Thanks to the recent very high-precision photometry of red giants from satellites such as Kepler, precise mass and radius values as well as accurate information of evolutionary stages are already established by asteroseismic approach for a large number of G-K giants. Based on the high-dispersion spectra of selected such 55 red giants in the Kepler field with precisely known seismic parameters (among which parallaxes are available for 9 stars), we checked the accuracy of the determination method of stellar parameters previously applied to many red giants by Takeda et al. (2008, PASJ, 60, 781), since it may be possible to discriminate their complex evolutionary status by using the surface gravity vs. mass diagram. We confirmed that our spectroscopic gravity and the seismic gravity satisfactorily agree with each other (to within ~0.1 dex) without any systematic difference. However, the mass values of He-burning red clump giants derived from stellar evolutionary tracks (~2-3 Msun) were found to be markedly larger by ~50% compared to the seismic values (~1-2 Msun) though such discrepancy is not seen for normal giants in the H-burning phase, which reflects the difficulty of mass determination from intricately overlapping tracks on the luminosity vs. effective temperature diagram. This consequence implies that the mass results of many red giants in the clump region determined by Takeda et al. (2008) are likely to be significantly overestimated. We also compare our spectroscopically established parameters with recent literature values, and further discuss the prospect of distinguishing the evolutionary status of red giants based on the conventional (i.e., non-seismic) approach.
Spectroscopic parameters (effective temperature, metallicity, etc) were determined for a large sample of ~100 red giants in the Kepler field, for which mass, radius, and evolutionary status had already been asteroseismologically established. These two kinds of spectroscopic and seismic information suffice to define the position on the "luminosity versus effective temperature" diagram and to assign an appropriate theoretical evolutionary track to each star. Making use of this advantage, we examined whether the stellar location on this diagram really matches the assigned track, which would make …
Given the implications for the origin of Li enhancement in red giants we have reviewed Li-rich classification of KIC~9821622, the only bonafide RGB giant with He inert-core till date, reported as a Li-rich giant by reanalyzing the high-resolution spectra. We have obtained $A(Li)_{LTE} = 1.42 \pm 0.05$ dex. After correcting for non-LTE it is $A(Li)_{NLTE} = 1.57 \pm 0.05 $ dex which is significantly less than the reported A(Li) = $1.80 \pm 0.2$~dex. We found the sub-ordinate line at 6103 Å is too weak or absent to measure Li abundance. The derived abundance is normal for red giants undergoing d…
The variable star known as NSVS 5597754 was identified as a short period RRc field star from ground-based photometry. This star happens to fall in the Kepler field of view and has been observed for 3 years with the Kepler spacecraft. Renamed KIC 9832227, this star was reidentified as an eclipsing binary with a periodicity of 0.457970 days. The NSVS ground based photometry yielded a period solution of 0.229 days. The Kepler photometry, which sampled the light cycle every 30 minutes, shows a peculiar behavior that is not purely eclipsing superposed over the primary variation. Follow-up ground ba…
We report the discovery of an Earth-sized planet in the habitable zone of a low-mass star called Kepler-1649. The planet, Kepler-1649 c, is 1.06$^{+0.15}_{-0.10}$ times the size of Earth and transits its 0.1977 +/- 0.0051 Msun mid M-dwarf host star every 19.5 days. It receives 74 +/- 3 % the incident flux of Earth, giving it an equilibrium temperature of 234 +/- 20K and placing it firmly inside the circumstellar habitable zone. Kepler-1649 also hosts a previously-known inner planet that orbits every 8.7 days and is roughly equivalent to Venus in size and incident flux. Kepler-1649 c was origin…
Today, asteroseismology is entering in its golden age thanks to the observations provided by the CoRoT and Kepler space missions. In particular, we will make significant progresses in the understanding of the structure and evolution of solar-like oscillating stars. These stars have acoustic modes stochastically excited by the near-surface convection. Thanks to the observations already provided by these two missions, we have detected several hundred of stars showing solar-like oscillations in the main sequence and several thousands in the red- giant branch. Here, I give an overview of the prese…
