## How to use MESA-Web

To construct and evolve a stellar model, MESA-Web allows you to choose the following parameters:

Here we describe the physics associated with the input parameters used in MESA. For a more in-depth review of these parameters please see Paper I, Paper II, or more recently Paper III.

__Mass__ The initial mass of the star, in solar masses. This value must be between 0.1 and 100.

__Nuclear Reactions__

*Network*A net is defined by a set of isotopes and a set of reactions.

Descriptions of some of the nuclear reaction networks provided by the README with mesa/data/net_data/nets/.

Below is a table of the nuclear reaction networks currently available for use with MESA-Web, a list of the isotopes they contain and examples of their use in different stellar environments:

Network | Isotopes | Uses |
---|---|---|

agb | n,^{1-2}H,^{3,4}He,^{7}Li,^{7}Be,^{8}B,^{12,13}C,^{13-15}N,^{16-18}O,^{19}F |
Evolved Low Mass Stars |

approx21 | n,p,^{1}H,^{4}He,^{12}C,^{14}N,^{16}O,^{20}Ne,^{24}Mg,^{28}Si,^{32}S,^{36}Ar,^{40}Ca,^{44}Ti,^{48}Cr,^{52,54,56}Fe,^{56}Ni |
Evolved Massive Stars |

basic | ^{1}H,^{3}He,^{4}He,^{12}C,^{14}N,^{16}O,^{20}Ne,^{24}Mg |
General H/He Burning |

co_burn | basic + ^{28}Si |
General H/He/C/O Burning |

h_burn | n,^{1,2}H,^{3,4}He,^{6,7}Li,^{7,9,10}Be,^{8,10,11}B,^{12,13}C,^{13-15}N,^{14-18}O,^{17-19}F,^{18-22}Ne,^{21-23}Na,^{22-26}Mg,^{25-27}Al,^{27,28}Si |
Complex H Burning |

mesa_49 | n,^{1,2}H,^{3,4}He,^{7}Li,^{7,9,10}Be,^{8}B,^{12,13}C,^{13-15}N,^{14-18}O,^{17-19}F,^{18-22}Ne,^{21-24}Na,^{23-26}Mg,^{25-27}Al,^{27-30}Si,^{30,31}P,^{31-34}S |
Complex H/He Burning |

sagb_NeNa_MgAl | n,^{1,2}H,^{3,4}He,^{7}Li,^{7}Be,^{8}B,^{12,13}C,^{13-15}N,^{16-18}O,^{19}F,^{20-22}Ne,^{21-23}Na,^{24-26}Mg,^{25-27}Al |
Evolved Intermediate Mass Stars |

*Custom Nuclear Reaction Rate*MESA-Web allows you to utilize MESA's ability to use custom nuclear reaction rates. Currently MESA-Web supports user input for the

^{14}N(

*p,γ*)

^{15}O,

^{12}C(

*α,γ*)

^{16}O, and Triple-α reaction rates. To utilize this feature, select the custom rate you wish to upload from the dropdown bar, then upload the custom reaction rate as a function of T8 (T/10

^{8}K). The first line of the input rate

**must**contain an integer corresponding to the number of T8, rate pairs in your uploaded file. A correct and incorrect example is shown below:

Correct Input File

Incorrect Input File

Currently MESA-Web only allows a user to change one reaction rate per run. To change multiple reactions at a time for a calculation, the full version of MESA is recommended.

__Composition__

*Metallicity (Z)*

The initial metallicity of the star.

__Mixing__

*Convection (α*_{MLT})The mixing length due to convection is computed as

*l = α*,

_{MLT}H_{p}where

*l*is the mixing length, α

_{MLT}is a free parameter, and

*H*is the pressure scale height.

