The distribution of OH in Taurus Molecular Cloud-1

The column density of the OH radical in Taurus Molecular Cloud-1 (TMC-I), r eaches its maximum close to the cyanopolyyne peak. A comparison with previo usly published maps of other molecules shows that OH has a similar distribu tion as HC3N and CCS, but differs largely from CS, SO, NH3 and N2H+. The OH /(CO)-O-18 column density ratio is, however, almost constant along the dens e ridge of TMC-1, suggesting that the fractional OH abundance does not chan ge much, the derived value being about 10(-7). This high number confirms th at in dense gas OH is mainly formed by the electron recombination of H3O+. The approximate constancy of the fractional OH abundance is in accordance w ith 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 pa rt of the cloud would be chemically less evolved than the ammonia maximum i n the northwest. Therefore we discuss the possibility that the OH maximum r epresents the so called 'radical peak', which occurs when freezing on to gr ain surfaces starts to be the dominant factor controlling the chemical comp osition and reactions (Brown & Millar 1984). 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 ne ighbourhood of the TMC-1 ridge.