Shock excited far-infrared molecular emission around T Tau

The first complete far-infrared spectrum of T Tau has been obtained with th e LWS spectrometer on-board the Infrared Space Observatory (ISO), which det ected strong emission from high-J (J=14-25) CO, para- and ortho-H2O and OH transitions over the wavelength range from 40 to 190 mu m. In addition the [OI]63 mu m, [OI]145 mu m and [CII]158 mu m atomic lines were also detected . Most of the observed molecular emission can be explained by a single emis sion region at T-300-900 K and nH(2) similar to 10(5-6)cm(-3), With a diame ter of about 2-3 arcsec. This corresponds to a very compact region of 300 - 400 AU at the distance of 140 pc. A higher temperature component seems to be needed to explain the highest excitation CO and H2O lines. We derive a w ater abundance of 1-7 . 10(-5) and an OH abundance of similar to 3.10(-5) w ith respect to molecular hydrogen, implying H2O and OH enhancements by more than a factor of 10 with respect to the expected ambient gas abundance. The observed cooling in the various species amounts to 0.04 L., comparable to the mechanical luminosity of the outflow, indicating that the stellar wi nds could be responsible of the line excitation through shocks. In order to explain the observed molecular cooling in T Tau in terms of C-t ype shock models, we hypothesise that the strong far-ultraviolet radiation field photodissociates water in favour of OH. This would explain the large overabundance of OH observed. The estimated relatively high density and compactness of the observed emiss ion suggest that it originates from the shocks taking place at the base of the molecular outflow emission, in the region where the action of the stell ar winds from the two stars of the binary system is important.