The Kepler space mission provided near-continuous and high-precision photometry of about 207 000 stars, which can be used for asteroseismology. However, for successful seismic modeling it is equally ...important to have accurate stellar physical parameters. Therefore, supplementary ground-based data are needed. We report the results of the analysis of high-resolution spectroscopic data of A- and F-type stars from the Kepler field, which were obtained with the HERMES spectrograph on the Mercator telescope. We determined spectral types, atmospheric parameters and chemical abundances for a sample of 117 stars. Hydrogen Balmer, Fe i, and Fe ii lines were used to derive effective temperatures, surface gravities, and microturbulent velocities. We determined chemical abundances and projected rotational velocities using a spectrum synthesis technique. The atmospheric parameters obtained were compared with those from the Kepler Input Catalogue (KIC), confirming that the KIC effective temperatures are underestimated for A stars. Effective temperatures calculated by spectral energy distribution fitting are in good agreement with those determined from the spectral line analysis. The analysed sample comprises stars with approximately solar chemical abundances, as well as chemically peculiar stars of the Am, Ap, and λ Boo types. The distribution of the projected rotational velocity, vsin i, is typical for A and F stars and ranges from 8 to about 280 km s−1, with a mean of 134 km s−1.
The nearly continuous light curves with micromagnitude precision provided by the space mission Kepler are revolutionizing our view of pulsating stars. They have revealed a vast sea of low-amplitude ...pulsation modes that were undetectable from Earth. The long time base of Kepler light curves allows for the accurate determination of the frequencies and amplitudes of pulsation modes needed for in-depth asteroseismic modeling. However, for an asteroseismic study to be successful, the first estimates of stellar parameters need to be known and they cannot be derived from the Kepler photometry itself. The Kepler Input Catalog provides values for the effective temperature, surface gravity, and metallicity, but not always with sufficient accuracy. Moreover, information on the chemical composition and rotation rate is lacking. We are collecting low-resolution spectra for objects in the Kepler field of view with the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST, Xinglong observatory, China). All of the requested fields have now been observed at least once. In this paper, we describe those observations and provide a useful database for the whole astronomical community.
We present an evaluation of the performance of an automated classification of the Hipparcos periodic variable stars into 26 types. The sub-sample with the most reliable variability types available in ...the literature is used to train supervised algorithms to characterize the type dependencies on a number of attributes. The most useful attributes evaluated with the random forest methodology include, in decreasing order of importance, the period, the amplitude, the V−I colour index, the absolute magnitude, the residual around the folded light-curve model, the magnitude distribution skewness and the amplitude of the second harmonic of the Fourier series model relative to that of the fundamental frequency. Random forests and a multi-stage scheme involving Bayesian network and Gaussian mixture methods lead to statistically equivalent results. In standard 10-fold cross-validation (CV) experiments, the rate of correct classification is between 90 and 100 per cent, depending on the variability type. The main mis-classification cases, up to a rate of about 10 per cent, arise due to confusion between SPB and ACV blue variables and between eclipsing binaries, ellipsoidal variables and other variability types. Our training set and the predicted types for the other Hipparcos periodic stars are available online.
ABSTRACT The LAMOST-Kepler project was designed to obtain high-quality, low-resolution spectra of many of the stars in the Kepler field with the Large Sky Area Multi Object Fiber Spectroscopic ...Telescope (LAMOST) spectroscopic telescope. To date 101,086 spectra of 80,447 objects over the entire Kepler field have been acquired. Physical parameters, radial velocities, and rotational velocities of these stars will be reported in other papers. In this paper we present MK spectral classifications for these spectra determined with the automatic classification code MKCLASS. We discuss the quality and reliability of the spectral types and present histograms showing the frequency of the spectral types in the main table organized according to luminosity class. Finally, as examples of the use of this spectral database, we compute the proportion of A-type stars that are Am stars, and identify 32 new barium dwarf candidates.
