jaff.physics

Physical constants and photochemical cross-section lookup for astrochemical calculations.

Classes

Class	Description
`Photochemistry`	Photo cross-section lookup from the bundled databases (see below)

Submodules

Submodule	Description
`constants`	Physical & astronomical constants as `astropy` Quantities (see below)

Photochemistry methods

jaff.physics.Photochemistry resolves a reaction's cross sections and shielding factors from jaff.db. Constructing it downloads the cross-section and line-shielding data files on first use (cached thereafter), so instantiate once and reuse:

from jaff.physics import Photochemistry

photo = Photochemistry()
photo.get_xsec(rxn)              # XsecsProps from the tabulated databases
photo.get_verner_xsec(rxn)       # analytic Verner σ(E) (sympy) or None
Photochemistry.shielding(rxn, net)  # symbolic shielding factor (sympy)

Method	Returns	Description
`get_xsec(reaction)`	`XsecsProps or None`	Tabulated cross sections (Leiden / NORAD): `photon_energy` (eV) plus `photo_absorption` and `photodecay` (cm²)
`get_verner_xsec(reaction)`	`sympy.Basic or None`	Analytic Verner (1996) σ(E) expression (symbol `E` in erg, σ in cm²)
`shielding(reaction, network)`	`sympy.Expr`	Dimensionless line-shielding factor; dispatches to the global/local shielding function named by the reaction metadata

Unit systems

jaff.physics.constants exposes physical constants as module-level astropy.units.Quantity objects. There are no separate cgs/si/gaussian/natural tables — select the unit system per quantity at the call site with .cgs, .si, or .to(...). See constants for the full list.

Example

from jaff.physics import constants as c

# Per-quantity unit selection
c.c.cgs              # speed of light [cm/s]
c.k_B.cgs            # Boltzmann constant [erg/K]
c.c.si               # speed of light [m/s]
c.k_B.to("eV / K")   # Boltzmann constant [eV/K]

# Elementary charge keeps astropy's EM-unit views
c.e.esu              # Gaussian / CGS-ESU [Fr]
c.e.si               # SI [C]

# Bare float (in the chosen unit) when a number is required
float(c.m_e.cgs.value)   # electron mass [g]