New stellar evolutionary models of massive stars with rotation: Expanding the BoOST project

Hanno Nicolas Stinshoff 1 , Dorottya Szécsi 1 , Richard Wünsch 2

  • 1 Nicolaus Copernicus University, Toruń, Poland, Toruń
  • 2 Astronomical Institute of the Czech Academy of Sciences

Abstract

Rotation enhances mixing in massive stars. This facilitates longer lifetimes and different surface abundances, and in extreme cases may lead to (complete or partial) chemically homogeneous evolution. As shown by (e.g.) Szécsi et al. (2015) (by means of models with metallicity of the metal-poor dwarf galaxy I Zwicky 18) this evolutionary behavior is more likely for higher masses and velocities.
Furthermore, Szécsi et al. (2022) published a grid of single star models in the so-called BoOST format (short for Bonn Optimized Stellar Tracks), providing tabulated stellar models for masses in the range of 9 − 500 M_{sol} and with metallicities between Solar and 0.02 times that of the Small Magellanic Cloud. However, in the current version of the project, mostly models with slow spins are published (100 km/s initial equatorial rotational velocity). This means that the current version needs to be expanded in order to cover rotational velocities facilitating chemically homogeneous evolution all over the mentioned metallicity range.
I have created new models of massive stars with the same set-up as in the BoOST project, but adding the much-needed coverage in rotational velocity with 0, 100, 200, 300, 400 and 500 km/s. I am using the "Bonn" stellar evolution code, following the models from birth till the end of core-helium burning, and convert them into the BoOST format.
The models show that the occurrence of chemically homogeneous evolution depends on all three input parameters (mass, rotation, metallicity), with high rotation, high mass and low metallicity increasing the probability of chemically homogeneous evolution.
The models are optimized to be applicable for further studies on globular cluster abundance anomalies (following up on Szécsi and Wünsch (2019)). Additionally, research on gravitational wave events (massive binary events with subsequent mergers) and studies on gamma ray bursts in the collapsar scenario will be possible using these optimized models.

Rotation enhances mixing

In contrast to the ‘normal’ evolution of stars (with core-envelope structure) in our vicinity, models show that a different kind of evolution is possible:
Rotation enhances the mixing in massive stars. This facilitates longer lifetimes and different surface abundances, and in extreme cases may lead to (complete or partial) so-called chemically homogeneous evolution (Yoon & Langer 2005). The chemical composition is no longer layered but instead homogeneous over the whole star.

Fig. 1: Models from Szécsi et al. (2015), Z=0.02 Z☉. Each dot represents one model sequence at the end of the main sequence, with a specific set of initial velocity and mass. The corresponding Helium Mass Fraction at the surface is displayed with color, with red color indicating low (around 0.25) and blue color a high (up to around 1.0) helium mass fraction. One can see that there are three areas differentiated, relating to a 'normal' evolution (red), a chemically homogeneous one (blue) and some transitional evolution with a mixture (yellow).

Szécsi et al. (2015) showed (by means of models with metallicity of the metal-poor dwarf galaxy I Zwicky 18) that this evolutionary behavior is more likely for higher masses and velocities (cf. Fig. 1). The resulting models have a higher (up to around 100%) Surface Helium Mass Fraction (blue in contrast to red color) and a higher temperature (among other attributes) at the end of the main sequence.

BoOST and my models

Moreover, Szécsi et al. (2022) published a grid of single star models in the so-called BoOST format (short for Bonn Optimized Stellar Tracks), providing tabulated stellar models for masses in the range of 9 − 500 M☉ and metallicities between Solar and 0.02 times that of the Small Magellanic Cloud. However, in the current version of the project, mostly models with slow spins are published (100 km/s initial equatorial rotational velocity).

The models I created cover Minit= 10, 20, 40, 80, 150, 300 and 500 M☉, vrot,init= 0, 100, 200, 300, 400 and 500 km/s and Zinit of the Milky way (Z☉), the Large Magellanic Cloud (ZLMC) and multiple fractions of that of the Small Magellanic Cloud (1, 0.5, 0.2, 0.1, 0.05 and 0.02 · ZSMC), summing up to 336 model sequences.

Fig. 2: My own models, displaying the Surface Helium Mass Fraction (YS) at the current end of the evolution. Each dot is one model with a specific set of initial mass, metallicity and rotational velocity. High YS values can indicate a chemically homogeneous evolution, while low ones are usually a sign for 'normal' evolution.


This graph is also displayed as animation on my website (cf. the QR-Code) to provide multiple perspectives.

The bifurcation of the evolutionary paths can be displayed by plotting the Surface Helium Mass Fraction of the models at the end of the main sequence (dots) or the end of their current lifetime if the main sequence was not yet completed (crossed dots) of each model (in a color diagram showing YS, cf. Fig. 2). 

The evolutionary pathway heavily impacts the helium levels; chemically homogeneous evolution leads to high surface helium abundances (up to 0.95, blue in the figure), whereas a normal evolution leaves the surface helium abundance more or less untouched at initial values (around 0.25, red in the figure). When plotting it against the initial parameters, one therefore can see the tendencies for such behaviors depending on the initial parameters.

 

HRD

Fig. 3: Selection of Hertsprung-Russell diagrams for masses 20-80 M☉ and metallicities 0.2-0.05 ZSMC (cf. caption of each diagram) of my models. The evolution starts in the bottom middle part of each diagram, moving upwards. The tracks evolving to the left (=hot) side indicates a chemically homogeneous evolution, those taking a turn indicate transitional evolution. A rightward movement is indicator for 'normal' evolution.

It can also be displayed with the Hertzsprung-Russell diagram (cf. the selection in Fig. 3); the homogeneously evolving models move leftwards (towards higher temperatures) in contrast to the normally evolving ones becoming cooler over their evolution.
One can even see transitional evolution (cf. for example the orange line of the 40 M☉, 0.1 · ZSMC model).

Future plans

The models can be used for multiple purposes in the future. One of them is the research on globular cluster abundance anomalies:

 

Fig. 4: Image by R. Wünsch (ASU).
Multiple generations of stars possibly result in the abundance anomalies in globular clusters. Massive stars can play an important role in that process, so having proper coverage for the parameter space (including low metallicities) is necessary.

A possible explanation for the globular cluster abundance anomalies is that of multiple generations of stars in that cluster (Szécsi & Wünsch 2019); a first generation of massive stars can synthesize the observed light element ratios during the CNO-Cycle. Due to stellar winds they disperse their material, from which the second generation of (low-mass) stars form. The resulting stars are not massive enough to create those abundance patterns themselves, but they can inherit them from their predecessors.

Contact

  • Homepage: https://astro.umk.pl/~hanno/Hanno/
  • Mail: hstinshoff@doktorant.umk.pl
  • Affiliation: Faculty of Physics, Astronomy and Informatics - Nicolaus Copernicus University, Grudzia̧dzka 5, 87-100 Toruń, Poland
  • Hanno Stinshoff is funded in part by the National Science Center (NCN), Poland under grant No. OPUS 2021/41/B/ST9/00757
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