ABSTRACT
Eclipsing binaries with (a) pulsating component(s) are remarkable objects to investigate the evolution and structure of stellar systems. Detailed studies of such systems are also important ...to reveal their pulsation properties. The largest sample of pulsating eclipsing binaries is the one containing δ Scuti variables. In this study, we present a comprehensive spectroscopic and photometric study of three such systems, GQ Dra, RR Lep, and TYC 683-640-1. Their orbital parameters were derived from their large-amplitude radial velocity variations. All systems were found to be single-lined eclipsing binaries. We determined the atmospheric parameters of the primary components, which are the more luminous ones. By using the results of the spectroscopic analysis, the binarity of the systems was modelled and the fundamental stellar parameters (mass, radius) of each system were obtained. In addition, the pulsations of the systems were analysed. Both GQ Dra and RR Lep exhibit frequencies that are modulated with the orbital period. Therefore, the pulsating components of these systems are identified as candidate tidally tilted pulsators.
Aims. A comprehensive and homogeneous determination of stellar parameters for the stars observed by the Kepler space telescope is necessary for statistical studies of their properties. As a result of ...the large number of stars monitored by Kepler, the largest and more complete databases of stellar parameters published to date are multiband photometric surveys. The LAMOST-Kepler survey, whose spectra are analyzed in the present paper, was the first large spectroscopic project, which started in 2011 and aimed at filling that gap. In this work we present the results of our analysis, which is focused on selecting spectra with emission lines and chromospherically active stars by means of the spectral subtraction of inactive templates. The spectroscopic determination of the atmospheric parameters for a large number of stars is a by-product of our analysis. Methods. We have used a purposely developed version of the code ROTFIT for the determination of the stellar parameters by exploiting a wide and homogeneous collection of real star spectra, namely the Indo US library. We provide a catalog with the atmospheric parameters (T sub(eff), logg, and Fe/H), radial velocity (RV), and an estimate of the projected rotation velocity (v sini). For cool stars (T sub(eff)< or = 6000K), we also calculated the Halpha and Caii-IRT fluxes, which are important proxies of chromospheric activity. Results. We have derived the RV and atmospheric parameters for 61753 spectra of 51385 stars. The average uncertainties, which we estimate from the stars observed more than once, are about 12kms super(-1), 1.3%, 0.05dex, and 0.06dex for RV, T sub(eff), logg, and Fe/H, respectively, although they are larger for the spectra with a very low signal-to-noise ratio. Literature data for a few hundred stars (mainly from high-resolution spectroscopy) were used to peform quality control of our results. The final accuracy of the RV is about 14 kms super(-1). The accuracy of the T sub(eff), logg, and Fe/H measurements is about 3.5%, 0.3dex, and 0.2dex, respectively. However, while the T sub(eff) values are in very good agreement with the literature, we noted some issues with the determination of Fe/H of metal poor stars and the tendency, for logg, to cluster around the values typical for main-sequence and red giant stars. We propose correction relations based on these comparisons and we show that this does not have a significant effect on the determination of the chromospheric fluxes. The RV distribution is asymmetric and shows an excess of stars with negative RVs that are larger at low metallicities. Despite the rather low LAMOST resolution, we were able to identify interesting and peculiar objects, such as stars with variable RV, ultrafast rotators, and emission-line objects. Based on the Halpha and Caii-IRT fluxes, we found 442 chromospherically active stars, one of which is a likely accreting object. The availability of precise rotation periods from the Kepler photometry allowed us to study the dependency of these chromospheric fluxes on the rotation rate for a very large sample of field stars.
We examine 4 yr almost continuous Kepler photometry of 115 B stars. We find that the light curves of 39 per cent of these stars can be described by a low-frequency sinusoid and its harmonic, usually ...with variable amplitude, which we interpret as rotational modulation. A large fraction (32 stars or 28 per cent) might be classified as ellipsoidal variables, but a statistical argument suggests that these are probably rotational variables as well. Ten stars (8 per cent of the total) have a peculiar periodogram feature which is common among A stars. The physical cause of this feature is unknown but very likely related to rotation. The large number of rotating variables suggests that star-spots are common in B stars and indicates that magnetic fields can be generated in radiative stellar envelopes. We find five stars (4 per cent) to be β Cep (BECP) variables, all of which have low frequencies with relatively large amplitudes. The presence of low frequencies in BECP stars is an unsolved problem. There are 21 stars (18 per cent) with multiple low frequencies which could be described as slowly pulsating B (SPB) variables. Other pulsating stars do not fit into the general definition of BECP or SPB variables in that they are cool but pulsate with high frequencies. We assume that these are further examples of the anomalous pulsating stars which in the past have been called ‘Maia’ variables. There are 11 stars (10 per cent) of this type. We also examined 300 B stars observed in the K2 Campaign 0 field. We find 12 BECP/Maia candidates and 15 SPB variables.
