Introduction to MESA. ZTF Summer Undergraduate Astronomy Institute, June 21, 2017 Bill Wolf, UCSB. This material prepared assuming MESA version 9793

Transcription

1 Introduction to MESA ZTF Summer Undergraduate Astronomy Institute, June 21, 2017 Bill Wolf, UCSB This material prepared assuming MESA version 9793

2 What is MESA? MESA: Modules for Experiments in Stellar Astrophysics Open source tool for 1-D modeling of stars Runs on a laptop, but makes use of multiple cores Modular different modules for different topics Experiments Alter the physics of stars and observe the effects

3 Use Cases Stars Supernovae Planets Binaries

4 Mesa is Open All source code is freely available All simulations should be fully reproducible Many users -> rapid development Bill Paxton, Lead Developer

5 MESA is Modular Roughly 20 fortran modules with different responsibilities Microphysics modules like kap (opacities) rates (nuclear reaction rates) Macrophysics modules like star (advance a stellar model in time) GYRE (search for oscillation modes in a stellar model)

6 What IS a stellar model? Model star as a perfect sphere (symmetry in polar and azimuthal directions) Slice star into many (hundreds to thousands) of spherical shells Each shell has a mass, temperature, density, composition, velocity, and rotation rate To be a good model, model must satisfy equations of stellar structure

7 All models are wrong, but some are useful. George Box

8 What MESA star does Start with a valid stellar model Determine a good timestep Time evolve each cell, re-meshing if necessary Check if new model is converged No: iterate using Newton-Raphson method until converged Yes: this is the starting model for the next tilmestep

9 All of Stellar Structure dr dm = 1 4 r(m) 2 (m) Mass Conservation dp dm = (m)g(m) Hydrostatic Equilibrium dl dm = nuc(m) neu (m) T (m) ds dt m Energy conservation dt dm = 3apple(m)L(m) r(m) 4 SBT (m) 3 Radiative Diffusion or Convection or Conduction Microphysics Equation of State Opacity Energy generation/loss P = P (,T,X i ) apple = apple(,t,x i ) = (,T,X i )

10 All of Stellar Evolution In thermal equilibrium energy generation is balanced by heat transport, and nothing would change. What causes stars to evolve? dx i dt Source = m i Chemical Evolution X j2sources r ji X k2sinks r ik 1 A Sink j1 + j2 i i +? k

11 MESA on Your Computer Only runs reliably on Mac and Linux machines Not a graphical application; you need to use a terminal Can make use of multiple cores by setting OMP_NUM_THREADS A simulation lives in a directory (folder), called a work directory

12 Ladies and Gentlemen, start your laptops (and your terminals) Key Information/Prompt Code you enter Expected output

13 Do You Haz MESA? Check that you have your MESA_DIR set: > echo $MESA_DIR Should print something like /Users/wmwolf/mesa-r9793 If not, try something like this: > export MESA_DIR=~/mesa-r9793 or wherever your mesa installation is. Ask a neighbor or me for help if you have problems.

14 Do You Haz the SDK? Make sure you have the MESA SDK running > gfortran -version Should print something like GNU Fortran (GCC) If not, see directions for your system at ref=mesasdk Ask your neighbor or me if you need help.

15 Create a Work Directory Copy the default work directory from MESA > cp $MESA_DIR/star/work ~/Desktop/ If you don t want it on your desktop, specify a different location, but I ll assume you used the desktop for now. Now enter the directory: > cd ~/Desktop/work Note: In general you NEVER edit any files in MESA_DIR. Instead, copy things from there and edit the copies.

16 [Selected] Structure of the > ls work LOGS inlist inlist_pgstar inlist_project mk photos re rn work directory List the files in your current directory: Key directory executable text file Holds output files Main configuration file Configuration of live plots Configuration of model Compiles directory Holds model snapshots Restarts from a snapshot Start simulation from beginning

17 Compile Your Work Directory While in your directory, execute >./mk Should print a bunch of lines of gfortran calls This compilation has created an executable, star, that is the program that will evolve a star.

