# Reaction Networks¶

## Summary of available nets¶

This is an incomplete summary of the nets included for use with mesa/net. See \$MESA_DIR/data/net_data/nets for the full list.

This lists the isotopes that are included in each net, but not the reactions. To check on those, it is best to have the code list them . In mesa/star you can do this by setting the star_job options show_net_reactions_info = .true.; similarly, you can get a list of the isotopes by setting show_net_species_info = .true..

### “no frills” hydrogen and helium burning¶

basic

h1, he3, he4, c12, n14, o16, ne20, mg24

Assumes T is low enough so can ignore advanced burning and hot cno issues.

### more complete coverage for hydrogen and helium burning¶

c13

basic + c13

o18_and_ne22

basic + o18 and ne22

o18_to_mg26

o18_and_ne22 + mg26

hot_cno

basic + c13, n13, n15, o15, o17, o18, f17, f18

for high temperatures where cno becomes beta limited.

cno_extras

hot_cno + o14, f19, ne18, ne19, mg22

for high temperatures where start to breakout of hot cno.

pp_and_cno_extras

pp_extras + cno_extras

pp_cno_extras_o18_ne22

pp_and_cno_extras + o18_and_ne22

### extensions of the basic net for C/O burning and alpha chains¶

co_burn

basic + si28

alpha_s32

co_burn + s32

alpha_ar36

alpha_s32 + ar36

alpha_ca40

alpha_ar36 + ca40

alpha_ti44

alpha_ca40 + ti44

alpha_cr48

alpha_ti44 + cr48

alpha_fe52

alpha_cr48 + fe52

alpha_ni56

alpha_fe52 + fe54, ni56, neut, and prot

approx19

same isotopes as alpha_ni56; similar to Frank Timmes’ APPROX19

approx20

approx19 + fe56

approx21

approx20 + cr56

ns_he

approx21 + (a,p)*(p,g) intermediates for alpha links + “bypass” for c12

## Creating a custom net¶

If you don’t find what you need here, it is fairly easy to create new nets. First, study o18_and_ne22.net to see how it works. The idea is to start from a net that is close to what you need, then add isotopes and reactions as needed; you may also need to remove some reactions. You can put your new net file in your work directory; mesa/net looks in the current directory before it checks data/net_data/nets. Before using your new net, get a list of isotopes and reactions and carefully check them.

For reference, the current set of isotopes is in mesa/data/chem_data/isotopes.data. The format of that file is described in a README file in the same directory. The current set of reactions is in mesa/net_data/reactions.list. The reaction names in net files must be defined in the reactions.list file. Those reactions are implemented by the mesa/rates module.

## Description of net format¶

A net is defined by a set of isotopes and a set of reactions. These sets are specified in a net definition file.

A net definition file contains a sequence of commands. It can have any number of the following commands in any order.

include 'net_filename'

This lets you break up a definition into several files (e.g., one net can defined as an extension of another). This first tries to open ‘net_filename’ in current directory. If that fails, it tries in mesa/data/net_data/nets/.

add_isos(isos_list)

For each iso in isos_list, add it to the current set of isos in the net. It really is a set, so multiple adds are okay.

remove_isos(isos_list)

For each iso in isos_list, remove it from the current set of isos in the net.

add_reactions(reactions_list)

For each reaction in reactions_list, add it to the current set of reactions in the net. It really is a set, so multiple adds are okay.

remove_reactions(reactions_list)

For each reaction in reactions_list, remove it from the current set of reactions in the net.

add_isos_and_reactions(isos_list)

For each iso in isos_list, add the iso to the current set, and add reactions linking it to other isos in current set.

The reactions (see below for descriptions of the reaction handle format) that automatically added by the add_isos_and_reactions command are:

• p,a,n capture reactions and photodisintegrations (e.g., r_x_pg_y)
• p,a,n exchanges (e.g., r_x_ap_y)
• standard 1-to-1 weak reactions (e.g., r_x_wk_y)
• other weak reactions (e.g., r_x_wk_h1_y)

It also checks if there are special cases to be added. Here’s an incomplete list.

