Tidally trapped pulsations in a close binary star system discovered by TESS

Abstract

It has long been suspected that tidal forces in close binary stars could modify the orientation of the pulsation axis of the constituent stars. Such stars have been searched for, but until now never detected. Here we report the discovery of tidally trapped pulsations in the ellipsoidal variable HD 74423 in Transiting Exoplanet Survey Satellite (TESS) space photometry data. The system contains a δ Scuti pulsator in a 1.6 d orbit, whose pulsation mode amplitude is strongly modulated at the orbital frequency, which can be explained if the pulsations have a much larger amplitude in one hemisphere of the star. We interpret this as an obliquely pulsating distorted dipole oscillation with a pulsation axis aligned with the tidal axis. This is the first time that oblique pulsation along a tidal axis has been recognized. It is unclear whether the pulsations are trapped in the hemisphere directed towards the companion or in the side facing away from it, but future spectral measurements can provide the solution. In the meantime, the single-sided pulsator HD 74423 stands out as the prototype of a new class of obliquely pulsating stars in which the interactions of stellar pulsations and tidal distortion can be studied.

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Fig. 1: TESS light curves of HD 74423.
Fig. 2: Fourier amplitude spectra of the TESS photometry of HD 74423.
Fig. 3: Run of the pulsation amplitude and phase over the orbital period.
Fig. 4: Comparison of the orbital amplitude and phase behaviour with the oblique pulsator model.

Data availability

TESS photometric data are publicly available at the Mikulski Archive for Space Telescopes (MAST, https://mast.stsci.edu/portal/Mashup/Clients/Mast/Portal.html). All relevant data are also available by request from the corresponding author.

Code availability

The codes for computing the discrete Fourier transform and to carry out the variability analyses are available on request from D.W.K. The SPECTRUM code used to compute synthetic spectra is publicly available from http://www.appstate.edu/~grayro/spectrum/spectrum.html.

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Acknowledgements

This paper includes data collected by the TESS mission. Funding for the TESS mission is provided by the NASA Explorer Program. Funding for the TESS Asteroseismic Science Operations Centre is provided by the Danish National Research Foundation (grant agreement DNRF106), ESA PRODEX (PEA 4000119301) and Stellar Astrophysics Centre (SAC) at Aarhus University. Some of the observations reported in this paper were obtained with the Southern African Large Telescope (SALT). Polish participation in SALT is funded by grant MNiSW DIR/WK/2016/07. D.W.K. acknowledges financial support from the STFC via grant ST/M000877/1. M.S. is supported by an Australian Government Research Training Program (RTP) Scholarship. G.H., S.C., F.K.A. and P.S. acknowledge financial support by the Polish NCN grant 2015/18/A/ST9/00578. 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. We thank the TESS team and staff and TASC/TASOC for their support of the present work and Allan R. Schmitt for making his light-curve examining software LcTools freely available. S.C. is grateful to C. Engelbrecht for introducing him to the use of the observing equipment. G.H. thanks E. Paunzen for helpful discussions on the spectra of λ Boötis stars. A.V. is a NASA Sagan Fellow.

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Contributions

G.H. provided the initial astrophysical interpretation for this object, coordinated the scientific analysis, analysed the photometric and spectroscopic data, and oversaw and contributed to the paper writing. D.W.K. carried out the frequency analysis and provided the interpretation in terms of the oblique pulsator model. S.A.R. initiated the collaboration, and oversaw and homogenized all aspects of the scientific analysis. H.S. modelled the pulsation amplitude and phase behaviour over the orbit. J.F. contributed the theoretical interpretation. D.J. and P.S. analysed the ellipsoidal variability. Z.G. provided expertise in the modelling of pulsators in close binary systems. S.C. and F.K.A. carried out auxiliary observations and computations. M.S. and S.J.M. independently noticed the star’s behaviour and provided their expertise. R.G. and T.L.J. originally pointed out the star to S.A.R. and A.V. who work with the citizen scientists to vet their findings.

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Correspondence to G. Handler.

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Supplementary Figs. 1 and 2.

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Handler, G., Kurtz, D.W., Rappaport, S.A. et al. Tidally trapped pulsations in a close binary star system discovered by TESS. Nat Astron (2020). https://doi.org/10.1038/s41550-020-1035-1

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