The H2O abundance and star formation history in rho Oph

Iso-Lws observations towards a large number of different positions in the c ore of the rho Oph cloud are used to place a limit to the abundance of wide spread H2O. From NLTE excitation and radiative transfer modelling the aver age line spectrum of the rho Oph cloud, we find X(H2O) < 10(-6) to 10(-5). We interprete these results within the framework of the dynamical chemistry models by Bergin, Melnick & Neufeld (1998), which describe the time evolut ion of molecular clouds being shocked by outflowing gas from young stellar objects, and conclude that the rho Oph cloud has been forming stars for les s than one million years (2-5 x 10(5) yr). Finally, we show that this limit ing result can be put to conclusive observational test by the ODIN satellit e mission in the near future.