A simple model for molecular hydrogen chemistry coupled to radiation hydrodynamics

Abstract

We introduce non-equilibrium molecular hydrogen chemistry into the radiation hydrodynamics code Ramses-RT. This is an adaptive mesh refinement grid code with radiation hydrodynamics that couples the thermal chemistry of hydrogen and helium to moment-based radiative transfer with the Eddington tensor closure model. The H 2 physics that we include are formation on dust grains, gas phase formation, formation by three-body collisions, collisional destruction, photodissociation, photoionization, cosmic ray ionization, and self-shielding. In particular, we implement the first model for H 2 self-shielding that is tied locally to moment-based radiative transfer by enhancing photodestruction. This self-shielding from Lyman-Werner line overlap is critical to H 2 formation and gas cooling. We can now track the non-equilibrium evolution of molecular, atomic, and ionized hydrogen species with their corresponding dissociating and ionizing photon groups. Over a series of tests we show that our model works well compared to specialized photodissociation region codes. We successfully reproduce the transition depth between molecular and atomic hydrogen, molecular cooling of the gas, and a realistic Strömgren sphere embedded in a molecular medium. In this paper we focus on test cases to demonstrate the validity of our model on small scales. Our ultimate goal is to implement this in large-scale galactic simulations.

Efficacy of early stellar feedback in low gas surface density environments

“…The self-shielding prescription is able to block most of the LW photons from entering into the molecular layer prducing a relatively sharp and thin H I layer between the fully ionized H II and fully molecular H 2 layers. This nicely matches with the results obtained by Nickerson et al (2018), thereby confirming the accuracy of our scheme. This paper has been typeset from a T E X/L A T E X file prepared by the author.…”

“…Unfortunately, our formulation of the RT equations does not track the optical depth of individual rays. Therefore, we resort to using the method described in Nickerson et al (2018) and boost the destruction of LW photons by a constant factor to incorporate the fact that only a fraction of LW photon absorption leads to H2 dissociation. As the LW photons propagate through gas cells, their repeated destruction mimics the column density variation of H2 destruction rates.…”

Towards the complete census of molecular hydrogen in a simulated disc galaxy

“…Considering the entire body of work between Nickerson et al (2018) and this present paper, we have shown that the laws that govern H 2 on the chemical scale give rise to relations that govern H 2 on the galactic scale. We have demonstrated that high resolution is critical in galaxy simulations to properly form H 2 without the need for a clumping factor.…”

“…What we do instead is to enhance LW destruction at each time-step but not H 2 dissociation, and thereby do not require a conversion to column density because the LW photons will travel through the column of cells with each timestep. We calibrate the factor of LW destruction enhance-ment with one-dimensional simulations of radiation dissociating an H 2 slab (see Nickerson et al 2018). We compare our transition between H 2 and Hi with its position as predicted by the analytical model of Bialy et al (2017), and find that enhancing the LW destruction by a factor of 400 gives good results for a range of incident fluxes, slab densities, and metallicities (Nickerson et al 2018).…”

“…We track the non-equilibrium chemistry of H 2 , Hi, Hii, Hei, Heii, and Heiii tied to the four dissociating and ionizing photon groups given in Table 2. Details of the molecular chemistry are given in Nickerson et al (2018) and the ionization chemistry in Rosdahl et al (2013) but we summarize the processes here.…”