1998A&A...335.1070S

The MPIfR 100m telescope, the NRAO 12m telescope, and the IRAM 30m telescope were used to search for low lying rotational transitions of the water dimer, (H₂O)₂, toward the Galactic cloud cores Orion-KL, W51d, W51e1e2, NGC7538, SgrB2(M) and L134N, and toward comet Hale-Bopp. The comet was observed on December 20, 1996 and March 25/26, 1997 (i.e. near perihelion on April 1) at 24GHz. No lines of (H₂O)₂ were found. Our limit for the abundance of (H₂O)₂ relative to that of water (from H₂¹⁸O data) is typically <10^–4 toward dense cloud cores. Toward Orion-KL an even more stringent limit of <8x10^–6 is derived. The abundance of (H₂O)₂ relative to H₂ 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 <5.7x10²⁹s^–1. This is less than 6% the production rate of H₂O.