The molecular cloud associated with SGR B2
A greater number of molecular species is seen towards the HIT region Sgr B2 than in the direction of any other galactic source. Many of the more complex molecules have however been observed only with large beams at a single position; such observations have not as yet shed much light on physical conditions inside the cloud. Detailed mapping has been carried out in the 6 cm absorption line of formaldehyde (Downes et al 1975) and in the 2 mm emission line of carbon monoxide (Scoville, Solomon and Jefferts 1974). These surveys reveal a complicated velocity structure with no evidence for uniform rotation. The strongest component has a velocity near +60 km s-1, close to the velocity of the ionized gas. The density in the cloud may be estimated from theories of the molecular excitation. Ammonia has proved particularly useful in this respect (Morris et al 1973), because transitions from both metastable and non-metastable levels can be seen; these allow a wide range of densities to be probed. In the central core densities of more than 106 cm-3 are found, while outside this core the density is around 103 cm-3. Comparison of the ortho and para transitions of ammonia may provide some information on the past history of the cloud.
KeywordsCarbon Monoxide Emission Line Past History Absorption Line Molecular Species
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