# Overview of rates module¶

The rates module implements nuclear reaction rates.

## Weak reactions¶

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 tabulations of 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 rates.)

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 variable 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 elements (when Z >= weaklib_blend_hi_Z and T9_weaklib_full_on_hi_Z < T < T9_weaklib_full_off_hi_Z).

Warning

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.

Physically, the weak_lowT_rate should be the rate for a neutral atom. In practice (see rates/private/rates_initialize.f90), these 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 or inappropriate.)