CO in absorption and emission toward compact extragalactic radio continuum sources

Details CO in absorption and emission toward compact extragalactic radio continuum sources CO in absorption and emission toward compact extragalactic radio continuum sources

Nov 1998

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1998a%26a...339..561l&link_type=abstract

Astronomy and Astrophysics, v.339, p.561-574 (1998)

Astronomy and Astrophysics

Astrophysics

59

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

Scientific paper

We have observed galactic <~mbda2.6mm and <~mbda1.3mm CO absorption and emission along nine lines of sight toward compact extragalactic mm-wave continuum sources, using the IRAM Plateau de Bure Interferometer and NRAO 12m telescopes. In absorption we detected some two dozen kinematic components, and nearly every feature known from {HCO(+) spectra \citep{LucLis96} has a direct CO counterpart: a few CO lines are missing or very weak in emission even when tau (CO) >~ 0.5. The column densities of CO and {HCO(+) are well correlated, but not linearly related. The widths of the CO lines are typically 15% smaller than those of {HCO(+) (on average 0.75 km s(-1) vs. 0.86 km s(-1) ). We derive (12) CO column densities 0.1 <~ N(12CO) <~ 20 x 10(15) pcc which are in all cases very small compared to the column of carbon nuclei expected for 1 magnitude of visual extinction, even allowing for substantial depletion. The partial thermal pressure of {H_2} is inferred to be 1000 <= n(HH)Tk <= 12,000 pccc K, with a median p/k = 3.2 x 10(3) pccc K. Thus the clouds are likely warm (Tsub K ~ tens of K), somewhat diffuse (n(HH) ~ 50-300 pccc), with the majority of the gas-phase carbon in the form of C(+) and perhaps even with a substantial fraction of H I in the thinnest cases. The isotope ratios in the CO usually differ strongly from the local interstellar ratio which we have separately measured in these clouds to be (12) C/(13) C = 60 \citep{LucLis98}; we find 15 <= N(12CO)/N(13CO) <= 54, declining with increasing N((12) CO). The (13) CO/C(18) O ratio seen in emission or absorption is typically 25 (instead of 8) and C(18) O is very difficult to detect in emission even when Tstar R (12CO)/Tstar R (13CO) < 10. Apparently, the relative abundance of (13) CO is typically greatly enhanced, even at very low extinction, and never diminished by selective photodissociation. One effect of this enhancement is that lines of (12) CO are substantially less optically thick than might otherwise have been inferred. There is little evidence for a general selective depletion of C(18) O.

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