Overview of rates module
rates module implements nuclear reaction rates.
The rates module provides MESA’s weak reaction rates.
This includes a compilation of tables (in
/data/rates_data/weakreactions.tables) suitable for the high
densities and temperatures encountered in late stages of stellar
evolution. These rates are based (in order of precedence) on the
Langanke & Martı́nez-Pinedo (2000),
Oda et al. (1994),
and Fuller et al. (1985).
(These were referred to as a separate
weaklib module in MESA I, but are now part of
These reaction rates are tabulated as function of \(\rho Y_e\) and \(T\). These tables cover 1 ≤ log(ρYe) ≤ 11 and 7 ≤ log(T) < 10.5, but are relatively coarse, with 11 points in the ρYe dimension (∆ log ρYe = 1) and 12 points in the T dimension (∆ log T ≈ 0.25). They are constructed assuming complete ionization (even if that is not appropriate at the given conditions).
For each isotope pair, the tables have:
positron emission rate
electron capture rate
total neutrino energy loss rate
electron emission rate
positron capture rate
total anti-neutrino energy loss rate
These tables do not cover the lower density, lower temperature conditions that are encountered in earlier stages of stellar evolution (e.g., main sequence) or in stellar envelopes. Their assumption of complete ionization means that they are not physically appropriate for conditions in which atoms are neutral or partially ionized (i.e., they ignore electron captures from bound electrons).
We have a separate set of low temperature weak rates (internal
weak_lowT_rate) that MESA will use when it is off the
weaklib tables. The blend from these to the table occurs over a
specified temperature range (i.e., when
T9_weaklib_full_on < T <
T9_weaklib_full_off) and there is a separate blend for higher Z
Z >= weaklib_blend_hi_Z and
T9_weaklib_full_on_hi_Z < T < T9_weaklib_full_off_hi_Z).
If you rely on any low temperature weak rates in your problem, you should carefully check them. The current MESA approach often yields unphysical values, both in the low temperature/density limit and in the blending region.
weak_lowT_rate should be the rate for a neutral
atom. In practice (see
rates are set by a call to
reaclib at \(T = 10^7\) K, or if
the rate is not in
reaclib, by consulting the file
data/rates_data/weak_info.list. Depending on what information is
included in those files and on the physical processes that set the
decay, this approach may or may not yield the desired result. (We
also must provide the average neutrino energies, which are not included in
reaclib and the values in
weak_info.list are mostly fantasies