From 2f511e2ed3e8628b28ca3bef872b76bb4c3d4a1d Mon Sep 17 00:00:00 2001
From: Aaron LI <aaronly.me@outlook.com>
Date: Fri, 24 Jun 2016 17:03:10 +0800
Subject: astro_params.py: add some constants

---
 astro_params.py | 25 ++++++++++++++++++++-----
 1 file changed, 20 insertions(+), 5 deletions(-)

diff --git a/astro_params.py b/astro_params.py
index b3b733b..bf7a0da 100644
--- a/astro_params.py
+++ b/astro_params.py
@@ -6,6 +6,9 @@
 #
 # Change logs:
 # 2016-06-24:
+#   * Add some constants from `astropy.constants`
+#   * Add parameter 'mu'
+#   * Fix reference
 #   * Add class 'ChandraPixel', moved from 'deproject_sbp.py'
 #
 
@@ -15,6 +18,7 @@ and astrophysics.
 """
 
 import astropy.units as au
+import astropy.constants as ac
 from astropy.cosmology import FlatLambdaCDM
 
 
@@ -22,20 +26,31 @@ class AstroParams:
     """
     The parameters/constants used in astronomy.
 
+    NOTE:
+    The 'cgs' unit system is used.
+
     References:
-    [1] ref. [4], eq.(9) below
+    [1] Ettori et al, 2013, Space Science Review, 177, 119-154
     """
+    # Gravitational constant
+    G = ac.G.cgs.value  # [ cm^3 g^-1 s^-2 ]
+    # Boltzmann constant
+    k_B = ac.k_B.cgs.value  # [ erg K^-1 ]
+    # Atomic mass unit (i.e., a.m.u)
+    m_atom = ac.u.cgs.value  # [ g ]
     # Hubble constant at z=0
     H0 = 71.0  # [ km/s/Mpc ]
     # density of non-relativistic matter in units of the critical density
     # at z=0
     OmegaM0 = 0.27
-    # ratio of electron density (n_e) to proton density (n_p) [1]
+    # ratio of electron density (n_e) to proton density (n_p)
+    # (Ref: [1], eq.(9) below)
     ratio_ne_np = 1.211
-    # molecular weight per electron (0.3 solar abundance; grsa table) [1]
+    # molecular weight per electron (0.3 solar abundance; grsa table)
+    # (Ref: [1], eq.(9) below)
     mu_e = 1.155
-    # atomic mass unit
-    m_atom = au.u.to(au.g)  # [ g ]
+    # mean molecular weight (unit: a.m.u) [1] (Ref: [1], eq.(6) below)
+    mu = 0.6
 
 
 class ChandraPixel:
-- 
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