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-rw-r--r--fg21sim/extragalactic/clusters/units.py66
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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.
+ """
+ Msun2g = au.solMass.to(au.g)
+ g2Msun = au.g.to(au.solMass)
+ Gyr2s = au.Gyr.to(au.s)
+ s2Gyr = au.s.to(au.Gyr)
+ 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)
+ 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]
+
+ # Mean molecular weight
+ # Ref.: Ettori et al, 2013, Space Science Review, 177, 119-154, Eq.(6)
+ mu = 0.6