First detection of CO lines in a water fountain star.

Context. Water fountain stars are very young post-AGB stars with high velocity water maser jets. They are the best objects in which to study the onset of bipolar jets from evolved stars due to their young dynamical ages. However, none of them has been observed in any thermal lines. Aims. We aim to search for CO lines in the water fountain star IRAS 16342-3814 and investigate the properties of its thermal gas. Methods. The proximity, peculiar stellar velocity and high Galactic latitude of IRAS 16342-3814 make a single dish observation possible. We use the Arizona Radio Observatory 10 m telescope to observe the CO J = 2-1 line and compare the line parameters with those of masers. Results. We report the detection of (12)CO and (13)CO J = 2-1 lines from IRAS 16342-3814. The inferred (12)CO mass loss rate is an order of magnitude lower than the infrared and OH mass loss rates, indicating a very cold and thick O-rich circumstellar envelope around the star. We also find a (12)CO expansion velocity of V(exp) = 46 +/- 1 km s(-1) that is too high for an AGB wind and confirm the systemic velocity of 44 +/- 1 km s(-1). In addition we measure a very low (12)CO/(13)CO line ratio of 1.7. Conclusions. The first detection of CO lines has provided a new way to investigate water fountain stars. Given the high expansion velocity of the CO gas and its relation to maser velocities, we infer that the CO emission region is co-located with the OH mainline masers in the warm base of the optical bipolar lobes, while the high velocity OH 1612 MHz and H(2)O masers are located in the side walls and at the farthest ends of the bipolar lobes, respectively. Further observations are highly desirable to understand the very low (12)CO/(13)CO line ratio.