Astronomy and Astrophysics – Astronomy
Scientific paper
May 1995
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1995adil...yp...02p&link_type=abstract
NCSA Astronomy Digital Image Library
Astronomy and Astrophysics
Astronomy
Scientific paper
The (29)SiO J=2-1 transition at 85.76 GHz and the HC(15)N J=1-0 transition at 86.05 GHz have been detected in absorption towards the middle and northern HII region complexes in Sgr B2 with the BIMA array at a resolution of 8.0 x 4.7 arcsec. The optical depth of the transitions are approximately one; although we observed only one transition of each species, we present arguments that the excitation temperature is lower than 5 K. The velocity and line width of the (29)SiO absorption line are similar to NH3 (8,8) and (9,9) absorption lines observed with the Bonn 100 m telescope. This and the low excitation temperature imply that the absorption arises in the same extended hot (175 K), low density (10(4) per cubic cm) envelope as the NH3. The SiO to HCN abundance ratio in this low-excitation gas averages approximately 0.07, much larger than in dark clouds and close to the ratio in the Orion-KL outflow. The lower limit on the fractional SiO abundance is [SiO]/[H2] > 5 x 10(-10), which is more than two orders of magnitude larger than in dark clouds. The HCN abundance is found to be 8 x 10(-9), between the values predicted by shock and quiescent cloud models. The line width of (29)SiO is narrower than HC(15)N and other species, which suggests that the observed SiO enhancement is not due to the destruction of dust grains by high-velocity shocks such as produced by molecular outflows, unlike the enhancement mechanism which appears to operate in other sources. We suggest that the SiO enhancement is due to the destruction of icy grain mantles by low velocity shocks produced by cloud-cloud collisions, which release a fraction of the Si into the gas phase. We have also observed the J=2-1 transition of the main SiO isotope and detected emission to the north of Sgr B2(N). This emission is not associated with known heating sources.
Carlstrom John E.
Peng Yejuan
Vogel Stuart N.
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