Solar-like oscillations in the G9.5 subgiant β Aquilae.

Context. An interesting asteroseismic target is the G9.5 IV solar-like star β Aql. This is an ideal target for asteroseismic investigations, because precise astrometric measurements are available from Hipparcos that greatly help in constraining the theoretical interpretation of the results. The star was observed during six nights in August 2009 by means of the high-resolution échelle spectrograph SARG operating with the TNG 3.58 m Italian telescope (Telescopio Nazionale Galileo) on the Canary Islands, exploiting the iodine cell technique. Aims. We present the result and the detailed analysis of high-precision radial velocity measurements, where the possibility of detecting individual p-mode frequencies for the first time and deriving their corresponding asymptotic values will be discussed. Methods. The Fourier analysis technique based on radial velocity time series and the fitting of asymptotic relation to the power spectrum were the main tools used for the detection of the asteroseismic parameters of the star. Results. The time-series analysis carried out from ∼800 collected spectra shows the typical p-mode frequency pattern with amaximum centered at 416 μHz. In the frequency range 300-600 μHz we identified for the first time six high signal-to-noise ratio (S/N >≳ 3.5) modes with l = 0, 2 and 11 < n < 16 and three possible candidates for mixed modes (l = 1), although the p-mode identification for this type of star appears to be quite difficult owing to a substantial presence of avoided crossings. The large frequency separation and the surface term from the set of identified modes by means of the asymptotic relation were derived for the first time. Their values are Δν = 29.56 ± 0.10 μHz and ϵ = 1.29 ± 0.04, consistent with expectations. The most likely value for the small separation is δν02 = 2.55 ± 0.71 μHz. [ABSTRACT FROM AUTHOR]