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# Copyright (c) 2017 Weitian LI <weitian@aaronly.me>
# MIT license
"""
Commonly used units and their conversions relations, as well as constants.
Astropy's units system is very powerful, but also very slow,
and may even be the speed bottleneck of the program.
This module provides commonly used units conversions by holding
them directly in a class, thus avoid repeated/unnecessary calculations.
"""
import astropy.units as au
import astropy.constants as ac
class Units:
"""
Commonly used units, especially in the CGS unit system.
"""
# Unit for electron momentum (p), thus its value is the Lorentz factor
# Unit: [g cm / s]
mec = ac.m_e.cgs.value * ac.c.cgs.value
# Energy of a still electron
# Unit: [erg]
mec2 = (ac.m_e * ac.c**2).to(au.erg).value
class UnitConversions:
"""
Commonly used units conversion relations.
Hold the conversion relations directly to avoid repeated/unnecessary
calculations.
"""
# Mass
Msun2g = au.solMass.to(au.g)
g2Msun = 1.0 / Msun2g
# Time
Gyr2s = au.Gyr.to(au.s)
s2Gyr = 1.0 / Gyr2s
# Length
kpc2m = au.kpc.to(au.m)
m2kpc = 1.0 / kpc2m
Mpc2m = au.Mpc.to(au.m)
m2Mpc = 1.0 / Mpc2m
kpc2cm = au.kpc.to(au.cm)
cm2kpc = 1.0 / kpc2cm
Mpc2cm = au.Mpc.to(au.cm)
cm2Mpc = 1.0 / Mpc2cm
Mpc2km = au.Mpc.to(au.km)
km2Mpc = 1.0 / Mpc2km
kpc2km = au.kpc.to(au.km)
km2kpc = 1.0 / kpc2km
km2cm = au.km.to(au.cm)
cm2km = 1.0 / km2cm
# Energy
keV2erg = au.keV.to(au.erg)
erg2keV = 1.0 / keV2erg
# Angle
rad2deg = au.rad.to(au.deg)
deg2rad = 1.0 / rad2deg
rad2arcsec = au.rad.to(au.arcsec)
arcsec2rad = 1.0 / rad2arcsec
rad2arcmin = au.rad.to(au.arcmin)
arcmin2rad = 1.0 / rad2arcmin
deg2arcmin = au.deg.to(au.arcmin)
arcmin2deg = 1.0 / deg2arcmin
deg2arcsec = au.deg.to(au.arcsec)
arcsec2deg = 1.0 / deg2arcsec
arcmin2arcsec = au.arcmin.to(au.arcsec)
arcsec2arcmin = 1.0 / arcmin2arcsec
# Temperature
eV2K = au.eV.to(ac.k_B*au.K)
K2eV = 1.0 / eV2K
keV2K = au.keV.to(ac.k_B*au.K)
K2keV = 1.0 / keV2K
class Constants:
"""
Commonly used constants, especially in the CGS unit system.
Astropy's constants are stored in SI units by default.
When request a constant in CGS unit system, additional (and slow)
conversions required.
"""
# Speed of light
c = ac.c.cgs.value # [cm/s]
# Atomic mass unit (i.e., a.m.u.)
u = ac.u.cgs.value # [g]
# Gravitational constant
G = ac.G.cgs.value # [cm^3/g/s^2]
# Electron charge
e = ac.e.gauss.value # [Fr] = [esu]
# Boltzmann constant
k_B = ac.k_B.cgs.value # [erg/K]
# Mean molecular weight
# Ref.: Ettori et al, 2013, Space Science Review, 177, 119-154, Eq.(6)
mu = 0.6
# Adiabatic index of ideal monatomic gas
gamma = 5.0/3.0
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