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Astron. Astrophys. 334, 646-658 (1998)

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6. Conclusions

Based on our measurements and analysis of 33 molecular density peaks in the Galactic center in the [FORMULA] and [FORMULA] transitions of [FORMULA], the [FORMULA] and [FORMULA] transitions of 28 SiO and the [FORMULA] transitions of 29 SiO and 30 SiO, we find:

  1. All sources are easily detected in all transitions of [FORMULA] searched for and in 28 SiO([FORMULA]), demonstrating that the properties of molecular peaks in the Galactic center region are markedly different from the Galactic disk, where thermal SiO emission is confined to very small regions in the vicinity of outflows associated with star formation. Local gas on the line-of-sight, distinguished from Galactic center gas by its narrow lines, is not detected in SiO.
  2. The rare isotopomers 29 SiO and 30 SiO are detected in all 12 studied sources. The [FORMULA] transition of the main isotopomers is seen in 7 out of 8 sources. The line intensity ratio of 29 SiO and 30 SiO shows that the terrestrial isotope ratio, 29 Si/30 Si [FORMULA] 1.5, holds for the Galactic center region.
  3. From LVG model calculations applied to the [FORMULA] line intensity ratios [FORMULA] and [FORMULA], cool ([FORMULA] [FORMULA] K) gas toward the molecular peaks has moderately high densities ([FORMULA]) [FORMULA] [FORMULA]), while high kinetic temperatures of [FORMULA] K correspond to H2 densities that are an order of magnitude lower. This is contrary to what is found in the disk, where the cores of GMCs are usually hot, and is an indication that the `cool cores' in `typical' Galactic center GMCs, away from Sgr A and Sgr B2, are not presently forming high mass stars.
  4. Combining the results of LVG models for [FORMULA] and SiO, and using [FORMULA] as a tracer of total H2 column density, a beam averaged SiO abundance is derived for all clouds. This varies significantly from source to source, ranging from [FORMULA] to [FORMULA]. The 28 SiO([FORMULA]) transitions are optically thick, with [FORMULA] ranging from [FORMULA].
  5. Including information on the temperature structure of the clouds from NH3, it is shown that for most clouds a self-consistent solution accounting for the properties of [FORMULA] and SiO is only possible if the bulk of the SiO emission arises in the cool, dense gas component.
  6. One source, M+1.31-0.13, is different from all the others: SiO arises in the hot, thin gas component. Since this is also the cloud with the highest SiO abundance in the entire sample, the SiO formation process, probably grain erosion by shocks, is likely to be still ongoing in this source.
  7. The SiO abundance in individual clouds is related to the large scale gas dynamics in the Galactic center region. The highest abundances are found at Galactic longitudes of [FORMULA], which can be identified with `collision regions' likely to encounter shocks in terms of the bar model of gas dynamics.
  8. As in the disk, SiO in the Galactic center region is likely to originate in shocks. Since the hot, thin post-shock gas forms dense, cool cores faster than SiO recondenses to dust grains, the SiO rich gas is cool for most of its lifetime. On a timescale of [FORMULA] yr, the SiO molecules freeze out on grain mantles and the clouds lose their chemical memory of the shock.
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© European Southern Observatory (ESO) 1998

Online publication: May 15, 1998