A High Resolution Study of the HI-H2 Transition across the Perseus Molecular Cloud

Abstract

To investigate the fundamental principles of H2 formation in a giant molecular cloud (GMC), we derive the HI and H2 surface density (Sigma_HI and Sigma_H2) images of the Perseus molecular cloud on sub-pc scales (~0.4 pc). We use the far-infrared data from the Improved Reprocessing of the IRAS Survey and the V-band extinction image provided by the COMPLETE Survey to estimate the dust column density image of Perseus. In combination with the HI data from the Galactic Arecibo L-band Feed Array HI Survey and an estimate of the local dust-to-gas ratio, we then derive the Sigma_H2 distribution across Perseus. We find a relatively uniform Sigma_HI ~ 6-8 Msun pc^-2 for both dark and star-forming regions, suggesting a minimum HI surface density required to shield H2 against photodissociation. As a result, a remarkably tight and consistent relation is found between Sigma_H2/Sigma_HI and Sigma_HI+Sigma_H2. The transition between the HI- and H2-dominated regions occurs at N(HI)+2N(H2) ~ (8-14) x 10^20 cm^-2. Our findings are consistent with predictions for H2 formation in equilibrium, suggesting that turbulence may not be of primary importance for H2 formation. However, the importance of a warm neutral medium for H2 shielding, an internal radiation field, and the timescale of H2 formation still remain as open questions. We also compare H2 and CO distributions and estimate the fraction of "CO-dark" gas, f_DG ~ 0.3. While significant spatial variations of f_DG are found, we do not find a clear correlation with the mean V-band extinction.

updated to match the final version published in April 2012

References in corpus (12)

• Theory of Star Formation
• The Role of Pressure in GMC Formation II: The H_2 - Pressure Relation
• The CO-to-H2 Conversion Factor and Dust-to-Gas Ratio on Kiloparsec Scales in Nearby Galaxies
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• Simulating the formation of molecular clouds. II. Rapid formation from turbulent initial conditions
• The COMPLETE Survey of Star-Forming Regions: Phase I Data
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• The Five College Radio Astronomy Observatory CO Mapping Survey of the Taurus Molecular Cloud
• The Structure of a Low-Metallicity Giant Molecular Cloud Complex
• The Transition from Atomic to Molecular Hydrogen in Interstellar Clouds: 21cm Signature of the Evolution of Cold Atomic Hydrogen in Dense Clouds
• The COMPLETE Nature of the Warm Dust Ring in Perseus
• Time-dependent H2 formation and protonation

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• The thermal dust emission in the N158-N159-N160 (LMC) star forming complex mapped by Spitzer, Herschel and LABOCA
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• The Relationship between CO Emission and Visual Extinction Traced by Dust Emission in the Magellanic Clouds
• Cosmic-rays, gas, and dust in nearby anticentre clouds : II -- Interstellar phase transitions and the Dark Neutral Medium
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• The Metallicity Dependence of the HI Shielding Layers in Nearby Galaxies
• The role of neutral hydrogen in setting the abundances of molecular species in the Milky Way's diffuse interstellar medium. I. Observational constraints from ALMA and NOEMA
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