On the magnetic activity cycle and rotation of the yellow subgiant β Aql

The cycles of activity similar to the solar one are thought to exist in other stars with outer convection zones. The long‐term monitoring of magnetic cycles in stars similar in structure to the Sun is a main diagnostic method for understanding how dynamo generation and amplification of magnetic fields occur. We have performed a search for solar‐like magnetic activity on the yellow subgiant β Aql. Direct measurements of the longitudinal magnetic field of β Aql were performed by measuring the Zeeman splitting in spectral lines, using the circularly polarized spectra obtained at the Crimean Astrophysical Observatory over 51 nights during 1997–2015. The magnetic field on β Aql was detected with the confidence level above 3 σ over 24 nights. The activity cycle of β Aql was found to be 969 ± 27 days. We assume that the activity of β Aql is similar to that of stars younger than the Sun. The most probable rotation period was found to be P rot = 5.08697 ± 0.00031 days. Assuming the global magnetic field of β Aql as a dipole, we estimated the polar field strength B pol = 24 G, the angle between the rotation axis and the line of sight i = 25 ° , and the angle between the rotation and dipole axes β = 96 ° .