Fundamental Constants

Fundamental constants are defined in constants/fundamental_constants.H. This file contains physical constants and unit conversion factors, such as Boltzmann’s constant, kB, and is automatically generated by the Python script write_fundamental_constants.py.

All constant values are obtained from scipy.constants to ensure consistency between Microphysics and pynucastro, the latter is used for generating the reaction networks used by Microphysics.

There are two exceptions:

  1. Constants under the Legacy namespace represent outdated values historically used in networks such as aprox13, aprox19, and aprox21. These constants are retained for backward compatibility and should NOT be used in other places.

  2. Newton’s gravitational constant continues to use an older value to preserve hydrostatic equilibrium in existing initial models used by Castro.

Note

The consistency of physical constants between pynucastro and Microphysics is particularly important when using inverse rates derived from detailed balance. Discrepancies in constant values can affect the agreement between mass fractions obtained from direct integration and those computed from nuclear statistical equilibrium (NSE).

This a subtle point, but important when using NSE evolution mode (i.e., when compiled with USE_NSE_NET=TRUE and USE_NSE_TABLE=TRUE). See NSE for more details.