A SEARCH FOR (H2O)(2) IN THE GALAXY AND TOWARD COMET HALE-BOPP

The MPIfR 100m telescope, the NRAO 12m telescope, and the IRAM 30 m te lescope were used to search for low lying rotational transitions of th e water dimer, (H2O)(2), toward the Galactic cloud cores Orion-KL, W51 d, W51e1e2, NGC7538, SgrB2(M) and L134N, and toward comet Hale-Bopp. T he comet was observed on December 20, 1996 and March 25/26, 1997 (i.e. near perihelion on April 1) at 24 GHz. No lines of(H2O)(2) were found . Our limit for the abundance of (H2O)(2) relative to that of water (f rom (H2O)-O-18 data) is typically < 10(-4) toward dense cloud cores. T oward Orion-KL an even more stringent limit of < 8 10(-6) is derived. The abundance of (H2O)(2) relative to H-2 is typically < 10(-10). Our data show that at perihelion the production rate of the water dimer of comet C/1995 O1 (Hale-Bopp) was < 5.7 10(29) s(-1). This is less than 6% the production rate of H2O.