2000A&A...355..327L

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 toward our sample of extragalactic continuum sources. SiO is present at a level N(SiO)/N(HCO⁺)≃0.01-0.1, or N(SiO)/N(H₂)≃2x10^–11-2x10^–10. N(SiO) declines with increasing N(HCO⁺) and with increasing thermal pressure measured in the J=1-0 lines of CO. SiO is grossly underabundant, even compared to the known gas-phase depletion of Si in diffuse clouds. To pursue the subject further, we mapped the H¹³CO⁺ J=1-0 and SiO J=2-1 lines toward the core of W49A: SiO and many other molecular absorption lines have been studied in `spiral-arm' clouds seen along the galactic plane at v=40 and 60km/s using single dishes. H<sup>13</sup>CO<sup>+</sup> absorbs quite strongly at these velocities, with column densities at least 3-4 times larger than in any of the clouds we have studied toward extragalactic sources. But SiO absorption is absent at 40km/s and perhaps at 60km/s as well since the latter is overlaid by a series of dimethyl ether lines originating 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 40km/s is consistent with the trends seen in the more diffuse gas at higher galactic latitudes toward the extragalactic sources.