Dust

JAFF ships an optional dust module that couples interstellar grains to the network's radiation field. Once enabled it does two things: it drives grain photoelectric heating (via the self-consistent chi_pe field) and it attenuates the radiation field as photons are absorbed and scattered by dust. This page describes the grain model, the tabulated cross sections, and how to turn the module on.

What dust does

The module contributes two distinct pieces of physics.

Photoelectric heating (chi_pe). Far-UV photons eject electrons from grain surfaces, heating the gas. The heating rate is parametrised by chi_pe: the local radiation field in the photoelectric band, scaled to a reference (Draine/ISRF) field. JAFF computes chi_pe from the network's own radiation bands, so photoelectric heating stays consistent with the radiation transport actually being solved. This is the same chi_pe symbol documented in Photochemistry → Self-consistent photoelectric field; the dust module is what supplies it.

Radiation extinction. Dust removes energy from the radiation field. When the radiation-transport ODEs are generated, each band's density (0th, "u") and flux (1st, "f") moments lose a dust-attenuation term of the form

and analogously for the flux moment with \(\langle\sigma\rangle_i^{\,(f)}\). Here \(n_\mathrm{H}\) is the total hydrogen-nucleus number density, \(c\) the (reduced) speed of light, \(Z_d\) a runtime dust-abundance scaling symbol supplied by the host code, and \(\langle\sigma\rangle_i\) the band-averaged dust cross section per H nucleus. Which cross-section kind attenuates the density and the flux moment is chosen independently by u_reduction and f_reduction (set either to "none"/None to drop that term).

Grain model and cross sections

Dust optical properties come from a bundled table (src/jaff/data/dust/dust.hdf5). The rv parameter selects a Milky-Way grain model by its total-to-selective extinction ratio \(R_V\), reading the matching mw_rv{rv} group (mw_rv3.1, mw_rv4.0, or mw_rv5.5).

The table stores a single extinction cross section per H nucleus, \(C_\text{ext}/H\) (cm²/H), along with the scattering albedo \(\omega\) and the scattering asymmetry \(\langle\cos\theta\rangle\) over a photon-energy grid (eV). From these, jaff.physics.dust.tabular.Tabular derives the four cross-section kinds:

For each radiation band, JAFF forms the band-averaged cross section per H by weighting with the radiation photon-density profile over the band edges:

This is the quantity that enters the extinction terms above, so the same band grid used for photochemistry also controls dust extinction.

Parameters

The dust module is configured with a DustProps model:

from jaff.physics import DustProps

DustProps(
    rv=3.1,
    u_reduction="absorption",
    f_reduction="transport",
    pe_threshold_low=6.0,
    pe_threshold_high=13.6,
)

The [pe_threshold_low, pe_threshold_high] window is the band over which chi_pe is integrated; the defaults span the classic 6–13.6 eV FUV band.

Enabling dust

From Python

Pass a DustProps instance as dust_props= to the Network constructor, together with a radiation_props= so the radiation field exists:

from jaff import Network
from jaff.physics import RadiationProps, DustProps

net = Network(
    "GOW++",
    radiation_props=RadiationProps(
        bands=[6.0, 11.2, 13.6],   # band edges in eV
        background_field="draine",  # reference field for chi_pe
    ),
    dust_props=DustProps(
        rv=3.1,
        u_reduction="extinction",
        f_reduction="extinction",
    ),
)

Omitting dust_props (leaving it None) leaves the dust module off.

From jaffgen.toml

For code generation, the mere presence of a [network.dust] table enables the module. Its keys mirror DustProps:

[network.radiation]
bands            = [6, 11.2, 13.6]
background_field = "draine"

[network.dust]
rv                = 3.1
u_reduction       = "extinction"
f_reduction       = "extinction"
pe_threshold_low  = 6      # eV
pe_threshold_high = 13.6   # eV

All keys are optional and fall back to the DustProps defaults; an empty [network.dust] table is enough to turn the module on. See the [network.dust] reference in the configuration guide for the full key list, and Photochemistry for the radiation setup that dust builds on.

Python API

from jaff import Network
from jaff.physics import RadiationProps, DustProps

net = Network(
    "GOW++",
    radiation_props=RadiationProps(bands=[6.0, 11.2, 13.6]),
    dust_props=DustProps(rv=3.1),
)

# The dust container and its sub-objects
net.dust                      # jaff.physics.dust.Dust (or None if disabled)
net.dust.rv                   # selected R_V grain model
net.dust.pe.chi               # symbolic chi_pe expression
net.dust.tabular.photon_energy  # eV grid backing the cross sections

# Band-averaged dust cross section per H (cm^2/H) over an energy window
net.dust.tabular.avg_cross_section_per_hnuc("absorption", (6.0, 13.6))