Stellar mergers as the origin of magnetic massive stars


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.

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.


All prices are NET prices.

Fig. 1: Dynamical evolution of the merger of two main-sequence stars.
Fig. 2: Long-term evolution of the merger product in the Hertzsprung–Russell diagram.

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 The MESA code is publicly available at


  1. 1.

    Donati, J.-F. & Landstreet, J. D. Magnetic fields of nondegenerate stars. Annu. Rev. Astron. Astrophys. 47, 333–370 (2009).

    ADS  CAS  Google Scholar 

  2. 2.

    Fossati, L. et al. B fields in OB stars (BOB): low-resolution FORS2 spectropolarimetry of the first sample of 50 massive stars. Astron. Astrophys. 582, A45 (2015).

    Google Scholar 

  3. 3.

    Grunhut, J. H. et al. The MiMeS survey of magnetism in massive stars: magnetic analysis of the O-type stars. Mon. Not. R. Astron. Soc. 465, 2432–2470 (2017).

    ADS  CAS  Google Scholar 

  4. 4.

    Ferrario, L., Pringle, J. E., Tout, C. A. & Wickramasinghe, D. T. The origin of magnetism on the upper main sequence. Mon. Not. R. Astron. Soc. 400, L71–L74 (2009).

    ADS  Google Scholar 

  5. 5.

    Wickramasinghe, D. T., Tout, C. A. & Ferrario, L. The most magnetic stars. Mon. Not. R. Astron. Soc. 437, 675–681 (2014).

    ADS  Google Scholar 

  6. 6.

    Podsiadlowski, P., Joss, P. C. & Hsu, J. J. L. Presupernova evolution in massive interacting binaries. Astrophys. J. 391, 246–264 (1992).

    ADS  Google Scholar 

  7. 7.

    de Mink, S. E., Sana, H., Langer, N., Izzard, R. G. & Schneider, F. R. N. The incidence of stellar mergers and mass gainers among massive stars. Astrophys. J. 782, 7 (2014).

    ADS  Google Scholar 

  8. 8.

    Carrier, F., North, P., Udry, S. & Babel, J. Multiplicity among chemically peculiar stars. II. Cool magnetic Ap stars. Astron. Astrophys. 394, 151–169 (2002).

    ADS  Google Scholar 

  9. 9.

    Alecian, E. et al. The BinaMIcS project: understanding the origin of magnetic fields in massive stars through close binary systems. In IAU Symposium Vol. 307, 330–335 (IAU, 2015).

  10. 10.

    Schneider, F. R. N., Podsiadlowski, P., Langer, N., Castro, N. & Fossati, L. Rejuvenation of stellar mergers and the origin of magnetic fields in massive stars. Mon. Not. R. Astron. Soc. 457, 2355–2365 (2016).

    ADS  CAS  Google Scholar 

  11. 11.

    Metzger, B. D., Berger, E. & Margalit, B. Millisecond magnetar birth connects FRB 121102 to superluminous supernovae and long-duration gamma-ray bursts. Astrophys. J. 841, 14 (2017).

    ADS  Google Scholar 

  12. 12.

    Obergaulinger, M., Janka, H. T. & Aloy, M. A. Magnetic field amplification and magnetically supported explosions of collapsing, non-rotating stellar cores. Mon. Not. R. Astron. Soc. 445, 3169–3199 (2014).

    ADS  CAS  Google Scholar 

  13. 13.

    Springel, V. E pur si muove: Galilean-invariant cosmological hydrodynamical simulations on a moving mesh. Mon. Not. R. Astron. Soc. 401, 791–851 (2010).

    ADS  Google Scholar 

  14. 14.

    Balbus, S. A. & Hawley, J. F. A powerful local shear instability in weakly magnetized disks. I—Linear analysis. II—Nonlinear evolution. Astrophys. J. 376, 214–233 (1991).

    ADS  Google Scholar 

  15. 15.

    Rembiasz, T., Obergaulinger, M., Cerdá-Durán, P., Müller, E. & Aloy, M. A. Termination of the magnetorotational instability via parasitic instabilities in core-collapse supernovae. Mon. Not. R. Astron. Soc. 456, 3782–3802 (2016).

