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Register a new userLithium Evolution of Giant Stars Observed by LAMOST and Kepler
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Mapping lithium evolution for evolved stars will provide restrictions and constraints on the fundamental stellar interior physical processes, which further shed light on our understanding of the theory of stellar structure and evolution. Based on a sample of 1,848 giants with known evolutionary phases and lithium abundances from the LAMOST-kepler{} and LAMOST-emph{K}2 fields, we construct mass-radius diagrams to characterize the evolutionary features of lithium. The stars at red giant branch (RGB) phase show natural depletion along with their stellar evolution, particularly, there is no obvious crowd stars with anomalously high Li abundances near the bump. Most of the low-mass stars reaching their zero-age sequence of core-helium-burning (ZAHeB) have Li abundances around $sim1.0$,dex, which show an increase of Li abundance by $sim0.6$,dex compared to the stars above the bump of RGB. This suggests the helium flash can be responsible for moderate Li production. While for super Li-rich stars, some special mechanisms should be considered during helium flash. Other scenarios, such as merger, could also be interpretations given the Li-rich stars can be found at anytime during the steady state phase of core He-burning. During the core He-burning (HeB) phase, there is no indication of obvious lithium depletion.
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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 emph{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 $sim$,20 per cent. The sample stars reveal two separate sequences in the age -- [$alpha$/Fe] relation -- a high--$alpha$ sequence with stars older than $sim$,8,Gyr and a low--$alpha$ 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 $sim$,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.
According to standard stellar evolution, lithium is destroyed throughout most of the evolution of low- to intermediate-mass stars. However, a number of evolved stars on the red giant branch (RGB) and the asymptotic giant branch (AGB) are known to contain a considerable amount of Li, whose origin is not always understood well. Here we present the latest development on the observational side to obtain a better understanding of Li-rich K giants (RGB), moderately Li-rich low-mass stars on the AGB, as well as very Li-rich intermediate-mass AGB stars possibly undergoing the standard hot bottom burning phase. These last ones probably also enrich the interstellar medium with freshly produced Li.
Tidal interaction governs the redistribution of angular momentum in close binary stars and planetary systems and determines the systems evolution towards the possible equilibrium state. Turbulent friction acting on the equilibrium tide in the convective envelope of low-mass stars is known to have a strong impact on this exchange of angular momentum in binaries. Moreover, theoretical modelling in recent literature as well as presented in this paper suggests that the dissipation of the dynamical tide, constituted of tidal inertial waves propagating in the convective envelope, is weak compared to the dissipation of the equilibrium tide during the red-giant phase. This prediction is confirmed when we apply the equilibrium-tide formalism developed by Zahn (1977), Verbunt & Phinney (1995), and Remus, Mathis & Zahn (2012) onto the sample of all known red-giant binaries observed by the NASA Kepler mission. Moreover, the observations are adequately explained by only invoking the equilibrium tide dissipation. Such ensemble analysis also benefits from the seismic characterisation of the oscillating components and surface rotation rates. Through asteroseismology, previous claims of the eccentricity as an evolutionary state diagnostic are discarded. This result is important for our understanding of the evolution of multiple star and planetary systems during advanced stages of stellar evolution.
The study of stellar activity is important because it can provide new constraints for dynamo models, when combined with surface rotation rates and the depth of the convection zone. We know that the dynamo mechanism, which is believed to be the main process to rule the magnetic cycle of solar-like stars at least, results from the interaction between (differential) rotation, convection, and magnetic field. The Kepler mission has been collecting data for a large number of stars during 4 years allowing us to investigate magnetic stellar cycles. We investigated the Kepler light curves to look for magnetic activity or even hints of magnetic activity cycles. Based on the photometric data we also looked for new magnetic indexes to characterise the magnetic activity of the stars. We selected a sample of 22 solar-like F stars that have a rotation period smaller than 12 days. We performed a time-frequency analysis using the Morlet wavelet yielding a magnetic proxy. We computed the magnetic index S_ph as the standard deviation of the whole time series and the index <S_ph> that is the mean of standard deviations measured in subseries of length five times the rotation period of the star. We defined new indicators to take into account the fact that complete magnetic cycles are not observed for all the stars, such as the contrast between high and low activity. We also inferred the Rossby number of the stars and studied their stellar background. This analysis shows different types of behaviours in the 22 F stars. Two stars show behaviours very similar to magnetic activity cycles. Five stars show long-lived spots or active regions suggesting the existence of active longitudes. Two stars of our sample seem to have a decreasing or increasing trend in the temporal variation of the magnetic proxies. Finally the last group of stars show magnetic activity (with presence of spots) but no sign of cycle.
The space-borne missions CoRoT and Kepler have provided a large amount of precise photometric data. Among the stars observed, red giants show a rich oscillation pattern that allows their precise characterization. Long-duration observations allow for investigating the fine structure of this oscillation pattern. A common pattern of oscillation frequency was observed in red giant stars, which corresponds to the second-order development of the asymptotic theory. This pattern, called the universal red giant oscillation pattern, describes the frequencies of stellar acoustic modes. We aim to investigate the deviations observed from this universal pattern, thereby characterizing them in terms of the location of the second ionization zone of helium. We also show how this seismic signature depends on stellar evolution. We measured the frequencies of radial modes with a maximum likelihood estimator method, then we identified a modulation corresponding to the departure from the universal oscillation pattern. We identify the modulation component of the radial mode frequency spacings in more than five hundred red giants. The variation in the modulation that we observe at different evolutionary states brings new constraints on the interior models for these stars. We also derive an updated form of the universal pattern that accounts for the modulation and provides highly precise radial frequencies.
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