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Astron. Astrophys. 355, 327-332 (2000)

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

R. Lucas 1 and H.S. Liszt 2

1 Institut de Radioastronomie Millimétrique, 300 Rue de la Piscine, 38406 Saint Martin d'Hères, France
2 National Radio Astronomy Observatory, 520 Edgemont Road, Charlottesville, VA 22903-2475, USA

Received 28 September 1999 / Accepted 5 January 2000


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+) [FORMULA], or N(SiO)/N([FORMULA]) [FORMULA]. 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.

Key words: ISM: abundances – ISM: clouds – ISM: molecules – ISM: structure – radio lines: ISM

Send offprint requests to: R. Lucas

Correspondence to: rlucas@iram.fr

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© European Southern Observatory (ESO) 2000

Online publication: March 17, 2000