    ADS  CAS  Google Scholar 

  16. 16.

    Braithwaite, J. & Nordlund, Å. Stable magnetic fields in stellar interiors. Astron. Astrophys. 450, 1077–1095 (2006).

    ADS  MATH  Google Scholar 

  17. 17.

    Paxton, B. et al. Modules for Experiments in Stellar Astrophysics (MESA). Astrophys. J. Suppl. Ser. 192, 3 (2011).

    ADS  Google Scholar 

  18. 18.

    Zhu, C., Pakmor, R., van Kerkwijk, M. H. & Chang, P. Magnetized moving mesh merger of a carbon-oxygen white dwarf binary. Astrophys. J. 806, L1 (2015).

    ADS  Google Scholar 

  19. 19.

    Price, D. J. & Rosswog, S. Producing ultrastrong magnetic fields in neutron star mergers. Science 312, 719–722 (2006).

    ADS  CAS  PubMed  PubMed Central  Google Scholar 

  20. 20.

    Ohlmann, S. T., Röpke, F. K., Pakmor, R., Springel, V. & Müller, E. Magnetic field amplification during the common envelope phase. Mon. Not. R. Astron. Soc. 462, L121–L125 (2016).

    ADS  Google Scholar 

  21. 21.

    Olausen, S. A. & Kaspi, V. M. The McGill Magnetar Catalog. Astrophys. J. Suppl. Ser. 212, 6 (2014).

    ADS  Google Scholar 

  22. 22.

    Keane, E. F. & Kramer, M. On the birthrates of Galactic neutron stars. Mon. Not. R. Astron. Soc. 391, 2009–2016 (2008).

    ADS  CAS  Google Scholar 

  23. 23.

    Diehl, R. et al. Radioactive 26Al from massive stars in the Galaxy. Nature 439, 45–47 (2006).

    ADS  CAS  Google Scholar 

  24. 24.

    Moriya, T. J., Sorokina, E. I. & Chevalier, R. A. Superluminous supernovae. Space Sci. Rev. 214, 59 (2018).

    ADS  Google Scholar 

  25. 25.

    Bucciantini, N. et al. Magnetized relativistic jets and long-duration GRBs from magnetar spin-down during core-collapse supernovae. Mon. Not. R. Astron. Soc. 396, 2038–2050 (2009).

    ADS  CAS  Google Scholar 

  26. 26.

    Kasen, D. & Bildsten, L. Supernova light curves powered by young magnetars. Astrophys. J. 717, 245–249 (2010).

    ADS  Google Scholar 

  27. 27.

    Tout, C. A., Wickramasinghe, D. T., Lau, H. H.-B., Pringle, J. E. & Ferrario, L. A common envelope binary star origin of long gamma-ray bursts. Mon. Not. R. Astron. Soc. 410, 2458–2462 (2011).

    ADS  Google Scholar 

  28. 28.

    Mokiem, M. R. et al. Spectral analysis of early-type stars using a genetic algorithm based fitting method. Astron. Astrophys. 441, 711–733 (2005).

    ADS  CAS  Google Scholar 

  29. 29.

    Simón-Díaz, S., Herrero, A., Esteban, C. & Najarro, F. Detailed spectroscopic analysis of the Trapezium cluster stars inside the Orion nebula. Rotational velocities, stellar parameters, and oxygen abundances. Astron. Astrophys. 448, 351–366 (2006).

    ADS  Google Scholar 

  30. 30.

    Nieva, M.-F. & Przybilla, N. Fundamental properties of nearby single early B-type stars. Astron. Astrophys. 566, A7 (2014).

    Google Scholar 

  31. 31.

    Pakmor, R., Bauer, A. & Springel, V. Magnetohydrodynamics on an unstructured moving grid. Mon. Not. R. Astron. Soc. 418, 1392–1401 (2011).

    ADS  Google Scholar 

  32. 32.

