Opacity¶

There are several options for calculating and controlling the opacity of the gas in the calculation.

Though we generally use the word “opacity”, the code actually calculates and stores an extinction coefficient. The two are related by

$\alpha = \kappa \rho$

where $\alpha$ is the extinction coefficient (units ${\rm cm}^{-1}$ ), $\kappa$ is the opacity (units ${\rm cm}^{2}$ ${\rm g}^{-1}$ ), and $\rho$ is the mass density

Grey Opacity¶

The grey opacity flags allow the user to specify a simple, wavelength-independent opacity. Note that setting grey opacity will override all other forms of opacity described below.

To set a uniform grey opacity at all points in space, set the parameter:

opacity_grey_opacity = 0.1

where, in this example, the code will set the value $\kappa_{g} = 0.1$ ${\rm cm}^{2}$ ${\rm g}^{-1}$ to all zones in the model. All other sources of opacity discussed below (e.g., free-free, bound-free) will be ignored.

The user can also set a spatially varying grey opacity, provided that a dataset named grey_opacity giving the value of kappa in every zone has been set in an input hdf5 model file. Then setting:

opacity_zone_specific_grey_opacity    = 1

will use the kappa defined in the model file

Continuum opacities¶

Electron Scattering Opacity¶

To turn on electron-scattering opacity:

opacity_electron_scattering = 1

This is calculated as

$\alpha_{\rm es} = \sigma_t n_e$

where $n_e$ is the free electron density

Options for include Klein-Nishina corrections, Comptonization etc…

Bound-Free Opacity¶

Free-Free Opacity¶

Line Opacity¶

There are various options for treating lines. In general, only one of these approaches should be used to treat lines, unless one can be certain that line opacity is not be multiply counted.

Resolved Bound-Bound Opacity¶

This approach is selecting by setting the runtime parameter:

opacity_bound_bound = 1

This approach treats the lines generally as Voigt profiles. The frequency good must be fine enough that there are multiple grid points across to resolve the line profile.

The widths of lines can be artificially broadened using the runtime parameter:

line_velocity_width = <real>

where <real> is a velocity (in cm/s) by which the lines should be Gaussian broadened.

Line Expansion Opacity¶

This approach is selected by setting the runtime parameter:

opacity_line_expansion = 1

This approach bins lines into frequency bins, assuming a homologous flow.

This approach implies the Sobolev approximation. For each line, the code calculates the Sobolev optical depth

$\tau_{\rm sob} = \frac{ \pi e^2}{m_e c} n_l f_{lu} \lambda_0 t_{\rm exp}$

where…

The expansion opacity is then calculated by binning lines

$\alpha_{\rm exp} = \frac{1}{c t_{\rm exp}} \frac{\lambda_0}{\Delta \lambda} \sum_i (1 - e^{-\tau_i})$

Some linelists (such as the Kurucz lists) are in a format that do not include as much detail about the atomic levels. These can be included as:

opacity_fuzz_expansion = 1

The physics here is identical to that of line expansion opacity, it is just that the lines are read from an independent file.

Resonant Line Scattering¶

This is how we would treat scattering in strong individual lines like Lyman alpha.

User Defined Opacity¶

You can write your own function

LTE and NLTE settings¶

Opacity Parameters¶

Opacity parameters¶
parameter	values	definition
opacity_grey_opacity	<real>	value of grey opacity to use (in cm^2/g). Will override all other opacity settings
opacity_zone_specific_grey_opacity	0 = no \| 1 = yes	Use a zone-specific grey opacity dataset that is set in an hdf5 input model file and named grey_opacity
opacity_user_defined	0 = no \| 1 = yes	Calculate opacities by calling the function
opacity_epsilon	<float>	The fraction of
opacity_atom_zero_epsilon	<int>
opacity_electron_scattering	0 = no \| 1 = yes	include electron scattering opacity
opacity_line_expansion	0 = no \| 1 = yes	include binned line expansion opacity
opacity_fuzz_expansion	0 = no \| 1 = yes	include binned line expansion opacity, taken from a fuzz file
opacity_bound_free	0 = no \| 1 = yes	include bound-free (photoionization) opacity
opacity_free_free	0 = no \| 1 = yes	include free-free opacity
opacity_bound_bound	0 = no \| 1 = yes	include bound-bound (resolved line) opacity
opacity_use_nlte	0 = no \| 1 = yes	include nlte opacity
opacity_atoms_in_nlte	<int vector>	A vector of atomic numbers of the species to be treated in NLTE
opacity_use_collisions_nlte	0 = no \| 1 = yes	only matters if use_nlte == 1, include collisions for nlte calculations
opacity_no_ground_recomb	0 = no \| 1 = yes	Suppress all recombination transitions to the ground state in the NLTE level population solve
opacity_minimum_extinction	<float>	Minimum value of the extinction coefficient (units 1/cm) in any zone
opacity_maximum_opacity	<float>	Minimum value of the extinction coefficient (units 1/cm) in any zone
opacity_no_scattering	0 = no \| 1 = yes	if = 1, will not include any kind of scattering opacity
dont_decay_composition
opacity_compton_scatter_photons
line_velocity_width	<float>	velocity in cm/s used to doppler broaden the (bound bound?) lines
line_x_extent