Evolution of OH and CO-dark molecular gas fraction across a molecular cloud boundary in Taurus

Journal Article · Tue Mar 01 04:00:00 EST 2016 · Astrophysical Journal

DOI:https://doi.org/10.3847/0004-637X/819/1/22· OSTI ID:22886981

Xu, Duo; Li, Di; Yue, Nannan ^[1]

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1. National Astronomical Observatories, Chinese Academy of Sciences, A20 Datun Road, Chaoyang District, Beijing 100012 (China)

We present observations of {sup 12}CO J = 1-0, {sup 13}CO 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]/[H{sub 2}] decreases from 8×10{sup −7} to 1×10{sup −7} as A{sub v} increases from 0.4 to 2.7 mag following an empirical law: 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: This trend of the DGF is consistent with our understanding that the DGF drops at low visual extinction due to photodissociation of H{sub 2} and drops at high visual extinction due to CO formation. The DGF peaks in the extinction range where H{sub 2} has already formed and achieved self-shielding but {sup 12}CO has not. Two narrow velocity components with a peak-to-peak spacing of ∼1 km s{sup −1} were clearly identified. Their relative intensity and variation in space and frequency suggest colliding streams or gas flows at the boundary region.

• OSTI ID: 22886981
• Journal Information: Astrophysical Journal, Journal Name: Astrophysical Journal Journal Issue: 1 Vol. 819; ISSN ASJOAB; ISSN 0004-637X
• Country of Publication: United Kingdom
• Language: English

Related Subjects

79 ASTRONOMY AND ASTROPHYSICS
CARBON MONOXIDE
CLOUDS
COSMIC GASES
DISSOCIATION
ELEMENT ABUNDANCE
EMISSION
GAS FLOW
GROUND STATES
HYDROGEN
HYDROXIDES
HYDROXYL RADICALS
MOLECULES
PHOTOLYSIS
RADIANT HEAT TRANSFER
SELF-SHIELDING
SPACE
STREAMS
T TAURI STARS
THERMAL EQUILIBRIUM
VELOCITY