EVOLUTION OF OH AND CO-DARK MOLECULAR GAS FRACTION ACROSS A MOLECULAR CLOUD BOUNDARY IN TAURUS
EVOLUTION OF OH AND CO-DARK MOLECULAR GAS FRACTION ACROSS A MOLECULAR CLOUD BOUNDARY IN TAURUS
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DOI：
10.3847/0004-637x/819/1/22
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发表时间：
2016-01
期刊：
影响因子：
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通讯作者：
Duo Xu;Di Li;N. Yue;P. Goldsmith
中科院分区：
文献类型：
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作者：
Duo Xu;Di Li;N. Yue;P. Goldsmith
我们目前的观察12 CO J = 1-0，13 CO J = 1-0，H i，和所有四个基态跃迁的羟基（OH）自由基对一个尖锐的边界区域的金牛座分子云。基于一个光解区（PDR）模型，再现CO和[C i]发射从同一区域，我们模拟了三个OH跃迁，1612，1665，和1667 MHz成功地通过逃逸概率非局部热平衡辐射传输模型计算。由于未建模的泵浦机制，我们无法重现1720 MHz的观测结果，其中最有可能的候选者是C-冲击。OH和CO暗分子气体的丰度受到很好的约束。OH丰度[OH]/[H2]从8 × 10 − 7？1 × 10 - 7？当Av从0.4增加到2.7 mag时，遵循经验定律：[ OH ] / [H2] = 1.5 × 10 − 7 + 9.0 × 10 − 7 × exp（− Av/ 0.81），？>这比PDR模型对低消光区的预测高80倍。在1等或低于1等的反射率下，OH的过剩可能是C-冲击的结果。暗气体部分（DGF，定义为没有可检测到的CO排放的分子气体部分）从80%下降到20%，遵循高斯分布：DGF = 0.90 × exp − A v − 0.79 0.71 2。?> DGF的这种趋势与我们的理解一致，即由于H2的光解，DGF在低视觉消光下下降，并且由于CO的形成，DGF在高视觉消光下下降。DGF在H2已经形成并实现自屏蔽但12 CO尚未形成的消光范围内达到峰值。清晰地识别出两个窄的速度分量，峰间间距为1.01 km s-1。它们的相对强度以及空间和频率的变化表明在边界区域有碰撞流或气流。
We present observations of 12CO J = 1-0, 13CO J = 1-0, H i, and all four ground-state transitions of the hydroxyl (OH) radical toward a sharp boundary region of the Taurus molecular cloud. Based on a photodissociation region (PDR) model that reproduces CO and [C i] emission from the same region, we modeled the three OH transitions, 1612, 1665, and 1667 MHz successfully through escape probability non-local thermal equilibrium radiative transfer model calculations. We could not reproduce the 1720 MHz observations, due to unmodeled pumping mechanisms, of which the most likely candidate is a C-shock. The abundance of OH and CO-dark molecular gas is well-constrained. The OH abundance [OH]/[H2] decreases from 8 × 10 − 7 ?> to 1 × 10 − 7 ?> as A v increases from 0.4 to 2.7 mag following an empirical law: [ OH ] / [ H 2 ] = 1.5 × 10 − 7 + 9.0 × 10 − 7 × exp ( − A v / 0.81 ) , ?> which is higher than PDR model predictions for low-extinction regions by a factor of 80. The overabundance of OH at extinctions at or below 1 mag is likely the result of a C-shock. The dark gas fraction (DGF, defined as the fraction of molecular gas without detectable CO emission) decreases from 80% to 20% following a Gaussian profile: DGF = 0.90 × exp − A v − 0.79 0.71 2 . ?> This trend of the DGF is consistent with our understanding that the DGF drops at low visual extinction due to photodissociation of H2 and drops at high visual extinction due to CO formation. The DGF peaks in the extinction range where H2 has already formed and achieved self-shielding but 12CO has not. Two narrow velocity components with a peak-to-peak spacing of ∼1 km s−1 were clearly identified. Their relative intensity and variation in space and frequency suggest colliding streams or gas flows at the boundary region.