Asteroseismic Modeling of 1,153 Kepler Red Giant Branch Stars: Improved Stellar Parameters with Gravity-Mode Period Spacings and Luminosity Constraints

This paper reports estimated stellar parameters of 1,153 Kepler red giant branch stars determined with asteroseismic modeling. We use radial-mode oscillation frequencies, gravity-mode period spacings, Gaia luminosities, and spectroscopic data to characterize these stars. Compared with previous studies, we find that the two additional observed constraints, i.e., the gravity-mode period spacing and luminosity, significantly improve the precision of fundamental stellar parameters. The typical uncertainties are 2.9% for the mass, 11% for the age, 1.0% for the radius, 0.0039 dex for the surface gravity, and 0.5\% for the helium core mass, making this the best-characterized large sample of red-giant stars available to date. With better characterizations for these red giants, we recalibrate the seismic scaling relations and study the surface term on the red-giant branch. We confirm that the surface term depends on the surface gravity and effective temperature, but there is no significant correlation with metallicity.Comment: Accepted by Ap

Solar-type Stars Observed by LAMOST and Kepler

Obtaining measurements of chromospheric and photometric activity of stars with near-solar fundamental parameters and rotation periods is important for a better understanding of solar-stellar connection. We select a sample of 2603 stars with near-solar fundamental parameters from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST spectra to measure their chromospheric activity and Kepler light curves to measure their photospheric activity (i.e., the amplitude of the photometric variability). While the rotation periods of 1556 of these stars could not be measured due to the low amplitude of the photometric variability and highly irregular temporal profile of light curves, 254 stars were further identified as having near-solar rotation periods. We show that stars with near-solar rotation periods have chromospheric activities that are systematically higher than stars with undetected rotation periods. Furthermore, while the solar level of photospheric and chromospheric activity appears to be typical for stars with undetected rotation periods, the Sun appears to be less active than most stars with near-solar rotation periods (both in terms of photospheric and chromospheric activity).Comment: 7 pages, 6 figure

Imprints of Sagittarius accretion event: Young O-rich stars and discontinuous chemical evolution in Milky Way disc

The Milky Way has undergone significant transformations in its early history, characterised by violent mergers and the accretion of satellite galaxies. Among these events, the infall of the satellite galaxy Gaia-Enceladus/Sausage is recognised as the last major merger event, fundamentally altering the evolution of the Milky Way and shaping its chemo-dynamical structure. However, recent observational evidence suggests that the Milky Way remains undergone notable events of star formation in the past 4 Gyr, which is thought to be triggered by the perturbations from Sagittarius dwarf galaxy (Sgr). Here we report chemical signatures of the Sgr accretion event in the past 4 Gyr, using the [Fe/H] and [O/Fe] ratios in the thin disc, which is reported for the first time. It reveals that the previously discovered V-shape structure of age-[Fe/H] relation varies across different Galactic locations and has rich substructures. Interestingly, we discover a discontinuous structure at z$_{\rm max}$ $<$ 0.3 kpc, interrupted by a recent burst of star formation from 4 Gyr to 2 Gyr ago. In this episode, we find a significant rise in oxygen abundance leading to a distinct [O/Fe] gradient, contributing to the formation of young O-rich stars. Combined with the simulated star formation history and chemical abundance of Sgr, we suggest that the Sgr is an important actor in the discontinuous chemical evolution of the Milky Way disc.Comment: 17 pages, 15 figures. Under review at Nature Communication

Asteroseismology of three evolved stars in M67:testing systematic biases in seismic masses and ages

Asteroseismology can precisely characterize stars, but the accuracy of seismic inference is still an open question. In this work, we use three evolved stars, including one late subgiant (EPIC 211411922) and two young red giants (EPIC 211409560 and EPIC 211416749) in the open cluster M67 to validate seismic inferences of stellar mass and age. We start by measuring oscillation mode frequencies and identifying acoustic and mixed modes in all three stars. We then fit the stars to determine their masses and ages with a detailed modelling approach. Our age estimates for all stars agree well with the literature consensus age range of M67 (3.0-5.0 Gyr). Mass estimates for EPIC 211411922 and EPIC 211409560 are sensible because they are slightly higher than the typical turn-off mass of the cluster. The inferred mass for EPIC 211416749 is underestimated by 5-10 per cent compared with the turn-off mass, but this could be caused by poor data quality. Our results indicate that the detailed modelling can determine sensible masses and ages for evolved stars near the base of the red giant branch

Asteroseismology of KIC 7107778: a binary comprising almost identical subgiants

We analyse an asteroseismic binary system: KIC 7107778, a non-eclipsing, unresolved target, with solar-like oscillations in both components. We used Kepler short cadence time series spanning nearly 2 yr to obtain the power spectrum. Oscillation mode parameters were determined using Bayesian inference and a nested sampling Monte Carlo algorithm with the DIAMONDS package. The power profiles of the two components fully overlap, indicating their close similarity. We modelled the two stars with MESA and calculated oscillation frequencies with GYRE. Stellar fundamental parameters (mass, radius, and age) were estimated by grid modelling with atmospheric parameters and the oscillation frequencies of l = 0, 2 modes as constraints. Most l = 1 mixed modes were identified with models searched using a bisection method. Stellar parameters for the two sub-giant stars are MA = 1.42 ± 0.06 M☉, MB = 1.39 ± 0.03 M☉, RA = 2.93 ± 0.05 R☉, RB = 2.76 ± 0.04 R☉, tA = 3.32 ± 0.54 Gyr and tB = 3.51 ± 0.33 Gyr. The mass difference of the system is ∼1 per cent. The results confirm their simultaneous birth and evolution, as is expected from binary formation. KIC 7107778 comprises almost identical twins, and is the first asteroseismic sub-giant binary to be detected