# Stellar mergers as the origin of magnetic massive stars

## Abstract

About ten per cent of ‘massive’ stars (those of more than 1.5 solar masses) have strong, large-scale surface magnetic fields1,2,3. It has been suggested that merging of main-sequence and pre-main-sequence stars could produce such strong fields4,5, and the predicted fraction of merged massive stars is also about ten per cent6,7. The merger hypothesis is further supported by a lack of magnetic stars in close binaries8,9, which is as expected if mergers produce magnetic stars. Here we report three-dimensional magnetohydrodynamical simulations of the coalescence of two massive stars and follow the evolution of the merged product. Strong magnetic fields are produced in the simulations, and the merged star rejuvenates such that it appears younger and bluer than other coeval stars. This can explain the properties of the magnetic ‘blue straggler’ star τ Sco in the Upper Scorpius association that has an observationally inferred, apparent age of less than five million years, which is less than half the age of its birth association10. Such massive blue straggler stars seem likely to be progenitors of magnetars, perhaps giving rise to some of the enigmatic fast radio bursts observed11, and their supernovae may be affected by their strong magnetic fields12.

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## Data availability

The data generated, analysed and presented in this study are available from the corresponding authors on reasonable request.

## Code availability

The AREPO code is publicly available at https://arepo-code.org. The MESA code is publicly available at http://mesa.sourceforge.net.

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## Acknowledgements

This work was supported by the Oxford Hintze Centre for Astrophysical Surveys, which is funded through generous support from the Hintze Family Charitable Foundation. S.T.O., F.K.R. and F.R.N.S. acknowledge funding from the Klaus Tschira foundation.

## Author information

Authors

### Contributions

F.R.N.S. initiated the project and carried out the 1D MESA computations. S.T.O. carried out the 3D AREPO simulations. F.R.N.S. and S.T.O. wrote most of the manuscript. P.P. and F.K.R. assisted with the 1D and 3D computations, respectively. S.A.B. in particular helped to analyse and understand the magnetic-field amplification process. V.S. and R.P. wrote the AREPO code and supported S.T.O. with the 3D simulations. All authors contributed to the analysis, discussion and writing of the paper.

### Corresponding authors

Correspondence to Fabian R. N. Schneider or Sebastian T. Ohlmann.

## Ethics declarations

### Competing interests

The authors declare no competing interests.

Publisher’s note Springer Nature remains neutral with regard to jurisdictional claims in published maps and institutional affiliations.

Peer review information Nature thanks Christopher Tout and the other, anonymous, reviewer(s) for their contribution to the peer review of this work.

## Extended data figures and tables

### Extended Data Fig. 1 Ratio of viscous and cooling timescales.

The τvisc/τcool ratio is shown as a function of radius (and mass) of the merger after 6 d for different disk thicknesses h and viscosity parameters α.

### Extended Data Fig. 2 Evolution of total magnetic field energy for different simulation setups.

Model 1 is the standard run shown in the main text. Models 2 and 3 have a lower resolution. Model 3 started with a larger initial separation. The times for all models are normalized with the time of merger set to 0.

### Extended Data Fig. 3 Ratio of magnetic and radial, kinetic (that is, turbulent) energy in our 3D MHD simulations.

The magnetic energies, EB, of our models approach equipartition with the turbulent energy, for which we use the kinetic energy of motions in the radial and z directions, Ekin,r,z, as a proxy. Inset, the ratio of magnetic and kinetic energy on a linear scale, showing that our models reach values of about 5%–30%. The dashed horizontal line indicates equipartition of magnetic and kinetic energy. The small, periodic wiggles in the curves before coalescence are caused by the orbital motion of the binary.

### Extended Data Fig. 4 Comparison of the final 3D and initial 1D merger product.

a, b, The entropy (a) and the hydrogen (H1), helium (He4), carbon (C12), nitrogen (N14) and oxygen (O16) mass fractions (b) of the 3D merger remnant (thick grey lines) and the 1D stellar model (dashed lines) are compared.

### Extended Data Fig. 5 Rotational velocity and internal mass readjustment of the 1D merger model.

a, Equatorial surface rotational velocity vrot (blue solid line) and rotational velocity in terms of critical Keplerian velocity vcrit (red dashed line) as a function of time after the merger. b, Radial location of various mass coordinates in steps of 5% of the total mass (see percentage labels for the black dotted and dashed lines) and moment of inertia factor $${r}_{{\rm{g}}}^{2}$$ (blue solid line) as a function of time. The black solid line indicates the stellar surface.

## Supplementary information

### Video 1

Magnetic-field evolution in the orbital plane. Similar to Fig. 1g–i, in this video the evolution of the absolute magnetic-field strength is colour-coded while the geometry of the magnetic field is visualised using line-integral convolution. This illustrates that the magnetic field is initially amplified locally before it organises itself on larger scales.

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Schneider, F.R.N., Ohlmann, S.T., Podsiadlowski, P. et al. Stellar mergers as the origin of magnetic massive stars. Nature 574, 211–214 (2019). https://doi.org/10.1038/s41586-019-1621-5

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