CH abundance gradient in TMC-1
  We observed the 9-cm Lambda-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. 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 1.0e-8, to the southeast where it reaches a value of 2.0e-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. 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.
