Asteroseismology of the G8 subgiant β Aquilae with SONG-Tenerife, SONG-Australia and TESS

Aims. We present time-series radial velocities of the G8 subgiant star β Aql obtained in 2022 and 2023 using SONG-Tenerife and, for the first time, SONG-Australia. We also analyse a sector of TESS photometry that overlapped with the 2022 SONG data. Methods. We processed the time series to assign weights and to remove bad data points. The resulting power spectrum clearly shows solar-like oscillations centred at 430 μHz. The TESS light curve shows the oscillations at lower signal-to-noise, reflecting the fact that photometric measurements are much more affected by the granulation background than are radial velocities. Results. The simultaneous observations in velocity and photometry represent the best such measurements for any star apart from the Sun. They allowed us to measure the ratio between the bolometric photometric amplitude and the velocity amplitude to be 26.6 ± 3.1 ppm/ms<SUP>‑1</SUP>. We measured this ratio for the Sun from published SOHO data to be 19.5 ± 0.7 ppm/ms<SUP>‑1</SUP> and, after accounting for the difference in effective temperatures of β Aql and the Sun, these values align with expectations. In both the Sun and β Aql, the photometry-to-velocity ratio appears to be a function of frequency. We also measured the phase shift of the oscillations in β Aql between SONG and TESS to be ‑113° ±7°, which agrees with the value for the Sun and also with a 3D simulation of a star with similar properties to β Aql. Importantly for exoplanet searches, we argue that simultaneous photometry can be used to predict the contribution of oscillations to radial velocities. We measured frequencies for 22 oscillation modes in β Aql and carried out asteroseismic modelling, yielding an excellent fit to the frequencies. We derived accurate values for the mass and age, and were able to place quite strong constraints on the mixing-length parameter. Finally, we show that the oscillation properties of β Aql are very similar to stars in the open cluster M67.