Abstract
Context. The atomic-to-molecular hydrogen (H/H2) transition has been extensively studied as it controls the fraction of gas in a molecular state in an interstellar cloud. This fraction is linked to star-formation by the Schmidt–Kennicutt law. While theoretical estimates of the column density of the H I layer have been proposed for static photodissociation regions (PDRs), Herschel and well-resolved Atacama Large Millimeter Array observations have revealed dynamical effects in star forming regions, caused by the process of photoevaporation. Aims. We extend the analytic study of the H/H2 transition to include the effects of the propagation of the ionization front, in particular in the presence of photoevaporation at the walls of blister H II regions, and we find its consequences on the total atomic hydrogen column density at the surface of clouds in the presence of an ultraviolet field, and on the properties of the H/H2 transition. Methods. We solved semi-analytically the differential equation giving the H2 column density profile by taking into account H2 formation on grains, H2 photodissociation, and the ionization front propagation dynamics modeled as advection of the gas through the ionization front. Results. Taking this advection into account reduces the width of the atomic region compared to static models. The atomic region may disappear if the ionization front velocity exceeds a certain value, leading the H/H2 transition and the ionization front to merge. For both dissociated and merged configurations, we provide analytical expressions to determine the total H I column density. Our results take the metallicity into account. Finally, we compared our results to observations of PDRs illuminated by O-stars, for which we conclude that the dynamical effects are strong, especially for low-excitation PDRs.
Highlights
In the atomic envelop surrounding a molecular cloud (Wannier et al 1983; Andersson et al 1991), the transition where molecular hydrogen turns into atomic hydrogen is called the atomic-tomolecular hydrogen (H/H2) transition, or the photodissociation front
Estimating the total H I column density at the surface of neutral interstellar clouds is a key problem for tackling several important astrophysical tasks: to deduce the H2 mass fraction intervening in the Schmidt–Kennicutt law of star-formation (Villanueva et al 2017), to deduce the CO-to-H2 conversion factor (Schruba et al 2017; Dessauges-Zavadsky et al 2017; Pineda et al 2017), or even to study the dynamics of a cloud (Valdivia et al 2016; Beuther et al 2020)
It compares the spatial profiles of atomic hydrogen and molecular hydrogen with and without advection for physical 0.5 conditions typical cm3 and ionization of a front bvreilgohctitPyDvIRF,=w1itkhmasG−10./InnHthriasticoasoef, we clearly see that the H/H2 transition is closer to the ionization front on the left than the static model
Summary
In the atomic envelop surrounding a molecular cloud (Wannier et al 1983; Andersson et al 1991), the transition where molecular hydrogen turns into atomic hydrogen is called the atomic-tomolecular hydrogen (H/H2) transition, or the photodissociation front. Knowledge about the location of the H/H2 transition gives us information on the total fraction of molecular gas in a cloud. It is in the cold and dense molecular medium that stars are formed through gravitational collapse of the gas. Estimating the total H I column density at the surface of neutral interstellar clouds is a key problem for tackling several important astrophysical tasks: to deduce the H2 mass fraction intervening in the Schmidt–Kennicutt law of star-formation (Villanueva et al 2017), to deduce the CO-to-H2 conversion factor (Schruba et al 2017; Dessauges-Zavadsky et al 2017; Pineda et al 2017), or even to study the dynamics of a cloud (Valdivia et al 2016; Beuther et al 2020). Some direct estimates of the total H I column density were performed through 21 cm observations (Kim et al 1998, 2003; Stanimirovicet al. 1999, 2014; Staveley-Smith et al 2003; Stil et al 2006; Peek et al 2011; Bihr et al 2015; Beuther et al 2016)
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