Tracing the Molecular Hydrogen Content of the Draco Nebula - Very Low H/H2/W/12CO/ Ratios or Varying Far Infrared Emissivities

Details Tracing the Molecular Hydrogen Content of the Draco Nebula - Very Low H/H2/W/12CO/ Ratios or Varying Far Infrared Emissivities Tracing the Molecular Hydrogen Content of the Draco Nebula - Very Low H/H2/W/12CO/ Ratios or Varying Far Infrared Emissivities

May 1993

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1993a%26a...272..514h&link_type=abstract

Astronomy and Astrophysics, Vol.272, NO. 2/MAYII, P. 514, 1993

Astronomy and Astrophysics

Astrophysics

27

Interstellar Medium: Abundances Interstellar Medium: Atoms, Interstellar Medium: Clouds, Interstellar Medium: Dust, Extinction, Interstellar Medium: Individual Objects: Draco Nebula, Interstellar Medium: Molecules

Scientific paper

We have analysed three complete data sets, the IRAS 100 μm intensity, I100, the H I column density, N(H I), and the integral of the J = 1 → 0 line of the 12C0 molecule, W(12CO), for the Draco nebula [=IVC G091.0+38.0 (VLSR = -21)] to trace the total column density of hydrogen nuclei, N(H), and to derive the extinction, AV, and the total gas mass of this nebula. Adopting the relations between these quantities which were set up by Heithausen & Mebold we have analysed the [I100, N(H I), W(12CO)] data cube by the method of the principal component transformation. The results are tested by the statistical F- and t-tests. We derive the FIR-emissivity per H I nucleon at 100 μm, α100, and the ratio χWCO≡N(H2)/W(12CO) for different regions of the Draco nebula. A difference of the FIR-emissivities for the predominantly molecular cloud cores and their predominantly atomic halos is established and taken into account for the determination of χWCO.
The FIR-emissivities found in the Draco nebula are larger than the mean value for the galactic cirrus of 110-20 MJy ster-1 em-2. They depend on the molecular content as well as on the 60 μm/l00 μm colour temperature. This dependence is probably due to screening of the interstellar radiation field, variations of the dust grain size, and/or a non-constant gas-to-dust ratio.
The χWCO values in the Draco nebula are typically in the range 0.02 1020 cm-2 < χWCO < 0.26 l02O (K km s-1)-1, i.e. more than a factor of 10 smaller than in the galactic plane and smaller than in most other cirrus clouds. The lowest values are found in regions where we suspect interaction with high velocity clouds. The low χWCO values are only partly due to optical depth effects of the 12CO J = 1 → 0 lines or excitation conditions of the CO molecules by hydrogen atoms. They result probably from the combined effects of high temperatures and an increase of the CO abundance in the shocks of the interaction zone.
Our results arc used to derive the atomic and the molecular content [M(H2)=45MsunD2 (500 pc)-2] and the total mass of the Draco nebula [M = 630MsunD2 (500 pc)-2] and its different molecular clumps. The consequences for the visual extinction and the distance, D, are discussed.

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