18 The Inlist File I Open the inlist with your editor of choice. (I ll use nano) > less inlist It s a small file, divided in three sections: &star_job / &controls / &pgstar / Controls about loading and saving models, and some initial conditions Controls about physics to be used in the simulation as well as output options Controls for plots displayed onscreen or saved to pngs as the simulation is running

19 The Inlist File II Each section starting with an ampersand and ending with a slash denotes a namelist. Each namelist can read from another inlist (ours punts to inlist_project for star_job and controls and inlist_pgstar for pgstar). Exit your viewer (type q in less) and open up inlist_project and look at its contents > less inlist_project

20 &star_job! begin with a pre-main sequence model create_pre_main_sequence_model =.true.! save a model at the end of the run save_model_when_terminate =.false. save_model_filename = '15M_at_TAMS.mod'! display on-screen plots pgstar_flag =.true. /!end of star_job namelist

21 &controls! starting specifications initial_mass = 15! in Msun units! stop when the star nears ZAMS!(Lnuc/L > 0.99) Lnuc_div_L_zams_limit = 0.99d0 stop_near_zams =.true.! stop when the center mass fraction of! h1 drops below this limit xa_central_lower_limit_species(1) = 'h1' xa_central_lower_limit(1) = 1d-3

22 Do your first MESA run! Close your viewer again (type q in less) Run your first MESA simulation: >./rn You should see a bunch of text fly across the screen and eventually two windows will pop up.

23 Output During a Run: Text On your terminal, you ll see something like this: Column headings match data below, so the step number is 270 and the mass of the star is 15. How often the headings are shown and how often the data are shown (every step, every ten steps, etc.) can be customized in the controls namelist

24 Output During a Run: Plots Currently inlist_pgstar sets up two real-time plots These plots are invaluable to understanding what s going on in your simulation. They can also be saved as pngs for later viewing or for assembling a movie Better tools exist for publication quality plots and post-processing

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27 Stopping and Restarting You may need to interrupt a run before it finishes. Stop (or restart and stop) your simulation now via > control-c MESA can quickly restart from an earlier point. Restart from model 100 via >./re x100

28 More on Photos Photos are saved in the photos directory. Every thousandth photo s name will be its full model number e.g. 1000, 2000 Off-thousands will have an x in the thousands place to prevent saving too many photos. E.g. photo for model number 1100 will be named x100. Photo for model number 2100 will also be x100 (it will overwrite older photos)

29 Photos vs Models Photos are great for quickly tweaking and restarting They do NOT persist across MESA versions They are NOT human readable (try opening one!) Saved models are less precise but more portable. Rely on models in the long-term. Photos are more convenient in the short term.

30 Examine Your Output I The whizzing text and plots are cool, but MESA saves much more useful output. Terminate your run and enter the LOGS directory: > cd LOGS See what s in the directory: > ls There should be one file called history.data, one file called profiles.index, and many that look like profilexx.data

31 Examine Your Output II history.data gives information about the whole stellar model as a function of time. Each row in the file gives data for a single timestep. Open up the history file: > less -S history.data (The -S option tells less not to wrap lines). The first few lines tell a little about the simulation as a whole, and then the sixth line gives column headings for the real history data. Scroll left and right to see what data is available. Scrolling up and down in one column will show you how a particular property changed in time.

32 Examine Your Output III The profiles give detailed information about one particular model; a snapshot in time. Here each line represents one cell in the grid of that particular model. Pick on profile to look at > less -S profile1.data The structure is similar to the history file. Here the first few lines give some general properties of interest, but the real action starts at line 6, where we get details about the structure of the model. Only 9 columns are output by default to keep the file size relatively small.

33 Examine Your Output IV Finally the profile index provides a link between the model numbers and the profile numbers (the number that comes after the word profile in the profile file). Open up this file. > less -S profiles.index There are only three columns here. The first and third relate the model number and profile number, respectively. The middle is the priority. MESA will only store a finite number of profiles to prevent overflowing your disk (like profiles), so it preferentially overwrites lower-priority profiles when it has reached its profile cap.

