Small-scale abundance variations in TMC-1: Dynamics and hydrocarbon chemistry

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 (CO)-O-18, 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 (similar to 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 T-crit similar to 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.