The HI Probability Distribution Function and the Atomic-to-Molecular Transition in Molecular Clouds

Abstract

We characterize the column density probability distributions functions (PDFs) of the atomic hydrogen gas, HI, associated with seven Galactic molecular clouds (MCs). We use 21 cm observations from the Leiden/Argentine/Bonn Galactic HI Survey to derive column density maps and PDFs. We find that the peaks of the HI PDFs occur at column densities ranging from ~1-2$\times 10^{21}$ cm${}^2$ (equivalently, ~0.5-1 mag). The PDFs are uniformly narrow, with a mean dispersion of ${\sigma }_{HI}\approx 10^{20}$ cm${}^2$ (~0.1 mag). We also investigate the HI-to-H${}_2$ transition towards the cloud complexes and estimate HI surface densities ranging from 7-16 $M_{\odot }$ pc${}^{-2}$ at the transition. We propose that the HI PDF is a fitting tool for identifying the HI-to-H${}_2$ transition column in Galactic MCs.

Accepted to ApJ

References in corpus (20)

• Theory of Star Formation
• The Role of Pressure in GMC Formation II: The H_2 - Pressure Relation
• The Density Probability Distribution in Compressible Isothermal Turbulence: Solenoidal versus Compressive Forcing
• The Atomic to Molecular Transition in Galaxies. II: HI and H_2 Column Densities
• On the Star Formation Efficiency of Turbulent Magnetized Clouds
• What determines the density structure of molecular clouds ? A case study of Orion B with Herschel
• On the Density Distribution in Star-forming Interstellar Clouds
• Density Fluctuations in MHD Turbulence: Spectra, Intermittency and Topology
• 2MASS wide field extinction maps - I. The Pipe nebula
• The Atomic to Molecular Transition in Galaxies. I: An Analytic Approximation for Photodissociation Fronts in Finite Clouds
• Herschel-Planck dust optical-depth and column-density maps: I. Method description and results for Orion
• Hipparcos distances of Ophiuchus and Lupus cloud complexes
• NICEST, a near-infrared color excess method tailored for small-scale structures
• Molecular clouds have power-law probability distribution functions
• The Turbulent Warm Ionized Medium: Emission Measure Distribution and MHD Simulations
• Understanding star formation in molecular clouds I. Effects of line-of-sight contamination on the column density structure
• The 21-SPONGE HI Absorption Survey I: Techniques and Initial Results
• Cold and warm atomic gas around the Perseus molecular cloud I: Basic Properties
• Evolving Molecular Cloud Structure and the Column Density Probability Distribution Function
• Rethinking a Mysterious Molecular Cloud

Cited by in corpus (3)

• The shapes of column density PDFs - The importance of the last closed contour
• The structure and statistics of interstellar turbulence
• The Razor's Edge of Collapse: The Transition Point from Lognormal to Powerlaw in Molecular Cloud PDFs