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TESS asteroseismology of the known planet host star λ2 Fornacis

M. B. Nielsen, W. H. Ball, M. R. Standing, A. H. M. J. Triaud, D. L. Buzasi, L. Carboneau, Stassun K. G., S. R. Kane, W. J. Chaplin, E. P. Bellinger, B. Mosser, I. W. Roxburgh, Z. «elik Orhan, M. Yildiz, S. ÷rtel, M. Vrard, A. Mazumdar, P. Ranadive, M. Deal, G. R. Davies, T. L. Campante, R. A. GarcŪa, S. Mathur, L. GonzŠlez-Cuesta, A. M. Serenelli

Abstract
Context. The Transiting Exoplanet Survey Satellite (TESS) is observing bright known planet-host stars across almost the entire sky. These stars have been subject to extensive ground-based observations, providing a large number of radial velocity measurements.
Aims. The objective of this work is to use the new TESS photometric observations to characterize the star λ2 Fornacis, and following this to update the parameters of the orbiting planet λ2 For b.
Methods. We measured the frequencies of the p-mode oscillations in λ2 For, and in combination with non-seismic parameters estimated the stellar fundamental properties using stellar models. Using the revised stellar properties and a time series of archival radial velocities from the UCLES, HIRES and HARPS instruments spanning almost 20 years, we refit the orbit of λ2 For b and searched the residual radial velocities for remaining variability.
Results. We find that λ2 For has a mass of 1.16 ± 0.03 M and a radius of 1.63 ± 0.04 R, with an age of 6.3 ± 0.9 Gyr. This and the updated radial velocity measurements suggest a mass of λ2 For b of 16.8−1.3+1.2 M, which is ∼5M less than literature estimates. We also detect an additional periodicity at 33 days in the radial velocity measurements, which is likely due to the rotation of the host star.
Conclusions. While previous literature estimates of the properties of λ2 For are ambiguous, the asteroseismic measurements place the star firmly at the early stage of its subgiant evolutionary phase. Typically only short time series of photometric data are available from TESS, but by using asteroseismology it is still possible to provide tight constraints on the properties of bright stars that until now have only been observed from the ground. This prompts a reexamination of archival radial velocity data that have been accumulated in the past few decades in order to update the characteristics of the planet hosting systems observed by TESS for which asteroseismology is possible.

Keywords
asteroseismology; stars: individual: HD 16417; planetary systems; techniques: radial velocities; techniques: photometric; Astrophysics - Solar and Stellar Astrophysics; Astrophysics - Earth and Planetary Astrophysics

Astronomy and Astrophysics
Volume 641, Article Number A25, Number of pages 11
2020 September

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Instituto de Astrof√≠sica e Ci√™ncias do Espa√ßo Universidade do Porto Faculdade de Ciências da Universidade de Lisboa
Fundação para a Ciência e a Tecnologia COMPETE 2020 PORTUGAL 2020 União Europeia