High abundance ratio of 13CO to C18O toward photon-dominated regions in the Orion-A giant molecular cloud⋆
CEA/DSM-CNRS-Universitlté Paris Diderot, IRFU/Service d’Astrophysique, CEA Saclay,
2 National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, 181-8588 Tokyo, Japan
3 Nobeyama Radio Observatory, 462-2 Nobeyama, Minamimaki, Minamisaku, 384-1305 Nagano, Japan
4 Institute of Space and Astronautical Science, Japan Aerospace Exploration Agency, 3-1-1 Yoshinodai, Chuo-ku, 252-5210 Sagamihara, Japan
5 Joint ALMA Observatory, Alonso de Cordova 3107 Vitacura, 763 0355 Santiago, Chile
6 Institute of Astrophysics and Planetary Sciences, Ibaraki University, 2-1-1 Bunkyo, Mito, 310-8512 Ibaraki, Japan
7 Department of Astronomy and Earth Sciences, Tokyo Gakugei University, Koganei, 184-8501 Tokyo, Japan
8 The University of Tokyo, 7-3-1 Hongo Bunkyo, 113-0033 Tokyo, Japan
9 Department of Physical Science, Osaka Prefecture University, Gakuen 1-1, Sakai, 599-8531 Osaka, Japan
Accepted: 24 February 2014
Aims. We derive physical properties such as the optical depths and the column densities of 13CO and C18O to investigate the relationship between the far ultraviolet (FUV) radiation and the abundance ratios between 13CO and C18O.
Methods. We have carried out wide-field (0.4 deg2) observations with an angular resolution of 25.8″ (~0.05 pc) in 13CO (J = 1–0) and C18O (J = 1–0) toward the Orion-A giant molecular cloud using the Nobeyama 45 m telescope in the on-the-fly mode.
Results. Overall distributions and velocity structures of the 13CO and C18O emissions are similar to those of the 12CO (J = 1–0) emission. The optical depths of the 13CO and C18O emission lines are estimated to be 0.05 < τ13CO < 1.54 and 0.01 <τC18O< 0.18, respectively. The column densities of the 13CO and C18O emission lines are estimated to be 0.2 × 1016< N13CO < 3.7 × 1017 cm-2 and 0.4 × 1015<NC18O< 3.5 × 1016 cm-2, respectively. The abundance ratios between 13CO and C18O, X13CO/XC18O, are found to be 5.7−33.0. The mean value of X13CO/XC18O in the nearly edge-on photon-dominated regions is found to be 16.47 ± 0.10, which is a third larger than that the solar system value of 5.5. The mean value of X13CO/XC18O in the other regions is found to be 12.29 ± 0.02. The difference of the abundance ratio is most likely due to the selective FUV photodissociation of C18O.
Key words: ISM: abundances / ISM: clouds / photon-dominated region / ISM: individual objects: Orion-A
The data and the FITS files of Figs. 1, 2a,b, and 3a are only available at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (22.214.171.124) or via http://cdsarc.u-strasbg.fr/viz-bin/qcat?J/A+A/564/A68
© ESO, 2014