We present a simulation showing that super-Nyquist frequencies may have periodic amplitude and frequency modulations, even if actually stable, in time series sampled like the Kepler data. These modulations are caused by the barycentric time correction, which destroys the evenly spaced time measurements, making the Nyquist frequency variable over the spacecraft orbit around the Sun. These modulations can easily be identified in pulsating stars from Kepler's photometric data.
Barycentric corrections made to the timing of Kepler observations, necessitated by variations in light arrival time at the satellite, break the regular time-sampling of the data -- the time stamps are periodically modulated. A consequence is that Nyquist aliases are split into multiplets that can be identified by their shape. Real pulsation frequencies are distinguishable from these aliases and their frequencies are completely recoverable, even in the super-Nyquist regime, that is, when the sampling interval is longer than half the pulsation period. We provide an analytical derivation of the p…
We consider the prospects for detecting solar-like oscillations in the "super-Nyquist" regime of long-cadence (LC) Kepler photometry, i.e., above the associated Nyquist frequency of approximately 283 μHz. Targets of interest are cool, evolved subgiants and stars lying at the base of the red-giant branch. These stars would ordinarily be studied using the short-cadence (SC) data, since the associated SC Nyquist frequency lies well above the frequencies of the detectable oscillations. However, the number of available SC target slots is quite limited. This imposes a severe restriction on the size …
We perform a search for rapidly oscillating Ap stars in the Kepler long-cadence data, where true oscillations above the Nyquist limit of 283.21 μHz can be reliably distinguished from aliases as a consequence of the barycentric time corrections applied to the Kepler data. We find evidence for rapid oscillations in six stars: KIC 6631188, KIC 7018170, KIC 10685175, KIC 11031749, KIC 11296437 and KIC 11409673, and identify each star as chemically peculiar through either pre-existing classifications or spectroscopic measurements. For each star, we identify the principal pulsation mode, and are abl…
The original Kepler mission achieved high photometric precision thanks to ultra-stable pointing enabled by use of four reaction wheels. The loss of two of these reaction wheels reduced the telescope's ability to point precisely for extended periods of time, and as a result, the photometric precision has suffered. We present a technique for generating photometric light curves from pixel-level data obtained with the two-wheeled extended Kepler mission, K2. Our photometric technique accounts for the non-uniform pixel response function of the Kepler detectors by correlating flux measurements with …
We present a simple phenomenological method for detrending of raw Kepler and MOST photometry, which is illustrated by means of photometric data processing of two periodically variable chemically peculiar stars, KIC 6950556 and HD 37633. In principle, this method may be applied to any type of periodically variable objects and satellite or ground based photometries. As a by product, we have identified KIC 6950556 as a magnetic chemically peculiar star with an ACV type variability.
We analysed solar-like oscillations in 1523 $\textit{Kepler}$ red giants which have previously been misclassified as subgiants, with predicted $ν_{\rm max}$ values (based on the Kepler Input Catalogue) between 280$μ$Hz to 700$μ$Hz. We report the discovery of 626 new oscillating red giants in our sample, in addition to 897 oscillators that were previously characterized by Hekker et al. (2011) from one quarter of $\textit{Kepler}$ data. Our sample increases the known number of oscillating low-luminosity red giants by $26\%$ (up to $\sim$ 1900 stars). About three quarters of our sample are classi…
We summarize our ground-based program of spectroscopic and photometric observations of the asteroseismic targets of the Kepler space telescope. We have already determined atmospheric parameters, projected velocity of rotation, and radial velocity of 62 Kepler asteroseismic targets and 33 other stars in the Kepler field of view. We discovered six single-lined and two double-lined spectroscopic binaries, we determined the interstellar reddening for 29 stars in the Kepler field of view, and discovered three delta Sct, two gamma Dor and 14 other variable stars in the field of NGC 6866.
One of the tasks of the Kepler Asteroseismic Science Operations Center (KASOC) is to provide asteroseismic analyses on Kepler Objects of Interest (KOIs). However, asteroseismic analysis of planetary host stars presents some unique complications with respect to data preprocessing, compared to pure asteroseismic targets. If not accounted for, the presence of planetary transits in the photometric time series often greatly complicates or even hinders these asteroseismic analyses. This drives the need for specialised methods of preprocessing data to make them suitable for asteroseismic analysis. In…
High-quality time series provided by space instrumentation such as CoRoT and Kepler, allow us to measure modulations in the light curves due to changes in the surface of stars related to rotation and activity. Therefore, we are able to infer the surface (possibly differential) rotation rate. However, instrumental perturbations can also produce artificial modulations in the light curves that would mimic those of truly stellar origin. In this work we will concentrate on Kepler observations in order to review an optimal way to extract reliable surface rotation rates.
