4.9. Molecular abundances in G1.6-0.025

Abstract

G1.6-0.025 is a dense molecular cloud, located near the Galactic Centre at l=1.6° and b=−0.025°. Lacking in star-forming or infra-red regions, the cloud was first detected as a prominent 5-GHz H2CO absorption feature (Whiteoak & Gardner 1979). NH3 observations by Gardner et al (1985) revealed a clumpy cloud with a diameter of about 30 parsec, average LSR velocity of 50 km s−1, and a kinetic temperature exceeding 50 K. In addition, a high-velocity component at 160 km s−1 has been detected towards one of the clumps (Gardner & Boes 1987). CH3OH in the cloud gives rise to a 12-GHz 20−3_1 E transition in absorption, representing the best example of rare population ‘anti-inversion’ for this transition (Whiteoak & Peng 1989; Peng & Whiteoak 1993). In contrast, the 36-GHz 4−1−30 E transition shows extended emission with superimposed knots containing maser emission (Haschick & Baan, 1993). Sobolev (1996) has suggested that a collision! has occurred between the main cloud and another cloudlet with relative velocity of 100 km s−1, resulting in high kinetic temperatures and shock waves that create the CH3OH masers.