Solar-like oscillations in γ Cephei A as seen through SONG and TESS

A seismic study of γ Cephei A

¹ Stellar Astrophysics Centre, Department of Physics and Astronomy, Aarhus University, Ny Munkegade 120, 8000 Aarhus C, Denmark
e-mail: emil@phys.au.dk
² Nordic Optical Telescope, Rambla José Ana Fernández Pérez 7, 38711 Breña Baja, Spain
³ Instituto de Astrofísica de Canarias, 38205 La Laguna, Tenerife, Spain
⁴ Universidad de La Laguna (ULL), Departamento de Astrofísica, 38206 La Laguna, Tenerife, Spain
⁵ School of Physics, The University of New South Wales, Sydney, NSW 2052, Australia
⁶ Sydney Institute for Astronomy (SIfA), School of Physics, University of Sydney, Camperdown, NSW 2006, Australia
⁷ Department of Astronomy, The Ohio State University, Columbus, OH 43210, USA

Received: 20 April 2023
Accepted: 12 June 2023

Abstract

Context. Fundamental stellar parameters such as mass and radius are some of the most important building blocks in astronomy, both when it comes to understanding the star itself and when deriving the properties of any exoplanet(s) they may host. Asteroseismology of solar-like oscillations allows us to determine these parameters with high precision.

Aims. We investigate the solar-like oscillations of the red-giant-branch star γ Cep A, which harbours a giant planet on a wide orbit.

Methods. We did this by utilising both ground-based radial velocities from the SONG network and space-borne photometry from the NASA TESS mission.

Results. From the radial velocities and photometric observations, we created a combined power spectrum, which we used in an asteroseismic analysis to extract individual frequencies. We clearly identify several radial and quadrupole modes as well as multiple mixed, dipole modes. We used these frequencies along with spectroscopic and astrometric constraints to model the star, and we find a mass of 1.27_−0.07^+0.05 M_⊙, a radius of 4.74_−0.08^+0.07 R_⊙, and an age of 5.7_−0.9^+0.8 Gyr. We then used the mass of γ Cep A and our SONG radial velocities to derive masses for γ Cep B and γ Cep Ab of 0.328_−0.012^+0.009 M_⊙ and 6.6_−2.8^+2.3 M_Jup, respectively.