2008A&A...488..559K

The determination of the gaseous water abundances in different intestellar environments is crucial for understanding the oxygen chemistry and its role in the molecular cloud evolution. The purpose of this study is to estimate water abundance in the protostellar core Cha-MMS1. The ground-state line of o-H₂0 at 557 GHz was observed with the ODIN telescope. Two observing runs performed in 2002 and 2003 resulted in an upper limit of T_A=16mK. A model for the core density and temperature structure was constructed using a 1.3 mm continuum map from SEST/SIMBA. The water abundance profile through the cloud was derived and water line intensities expected from this model were calculated. An upper limit of 7x10^–9 was derived for the average fractional o-H₂O abundance (relative to H₂). The non-detection is consistent with an abundance profile where a high fractional abundance (∼10^–7) is reached in the low-density envelope of the core. According to our radiative transfer calculations, the detection of the 557GHz line from a quiescent core should severely be hampered by self-absorption. Therefore the present observations do not put hard contrainst on the H₂O abundance.