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Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars

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Abstract

A knowledge of stellar ages is crucial for our understanding of many astrophysical phenomena, and yet ages can be difficult to determine. As they become older, stars lose mass and angular momentum, resulting in an observed slowdown in surface rotation1. The technique of ‘gyrochronology’ uses the rotation period of a star to calculate its age2,3. However, stars of known age must be used for calibration, and, until recently, the approach was untested for old stars (older than 1 gigayear, Gyr). Rotation periods are now known for stars in an open cluster of intermediate age4 (NGC 6819; 2.5 Gyr old), and for old field stars whose ages have been determined with asteroseismology5,6. The data for the cluster agree with previous period–age relations4, but these relations fail to describe the asteroseismic sample7. Here we report stellar evolutionary modelling5,6,8,9,10, and confirm the presence of unexpectedly rapid rotation in stars that are more evolved than the Sun. We demonstrate that models that incorporate dramatically weakened magnetic braking for old stars can—unlike existing models—reproduce both the asteroseismic and the cluster data. Our findings might suggest a fundamental change in the nature of ageing stellar dynamos, with the Sun being close to the critical transition to much weaker magnetized winds. This weakened braking limits the diagnostic power of gyrochronology for those stars that are more than halfway through their main-sequence lifetimes.

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Figure 1: The period–age plane as predicted by gyrochronology, compared with observed periods.
Figure 2: Ratios of the predicted rotation period14 to the observed period.
Figure 3: The effects of a Rocrit threshold on rotational evolution.

Change history

  • 13 January 2016

    A citation was amended in the Abstract, and Extended Data Fig. 4 was corrected.

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Acknowledgements

We wish to thank the Kavli Institute for Theoretical Physics and the organizers of the ‘Galactic Archaeology and Precision Stellar Astrophysics’ program held from January to April 2015. We thank B. Shappee for useful discussions. J.L.v.S. is a Carnegie-Princeton Fellow. T.S.M acknowledges the adopt-a-star crowdfunding program administered by White Dwarf Research Corp. G.R.D. acknowledges the support of the UK Science and Technology Facilities Council (STFC). M.H.P, T.S.M. and S.M. acknowledge support from NASA grant NNX15AF13G and National Science Foundation (NSF) grant AST-1411685. T.C. and R.A.G. received grants from the Centre National d’Études Spatiales (CNES) at the French Alternative Energies and Atomic Energy Commission (CEA). R.A.G. also acknowledges funding from the European Community Seventh Framework Programme (FP7/2007−2013) under grants 312844 (SPACEINN) and 269194 (IRSES/ASK). This research was supported in part by the NSF under grant NSF PHY11-25915. Funding for the Stellar Astrophysics Centre is provided by The Danish National Research Foundation (grant agreement DNRF106). This research is supported by the ASTERISK project (ASTERoseismic Investigations with SONG and Kepler) funded by the European Research Council (grant agreement 267864). V.S.A. acknowledges support from VILLUM FONDEN (research grant 10118).

Author information

Affiliations

Authors

Contributions

J.L.v.S. provided the physical interpretation of the rapid rotation and the rotational and stellar modelling. M.H.P. contributed to the design of the rotation tracer code, to ongoing YREC (Yale rotating stellar evolution code) development, and to interpretation of the results. T.S.M. carried out asteroseismic modelling portal (AMP) modelling of all targets. V.S.A. carried out Bayesian stellar algorithm (BASTA)–Garching stellar evolution code modelling. T.C., R.A.G. and S.M. developed and implemented the analysis of rotational modulation in Kepler data. G.R.D provided updated asteroseismic mode frequencies used in the modelling.

Corresponding authors

Correspondence to Jennifer L. van Saders or Rafael A. García.

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Competing interests

The authors declare no competing financial interests.

Extended data figures and tables

Extended Data Figure 1 The positions of all 21 Kepler stars on the Hertzsprung–Russell diagram.

We plot spectroscopic Teff (a proxy for mass) versus seismic log(g) (surface gravity), with 1σ observational error bars; the symbol size is proportional to the period ratio (AMP ages, fiducial models14). Colours and symbol conventions are as in Fig. 2. Evolutionary tracks are overplotted for [Z/H] = +0.3 (dotted lines) and [Z/H] = −0.1 (solid lines), for masses 0.8–1.3 M in increments of 0.1 M. ([Z/H] = +0.3, M = 0.8 M is beyond the plotted area.)

Extended Data Figure 2 Period–age plot of sample stars.

The 21-star sample, with observed rotational periods plotted against AMP asteroseismic ages. Symbol conventions are as in Fig. 2. The solid line denotes the empirical relation2 for Teff = 5,800 K (approximately equal to the mean sample Teff). All error bars represent 1σ.

Extended Data Figure 3 Period ratios using empirical gyrochronology relations.

Ratios of predicted periods2 to observed periods are plotted as a function of the AMP asteroseismic age, and divided according to AMP ZAMS Teff (a, ZAMS Teff = 5,900–6,200 K; b, ZAMS Teff = 5,600–5,900 K; c, ZAMS Teff = 5,100–5,400 K.) Error bars represent 1σ. Symbol conventions are as in Fig. 2.

Source data

Extended Data Figure 4 Detectability of stars in spot modulation.

Detection fractions for the 750 stars with noise in the hound-and-hare exercise of ref. 17, as a function of activity level A (where the activity level of the Sun is defined as A = 1) and rotation period. The total number of light curves searched for periodicity in each cell is overplotted. The dashed black line at P = 35 days represents the expected period for stars like the Sun under traditional gyrochronology relations found in the literature.

Source data

Extended Data Figure 5 Predicted versus observed rotation periods using ages determined with BASTA.

a, c, e, Plotted are the ratios of the periods predicted using the fiducial models14 to the observed rotation periods, as a function of stellar age. The grey band represents the offset expected from models in which Rocrit = 2.16. All error bars represent 1σ. b, d, f, Ratios of the predicted periods obtained from the empirical relation2 to the observed periods, plotted against stellar age. Stars are divided according to ZAMS Teff, using BASTA ZAMS Teff values: a, b, 5,900–6,200 K; c, d, 5,500–5,900 K; e, f, 5,100–5,400 K. All symbol conventions are as in Fig. 2.

Extended Data Figure 6 The shift in the period ratios induced by changing the stellar model input physics.

Circles are colour-coded according to ZAMS Teff as in Fig. 2. Ratios of the periodicity expected from the fiducial model14 to the observed periodicity are plotted against age. Arrows denote the shift in the period ratio that occurs when YREC models14 are run to match the AMP–ASTEC physics.

Extended Data Table 1 Rotation periods, asteroseismic data and spectroscopic quantities for sample stars

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van Saders, J., Ceillier, T., Metcalfe, T. et al. Weakened magnetic braking as the origin of anomalously rapid rotation in old field stars. Nature 529, 181–184 (2016). https://doi.org/10.1038/nature16168

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