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

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

Mar 2000

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000a%26a...355..327l&link_type=abstract

Astronomy and Astrophysics, v.355, p.327-332 (2000)

Astronomy and Astrophysics

Astrophysics

20

Ism: Abundances, Ism: Clouds, Ism: Molecules, Ism: Structure, Radio Lines: Ism

Scientific paper

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(H_2) ~ 2 x 10-11 - 2x 10-10. 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 60 km s-1 using single dishes. {H13CO+ 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 40 km s-1 and perhaps at 60 km s-1 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 40 km s-1 is consistent with the trends seen in the more diffuse gas at higher galactic latitudes toward the extragalactic sources.

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