NCSA Astronomy Digital Image Library
Project Abstract and Description
A Study of the Physics and Chemistry of TMC-1
P. Pratap, J.E, Dickens, R.L. Snell, M.P. Miralles, E.A.Bergin, W.M.Irvine, F.P. Schloerb
Contact: Preethi Pratap
Email: preethi@newton.haystack.edu
Number of images in project: 24
References: 1997ApJ...486..862P
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 multi-transition 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 years. 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 years 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 and abundance gradient
similar to that observed.
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