A Study of the Physics and Chemistry of TMC‐1

Abstract

We present a comprehensive study of the physical and chemical conditions along the TMC-1 ridge. Temperatures were estimated from observations of CH3CCH, NH3, and CO. Densities were obtained from a multitransition study of HC3N. The values of the density and temperature allow column densities for 13 molecular species to be estimated from statistical equilibrium calculations, using observations of rarer isotopomers where possible, to minimize opacity effects. The most striking abundance variations relative to HCO+ along the ridge were seen for HC3N, CH3CCH, and SO, while smaller variations were seen in CS, C2H, and HCN. On the other hand, the NH3, HNC, and N2H+ abundances relative to HCO+ were determined to be constant, indicating that the so-called NH3 peak in TMC-1 is probably a peak in the ammonia column density rather than a relative abundance peak. In contrast, the well-studied cyanopolyyne peak is most likely due to an enhancement in the abundance of long-chain carbon species. Comparisons of the derived abundances to the results of time-dependent chemical models show good overall agreement for chemical timescales around 10(5) yr. We find that the observed abundance gradients can be explained either by a small variation in the chemical timescale from 1.2 x 10(5) to 1.8 x 10(5) yr or by a factor of 2 change in the density along the ridge. Alternatively, a variation in the C/O ratio from 0.4 to 0.5 along the ridge produces an abundance gradient similar to that observed.

The interstellar gas-phase chemistry of HCN and HNC

“…Taking into account the various detection uncertainties, the simulated HCN abundance is compatible with a cloud age between 310 5 yr and 610 5 yr varying from (2-6)10 -8 relative to H 2 . This value compares with the observed HCN abundance of between 7.110 -9 to 3.410 -8 relative to H 2 (Pratap et al 1997, Hirota et al 1998, Irvine & Schloerb 1984. As HCN is a quite unreactive molecule there is not many species directly correlated to it, more precisely there are the HNC, HCNH + and CH 3 CN species.…”

“…In their study, they used the rotational excitation rate constants for HCN with He calculated by Green & Thaddeus (1974) to deduce the rate constants for the rotational excitation of HCN by H 2 , assuming that the values for HNC-He and HNC-H 2 collisions were identical to the HCN ones. (Hirota et al 1998) performed detailed HCN and HNC observations in various dense molecular clouds leading to similar results for TMC-1 to (Pratap et al 1997) despite the lower abundances derived from Hirota et al. Taking into account the various detection uncertainties, the simulated HCN abundance is compatible with a cloud age between 310 5 yr and 610 5 yr varying from (2-6)10 -8 relative to H 2 .…”

“…A 12 C/ 13 C abundance ratio of between 64 and 68 is usually assumed, although this value has varied over time, with the most recent estimate being 68 (Milam et al 2005). The HCN and HNC detection in TMC-1 by (Pratap et al 1997) show that even for H 13 CN and HN 13 C, the lines are not optically thin and self-absorption plays a role. In their study, they used the rotational excitation rate constants for HCN with He calculated by Green & Thaddeus (1974) to deduce the rate constants for the rotational excitation of HCN by H 2 , assuming that the values for HNC-He and HNC-H 2 collisions were identical to the HCN ones.…”

Models and Observations of the Chemistry Near Young Stellar Objects

“…If this value is increased from the canonical ratio of 0.4 to higher values, improvement with observations for the complex molecules is obtained . Pratap et al (1997) This point is further illustrated by high spatial resolution observations of the L1498 pre-stellar core by Kuiper et al (1996), which show a chemically differentiated structure with NH 3 peaking in the inner region and C 2 S in the atomic-carbon rich (clumpy) outer part. Further chemical studies of such cores, especially those which have a more centrally concentrated density structure and appear to be on the verge of collapse (e.g., Ward-Thompson et al 1994, Motte et al 1998, are warranted.…”

“…This clump shows a particularly rich chemistry, with a large chemical gradient across the core. NH 3 and other 'late' molecules peak in the northern part, whereas long carbon-chain molecules such as HC 7 N and C 4 H are most abundant in the southern part (Olano et al 1988, Pratap et al 1997. High spectral and spatial resolution observations show that there are at least three different velocity components in this region on less than 10,000 AU scales, with different intensity ratios among the molecules .…”