Detection of solar-like oscillations, observational constraints, and stellar models for theta Cyg, the brightest star observed by the Kepler Mission

Abstract

[Theta] Cygni is an F3 spectral type magnitude V=4.48 main-sequence star that was the brightest star observed by the original Kepler spacecraft mission. Short-cadence (58.8 s) photometric data using a custom aperture were first obtained during Quarter 6 (2010 June–September) and subsequently in Quarters 8 and 12–17. We present analyses of solar-like oscillations based on Q6 and Q8 data, identifying angular degree l = 0, 1, and 2 modes with frequencies of 1000–2700 [mu]Hz, a large frequency separation of 83.9 [plus or minus] 0.4 [mu]Hz, and maximum oscillation amplitude at frequency ν[subscript max] = 1829 [plus or minus] 54 [mu]Hz. We also present analyses of new ground-based spectroscopic observations, which, combined with interferometric angular diameter measurements, give T[subscript eff] = 6697 [plus or minus] 78 K, radius 1.49 [plus or minus] 0.03 R[circled dot], [Fe/H] = −0.02 [plus or minus] 0.06 dex, and log g = 4.23 [plus or minus] 0.03. We calculate stellar models matching these constraints using the Yale Rotating Evolution Code and the Asteroseismic Modeling Portal. The best-fit models have masses of 1.35–1.39 M[circled dot] and ages of 1.0–1.6 Gyr. [theta] Cyg’s T[subscript eff] and log g place it cooler than the red edge of the [gamma] Doradus instability region established from pre-Kepler ground-based observations, but just at the red edge derived from pulsation modeling. The pulsation models show [gamma] Dor gravity modes driven by the convective blocking mechanism, with frequencies of 1–3 cycles per day (11 to 33 [mu]Hz). However, gravity modes were not seen in Kepler data; one signal at 1.776 cycles per day (20.56 [mu]Hz) may be attributable to a faint, possibly background, binary.