Asteroseismic analysis of solar-mass subgiants KIC 6442183 and KIC 11137075 observed by Kepler

Context. Asteroseismology provides a powerful way to constrain stellar parameters. Solar-like oscillations have been observed on subgiant stars with the Kepler mission. The continuous and high-precision time series enables us to carry out a detailed asteroseismic study for these stars. Aims. We carry out data processing of two subgiants of spectral type G: KIC 6442183 and KIC 11137075 observed with the Kepler mission, and perform seismic analysis for the two evolved stars. Methods. We estimated the global asteroseismic parameters Lambda nu = 64.9 +/- 0.2 mu Hz and nu(max) = 1225 +/- 17 mu Hz for KIC 6442183, Delta nu = 65.5 +/- 0.2 mu Hz and nu(max) = 1171 +/- 8 mu Hz for KIC 11137075, respectively. In addition, we extracted the individual mode frequencies of the two stars. We compared stellar models and observations, including mode frequencies and mode inertias. The mode inertias of mixed modes, which are sensitive to the stellar interior, were used to constrain stellar models. We defined a quantity d nu(m-p) that measures the difference between the mixed modes and the expected pure pressure modes, which is related to the inertia ratio of mixed modes to radial modes. Results. Asteroseismic together with spectroscopic constraints provide the estimates of the stellar parameters: M = 1.04(-0.04)(+0.01) M-circle dot, R = 1.66(-0.02)(+0.01) R-circle dot and t = 8.65(-0.06)(+1.12) Gyr for KIC 6442183, and M = 1.00(-0.01)(+0.01) M-circle dot, R = 1.63(-0.01)(+0.01) R-circle dot and t = 10.36(-0.20)(+0.01) Gyr for KIC 11137075. Either mode inertias or d nu(m-p) can be used to constrain stellar models.