hih2

Atomic-to-molecular hydrogen transition models for astrophysical simulations.

To install:

cd ~

git clone https://github.com/avapolzin/hih2.git

cd hih2

pip install .

or

pip install hih2

Included models are:

• Krumholz, McKee, & Tumlinson (2009b) -- KMT09b
• Gnedin & Kravtsov (2011) -- GK11
• Krumholz (2013) -- K13
• Sternberg et al. (2014) -- S14
• Gnedin & Draine (2014) -- GD14
• Polzin et al. (2024a) -- P24

Volumetric models are in hih2.vol and projected models are in hih2.proj. All syntax is the same between those modules. It's very simple, so the docstrings are largely sufficient and limited documentation follows here:

All models take as arguments nh (hydrogen number density if hih2.vol -- cm^-3 by default -- or hydrogen column density if hih2.proj -- cm^-2 by default) and met (solar-scaled metallicity). gk11(), k13(), s14(), and p24() require uv (UV field strength). hih2.proj.gd14() and p24() also require an scale argument, which is the size of the grid cells (by default in pc). hih2.vol.gd14(), kmt09b, k13, and s14 also take a scale argument, which follows the convention of hih2.proj.

Both k13() and s14() functions also take a clumping factor argument, fc, and k13() additionally takes rho_sd (density of stars and dark matter, by default in M_☉ pc^-3), iterate (toggles whether values are computed iteratively), and niter (if iterate = True, sets number of interations).

All functions take dens_unit, which takes astropy.units objects to set the units in which nh is reported. k13() has an equivalent for rho_sd called sddens_unit. All of the functions that require scale take a scale_unit argument.

Each function returns the molecular hydrogen fraction (defined as H₂/(HI + H₂)) for the input gas density, metallicity, and UV field strength. The implicit assumption of these models is that all gas is neutral and that any ionized component is negligible.

Required arguments are summarized in the tables below.

Why is this a package? Good question. It's very easy to implement these HI-H₂ models yourself either from the literature or from code examples (see compare_models.py for functions on their own), but some people may prefer a quick install/import. See the Appendix in Polzin et al. (2024a) for details of how the models included here are implemented.

Summary of required and optional parameters in models included in hih2.vol

Summary of required and optional parameters in models included in hih2.proj

How can I access the models in a simple script? Just like hih2, compare_models.py includes functions for each of the models on a cell-by-cell basis and projected on different scales; models are stored for use in Python.

I will add C/C++ code in the near future. As always, the idea is for these functions to be easily callable. One can always refer to the original papers to examine the details of the different HI-H₂ models.

Do you want to see a model added? Feel free to open a PR including a new model or open an issue requesting one. In each case, please include a link to the paper in which the model is published.