2) grain impacts during clump-clump collisions. In the quiescent region far from any protostar, the MHD activity can be generated locally by clump collisions. The passage of MHD waves helps maintain early-time chemistry in the clumps. Both mechanisms provide enough energy to raise grain temperatures from 10 K to Tcrit∼30 K, sufficient to cause reactive radical explosions in grain mantles and thermal desorption. In this manner, the mantle injection causes TMC-1 to exhibit some aspects of ``hot-core'' chemistry, as seen in more massive star-forming regions. The transient nature of the clumps and the mantle-driven chemistry make TMC-1 a good target for future searches of complex molecular species.
Astrochemistry - ISM: Clouds - ISM: individual (TMC-1) - ISM: Molecules - Radio Lines: ISM
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