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Register a new userMagnetic variability in the young solar analog KIC 10644253: Observations from the Kepler satellite and the HERMES spectrograph
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The continuous photometric observations collected by the Kepler satellite over 4 years provide a whelm of data with an unequalled quantity and quality for the study of stellar evolution of more than 200000 stars. Moreover, the length of the dataset provide a unique source of information to detect magnetic activity and associated temporal variability in the acoustic oscillations. In this regards, the Kepler mission was awaited with great expectation. The search for the signature of magnetic activity variability in solar-like pulsations still remained unfruitful more than 2 years after the end of the nominal mission. Here, however, we report the discovery of temporal variability in the low-degree acoustic frequencies of the young (1 Gyr-old) solar analog KIC 10644253 with a modulation of about 1.5 years with significant temporal variations along the duration of the Kepler observations. The variations are in agreement with the derived photometric activity. The frequency shifts extracted for KIC 10644253 are shown to result from the same physical mechanisms involved in the inner sub-surface layers as in the Sun. In parallel, a detailed spectroscopic analysis of KIC 10644253 is performed based on complementary ground-based, high-resolution observations collected by the HERMES instrument mounted on the MERCATOR telescope. Its lithium abundance and chromospheric activity S-index confirm that KIC 10644253 is a young and more active star than the Sun.
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Stars similar to the Sun, known as solar analogues, provide an excellent opportunity to study the preceding and following evolutionary phases of our host star. The unprecedented quality of photometric data collected by the Kepler NASA mission allows us to characterise solar-like stars through asteroseismology and study diagnostics of stellar evolution, such as variation of magnetic activity, rotation and the surface lithium abundance. In this project, presented in a series of papers by Salabert et al. (2016a,b) and Beck et al (2016a,b), we investigate the link between stellar activity, rotation, lithium abundance and oscillations in a group of 18 solar-analogue stars through space photometry, obtained with the NASA Kepler space telescope and from currently 50+ hours of ground-based, high-resolution spectroscopy with the Hermes instrument. In these proceedings, we first discuss the selection of the stars in the sample, observations and calibrations and then summarise the main results of the project. By investigating the chromospheric and photospheric activity of the solar analogues in this sample, it was shown that for a large fraction of these stars the measured activity levels are compatible to levels of the 11-year solar activity cycle 23. A clear correlation between the lithium abundance and surface rotation was found for rotation periods shorter than the solar value. Comparing the lithium abundance measured in the solar analogues to evolutionary models with the Toulouse-Geneva Evolutionary Code (TGEC), we found that the solar models calibrated to the Sun also correctly describe the set of solar/stellar analogs showing that they share the same internal mixing physics. Finally, the star KIC 3241581 and KIC 10644353 are discussed in more detail.
The preliminary results of an analysis of the KIC 5390438 and KIC 5701829 light curves are presented. The variations of these stars were detected by Baran et al. (2011a) in a search for pulsating M dwarfs in the Kepler public database. The objects have been observed by the Kepler spacecraft during the Q2 and Q3 runs in a short-candence mode (integration time of $sim$ 1 min). A Fourier analysis of the time series data has been performed by using the PERIOD04 package. The resulting power spectrum of each star shows a clear excess of power in the frequency range 100 and 350 $mu$Hz with a sequence of spaced peaks typical of solar-like oscillations. A rough estimation of the large and small separations has been obtained. Spectroscopic observations secured at the Observatorio Astronomico Nacional in San Pedro Martir allowed us to derive a spectral classification K2III and K0III for KIC 5390438 and KIC 5701829, respectively. Thus, KIC 5390438 and KIC 5701829 have been identified as solar-like oscillating red giant stars.