Context: the space-borne missions CoRoT and Kepler have provided photometric observations of unprecedented quality. The study of solar-like oscillations observed in red giant stars by these satellites allows a better understanding of the different physical processes occurring in their interiors. In particular, the study of the mode excitation and damping is a promising way to improve our understanding of stellar physics that has, so far, been performed only on a limited number of targets. Aims: the recent asteroseismic characterization of the evolutionary status for a large number of red giant…
Asteroseismology is a powerful tool to precisely determine the evolutionary status and fundamental properties of stars. With the unprecedented precision and nearly continuous photometric data acquired by the NASA Kepler mission, parameters of more than 10$^4$ stars have been determined nearly consistently. However, most studies still use photometric effective temperatures (Teff) and metallicities ([Fe/H]) as inputs, which are not sufficiently accurate as suggested by previous studies. We adopted the spectroscopic Teff and [Fe/H] values based on the LAMOST low-resolution spectra (R~1,800), and …
We present the latest results of our spectroscopic observations and refined modelling of a sample of detached eclipsing binaries (DEBs), selected from the $Kepler$} Eclipsing Binary Catalog, that are also double-lined spectroscopic binaries (SB2). New high resolution spectra obtained with the HIDES spectrograph, attached to the 1.88-m telescope of the Okayama Astrophysical Observatory supplemented the previous observations, allowing to improve physical parameters of some systems, and characterize three previously omitted. All the obtained radial velocities (RVs) were combined with $Kepler$ pho…
49 new eclipsing twin binary candidates are identified and analyzed based on Kepler eclipsing binary light curves. Their colours and spectral types are calculated according to our classification. A comparison of the spectral type distribution of eclipsing twin binary systems showed that F-type twins dominate among others, which agrees well with recent studies. The distance of eclipsing twin binaries from the galactic plane shows that F and G-type twins can be seen at any distance from the galactic plane and most of the known eclipsing binary twins are located within 200 pc of the galactic plan…
The spectroscopic class of subdwarf A-type (sdA) stars has come into focus in recent years because of their possible link to extremely low-mass white dwarfs, a rare class of objects resulting from binary evolution. Although most sdA stars are consistent with metal-poor halo main-sequence stars, the formation and evolution of a fraction of these stars are still matters of debate. The identification of photometric variability can help to put further constraints on the evolutionary status of sdA stars, in particular through the analysis of pulsations. Moreover, the binary ratio, which can be dedu…
We report the discovery of a new exceptional young lithium-rich giant, KIC 9821622, in the \textit{Kepler} field that exhibits an unusually large enhancement of $α$, Fe-peak, and \textit{r}-process elements. From high-resolution spectra obtained with GRACES at Gemini North, we derived fundamental parameters and detailed chemical abundances of 23 elements from equivalent widths and synthesis analysis. By combining atmospheric stellar parameters with available asteroseismic data, we obtained the stellar mass, radius, and age. The data analysis reveals that KIC 9821622 is a Li-rich (A(Li)$_{NLTE}…
We present the results of our spectroscopic observations of eight detached eclipsing binaries (DEBs), selected from the $Kepler$ Eclipsing Binary Catalog. Radial velocities (RVs) were calculated from high resolution spectra obtained with the HIDES pectrograph, attached to the 1.88-m telescope of the Okayama Astrophysical Observatory, and were used to characterize the targets in combination with the $Kepler$ light curves. For each binary we obtained a full set of orbital and physical parameters, reaching precision below 3% in masses and radii for 5 pairs. By comparing our results with theoretic…
We present results of our spectroscopic observations of nine detached eclipsing binaries (DEBs), selected from the $Kepler$ Eclipsing Binary Catalog, that only show one set of spectral lines. Radial velocities (RVs) were calculated from the high resolution spectra obtained with the HIDES instrument, attached to the 1.88-m telescope at the Okayama Astrophysical Observatory, and from the public APOGEE archive. In our sample we found five single-lined binaries, with one component dominating the spectrum. The orbital and light curve solutions were found for four of them, and compared with isochron…
