SILICON MONOXIDE IN SAGITTARIUS B2

The (SiO)-Si-29 J = 2 --> 1 transition at 85.76 GHz and the (HCN)-N-15 J = 1 --> 0 transition at 86.05 GHz have been detected in absorption toward the middle and northern H II region complexes in Sgr B2 with th e BIMA array at a resolution of 8 ''.0 x 4 ''.7. The optical depth of the transitions are approximately one; although we observed only one t ransition of each species, we present arguments that the excitation te mperature is lower than 5 K. The velocity and line width of the (SiO)- Si-29 absorption line are similar to NH3 (8, 8) and (9, 9) absoption l ines observed with the Bonn 100 m telescope. This and the low-excitati on temperature imply that the absorption arises in the same extended h ot (175 K), low-density (10(4) cm(-3)) envelope as the NH3. The SiO to HCN abundance ratio in this low-excitation gas averages similar to 0. 07, much larger than in dark clouds and close to the ratio in the Orio n-KL outflow. The lower limit on the fractional SiO abundance is [SiO] /[H-2] > 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), bet ween the values predicted by shock and quiescent cloud models. The lin e width of (SiO)-Si-29 is narrower than (HCN)-N-15 and other species, which suggests that the observed SiO enhancement is not due to the des truction of dust grains by high-velocity shocks such as produced by mo lecular outflows, unlike the enhancement mechanism which appears to op erate in other sources. We suggest that the SiO enhancement is due to the destruction of icy grain mantles by low-velocity shocks produced b y cloud-cloud collisions, which release a fraction of the Si into the gas phase.