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- Suutarinen, A., et al.
(författare)
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CH abundance gradient in TMC-1
- 2011
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Ingår i: Astronomy and Astrophysics. - : EDP Sciences. - 0004-6361 .- 1432-0746. ; 531, s. A121-
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Tidskriftsartikel (refereegranskat)abstract
- Aims. The aim of this study is to examine if the well-known chemical gradient in TMC-1 is reflected in the amount of rudimentaryforms of carbon available in the gas-phase. As a tracer we use the CH radical which is supposed to be well correlated with carbonatoms 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 comparedwith the total H2 column densities derived using the 2MASS NIR data and previously published SCUBA maps and with OH columndensities derived using previous observations with Effelsberg. We also modelled the chemical evolution of TMC-1 adopting physicalconditions typical of dark clouds using the UMIST Database for Astrochemistry gas-phase reaction network to aid the interpretationof 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 CHabundance relative to H2 increases steadily from the northwestern end of the filament where it lies around 1.0 × 10−8, to the southeastwhere it reaches a value of 2.0 × 10−8. The OH and CH column densities are well correlated, and we obtained OH/CH abundanceratios of ∼16–20. These values are clearly larger than what has been measured recently in diffuse interstellar gas and is likely to berelated to C to CO conversion at higher densities. The good correlation between CH and OH can be explained by similar productionand destruction pathways. We suggest that the observed CH and OH abundance gradients are mainly due to enhanced abundances ina 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 alarge 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.
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