    Pakmor, R. & Springel, V. Simulations of magnetic fields in isolated disc galaxies. Mon. Not. R. Astron. Soc. 432, 176–193 (2013).

    ADS  Google Scholar 

  33. 33.

    Powell, K. G., Roe, P. L., Linde, T. J., Gombosi, T. I. & De Zeeuw, D. L. A solution-adaptive upwind scheme for ideal magnetohydrodynamics. J. Comput. Phys. 154, 284–309 (1999).

    ADS  MathSciNet  CAS  MATH  Google Scholar 

  34. 34.

    Asplund, M., Grevesse, N., Sauval, A. J. & Scott, P. The chemical composition of the Sun. Annu. Rev. Astron. Astrophys. 47, 481–522 (2009).

    ADS  CAS  Google Scholar 

  35. 35.

    Paxton, B. et al. Modules for Experiments in Stellar Astrophysics (MESA): planets, oscillations, rotation, and massive stars. Astrophys. J. Suppl. Ser. 208, 4 (2013).

    ADS  Google Scholar 

  36. 36.

    Paxton, B. et al. Modules for Experiments in Stellar Astrophysics (MESA): binaries, pulsations, and explosions. Astrophys. J. Suppl. Ser. 220, 15 (2015).

    ADS  Google Scholar 

  37. 37.

    Paxton, B. et al. Modules for Experiments in Stellar Astrophysics (MESA): convective boundaries, element diffusion, and massive star explosions. Astrophys. J. Suppl. Ser. 234, 34 (2018).

    ADS  Google Scholar 

  38. 38.

    Ohlmann, S. T., Röpke, F. K., Pakmor, R. & Springel, V. Constructing stable 3D hydrodynamical models of giant stars. Astron. Astrophys. 599, A5 (2017).

    ADS  Google Scholar 

  39. 39.

    Górski, K. M. et al. HEALPix: a framework for high-resolution discretization and fast analysis of data distributed on the sphere. Astrophys. J. 622, 759–771 (2005).

    ADS  Google Scholar 

  40. 40.

    Spitzer, L. Physics of Fully Ionized Gases 2nd edn (Interscience, 1962).

  41. 41.

    Donati, J.-F. et al. The surprising magnetic topology of τ Sco: fossil remnant or dynamo output? Mon. Not. R. Astron. Soc. 370, 629–644 (2006).

    ADS  CAS  Google Scholar 

  42. 42.

    Ud-Doula, A., Owocki, S. P. & Townsend, R. H. D. Dynamical simulations of magnetically channelled line-driven stellar winds—III. Angular momentum loss and rotational spin-down. Mon. Not. R. Astron. Soc. 392, 1022–1033 (2009).

    ADS  CAS  Google Scholar 

  43. 43.

    Lamers, H. J. G. L. M., Snow, T. P. & Lindholm, D. M. Terminal velocities and the bistability of stellar winds. Astrophys. J. 455, 269 (1995).

    ADS  CAS  Google Scholar 

  44. 44.

    Shen, K. J., Bildsten, L., Kasen, D. & Quataert, E. The long-term evolution of double white dwarf mergers. Astrophys. J. 748, 35 (2012).

    ADS  Google Scholar 

  45. 45.

    Schwab, J., Shen, K. J., Quataert, E., Dan, M. & Rosswog, S. The viscous evolution of white dwarf merger remnants. Mon. Not. R. Astron. Soc. 427, 190–203 (2012).

    ADS  CAS  Google Scholar 

  46. 46.

    Ji, S. et al. The post-merger magnetized evolution of white dwarf binaries: the double-degenerate channel of sub-Chandrasekhar type Ia supernovae and the formation of magnetized white dwarfs. Astrophys. J. 773, 136 (2013).

    ADS  Google Scholar 

Download references


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




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.

Additional information

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.

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

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).

Download citation

Further reading


By submitting a comment you agree to abide by our Terms and Community Guidelines. If you find something abusive or that does not comply with our terms or guidelines please flag it as inappropriate.


Quick links

Sign up for the Nature Briefing newsletter for a daily update on COVID-19 science.
Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing