Part 2 of Hackathon: Rotating stars

Scientific background: should black holes spin?

In 2015, astronomy had a big break through: this was the first time that we managed to observe black holes using gravitational waves! Gravitational waves are emitted when 2 black holes circle around each other and eventually merge. We can measure the masses and spins of the inspiraling black holes from gravitational waves signals.

An overview of the observed stellar mass black holes and neutron stars. An interactive version is available here. Credit: LIGO-Virgo-KAGRA / Aaron Geller / Northwestern

It came as a surprise to many astronomers that these black holes appear to be spinning very slowly. This is because of observations of X-ray binaries: binaries consisting of a star and a black hole where the star is transferring mass to it's black hole companion. Before we had gravitational waves, we had exclusively detected stellar-mass black holes through Xray binaries. And all these Xray binaries have been observed to be spinning rapidly.

Cummulative distribution function of the spins of black holes in XRB (pink and blue), and GW-detected black holes (yellow). The black holes in GW sources are spinning much slower than the black holes in XRB (Fishbach & Kalogera 2022).

To date this is a hot research topic: Do we expect to form many spinning black holes or not? Do the black holes in XRB and gravitational-wave sources form differently? Are the observed XRB spins overestimated?

The first step to understanding this problem is to ask ourself if we expect the core of the star that came before the black hole is rotating. In part 2 of this hackathon, we will look at the effect of different assumptions on the rotation of stars and their core. We will explore three different cases that have an effect on the final rotation of the star and its core: its birth rotation, angular momentum transport inside the star, and stellar winds.

Creating a ZAMS model

We will start with creating a 15Msun Zero Age Main Sequence star at solar metallicity (Z=0.02), and safe this ZAMS model. To do this, it is useful to start from the $MESA_DIR/star/test_suite/ (just like we did in Part 1). Note how MESA has a lot of test suites available that can be very useful to start from when setting up a new model!

There are two test_suites that are of interest to us today: make_zams, and 15M_Dynamo. The latter is a bit more complicated than what we will do today, but it does contain some useful files that we will use in our own project.

Similar to day one, we will start by copying the test suite to our home directory.

Task 2.1:

Copy the make_zams directory to your home dir.
cp -r $MESA_DIR/star/test_suite/make_zams ~/rotating_star_proj
Change inlist_zams to run a 15Msun star, at a metallicity of 0.02 (for the latter, change both Zbase and initial_z).
Go into ~/home/rotating_star_proj, and ./clean ./mk and ./rn your code (it should take about a minute to create zams.mod)

Check that file called zams.mod has appeared. Congratulations, you just created a ZAMS model!

Loading our ZAMS model

We now want to load our zams model in a new inlist.

task 2.2: Creating a new inlist

Make a new file called 'inlist_rotating_star' in your in your rotating_star_proj directory (note, don't add .txt to the end of the file name). Copy over the contents of the inlist file inlist_rotating_star to your new file with the same name. Note that all that this inlist does is reset the age and module_number to zero at the start, and wants to run a $15\,M_{\odot}$ star at a metallicity of $Z= 0.02$.
Limit your model to only run for 250 models by adding max_model_number = 250 to the &controls.
To load the ZAMS model you just created, add the following to the &star_job
```
load_saved_model = .true.
load_model_filename = 'zams.mod' 
```

task 2.3: Setting up your run

We want to point the main inlist to our newly created inlist_rotating_star. In the file inlist, find and replace all "inlist_zams" to "inlist_rotating_star".
In your folder ~/rotating_star_proj there is a folder called src, with a file called 'run_star_extras.f90'. Replace the contents of this file with the contents of the file run_star_extras.f90
Note: We have made a small change in this file to save some extra output that will be useful for the next exercises. In the last section on Winds, we will get back to 'run_star_extras.f90' and learn how to add your own extra output.
The rn file determines which inlist is called first. We want to make sure that this now calls inlist. You can achieve this by commenting out 'do_one inlist_zams_header zams.mod' and adding 'do_one inlist'.
./clean ./mk and ./rn when you finished all steps!

Solution 2.3

Your rn file should look like


	#!/bin/bash

	# this provides the definition of do_one (run one part of test)
	# do_one [inlist] [output model] [LOGS directory]
	source "${MESA_DIR}/star/test_suite/test_suite_helpers"
	
	date "+DATE: %Y-%m-%d%nTIME: %H:%M:%S"
	
	#do_one inlist_zams_header zams.mod
	do_one inlist
	
	
	date "+DATE: %Y-%m-%d%nTIME: %H:%M:%S"
	
	echo 'finished'

And your inlist should look like

	
&star_job


read_extra_star_job_inlist(2) = .true.
extra_star_job_inlist_name(2) = 'inlist_rotating_star'

/ ! end of star_job namelist


&eos

read_extra_eos_inlist(1) = .true.
extra_eos_inlist_name(1) = 'inlist_rotating_star'

/ ! end of eos namelist


&kap

read_extra_kap_inlist(1) = .true.
extra_kap_inlist_name(1) = 'inlist_rotating_star'

/ ! end of kap namelist



&controls

read_extra_controls_inlist(1) = .true.
extra_controls_inlist_name(1)= 'inlist_rotating_star'

/ ! end of controls namelist


&pgstar

read_extra_pgstar_inlist(1) = .true.
extra_pgstar_inlist_name(1)= 'inlist_rotating_star'

/ ! end of pgstar namelist

Setting output and pgplot

For the next few exercises, we want to save more output than what is set by default. For this the output in the 15Msun model is a good example.

Moreover, we know it's fun to see a bunch of numbers running across your screen, but it's even better to have pictures show up while we are running our model! We will use the PGstar plot that comes from the 15Msun_dynamo model

task 2.4

copy over the history_columns.list, and profile_columns.list from $MESA_DIR/star/test_suite/15M_dynamo to your rotating_star_proj directory.

Enable pgstar by setting pgstar_flag = .true. in your inlist_rotating_star, and add the following code to the end of the same file:


&pgstar
	
	file_white_on_black_flag = .true. ! white_on_black flags -- true means white foreground color on black background
	file_device = 'png'            ! png
	file_extension = 'png'
	
	!file_device = 'vcps'          ! postscript
	!file_extension = 'ps'
	
	pgstar_interval = 50
	file_digits = 8

	Dynamo_legend_txt_scale_factor = 0.7
	Mixing_legend_txt_scale_factor = 0.55
	Profile_Panels4_txt_scale = 0.8

	Profile_Panels4_win_flag = .true.
	Profile_Panels4_win_width = 7

	Profile_Panels4_file_flag = .true.
	Profile_Panels4_file_dir = 'png'
	Profile_Panels4_file_prefix = 'profile_Panels4_'
	Profile_Panels4_file_interval = 50
	Profile_Panels4_file_width = 10
	
/ ! end of pgstar namelist

When you have successfully activated the pgstar plot, run the star again (i.e. type ./rn in your terminal) and a screen will pop up that looks like:

Example PGstar for the 15Msun dynamo model

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