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 HC 7 N 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 C 18 O, NH 3 , CS, C 3 S, C 3 H 2 , H 2 C 3 , H 2 C 4 , H 2 C 6 , HC 3 N, HC 5 N, DC 5 N, HC 9 N, C 4 H, C 5 H, C 6 H, and a successful detection of the rare C 8 H molecule. In addition, we have searched for and set meaningful abundance limits on several carbon chain and ring molecules such as C 7 H, H 2 C 5 , c-H 2 C 5 , 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 HC 7 N 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 ~ 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.