34 End of First Interactive Section

35 Bending MESA to Your Will Valid controls you can use in your inlists are detailed in $MESA_DIR/star/defaults/star_job.defaults $MESA_DIR/star/defaults/controls.defaults $MESA_DIR/star/defaults/pgstar.defaults Also on the web: mesa.sourceforge.net ( defaults sections) Perusing these files is how you learn what you can do with MESA WARNING: If unchanged, default values in these files are used. THIS MAY OR MAY NOT BE REASONABLE

36 Customizing Output Can customize data appearing in the history and profile files All options in $MESA_DIR/star/defaults/history_columns.list $MESA_DIR/star/defaults/profile_columns.list To use: copy these files to your work directory and then comment/uncomment individual rows to deactivate/activate different column headings Do NOT change the originals (never touch anything in MESA_DIR)

37 Customizing Live Plots You can make arbitrary profile plots from anything that CAN be saved in in a profile file, and history plots from anything that IS beings saved in the history file Can combine history plots into history panels with multiple axes and similarly for profile plots Finally, can make an entire dashboard with all plots in one window. See excellent screencasts by Frank Timmes. For more details, see $MESA_DIR/star/defaults/pgstar.README

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39 REALLY Customizing MESA MESA allows you to add new physics In your work directory, there s a directory called src Within that, run_star_extras.f contains user-editable functions and routines that allow you to do things like Add custom history and profile columns Make custom stopping conditions Add new heating sources, mass loss schemes, etc. This requires coding in fortran beyond the scope of a boot camp

40 BREAK

41 Lab: Convective Overshoot on the Main Sequence Adapted from Lars Bildsten's material from the 2016 MESA Summer School

42 Overshooting

43 From Space

44 Overshoot and Core Convection Stars more massive than the sun generate energy via the CNO cycle. CNO cycle is very temperature sensitive, radiative diffusion is insufficient to transport heat, so the core is convective. Convective core is fully mixed, so star runs out of core hydrogen suddenly. Our question: how big is the He core at the end of H-burning?

45 Who Cares? This matters! Without overshoot, main sequence (MS) lifetimes from models don t match observational evidence The final mass of the He core matters for understanding later phases of evolution Also just a fun physics problem

46 Convection Background Fluid is convectively unstable if a blob that is pushed upwards becomes buoyant. For an ideal b,2,2 gas, this condition is d ln T d ln P > 2 5 b,1,1 In the core, this is satisfied, but at some radius, the temperature gradient becomes more shallow and radiation takes over. b,1,1 convective if: b,2 <,2

47 Why Would the Blob Stop? Some form of overshoot should occur, but how precisely it happens and how it should be modeled in 1D is less certain.

48 One Approach: Exponential Overshoot Take the diffusion coefficient near the edge of a convective zone and let it decay exponentially on some length scale. It s a two-parameter (f and f 0 ) model. Mathematically: r r 0 D(r) =D(r 0 )e fh P r 0 is the location a distance f 0 * H before the end of the convection zone H is the pressure scale height: H dr d ln P

49 Lab(s) Goal: Understand how overshooting affects main sequence evolution (time spent doing core hydrogen burning) The lab comes in two parts: Generate a Zero-age main sequence (ZAMS) model of a star and measure its properties for comparison Evolve the star to core H exhaustion with and without overshoot and compare the differences

50 Your Tasks: Download: mesastar.org -> Education -> 2016 Lars Bildsten Move 2016_bildsten.zip to wherever you like and unzip: > unzip 2016_bildsten.zip In new 2016_bildsten directory, unzip lars_minilabs.tgz > tar -xvzf lars_minilabs.tgz cd lars_minilabs and follow directions for the first two minilabs (mini1_instructions.pdf and mini2_instructions.pdf) Post results on