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A plague of magnetic spots among the hot stars of globular clusters

Abstract

For more than six decades, the quest to understand the formation of hot (about 20,000−30,000 K) extreme horizontal branch (EHB) stars in Galactic globular clusters has remained one of the most elusive in stellar evolutionary theory. Here we report on two discoveries that challenge the idea of the stable luminosity of EHB stars. The first mode of EHB variability is periodic and cannot be ascribed to either binary evolution or pulsation. Instead, we attribute it here to the presence of magnetic spots: superficial chemical inhomogeneities whose projected rotation induces the variability. The second mode of EHB variability is aperiodic and manifests itself on timescales of years. In two cases, six-year-long light curves display superflare events that are several million times more energetic than solar analogues. We advocate a scenario in which the two EHB variability phenomena are different manifestations of diffuse, dynamo-generated, weak magnetic fields. Magnetism is therefore a key player driving the formation and evolution of EHB clusters stars and, likewise, operating in the Galactic field counterparts. Our conclusions bridge similar variability/magnetism phenomena in all radiative-enveloped hot-stars: young main-sequence stars, old EHBs and defunct white dwarfs.

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Fig. 1: The phased near-ultraviolet light curves of the EHB variable stars in three globular clusters.
Fig. 2: The EHB variable stars in the near-ultraviolet colour–magnitude diagrams.
Fig. 3: Modelling of the stellar spot in the NGC2808 EHB variable star vEHB-12.
Fig. 4: The uSDSS light curve of the EHB Padua-1 showing a complete superflare event.

Data availability

All the raw data (and associated calibrations) used in this paper are available for download in the ESO Science archive under the respective programme ID (see Methods), at http://archive.eso.org. Processed data supporting the findings of this study are available from the corresponding author upon request.

Code availability

All the codes used in this study are available at: Phoebe, http://phoebe-project.org/; KSint, http://eduscisoft.com/KSINT/index.php; EXOFAST, http://astroutils.astronomy.ohio-state.edu/exofast/limbdark.shtml; ISIS, http://www2.iap.fr/users/alard/package.html; VARTOOL, https://www.astro.princeton.edu/~jhartman/vartools.html; SM, https://www.astro.princeton.edu/~rhl/sm/; ALAMBIC, https://esosoft.univie.ac.at/software/esomvm/; DAOPHOT, http://www.star.bris.ac.uk/~mbt/daophot/; IRAF, https://iraf-community.github.io/.

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Acknowledgements

We acknowledge discussions with S. Bagnulo, A. Bressan, A. Bianchini, A. Renzini and P. Ochner, and we thank M. Dima for help in producing movies of the stellar spots. D.J. acknowledges support from the State Research Agency (AEI) of the Spanish Ministry of Science, Innovation and Universities (MCIU) and the European Regional Development Fund (FEDER) under grant AYA2017-83383-P. D.J. also acknowledges support under grant P/308614 financed by funds transferred from the Spanish Ministry of Science, Innovation and Universities, charged to the General State Budgets and with funds transferred from the General Budgets of the Autonomous Community of the Canary Islands by the Ministry of Economy, Industry, Trade and Knowledge.

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Contributions

Y.M. and S.Z. designed the study and coordinated the activity. Y.M., S.Z., M.M., H.M.J.B., D.J., M.G., I.S., L.M., C.M.B., V.D’O. and H.L. reduced and analysed the data. M.M. and S.Z. developed the spot modelling programme and related simulations. S.C., L.G. and D.J. provided theoretical modelling. G.P., A.P.M., P.B.S., Y.B. and E.M. contributed to the assembly of the photometric catalogues. Y.M. wrote the paper. S.Z., D.J., H.M.J.B., I.S., S.C., L.G. and H.L. contributed to the discussion and presentation of the paper. All authors contributed to the discussion of the results and commented on the manuscript.

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Correspondence to Y. Momany.

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Extended data

Extended Data Fig. 1 Estimating the EHB variable frequency.

Upper-left panel displays our NGC2808 VIMOS diagram highlighting all identified variables, the box delimits the EHB sample used to normalize the EHB variable stars frequency, and a ZAHB model is used to confirm the EHB variable stars temperature range (~17,500–24,500 K). The two EHB variable stars with open orange symbols are confirmed EHB stars, as identified in the higher resolution HST catalogue (upper-right panel). Lower panels show the position of all EHB variable stars identified in HST diagrams.

Extended Data Fig. 2 No binarity signature detected in the NGC2808 vEHB-1 variable.

Upper panel displays the phased Hα radial velocity curve of a comparison RR Lyrae star proving a successful detection of velocity variations in the data-set. The lower panel displays the velocity curve of our photometric variable vEHB-1 present in the same data-set. The error bars display the 1-σ error (~3.5 km/s) estimated at the vEHB-1 luminosity. No substantial velocity variations for vEHB-1 are observed. For clarity, the NGC2808 average radial velocity has been subtracted.

Extended Data Fig. 3 No binarity signature detected in the NGC6752 vEHB-1/2 variables.

Upper panel displays the phased Hγ,δ radial velocity curve of a comparison SX Phoenicis star59 proving a successful detection of velocity variations in the data-set. The lower panels display the velocity curves of the 2 EHB photometric variables (and the candidate EHB photometric59 variable vEHB-4/V17) present in the same data-set. The error bars display the 1-σ error (~3.0 km/s estimated at the vEHB-1 luminosity. No substantial velocity variations for the vEHB-1/2 are observed. For clarity, the NGC6752 average radial velocity has been subtracted.

Extended Data Fig. 4 The long-term stable variability of vEHB-1 in NGC6752.

Bottom plot shows all the uSDSS OmegaCAM measurements of vEHB-1 collected over a six-year period. The upper plots show the phased light curves sub-divided over six years. A typical 1-σ photometric error bar is plotted. The solid light-blue line is the best fitting model (Period  19.5 days) calculated using the six years’ measurements.

Extended Data Fig. 5 The aperiodic long-term Padua variables in NGC6752.

Light blue squares display the six-year archival OmegaCAM@VST data, while black squares display those originating from our three-year monitoring. A typical 1-σ photometric error bar is plotted. The Padua-2 mini-burst is incomplete but discernible.

Extended Data Fig. 6 Rotational variability and superflare event in a Galactic field sdB star.

Upper panel displays the folded TESS light curve of a Galactic field sdB star showing α2CVn spot-induced variability. Filled squares are the 2.5-σ clipped median values every 300 data points, while the error bars reflect the 1-σ RMS of the clipped flux values. Lower panel proves the occurrence of an energetic (~1035 erg) superflare event in this field sdB star. Both phenomena necessitate the presence of magnetic fields.

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Momany, Y., Zaggia, S., Montalto, M. et al. A plague of magnetic spots among the hot stars of globular clusters. Nat Astron 4, 1092–1101 (2020). https://doi.org/10.1038/s41550-020-1113-4

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