_{p}

*Mixing Length Theory Implementation*-

**- Gives radiative values with no mixing.**

*none*-

**- MLT as developed in Cox & Giuli 1968, Chapter 14.**

*Cox*-

**- Bohm-Vitense 1958.**

*ML1*-

**- Bohm and Cassinelli 1971.**

*ML2*-

**- Mihalas 1978, Kurucz 1979.**

*Mihalas*-

**- Henyey, Vardya, and Bodenheimer 1965.**

*Henyey*

*Convective Overshoot (f*_{OV,0},f_{OV})The overshooting diffusion coefficient

*D*is given by

_{OV}*D*=

_{OV}*D*exp(

_{conv,0}*-2z/f*),

_{OV}λ_{P,0}where

*D*is the convective diffusion coefficient measured a distance f

_{conv,0}_{OV,0}inside the convective zone,

*z*is the radial distance and λ

_{P,0}is the local pressure scale height. Overshooting then extends beyond the edge of the convective zone a distance f

_{OV}. Both f

_{OV,0}and f

_{OV}are assumed to be given as fractions of the local pressure scale height.

*Semiconvection (α*_{semi})Semiconvective mixing is taken to occur in regions where ∇

_{ad}< ∇

_{T}< ∇

_{L}, where ∇

_{L}= ∇

_{ad}+ B, and

*B*is the Brunt composition gradient.

*Thermohaline (α*_{th})Thermohaline mixing is taken to occur in regions where ∇

_{T}- ∇

_{ad}≤

*B*≤ 0, where again

*B*is the Brunt composition gradient.

*Thermohaline Mixing Implementation*-

**- Kippenhahn, R., Ruschenplatt, G., & Thomas, H.-C. 1980, A&A, 91, 175.**

*Kippenhahn*-

**- Traxler, Garaud, & Stellmach, ApJ Letters, 728:L29 (2011).**

*Traxler_Garaud_Stellmach_11*-

**- Brown, Garaud, & Stellmach, (2013).**

*Brown_Garaud_Stellmach_13*For more detailed information about the various mixing processes and calibration of the free parameters, see Farmer et al. 2016 and references therein.

__Mass Loss__ *RGB/AGB Wind Scheme*

Stellar wind scheme to be used during RGB/AGB.

*Wind Scheme*-

**- D. Reimers, Baschek, Kegel, Traving (eds), Springer, Berlin, 1975, p. 229.**

*Reimers*-

**- T. Blocker A&A 297, 727-738 (1995).**

*Blocker*-

**- Kudritzki et al, Astron. Astrophys. 219, 205-218 (1989).**

*Kudritzki*-

**- Nieuwenhuijzen, H.; de Jager, C. 1990, A&A, 231, 134.**

*Nieuwenhuijzen*-

**- Vink, J.S., de Koter, A., & Lamers, H.J.G.L.M., 2001, A&A, 369, 574.**

*Vink*-

**- Grafener, G. & Hamann, W.-R. 2008, A&A 482, 945.**

*Grafener*-

**- Glebbeek, E., et al, A&A 497, 255-264 (2009).**

*Dutch*** RGB/AGB Wind Scaling Factor**

Parameter for mass loss by RGB/AGB wind prescription.

Mass loss efficiency, varies with wind scheme. See Renzo et al. 2017 for a recent review.

__Rotation__

*Initial Rotation*Rotational value is set at the zero-age main sequence as a fraciton of the critical angular velocity given by

*Ω*,

^{2}_{crit}= (1-L/L_{edd})GM/R^{3}where

*L*is the luminosity,

*L*the Eddington luminosity,

_{edd}*G*the universal gravitational constant,

*M*the stellar mass, and

*R*the stellar radius.

__Resolution__

*Variance Control Target*This is the target value for relative variation in the structure from one model to the next. The default timestep adjustment is to increase or reduce the timestep depending on whether the actual variation was smaller or greater than this value.

*Mesh Delta Coefficient*A larger value increases the max allowed deltas and decreases the number of grid points. and a smaller does the opposite. E.g., you'll roughly double the number of grid points if you cut `mesh_delta_coeff` in half. Don't expect it to exacly double the number however since other parameters in addition to gradients also influence the details of the grid spacing.

*dX_nuc_drop_min_X_limit*Limits on max drop in abundance mass fraction from burning with possible mixing inflow. `dX_nuc_drop_limit` only for `X > dX_nuc_drop_min_X_limit`.

Each calculation runs for

**4 hours**, or until the stellar model reaches iron core collapse (in the case of massive stars), whichever comes first.

After a calculation has completed, the output files are packaged into a zip file. Then, a notification email is sent to the specified address, containing a link which may be used to download the zip file from the server.

We encourage those who wish to go beyond MESA-Web to download to full version of MESA here.