CH abundance gradient in TMC-1
Journal article, 2011
Aims. The aim of this study is to examine if the well-known chemical gradient in TMC-1 is reflected in the amount of rudimentary
forms of carbon available in the gas-phase. As a tracer we use the CH radical which is supposed to be well correlated with carbon
atoms and simple hydrocarbon ions.
Methods. We observed the 9-cm Λ-doubling lines of CH along the dense filament of TMC-1. The CH column densities were compared
with the total H2 column densities derived using the 2MASS NIR data and previously published SCUBA maps and with OH column
densities derived using previous observations with Effelsberg. We also modelled the chemical evolution of TMC-1 adopting physical
conditions typical of dark clouds using the UMIST Database for Astrochemistry gas-phase reaction network to aid the interpretation
of the observed OH/CH abundance ratios.
Results. The CH column density has a clear peak in the vicinity of the cyanopolyyne maximum of TMC-1. The fractional CH
abundance relative to H2 increases steadily from the northwestern end of the filament where it lies around 1.0 × 10−8, to the southeast
where it reaches a value of 2.0 × 10−8. The OH and CH column densities are well correlated, and we obtained OH/CH abundance
ratios of ∼16–20. These values are clearly larger than what has been measured recently in diffuse interstellar gas and is likely to be
related to C to CO conversion at higher densities. The good correlation between CH and OH can be explained by similar production
and destruction pathways. We suggest that the observed CH and OH abundance gradients are mainly due to enhanced abundances in
a low-density envelope which becomes more prominent in the southeastern part and seems to continue beyond the dense filament.
Conclusions. An extensive envelope probably signifies an early stage of dynamical evolution, and conforms with the detection of a
large CH abundance in the southeastern part of the cloud. The implied presence of other simple forms of carbon in the gas phase provides a natural explanation for the observation of "early-type" molecules in this region.
individual objects: TMC-1