The CoRoT and Kepler missions provide us with thousands of red-giant light curves that allow a very precise asteroseismic study of these objects. Before CoRoT and Kepler, the red-giant oscillation patterns remained obscure. Now, these spectra are much more clear and unveil many crucial interior structure properties. For thousands of red giants, we can derive from the seismic data precise estimates of the stellar mass and radius, the evolutionary status of the giants (with a clear difference between clump and RGB stars), the internal differential rotation, the mass loss, the distance of the sta…
We present a first determination of distances and extinctions for individual stars in the first release of the APOKASC catalogue, built from the joint efforts of the Apache Point Observatory Galactic Evolution Experiment (APOGEE) and the Kepler Asteroseismic Science Consortium (KASC). Our method takes into account the spectroscopic constraints derived from the APOGEE Stellar Parameters and Chemical Abundances Pipeline, together with the asteroseismic parameters from KASC. These parameters are then employed to estimate intrinsic stellar properties, including absolute magnitudes, using the Bayes…
We aim to describe the oscillation power excess observed in Kepler red giants, and to investigate empirical scaling relations governing these parameters. From these scalings relations, we derive new physical properties of red giant oscillations. Various different methods were compared in order to validate the processes and to derive reliable output values. For consistency, a single method was then used to determine scaling relations for the relevant global asteroseismic parameters: mean mode height, mean height of the background signal superimposed on the oscillation power excess, width of the…
To date, only 18 exoplanets with radial velocity (RV) semi-amplitudes $<2$ m/s have had their masses directly constrained. The biggest obstacle to RV detection of such exoplanets is variability intrinsic to stars themselves, e.g. nuisance signals arising from surface magnetic activity such as rotating spots and plages, which can drown out or even mimic planetary RV signals. We use Kepler-37 - known to host three transiting planets, one of which, Kepler-37d, should be on the cusp of RV detectability with modern spectrographs - as a case study in disentangling planetary and stellar activity sign…
In order to clarify the properties of the secondary clump star HD 226808 (KIC 5307747), we combined four years of data from Kepler space photometry with high-resolution spectroscopy of the High Efficiency and Resolution Mercator Échelle Spectrograph mounted on the Mercator telescope. The fundamental atmospheric parameters, radial velocities, rotation velocities, and elemental abundance for Fe and Li were determined by analyzing line strengths and fitting-line profiles, based on a 1D local thermodynamic equilibrium model atmosphere. Second, we analyzed a photometric light curve obtained by Kepl…
The CoRoT mission has provided thousands of red-giant light curves. The analysis of their solar-like oscillations allows us to characterize their stellar properties. Up to now, the global seismic parameters of the pressure modes remain unable to distinguish red-clump giants from members of the red-giant branch. As recently done with Kepler red giants, we intend to analyze and use the so-called mixed modes to determine the evolutionary status of the red giants observed with CoRoT. We also aim at deriving different seismic characteristics depending on evolution. The complete identification of th…
The mass (M) of a star can be evaluated from its spectroscopically determined effective temperature (T _eff) and metallicity ([Fe/H]) along with the luminosity (L; derived from parallax), while comparing them with grids of theoretical evolutionary tracks. It has been argued, however, that such a track-based mass (M_trk) may tend to be overestimated for the case of red giants. Meanwhile, there is an alternative approach of evaluating mass (M_gLT) directly from surface gravity (g), L, and T_eff. The practical reliability of M_gLT was examined for ~100 benchmark giants in the Kepler field, for wh…
In this paper we report two new super Li-rich K giants: KIC 2305930 and KIC 12645107 with Li abundances exceeding that of ISM (A(Li) $\geq$ 3.2 dex). Importantly, both the giants have been classified as core He-burning red clump stars based on asteroseismic data from Kepler mission. Also, both the stars are found to be low mass (M $\approx$ 1.0 M$_{\odot}$) which, together with an evidence of their evolutionary status of being red clump imply that the stars have gone through both the luminosity bump and He-flash during their RGB evolution. Stars' large Li abundance and evolutionary phase sugge…