Lithium abundance A(Li) and surface rotation are good diagnostic tools to probe the internal mixing and angular momentum transfer in stars. We explore the relation between surface rotation, A(Li) and age in a sample of seismic solar-analogue (SA) stars and study their possible binary nature. We select a sample of 18 SA observed by the NASA Kepler satellite for an in-depth analysis. Their seismic properties and surface rotation are well constrained from previous studies. About 53 hours of high-resolution spectroscopy were obtained to derive fundamental parameters and A(Li). These values were combined and confronted with seismic masses, radii and ages, as well as surface rotation periods. We identify a total of 6 binary systems. A well-defined relation between A(Li) and rotation was obtained. With models constrained by the characterisation of the individual mode frequencies for single stars, we identify a sequence of three SA with similar mass (~1.1Mo) and stellar ages ranging between 1 to 9 Gyr. Within the realistic estimate of ~7% for the mass uncertainty, we find a good agreement between the measured A(Li) and the predicted A(Li) evolution from a grid of models calculated with the Toulouse-Geneva stellar evolution code, which includes rotational internal mixing, calibrated to reproduce solar chemical properties. We present A(Li) for a consistent spectroscopic survey of SA with a mass of 1.00+/-0.15Mo, and characterised through asteroseismology and surface rotation rates based on Kepler observations. The correlation between A(Li) and P_rot supports the gyrochronological concept for stars younger than the Sun. The consensus between measured A(Li) for solar analogues with model grids, calibrated onto the Suns chemical properties suggests that these targets share the same internal physics. In this light, the solar Li and rotation rate appear to be normal for a star like the Sun.
We identify a set of 18 solar analogs among the seismic sample of solar-like stars observed by the Kepler satellite rotating between 10 and 40 days. This set is constructed using the asteroseismic stellar properties derived using either the global oscillation properties or the individual acoustic frequencies. We measure the magnetic activity properties of these stars using observations collected by the photometric Kepler satellite and by the ground-based, high-resolution Hermes spectrograph mounted on the Mercator telescope. The photospheric (Sph) and chromospheric (S index) magnetic activity levels of these seismic solar analogs are estimated and compared in relation to the solar activity. We show that the activity of the Sun is comparable to the activity of the seismic solar analogs, within the maximum-to-minimum temporal variations of the 11-year solar activity cycle 23. In agreement with previous studies, the youngest stars and fastest rotators in our sample are actually the most active. The activity of stars older than the Sun seems to not evolve much with age. Furthermore, the comparison of the photospheric, Sph, with the well-established chromospheric, S index, indicates that the Sph index can be used to provide a suitable magnetic activity proxy which can be easily estimated for a large number of stars from space photometric observations.
We present a detailed study of the two Sun-like stars KIC 7985370 and KIC 7765135, aimed at determining their activity level, spot distribution, and differential rotation. Both stars were discovered by us to be young stars and were observed by the NASA Kepler mission. The stellar parameters (vsini, spectral type, Teff, log g, and [Fe/H]) were derived from optical spectroscopy which allowed us also to study the chromospheric activity from the emission in the core of Halpha and CaII IRT lines. The high-precision Kepler photometric data spanning over 229 days were then fitted with a robust spot model. Model selection and parameter estimation are performed in a Bayesian manner, using a Markov chain Monte Carlo method. Both stars came out to be Sun-like with an age of about 100-200 Myr, based on their lithium content and kinematics. Their youth is confirmed by the high level of chromospheric activity, comparable to that displayed by the early G-type stars in the Pleiades cluster. The flux ratio of the CaII-IRT lines suggests that the cores of these lines are mainly formed in optically-thick regions analogous to solar plages. The model of the light curves requires at least seven enduring spots for KIC 7985370 and nine spots for KIC 7765135 for a satisfactory fit. The assumption of longevity of the star spots, whose area is allowed to evolve in time, is at the heart of our approach. We found, for both stars, a rather high value of the equator-to-pole differential rotation (dOmega~0.18 rad/day) which is in contrast with the predictions of some mean-field models of differential rotation for fast-rotating stars. Our results are instead in agreement with previous works on solar-type stars and with other models which predict a higher latitudinal shear, increasing with equatorial angular velocity.
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