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تسجيل مستخدم جديدAsteroseismic study of solar-like stars: A method of estimating stellar age
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Asteroseismology, as a tool to use the indirect information contained in stellar oscillations to probe the stellar interiors, is an active field of research presently. Stellar age, as a fundamental property of star apart from its mass, is most difficult to estimate. In addition, the estimating of stellar age can provide the chance to study the time evolution of astronomical phenomena. In our poster, we summarize our previous work and further present a method to determine age of low-mass main-sequence star.
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textbf{Scaling formulas were} deduced to describe the relations between the fundamental stellar parameters and the mean textbf{linewidth and lifetime} of solar-like oscillations of stars. The mean textbf{linewidth and} lifetime of solar-like oscillat
ions textbf{are dependent on the large frequency separation, the effective temperature, and the acoustic impedance ($rho c$) in the photosphere} of stars. The mean lifetime textbf{can be} approximate to the lifetime of the mode with $ usim u_{max}$. We compared the results of the scaling relations with the mean lifetimes of solar-like oscillations of stars observed by textit{Kepler} and textit{CoRoT}, which shows that the observed mean lifetimes are reproduced well by the scaling relations. The dependence of the mean lifetime on textbf{the} large frequency separation, the effective temperature, and the acoustic impedance of stars textbf{indicates} that lifetimes of solar-like oscillations rely on the mass and evolutionary phase of stars. Moreover, our calculations show that the mean lifetimes of $p$-modes of stars can be affected by metallicity abundances.
Context: The detection and identification of oscillation modes (in terms of their $ell$, $m$ and successive $n$) is a great challenge for present and future asteroseismic space missions. The peak tagging is an important step in the analysis of these
data to provide estimations of stellar oscillation mode parameters, i.e., frequencies, rotation rates, and further studies on the stellar structure. Aims: To increase the signal-to-noise ratio of the asteroseismic spectra computed from time series representative of MOST and CoRoT observations (30- and 150-day observations). Methods: We apply the curvelet transform -- a recent image processing technique which looks for curved patterns -- to echelle diagrams built using asteroseismic power spectra. In this diagram the eigenfrequencies appear as smooth continuous ridges. To test the method we use Monte Carlo simulations of several sun-like stars with different combinations of rotation rates, rotation-axis inclination and signal-to-noise ratios. Results: The filtered diagrams enhance the contrast between the ridges of the modes and the background allowing a better tagging of the modes and a better extraction of some stellar parameters. Monte Carlo simulations have also shown that the region where modes can be detected is enlarged at lower and higher frequencies compared to the raw spectra. Even more, the extraction of the mean rotational splitting from modes at low frequency can be done more easily than using the raw spectrum.
Stellar magnetic activity decays over the main-sequence life of cool stars due to the stellar spin-down driven by magnetic braking. The evolution of chromospheric emission is well-studied for younger stars, but difficulties in determining the ages of
older cool stars on the main sequence have complicated such studies for older stars in the past. Here we report on chromospheric Ca II H and K line measurements for 26 main-sequence cool stars with asteroseismic ages older than a gigayear and spectral types F and G. We find that for the G stars and the cooler F-type stars which still have convective envelopes the magnetic activity continues to decrease at stellar ages above one gigayear. Our magnetic activity measurements do not show evidence for a stalling of the magnetic braking mechanism, which has been reported for stellar rotation versus age for G and F type stars. We also find that the measured RHK indicator value for the cool F stars in our sample is lower than predicted by common age-activity relations that are mainly calibrated on data from young stellar clusters. We conclude that, within individual spectral type bins, chromospheric magnetic activity correlates well with stellar age even for old stars.
Asteroseismology with the Kepler space telescope is providing not only an improved characterization of exoplanets and their host stars, but also a new window on stellar structure and evolution for the large sample of solar-type stars in the field. We
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Motivated by the recent detection of stochastically excited modes in the massive star V1449 Aql (Belkacem et al., 2009b), already known to be a $beta$ Cephei, we theoretically investigate the driving by turbulent convection. By using a full non-adiab
atic computation of the damping rates, together with a computation of the energy injection rates, we provide an estimate of the amplitudes of modes excited by both the convective region induced by the iron opacity bump and the convective core. Despite uncertainties in the dynamical properties of such convective regions, we demonstrate that both are able to efficiently excite $p$ modes above the CoRoT observational threshold and the solar amplitudes. In addition, we emphasise the potential asteroseismic diagnostics provided by each convective region, which we hope will help to identify the one responsible for solar-like oscillations, and to give constraints on this convective zone. A forthcoming work will be dedicated to an extended investigation of the likelihood of solar-like oscillations across the Hertzsprung-Russell diagram.
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