High abundance ratio of ¹³CO to C¹⁸O toward photon-dominated regions in the Orion-A giant molecular cloud^⋆

¹ Laboratoire AIM, CEA/DSM-CNRS-Universitlté Paris Diderot, IRFU/Service d’Astrophysique, CEA Saclay, 91191 Gif-sur-Yvette, France
e-mail: Yoshito.Shimajiri@cea.fr
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

Received: 25 October 2013
Accepted: 24 February 2014

Abstract

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 < τ_¹³CO < 1.54 and 0.01 <τ_C¹⁸O< 0.18, respectively. The column densities of the ¹³CO and C¹⁸O emission lines are estimated to be 0.2 × 10¹⁶< N_¹³CO < 3.7 × 10¹⁷ cm^-2 and 0.4 × 10¹⁵<N_C¹⁸O< 3.5 × 10¹⁶ cm^-2, respectively. The abundance ratios between ¹³CO and C¹⁸O, X_¹³CO/X_C¹⁸O, are found to be 5.7−33.0. The mean value of X_¹³CO/X_C¹⁸O 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 X_¹³CO/X_C¹⁸O 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.