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Astron. Astrophys. 353, 1065-1073 (2000)

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The distribution of OH in Taurus Molecular Cloud-1

J. Harju 1, A. Winnberg 2 and J.G.A. Wouterloot 3

1 Observatory, P.O. Box 14, University of Helsinki, 00014 Helsinki, Finland
2 Onsala Space Observatory, 43992 Onsala, Sweden
3 Radioastronomisches Institut, Universität Bonn, Auf dem Hügel 71, 53121 Bonn, Germany

Received 26 April 1999 / Accepted 9 November 1999


The column density of the OH radical in Taurus Molecular Cloud-1 (TMC-1), reaches its maximum close to the cyanopolyyne peak. A comparison with previously published maps of other molecules shows that OH has a similar distribution as HC3N and CCS, but differs largely from CS, SO, NH3 and N2H+. The OH/C18O column density ratio is, however, almost constant along the dense ridge of TMC-1, suggesting that the fractional OH abundance does not change much, the derived value being about [FORMULA]. This high number confirms that in dense gas OH is mainly formed by the electron recombination of H3O+. The approximate constancy of the fractional OH abundance is in accordance with a flat density distribution as has been derived recently (Pratap et al. 1997).

Previous SO/CS maps together with some recent modelling results seem to be in conflict with the idea that the cyanopolyyne peak in the southeastern part of the cloud would be chemically less evolved than the ammonia maximum in the northwest. Therefore we discuss the possibility that the OH maximum represents the so called `radical peak', which occurs when freezing on to grain surfaces starts to be the dominant factor controlling the chemical composition and reactions (Brown & Millar 1989). It turns out that the greater part of the data accumulated so far, including the present OH observations, fit the `old' picture where a slightly higher density and a more advanced chemical state prevail in the northwestern part of the TMC-1 ridge.

The `satellite' lines towards two locations in the cloud show enhanced 1612 MHz emission. We suggest that this is due to non-thermal excitation by far-infrared radiation from dust, heated by the embedded young stars in the neighbourhood of the TMC-1 ridge.

Key words: molecular processes – ISM: abundances – ISM: clouds – ISM: molecules – ISM: individual objects: TMC-1

Send offprint requests to: J. Harju (jorma.harju@astro.helsinki.fi)

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© European Southern Observatory (ESO) 2000

Online publication: January 18, 2000