Abstract
We present the first results from the Transiting Exoplanet Survey Satellite (TESS) on the rotational and pulsational variability of magnetic chemically peculiar A-type stars. We analyse TESS ...2-min cadence data from sectors 1 and 2 on a sample of 83 stars. Five new rapidly oscillating Ap (roAp) stars are announced. One of these pulsates with periods around 4.7 min, making it the shortest period roAp star known to date. Four out of the five new roAp stars are multiperiodic. Three of these and the singly periodic one show the presence of rotational mode splitting. Individual frequencies are provided in all cases. In addition, seven previously known roAp stars are analysed. Additional modes of oscillation are found in some stars, while in others we are able to distinguish the true pulsations from possible aliases present in the ground-based data. We find that the pulsation amplitude in the TESS filter is typically a factor of 6 smaller than that in the B filter, which is usually used for ground-based observations. For four roAp stars we set constraints on the inclination angle and magnetic obliquity, through the application of the oblique pulsator model. We also confirm the absence of roAp-type pulsations down to amplitude limits of 6 and 13 $\mu$mag, respectively, in two of the best characterized non-oscillating Ap (noAp) stars. We announce 27 new rotational variables along with their rotation periods, and provide different rotation periods for seven other stars. Finally, we discuss how these results challenge state-of-the-art pulsation models for roAp stars.
ABSTRACT
Oscillating eclipsing Algols (oEAs) are remarkable systems that allow us to determine accurate fundamental stellar parameters (mass, radius) and probe the stellar interiors through ...pulsations. TZ Dra is an oEA system containing a δ Scuti component. To examine particular characteristics of such close systems including pulsations and mass transfer, we present a detailed photometric and spectroscopic study of TZ Dra. With the analysis of high-resolution spectra, the orbital parameters were determined by the radial velocity analysis and the atmospheric parameters were derived for the primary component. The binary modelling and the pulsational frequency analysis was carried out using the Transiting Exoplanet Survey Satellite data set. The H α line profiles show the signature of mass transfer from the cool to the hot binary component. The conclusion of mass transfer/mass-loss in the system was supported by the analysis of the orbital period changes. As a result, it was found that there is $3.52 \times 10^{-9}\, {\rm M}_\odot$ yr−1 mass-loss from the system most probably through the hotspot and stellar winds. Additionally, most pulsation frequencies originating from the primary component were found to be spaced by harmonics of the orbital frequencies in particular, 12 doublets spaced by 2forb were detected from which we infer that this star is a tidally tilted pulsator. A mean p-mode frequency spacing of ≈7.2 d−1 was found as well.
We present an 80-d long uninterrupted high-cadence K2 light curve of the B1Iab supergiant ρ Leo (HD 91316), deduced with the method of halo photometry. This light curve reveals a dominant frequency ...of f rot = 0.0373 d −1 and its harmonics. This dominant frequency corresponds with a rotation period of 26.8 d and is subject to amplitude and phase modulation. The K2 photometry additionally reveals multiperiodic low-frequency variability (<1.5 d −1) and is in full agreement with low-cadence high-resolution spectroscopy assembled during 1800 d. The spectroscopy reveals rotational modulation by a dynamic aspherical wind with an amplitude of about 20 km s −1 in the H α line, as well as photospheric velocity variations of a few km s −1 at frequencies in the range 0.2–0.6 d −1 in the Si III 4567 Å line. Given the large macroturbulence needed to explain the spectral line broadening of the star, we interpret the detected photospheric velocity as due to travelling superinertial low-degree large-scale gravity waves with dominant tangential amplitudes and discuss why ρ Leo is an excellent target to study how the observed photospheric variability propagates into the wind.
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