MINILAB 1 - Superadiabatic Reduction

Our first MESA task involves the evolution of a massive star model until the end of core Helium burning. For this, go to the google spreadsheet (and navigate to the tab Day2_Massive_lab1) and select an initial mass and a parameter set for superad_reduction (either TRUE, FALSE, or MODIFIED) by writing your name in the appropriate column.

NOTE 1: For the collaborative/group work part of this lab, you will be entering the results of one of your two runs into the spreadsheet. Check whether you will be recording a run with superad_reduction TRUE, FALSE or MODIFIED. If you have chosen a parameter set with FALSE, then you will be using the values from the end of this Section's run. If it is TRUE, or MODIFIED, then you will be using the values from the end of your run in Section 1.4.

VERY IMPORTANT NOTE 2: The higher the value of M, the longer the run will take. If you are concerned about the time it will take for your computer to run higher mass models, choose a lower initial mass. Conversely, if you know your computer can run MESA faster, please select a higher initial mass. Finally, if your computer is extremely fast or if you finish early, feel free to to run multiple parameters if any are unclaimed after your first run is complete.

Expected speed roughly corresponds to the number of threads available as follows:

• < 4 threads: slow

• 4-8 threads: average

• 8-16 threads: fast

• > 16 threads: not that much faster than 16.

And not all CPU's are equally fast. 

NOTE 3: Throughout these tasks we will need to edit the following;

• inlist

• inlist_mass_Z_wind_rotation

• inlist_to_end_core_he_burn

As was the case during the monday labs, inlist is the main inlist file which is often used to point to other inlists we may wish to use - this allows users to systematically change a base model setup. For example, we can keep the initial mass value in a separate inlist, which in our case is inlist_mass_Z_wind_rotation, and change only that file for any other masses we wish to test. The final inlist we wish to change for this lab, inlist_to_core_he_burn, will contain our stopping condition and any relevant physics for this stage of evolution. Meanwhile, inlist_common, which we will not change, contains all of the core physics we wish to implement in our model. You may wish to take a look through this inlist if you have the time.

NOTE 4: Throughout this minilab, you will be presented with three ways of going through the lab, which are analagous to difficulty. 1) "Do it myself" - only the question is given with no hints, except a link to the MESA documentation. 2) "Hints" - Several hints are given in order to direct you in to the right path. 3) "Basic solution" - The code block is given, however there may still be some things which need to be edited. If you are really stuck, then there are full solutions available in Section 1.6. 

NOTE 5: Each part where MESA code will be added to our inlists will have a question(s) associated with it. These should help you if you are stuck to figure out where this code should go, and what values are needed to put into the inlists. Each question has a Hint and an Answer which can be revealed by clicking the drop down menu option.

Modeling up through the core Helium burning phase can take a while to model appropriately, so we provide a basis to start from - XX_M_TAMS.mod, where XX is the mass of the model you have chosen - in a subfolder within your working directory; TAMS_models/XX_M_TAMS.mod. 

From the options available, choose the mod file which corresponds to your chosen initial mass. This model has finished core Hydrogen burning (the Terminal Age Main Sequence, or TAMS), and we will now continue its evolution by modifying inlist_to_end_core_he_burn. Then, load it in to the appropriate inlist (more hints below). 

If you would like to attempt this yourself, stop reading here and go for it!

If you would like some hints to help direct you, check out the questions below. If you would like the code required for this section, look at the grey minted code block - both of which are in the drop down menus.

Hinting Questions:

1. Which inlist do we want to edit?

2. Which namelist section does this go into?

3. What is the name of the file we wish to load?

4. Where would we look on the MESA documentation  website for this?

Our model starts at the end of the main sequence (central hydrogen mass fraction < 0.001) and we want to evolve the star through core Helium burning. MESA includes a suite of stopping conditions for us to use, including maximum model number, mass fraction in the core/envelope, core/surface temperature, and many more.

Hinting Questions

1. Which namelist section does this go into?

2. How do we write '0.001' in double precision in a MESA inlist?

Now when we run the model later in the lab, it will eventually stop once the fraction of central Helium-4 drops below the value set in your inlist.

