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Published March 2023 | Published
Journal Article Open

Tidally perturbed gravity-mode pulsations in a sample of close eclipsing binaries


Context. Thanks to the high-precision photometry from space missions such as Kepler and TESS, tidal perturbations and tilting of pulsations have been detected in more than a dozen binary systems. However, only two of these were gravity-mode (g-mode) pulsators. Aims. We aim to detect tidally perturbed g modes in additional binary systems and characterise them observationally. Methods. We performed a custom data reduction of the available Kepler and TESS photometry of a well-studied, published sample of 35 binary systems with γ Doradus (γ Dor) pulsators. For each target, we modelled the binary signal using a sum of 100 sine waves with frequencies at orbital harmonics and measured significant pulsation frequencies in an iterative pre-whitening analysis of the residual light curve. Pulsations are labelled as tidally perturbed g modes if they are part of both period-spacing patterns and multiplets spaced by integer multiples of the orbital frequency. After visual inspection and confirmation, the properties of these targets and g modes were characterised. Results. We detect tidally perturbed g-mode pulsations for five short-period binaries that are circularised and (almost) synchronously rotating: KIC 3228863, KIC 3341457, KIC 4947528, KIC 9108579, and KIC 12785282. Tidally perturbed g modes that occur within the same star and have the same mode identification (k, m), are found to have near-identical relative amplitude and phase modulations, which are within their respective 1 − σ uncertainties and also identical for the Kepler and TESS photometric passbands. By contrast, pulsations with different mode identifications (k, m) are found to exhibit different modulations. Moreover, the observed amplitude and phase modulations are correlated, indicating that the binary tides primarily distort the g-mode amplitudes on the stellar surface. The phase modulations are then primarily a geometric effect of the integration of the stellar flux over the visible stellar surface. All selected binaries also exhibit signal that resembles rotational modulation in the Fourier domain. In the case of KIC 3228863, this is caused by the presence of the known tertiary component, and for the other systems we hypothesise that it is caused by temperature variations on the stellar surface. Alternatively, the signal can be made up of overstable convective modes in the stellar core or may belong to the non-pulsating companion. Conclusions. While g-mode pulsation periods are known to be a direct probe of the deep interior stellar structure, the binary tides that cause the pulsation modulations are dominant in the outer stellar layers. Hence, in the future, tidally perturbed g modes may allow us to carry out core-to-surface asteroseismic modelling of tidally distorted stars.

Additional Information

© The Authors 2023. Open Access article, published by EDP Sciences, under the terms of the Creative Commons Attribution License (https://creativecommons.org/licenses/by/4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited. This article is published in open access under the Subscribe to Open model. Subscribe to A&A to support open access publication. Combinations and distortions of different pulsation mode geometries can be complicated. TVR is very grateful to Marrick Braam, Sarah Gebruers, Mathias Michielsen, and Joey S. G. Mombarg for useful discussions that have helped him to make sense of everything, or at least get confused consistently. We also thank the anonymous referee for useful comments which have improved the quality of the manuscript. TVR and DMB gratefully acknowledge funding from the research foundation Flanders (FWO) by means of junior and senior postdoctoral fellowships with grant agreements N° 12ZB620N and 1286521N, respectively, and FWO long stay travel grants N° V414021N and V411621N, respectively. JVB acknowledges receiving support from the Research Foundation Flanders (FWO) under grant agreement N° V421221N. The research leading to these results also received partial funding from the KU Leuven Research Council (grant C16/18/005: PARADISE), and was supported in part by the National Science Foundation under Grant No. NSF PHY-1748958. CJ gratefully acknowledges support from the Netherlands Research School of Astronomy (NOVA). This paper includes data collected with the Kepler and TESS missions, obtained from the MAST data archive at the Space Telescope Science Institute (STScI). Funding for the Kepler and TESS missions are provided by NASA's Science Mission Directorate and the NASA Explorer Program, respectively. We thank the whole teams for the development and operations of these missions. STScI is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. Support to MAST for these data is provided by the NASA Office of Space Science via grant NAG5-7584 and by other grants and contracts. This research also made use of the SIMBAD database, operated at CDS, Strasbourg, France, the SAO/NASA Astrophysics Data System, and the VizieR catalogue access tool, CDS, Strasbourg, France. The data analysis was done using Astropy (http://www.astropy.org; a community-developed core Python package for Astronomy; Astropy Collaboration 2013, 2018), Astroquery (https://astroquery.readthedocs.io/en/latest/; Ginsburg et al. 2019), Lightkurve (https://docs.lightkurve.org/; a Python package for Kepler and TESS data analysis; Lightkurve Collaboration 2018), lmfit (Newville et al. 2019), Matplotlib (the Python library for publication quality graphics; Hunter 2007), Numpy (Harris et al. 2020), and Scipy (Virtanen et al. 2020).

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August 22, 2023
October 20, 2023