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-# Copyright (c) 2017 Weitian LI <liweitianux@live.com>
-# 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
- mec = ac.m_e.cgs.value*ac.c.cgs.value # [g cm / s]
-
-
-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 = au.g.to(au.solMass)
- # Time
- Gyr2s = au.Gyr.to(au.s)
- s2Gyr = au.s.to(au.Gyr)
- # Length
- kpc2cm = au.kpc.to(au.cm)
- cm2kpc = au.cm.to(au.kpc)
- Mpc2cm = au.Mpc.to(au.cm)
- cm2Mpc = au.cm.to(au.Mpc)
- Mpc2km = au.Mpc.to(au.km)
- km2Mpc = au.km.to(au.Mpc)
- kpc2km = au.kpc.to(au.km)
- km2kpc = au.km.to(au.kpc)
- km2cm = au.km.to(au.cm)
- # Energy
- keV2erg = au.keV.to(au.erg)
-
-
-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]
-
- # Mean molecular weight
- # Ref.: Ettori et al, 2013, Space Science Review, 177, 119-154, Eq.(6)
- mu = 0.6