At the SIMULATING STARS SUMMER SCHOOL, each day will focus on a specific research subject and will be led by one of our lecturers with the help of the teaching assistants. Every day will be divided into theory lectures and  hands on labs with MESA. In this page you can find a description of each topic, and some reading material. You can take a look at the references either before or after the school to dig deeper into the topics, but we won't expect the student to have read them for the school. We do expect everyone to have installed the requred software (so if you haven't already, check  Getting started)

The topics will be:

• Introduction to stellar evolution with MESA
• Globular clusters and white dwarfs: from simulations to data
• Orbital evolution and stability of mass transfer in binary stars
• White dwarf novae
• Massive Stars with MESA: Above and beyond

On the first day, we will first make sure that everyone has access to a laptop or a desktop with the right version of MESA and with ANACONDA installed (see the page Getting Started). Then, we will go over the basics of stellar evolution and of MESA.

We will take a look at the test_suite 1M_pre_ms_to_wd, which simulates the evolution of a solar mass star from the pre-main-sequence phase all the way to its final stages as a white dwarf. We will take inspiration from it to look at the different phases of stellar evolution and to understand how MESA simulates a star.  We will also learn how to use PGPLOT, which is a very useful tool that allows you to monitor how your run is going in real time.

Finally, we will get a first taste on how to customize MESA with run_star_extra.

REFERENCES

• In order to refresh some stellar evolution, just choose the textbook you prefer.
• If you are new to MESA, a good start would be to take a look a the MESA instrumentation papers:
  • Paxton et al (2011), APJS 192, 1, 4
  • Paxton et al (2013), APJS 208, 1, 3
  • Paxton et al (2015), APJS 220, 1, 15
  • Paxton et al (2018), APJS 234, 2, 34
• You can also take a look at Ed Brown's book on Stellar Astrophysics with MESA exercises: https://github.com/Open-Astrophysics-Bookshelf/stellar-physics-notes

The study of stellar populations in globular clusters provide the most detailed tests of our understanding of stellar evolution. All of the stars in a globular cluster started their lives at approximately the same time, so observations of clusters today provide a snapshot of how a group of stars of different masses look at a fixed age.  If we can also assume that all of the stars have the same initial composition, the only difference among the stars is the single parameter of mass. From a theoretical point of view we call a coeval group of stars with the same composition an isochrone, and with an understanding of stellar atmospheres we can compare our theoretical isochrones with the observed clusters.

Globular clusters are sufficiently old that many stars have already exhausted their fuel and have become white dwarfs.  We can use white dwarfs to measure the age of the cluster independently of the stars that are still fusing hydrogen and also constrain the physics of neutrinos, axions and dense matter.

In this session we will learn how to construct isochrones with MESA and how to approximate isochrones with MESA history files. We will calculate the evolution of white dwarfs with standard physics and extensions to standard model.  We will also use stellar atmospheres to put our theoretical results into the observational plane.

REFERENCES

Here is some reading material about globular clusters in general and about white dwarfs in globular clusters:

• Bastian & Lardo (2018),  ARAA,  56
• Bailyn (1955), ARAA, 33:133-162
• Renzini & Pecci (1988), ARAA, 26:199-244
• Obertas et al (2018),  MNRAS, 474, 677
• Goldsbury et al (2016), ApJ, 821, 27

In recent times, binary evolution has been recognized as a fundamental aspect in the lives of massive stars, with observations indicating that more than half of them undergo strong interactions with a companion. Various observations of X-ray binaries, transient events, and the historic detection of gravitational waves from merging binary black holes and neutron stars puts significant pressure on our understanding of binaries. Two important uncertainties that remain concern the stability of mass transfer and the outcome of common envelope events.

In this session, we will investigate the interplay between orbital evolution and mass transfer, studying the different timescales involved in mass transferring systems, their stability, and the outcomes of common envelope evolution.

To illustrate these concepts, we will learn how to use MESA to model systems undergoing stable mass transfer, consisting of either two non-degenerate stars, or including a compact object modeled as a point mass. We will then explore how to extend MESA to account for unstable mass transfer leading to common envelope events by implementing a simple prescription commonly used in population synthesis calculations.

REFERENCES

• A detailed study on the conditions for stability in mass transferring systems is: Soberman et al. (1997), A&A, 327, 620-635
• The formation model for BH X-ray binaries through a common envelope phase is discussed in the population synthesis study of Podsiadlowski et al. (2003), MNRAS, 341, 385-404
• A recent review on the formation and evolution of X-ray binaries: Tauris & van den Heuvel (2006), In: Compact stellar X-ray sources. Edited by Walter Lewin & Michiel van der Klis. Cambridge Astrophysics Series, No. 39
• And for a recent review on common envelope evolution, see Ivanova et al. (2013), A&ARv, 21, 59

We will use MESA to explore nuclear burning in the surface layers of accreting white dwarfs and build models of novae. White dwarfs in close binaries accumulate hydrogen and helium on their surface for thousands of years, in some cases before it rapidly burns, giving a bright transient that we see as a nova. Novae eject significant amounts of mass back into the interstellar medium and so play an important role in nucleosynthesis. The mass ejection may also prevent the white dwarf mass becoming large enough to trigger carbon burning and a Type Ia supernova. At rapid accretion rates, the burning can be stable, so that the incoming light elements are processed smoothly into a thickening layer of helium that itself can later ignite.

In the lectures and labs, we will cover an introduction to novae, and the physics of unstable and stable nuclear burning, including the important nuclear reactions. We will discuss open issues such as the mechanism of mass loss, the fate of the helium produced by hydrogen burning, and enrichment of the accreting light element layers with carbon from the underlying white dwarf. The models will take advantage of MESA’s ability to implement accretion, mass loss, and different nuclear reaction networks.

REFERENCES

• For a recent review of classical novae, see Starrfield, Iliadis, & Hix (2016), PASP 128, 051001
• Studies of classical novae with MESA are reported in
  • Wolf et al. (2013), ApJ 777, 136
  • Denissenkov et al. (2013), ApJ 762, 8
• For a sense of the range of systems that show novae: Townsley & Bildsten (2005), ApJ 628, 400
• Winds during novae: Kato & Hachisu (1994), ApJ 437, 802

We will learn how to use and extend the Modules for Experiments for Stellar Astrophysics (MESA) code to simulate the evolution of massive stars, 15 to 40 times the mass of our sun.

We will investigate the effects of spatial and temporal resolution as well as nuclear reaction network size on final model parameters (such helium and carbon/oxygen core mass and core compactness) and use these results to make predictions about the ultimate fates of massive stars (which models will explode successfully and which might not).

We will also investigate the effects of rotationally-induced mixing and magnetic fields on the evolution of massive stars by using the provided MESA prescriptions but we will ultimately add our own subroutine to study the effects of the Magneto-Rotational Instability (MRI) on stellar chemical evolution.

The main objective of these lectures and lab is to understand the chaotic - and highly parametric and approximate nature - of theoretical stellar evolution and discuss implications for observations of massive stars and supernovae.

REFERENCES

These are a few references we will be using in class:

• Sukhbold et al. (2016), ApJ, 821, 38
• Ugliano et al. (2012), ApJ, 757, 69
• Heger et al. (2000), ApJ, 528, 368
• Wheeler et al. (2015), ApJ, 799, 85