TESS asteroseismology of the known planet host star λ² Fornacis

¹ School of Physics and Astronomy, University of Birmingham, Birmingham B15 2TT, UK
e-mail: m.b.nielsen.1@bham.ac.uk
² Stellar Astrophysics Centre (SAC), Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
³ Center for Space Science, NYUAD Institute, New York University Abu Dhabi, PO Box 129188, Abu Dhabi, UAE
e-mail: mbn4@nyu.edu
⁴ Department of Chemistry and Physics, Florida Gulf Coast University, 10501 FGCU Blvd., Fort Myers, FL 33965, USA
⁵ Department of Physics & Astronomy, Vanderbilt University, Nashville, TN 37235, USA
⁶ Department of Earth and Planetary Sciences, University of California, Riverside, CA 92521, USA
⁷ School of Physics, University of New South Wales, Kensington, NSW 2033, Australia
⁸ LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 92195 Meudon, France
⁹ Department of Astronomy and Space Sciences, Science Faculty, Ege University, 35100 Bornova, İzmir, Turkey
¹⁰ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
¹¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, Rua das Estrelas, 4150-762 Porto, Portugal
¹² Departamento de Física e Astronomia, Faculdade de Ciências da Universidade do Porto, Rua do Campo Alegre, s/n, 4169-007 Porto, Portugal
¹³ IRFU, CEA, Université Paris-Saclay, 91191 Gif-sur-Yvette, France
¹⁴ AIM, CEA, CNRS, Université Paris-Saclay, Université Paris Diderot, Sorbonne Paris Cité, 91191 Gif-sur-Yvette, France
¹⁵ Instituto de Astrofísica de Canarias, La Laguna, Tenerife, Spain
¹⁶ Dpto. de Astrofísica, Universidad de La Laguna, La Laguna, Tenerife, Spain
¹⁷ Institute of Space Sciences (ICE, CSIC), Carrer de Can Magrans S/N, 08193 Bellaterra, Spain
¹⁸ Institut d’Estudis Espacials de Catalunya (IEEC), Carrer Gran Capita 2, 08034 Barcelona, Spain
¹⁹ Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA
²⁰ Homi Bhabha Centre for Science Education, TIFR, V. N. Purav Marg, Mankhurd, Mumbai 400088, India

Received: 8 January 2020
Accepted: 15 June 2020

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.