• r_h1_h1_wk_h2
• r_h1_h1_ec_h2
• r_h2_h2_to_h1_h3, r_h1_h3_to_h2_h2
• r_he3_ec_h3
• r_h2_h2_to_neut_he3, r_neut_he3_to_h2_h2
• r_neut_neut_he4_to_h3_h3, r_h1_h1_he4_to_he3_he3
• r_h3_he3_to_h2_he4, r_h2_he4_to_h3_he3
• r_li6_to_neut_h1_he4, r_neut_h1_he4_to_li6
• r_h1_li7_to_h2_li6, r_h2_li6_to_h1_li7
• r_he4_he4_he4_to_c12, r_c12_to_he4_he4_he4
• r_c12_c12_to_he4_ne20, r_he4_ne20_to_c12_c12
• r_neut_mg23_to_c12_c12, r_c12_c12_to_neut_mg23
• r_c12_ne20_to_h1_p31, r_h1_p31_to_c12_ne20
• r_s27_wk_h1_h1_al25
• r_he4_ca36_to_h1_h1_ca38, r_h1_h1_ca38_to_he4_ca36
• r_pd89_to_h1_h1_ru87, r_h1_h1_ru87_to_pd89

The arguments to these commands have the following form:

isos_list

A sequence of iso_spec values optionally separated by commas

iso_spec

Either iso_name or element_name A_lo A_hi.

iso_name is something like h1 or he4 defined in chem.

element_name is something like h or he defined in chem. A_lo and A_hi define the range of A values to add for the element e.g., if iso_spec is he 3 4, then adds he3 and he4.

reactions_list

A sequence of reaction_handle values optionally separated by commas.

reaction_handle

A reaction name defined in reactions.list

or

A reaction name derived from the input and output isos as follows, where x and y are iso_names; e.g., r_c12_pg_n13.

• p,a,n capture reactions and photodisintegrations

• r_x_pg_y
• r_y_gp_x
• r_x_ag_y
• r_y_ga_x
• r_x_ng_y
• r_y_gn_x
• p,a,n exchanges

• r_x_ap_y
• r_y_pa_x
• r_x_np_y
• r_y_pn_x
• r_x_na_y
• r_y_an_x
• standard 1-to-1 weak reactions

• r_x_wk_y (positron emission or electron capture)
• r_x_wk-minus_y (electron emission or positron capture)
• other weak reactions (not in weaklib)

• r_x_wk_h1_y (* beta decay with products h1 and y*)
• r_x_wk_he4_y (beta decay with products he4 and y)
• r_h1_h1_ec_h2 (electron capture)
• r_h1_h1_wk_h2 (beta decay)
• r_he3_ec_h3
• r_be7_ec_li7
• r_h1_h1_wk_h2
• r_h1_he3_wk_he4
• other reactions

• r_<inputs>_to_<outputs>

where <inputs> and <outputs> are iso_name values separated by ‘_’ If the same iso appears 2 or more times, repeat the name that many times. e.g., triple alpha is r_he4_he4_he4_to_c12. iso values are ordered by increasing Z and N. e.g., r_h3_be7_to_neut_h1_he4_he4.

## Changing/selecting reaction rates¶

Warning

This section needs work.

### change rates for existing reactions¶

create a file of (T8, rate) pairs as in data/rates_data/rates add the file name to reactions.list (you can have a local copy of reactions.list as well as rates directory)

### add a new reaction with rate give by table of (T8, rate) pairs¶

get a name for the reaction by calling reaclib_create_handle then proceed same as for changing rate of an existing reaction use the reaction in a net in same way as for existing reactions.

### selecting rates for reactions¶

reactions in reactions.list have various rates available such as NACRE, REACLIB, or CF88. several reactions have other options as well: see rates_def.f for listing of choices

other reactions use rates from weaklib or reaclib.

also can provide a table of values for any reaction. arbitrarily many pairs of temperatures and rates; piecewise monotonic cubic interpolation. specify files for rate tables in rate_list.txt