SiO in diffuse, translucent and 'spiral-arm' clouds

Using the Plateau de Bure interferometer, we searched for thermal SiO J=1-2 absorption at 86 GHz from the diffuse and translucent clouds which lie tow ard our sample of extragalactic continuum sources. SiO is present at a leve l N(SiO)/N(HCO+) approximate to 0.01 - 0.1, or N(SiO)/N(H-2) approximate to 2 x 10(-11) - 2 x 10(-10). N(SiO) declines with increasing N(HCO+) and wit h increasing thermal pressure measured in the J=1-0 lines of CO. SiO is gro ssly underabundant, even compared to the known gas-phase depletion of Si in diffuse clouds. To pursue the subject further, we mapped the (HCO+)-C-13 J=1-0 and SiO J=2- 1 lines toward the core of W49A: SiO and many other molecular absorption li nes have been studied in 'spiral-arm' clouds seen along the galactic plane at v = 40 and 60 km s(-1) using single dishes. (HCO+)-C-13 absorbs quite st rongly at these velocities, with column densities at least 3-4 times larger than in any of the clouds we have studied toward extragalactic sources. Bu t SiO absorption is absent at 40 km s(-1) and perhaps at 60 km s(-1) as wel l since the latter is overlaid by a series of dimethyl ether lines originat ing in the dense core of the W49A molecular gas: the dimethyl ether was not recognized as such in singledish absorption profiles. Our upper limit for SiO in the spiral-arm' cloud at 40 km s(-1) is consistent with the trends s een in the more diffuse gas at higher galactic latitudes toward the extraga lactic sources.