Obtaining accurate and precise masses and ages for large numbers of giant stars is of great importance for unraveling the assemblage history of the Galaxy. In this paper, we estimate masses and ages of 6940 red giant branch (RGB) stars with asteroseismic parameters deduced from Kepler photometry and stellar atmospheric parameters derived from LAMOST spectra. The typical uncertainties of mass is a few per cent, and that of age is ∼ 20 per cent. The sample stars reveal two separate sequences in the age -[α/Fe] relation -a high-α sequence with stars older than ∼ 8 Gyr and a low-α sequence composed of stars with ages ranging from younger than 1 Gyr to older than 11 Gyr. We further investigate the feasibility of deducing ages and masses directly from LAMOST spectra with a machine learning method based on kernel based principal component analysis, taking a sub-sample of these RGB stars as a training data set. We demonstrate that ages thus derived achieve an accuracy of ∼ 24 per cent. We also explored the feasibility of estimating ages and masses based on the spectroscopically measured carbon and nitrogen abundances. The results are quite satisfactory and significantly improved compared to the previous studies.

No abstract

Monitoring chromospheric and photospheric indexes of magnetic activity can provide valuable information, especially the interaction between different parts of the atmosphere and their response to magnetic fields. We extract chromospheric indexes, S and + R HK , for 59,816 stars from LAMOST spectra in the LAMOST-Kepler program, and photospheric index, R eff , for 5575 stars from Kepler light curves. The log R eff shows positive correlation with log + R HK . We estimate the power-law indexes between R eff and + R HK for F-, G-, and K-type stars, respectively. We also confirm the dependence of both chromospheric and photospheric activity on stellar rotation. Ca II H and K emissions and photospheric variations generally decrease with increasing rotation periods for stars with rotation periods exceeding a few days. The power-law indexes in exponential decay regimes show different characteristics in the two activity-rotation relations. The updated largest sample including the activity proxies and reported rotation periods provides more information to understand the magnetic activity for cool stars.

The ratios r 01 and r 10 of small to large separations of KIC 2837475 primarily exhibit an increase behavior in the observed frequency range. The calculations indicate that only the models with overshooting parameter δ ov between approximately 1.2 and 1.6 can reproduce the observed ratios r 01 and r 10 of KIC 2837475. The ratios r 01 and r 10 of the frequency separations of p-modes with inner turning points that are located in the overshooting region of convective core can exhibit an increase behavior. The frequencies of the modes that can reach the overshooting region decrease with the increase in δ ov . Thus the ratio distributions are more sensitive to δ ov than to other parameters. The increase behavior of the KIC 2837475 ratios results from a direct effect of the overshooting of convective core. The characteristic of the ratios provides a strict constraint on stellar models. Observational constraints point to a star with M = 1.490 ± 0.018 M ⊙ , R = 1.67 ± 0.01 R ⊙ , age = 2.8 ± 0.4 Gyr, and 1.2 δ ov 1.6 for KIC 2837475.

We analyse five seasons of short-cadence data of a solar-type star of spectral type F: KIC 6225718 observed by Kepler. We obtain the power spectrum of this star by applying the Lomb-Scargle periodogram to the smoothed time series. By applying the autocorrelation technique to the power spectrum, we derive the large-frequency separation ν = 105.78 ± 0.65 μHz and the frequency of maximum power ν max = 2301 ± 21 μHz. We identify 33 p modes with angular degrees of l = 0-2 in the frequency range 1600-2800 μHz of the power spectrum with Bayesian Markov Chain Monte Carlo algorithms. In order to determine the parameters of the star accurately, we construct a grid of stellar models with core overshooting using the Yale stellar evolution code and then perform preliminary seismological analysis. With both asteroseismic and non-asteroseismic constraints, the following range of stellar parameters is estimated: mass M = 1.10 +0.04 −0.03 M , radius R = 1.22 +0.01 −0.01 R and age t = 3.35 +0.36 −0.75 Gyr for this star. In addition, we analyse the effects of overshooting on stellar interiors and find that the upper limit of the overshooting parameter α ov is approximately 0.2 for this star.