High abundance ratio of 13CO to C18O toward photon-dominated regions in the Orion-A giant molecular cloud
High abundance ratio of 13CO to C18O toward photon-dominated regions in the Orion-A giant molecular cloud
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DOI：
10.1051/0004-6361/201322912
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发表时间：
2014-03
影响因子：
6.5
通讯作者：
Y. Shimajiri;Y. Kitamura;M. Saito;M. Momose;F. Nakamura;K. Dobashi;T. Shimoikura;H. Nishitani;A. Yamabi;C. Hara;S. Katakura;T. Tsukagoshi;Tomohiro Tanaka;R. Kawabe
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文献类型：
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作者：
Y. Shimajiri;Y. Kitamura;M. Saito;M. Momose;F. Nakamura;K. Dobashi;T. Shimoikura;H. Nishitani;A. Yamabi;C. Hara;S. Katakura;T. Tsukagoshi;Tomohiro Tanaka;R. Kawabe
目标。通过计算13 CO和C18 O的光学深度和柱密度等物理性质，研究了远紫外辐射与13 CO和C18 O丰度比的关系。利用Nobeyama 45 m望远镜在飞行模式下对Orion-A巨分子云进行了角分辨率为25.8″（~0.05 pc）的13 CO（J = 1-0）和C18 O（J = 1-0）宽视场（0.4 deg 2）观测。13 CO和C18 O排放的总体分布和速度结构与12 CO（J = 1-0）排放的分布和速度结构相似。~（13）CO和~（18）O发射线的光学深度分别为0.05 < τ ~（13）CO < 1.54和0.01 <τ ~（18）O < 0.18。13 CO和C18 O发射线的柱密度分别为0.2 × 1016<N13 CO < 3.7 × 1017 cm-2和0.4 × 1015<NC 18 O < 3.5 × 1016 cm-2。13 CO和C18 O的丰度比X13 CO/XC 18 O为5.7−33.0。在近边光子主导区，X ~（13）CO/XC ~（18）O的平均值为16.47 ± 0.10，比太阳系的5.5大三分之一。其它地区的X ~（13）CO/XC ~（18）O平均值为12.29 ± 0.02。丰度比的差异很可能是由于C18 O的选择性FUV光解。
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.