Detection of 6 K gas in Ophiuchus D

\n Context. Cold cores in interstellar molecular clouds represent the very first\n phase in star formation. The physical conditions of these objects \n are studied in order to understand how molecular clouds evolve and how\n stellar masses are determined.\n Aims. The purpose of this study is to probe conditions in the dense, starless\nclump Ophichus D ( Oph D ).\n Methods. The ground-state ($1_{10}\\rightarrow1_{11}$) rotational transition \nof ortho-H2D+ was observed with APEX towards the density peak\nof Oph D .\n Results. The width of the H2D+ line indicates that the kinetic\ntemperature in the core is about 6 K. So far, this is the most direct evidence\nof such cold gas in molecular clouds.\nThe observed H2D+ spectrum can be reproduced with\na hydrostatic model with the temperature increasing from about 6 K in\nthe centre to almost 10 K at the surface. The model is unstable against\nany increase in the external pressure, and the core is likely to form a \nlow-mass star.\n Conclusions. The results suggest that an equilibrium configuration is a feasible\nintermediate stage of star formation even if the larger scale\nstructure of the cloud is thought to be determined by turbulent\nfragmentation. In comparison with the isothermal case, the inward\ndecrease in the temperature makes smaller, i.e. less massive, cores\nsusceptible to externally triggered collapse.\n