2000A&A...353.1065H

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 10^–7. 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., 1997ApJ...486..862P). 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, 1989MNRAS.237..661B). 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 1612MHz 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.