The star HII 2407 is a member of the relatively young Pleiades star cluster and was previously discovered to be a single-lined spectroscopic binary. It is newly identified here within $Kepler$/$K2$ photometric time series data as an eclipsing binary system. Mutual fitting of the radial velocity and photometric data leads to an orbital solution and constraints on fundamental stellar parameters. While the primary has arrived on the main sequence, the secondary is still pre-main-sequence and we compare our results for the $M/M_\odot$ and $R/R_\odot$ values with stellar evolutionary models. We als…
The space-borne missions CoRoT and Kepler opened up a new opportunity for better understanding stellar evolution by probing stellar interiors with unrivalled high-precision photometric data. Kepler has observed stellar oscillation for four years, which gave access to excellent frequency resolution that enables deciphering the oscillation spectrum of evolved red giant branch and asymptotic giant branch stars. The internal structure of stars in the upper parts of the red and asymptotic giant branches is poorly constrained, which makes the distinction between red and asymptotic giants difficult. …
Solar-analog stars provide an excellent opportunity to study the Sun's evolution, i.e. the changes with time in stellar structure, activity, or rotation for solar-like stars. The unparalleled photometric data from the NASA space telescope Kepler allows us to study and characterise solar-like stars through asteroseismology. We aim to spectroscopically investigate the fundamental parameter and chromospheric activity of solar analogues and twins, based on observations obtained with the HERMES spectrograph and combine them with asteroseismology. Therefore, we need to build a solar atlas for the sp…
We present our results of a combined analysis of radial velocity and light curves of a double-lined spectroscopic and eclipsing binary KIC 09246715, observed photometrically by the $Kepler$ satellite, and spectroscopically with the OAO-1.88m telescope with the HIgh-Dispertion Echelle Spectrograph (HIDES). The target was claimed to be composed of two red giants, one of which is showing solar-like oscillations. We have found that the mass and radius of the primary are $M_1=2.169\pm0.024$ M$_\odot$ and $R_1=8.47\pm0.13$ R$_\odot$, and of the secondary: $M_2=2.143\pm0.025$ M$_\odot$ and $R_2=8.18\…
We present an asteroseismic analysis of the Kepler light curve of KIC 8263801, a red-giant star in the open cluster NGC 6866 that has previously been reported to be a helium-burning red-clump star. We extracted the frequencies of the radial and quadrupole modes from its frequency power spectrum and determined its properties using a grid of evolutionary models constructed with MESA. The oscillation frequencies were calculated using the GYRE code and the surface term was corrected using the Ball & Gizon(2014) prescription. We find that the star has a mass of $M/M_{\odot} = 1.793\pm 0.072$, age $…
The accuracy of stellar masses and radii determined from asteroseismology is not known! We examine this issue for giant stars by comparing classical measurements of detached eclipsing binary systems (dEBs) with asteroseismic measurements from the Kepler mission. For star clusters, we extrapolate measurements of dEBs in the turn-off region to the red giant branch and the red clump where we investigate the giants as an ensemble. For the field stars, we measure dEBs with an oscillating giant component. These measurements allow a comparison of masses and radii calculated from a classical eclipsing…
We give here the Table of Contents of the proceedings from the workshop "Red Giants as Probes of the Structure and Evolution of the Milky Way", held in Roma, 15-17 November 2010. Exciting results are blooming, thanks to a convergence between unprecedented asteroseismic data obtained by the satellites CoRoT and Kepler, and state-of-the-art models of the internal structure of red giants and of galactic evolution. The pulsation properties now available for thousands of red giants promise to add valuable and independent constraints to current models of structure and evolution of our galaxy. Such a…
Asteroseismic analysis of solar-like stars allows us to determine physical parameters such as stellar mass, with a higher precision compared to most other methods. Even in a well-studied cluster such as the Hyades, the masses of the red giant stars are not well known, and previous mass estimates are based on model calculations (isochrones). The four known red giants in the Hyades are assumed to be clump (core-helium-burning) stars based on their positions in colour-magnitude diagrams, however asteroseismology offers an opportunity to test this assumption. Using asteroseismic techniques combine…