A word of warning: This condition does not differentiate between phases of stellar evolution, only fractional abundances. So, it is possible to have a model stop with this condition during, say, the early main sequence. If you would like to test this yourself, try running your model at TAMS to center c12 < 0.1.

Hinting Questions

1. Which namelist section does this go into?

2. Is it possible to add in multiple overshooting settings? If so, how can this be done?

Here, it would be a good idea to re-run the model quickly for a few timesteps just to check if you can see a white region on the Kippenhahn diagram in pgstar, corresponding to overshooting (white) surrounding the convective regions (light blue), as in Fig. 1. Then terminate the run (CTRL+C). Again, this should not take many timesteps, only 10 or 20.

A Kippenhahn diagram is a particularly useful and ubiquitous diagram in stellar evolution which simultaneously provides a history and profile of the stellar model. This is effectively a space-time diagram for stars. In Figure 1, we can trace the history on the X-axis (left is younger), in this case tracked by model_number (a time step in MESA), and see the time-evolution of stellar mass, core mass, and convective regions. Additionally, we can trace the profile of the model at each time through the Y-axis, in this case a mass coordinate, where we can examine the location of convective regions, nuclear burning regions, et cetera.

Now that we have the desired physics present in our model, we will run our star from TAMS until the end of core Helium burning.

While the model is running, you will start seeing two sets of output. One is the terminal output (Fig. 2), which provides a text-based summary of stellar properties, and the other is graphical through pgplot (Fig. 3) which we have enabled in inlist_pgstar. Note that you find the Kippenhahn diagram and H-R Diagram in different windows within pgplot.

If you have time at the end, you may wish to edit inlist_pgstar to change which plots you see, remove some, or add others. For the full list of pgplot options, see the documentation here.

If you put your name next to a row in the Google spreadsheet that corresponds to Superad_reduction = FALSE then don’t forget to add your final values of the effective temperature and radius to the spreadsheet (see 1.3.6 for instructions).

1. What is the Effective (surface) Temperature of the model at the final step?

2. What is the radius of the model at the final step?

3. Bonus: Pgstar exits once the model is completed. What could we put into our inlists to preserve the Pgstar output on our screen? Alternatively, what could we put into our inlists to save the output of Pgstar as a .png?

Here, α1, α2, Γc, δc are free parameters which control the energy enhancement, while g and h are functions which also control the amount of energy enhancement. β is the gas pressure ratio β = Pgas/(Prad + Pgas), and Γinv = 4(1 − β)/(4 − 3β) is the Eddington factor required for a density inversion.

Now that we have a model from the previous section that ran without superad_reduction, we want to test what happens if we put superad_reduction on with default values. 

If you have chosen to do a model with the TRUE option for superad_reduction in the spreadsheet, then keep superad_reduction_Gamma_limit_scale = 5.0d0. 

If you have chosen to do a model with the MODIFIED option for superad_reduction in the spreadsheet, then change superad_reduction_Gamma_limit_scale = 2.5d0. 

If, however, you have chosen to do a model with the FALSE option for superad_reduction in the spreadsheet, then you have a choice between either superad_reduction_Gamma_limit_scale = 2.5d0 or 5.0d0. 

The inlist options we will want to use are given under Partial Solutions, but you may wish to find them yourself in the documentation (linked here).

Pensive Questions: These are open-ended questions for thought provocation, as opposed to assistance in building the MESA inlist.

1. What are you expecting to see that is different between this run and the previous?

2. What effects could superad_reduction have on the post-Helium burning evolution of this model?

When you're done, enter the required values of Temperature and Radius into the spreadsheet if you are running a model with superad_reduction as TRUE or MODIFIED. 

1. What is the Effective (surface) Temperature of the model at the final step?

2. What is the radius of the model at the final step?

3. How do these quantities compare to the same mass model without superad_reduction?

Congratulations! Once you are comfortable with what has been presented here and with what you have done with your models, feel free to try the Bonus Tasks.