Multiwavelength observations of cirrus clouds in the North Celestial Loop: the transition from atomic to molecular gas

L. Barriault (1) , Gilles Joncas (1) , Edith Falgarone (2, 3) , D. J. Marshall (1) , M. Heyer (4) , Francois Boulanger (5) , Tyler Foster (6) , C. Brunt (7) , Marc-Antoine Miville-Deschenes (5) , K. Blagrave (8) , R. Kothes (9) , T. L. Landecker (9) , Peter G. Martin (8) , Douglas Scott (10) , J. M. Stil (11) , A. R. Taylor (11)

1 CRAQ - Centre de recherche en astrophysique du Québec, Université Laval
2 LERMA - Laboratoire d'Etude du Rayonnement et de la Matière en Astrophysique
3 Dynamique des milieux interstellaires et plasmas stellaires
4 Department of Astronomy, University of Massachusetts
5 IAS - Institut d'astrophysique spatiale
6 Department of Physics and Astronomy, Brandon University
7 University of Exeter
8 CITA - Canadian Institute for Theoretical Astrophysics
9 National Research Council of Canada, Herzberg Institute of Astrophysics, Dominion Radio Astrophysical Observatory
10 University of British Columbia [Vancouver]
11 University of Calgary

Two potential sites of H₂ formation have been discovered in diffuse gas at high Galactic latitude through examining the far-infrared (FIR) HI ratio and looking for an excess over that expected from an atomic medium. We call these the Spider and Ursa Major fields. New ¹²CO and ¹³CO Five College Radio Astronomical Observatory observations are presented for both regions (53936 spectra in the Spider and 23517 spectra in Ursa Major). Although there is a correlation between FIR excess and CO emission, we find that the FIR excess peaks do not coincide with the ¹²CO emission peaks, indicating that CO might be a poor tracer of H₂ in diffuse regions. This implies (i) that the density is too small to allow CO excitation, (ii) that the CO self-shielding is insufficient or (iii) local variations of the dust properties. The ¹²CO observations are compared with HI observations from the Dominion Radio Astrophysical Observatory. We decompose the 10000 HI profiles of the Spider and the 20 302 HI profiles of Ursa Major into Gaussian components. We always find at most two narrow components and one broad component. CO always seems to appear where two HI velocity components merge or where there is a HI velocity-shear.

Domaines: Astrophysique [astro-ph]; Planète et Univers [physics]

L. Barriault, Gilles Joncas, Edith Falgarone, D. J. Marshall, M. Heyer, et al.. Multiwavelength observations of cirrus clouds in the North Celestial Loop: the transition from atomic to molecular gas. Monthly Notices of the Royal Astronomical Society, 2010, 406, pp.2713-2731. ⟨10.1111/j.1365-2966.2010.16871.x⟩. ⟨hal-03733347⟩