This work brings a wavelet analysis for 14 Kepler white dwarf stars, in order to confirm their photometric variability behavior and to search for periodicities in these targets. From the observed Kepler light curves we obtained the wavelet local and global power spectra. Through this procedure, one can perform an analysis in time-frequency domain rich in details, and so to obtain a new perspective on the time evolution of the periodicities present in these stars. We identified a photometric variability behavior in ten white dwarfs, corresponding to period variations of ~ 2 h to 18 days: among …
Reported are UBV and uvbybeta observations of 15 candidates for Kepler primary astero- seismic targets and 14 other stars in the Kepler field, carried out at the M.G. Fracastoro station of the Catania Astrophysical Observatory. These data serve to plot the 29 stars in two-parameter diagrams with the photometric indices (such as B-V or delta m1 and the atmospheric parameters (such as the MK type or [Fe/H]) as coordinates. The two-parameter diagrams show no evidence of interstellar reddening. The photometric indices B-V and beta are then used to derive photometric effective temperatures, Teff(B-…
The aim of this work is to evaluate the performance of photometric metallicity [Fe/H], determined based on V-band light-curves photometrically transformed from the gr-band light-curves. We tested this by using a set of homogeneous sample of fundamental mode RR Lyrae located in the Kepler field. It was found that the color-term is necessary in such photometric transformation. We demonstrated that including the color-term the determined photometric [Fe/H] are in good agreement to the spectroscopic [Fe/H], either based on the calibrated or the transformed V-band light-curves. We also tested the i…
We utilize Kepler data to study the precision differential photometric variability of solar-type and cooler stars at different timescales, ranging from half an hour to 3 months. We define a diagnostic that characterizes the median differential intensity change between data bins of a given timescale. We apply the same diagnostics to SOHO data that has been rendered comparable to Kepler. The Sun exhibits similar photometric variability on all timescales as comparable solar-type stars in the Kepler field (it is not unusually quiet). The previously-defined photometric "range" serves as our activit…
We propose a photometric technique for future space missions that overcomes the problem of Nyquist aliases. These aliases result from typically long cadences of observation imposed by telemetry constraints. The proposed method is to introduce a periodic modulation to the sampling rate. Suitable combinations of the frequency and the amplitude of this modulation allow the true peaks to be distinguished from the aliases. We provide an analytical proof of the validity of this method and some demonstrations with simulated data. We also propose to divide a long cadence into two unequal parts, aiming…
The characterisation of detached eclipsing binaries with low mass components has become important when verifying the role of convection in stellar evolutionary models, which requires model-independent measurements of stellar parameters with great precision. However, spectroscopic characterisation depends on single-target radial velocity observations and only a few tens of well-studied low-mass systems have been diagnosed in this way. We characterise eclipsing detached systems from the {\it Kepler} field with low mass components by adopting a purely-photometric method. Based on an extensive mul…
We present the motivations for and methods we used to create a new ground-based photometric survey of the field targeted by the NASA Kepler Mission. The survey contains magnitudes for 4414002 sources in one or more of the UBV filters, including 1862902 sources detected in all three filters. The typical completeness limit is U~18.7, B~19.3, and V~19.1 magnitudes, but varies by location. The area covered is 191 square degrees and includes the areas on and between the 42 Kepler CCDs as well as additional areas around the perimeter of the Kepler field. The major significance of this survey is our …
We perform photometric (the Lomb-Scargle periodogram, autocorrelation, and wavelet) and asteroseismic analyses of 92 Kepler solar-like main-sequence stars to understand the reliability of the measured stellar rotation periods. We focus on the 70 stars without reported stellar companions, and classify them into four groups according to the quarter-to-quarter variance of the Lomb-Scargle period and the precision of the asteroseismic period. We present detailed individual comparison among photometric and asteroseismic constraints for these stars. We find that most of our targets exhibit significa…
Limb darkening is an important stellar phenomenon and must be accounted for in the study of stellar spectra, eclipsing binaries, transiting planetary systems, and microlensing events. The power-2 limb-darkening law provides a good match to the specific intensities predicted by stellar atmosphere models: it is better than other two-parameter laws and is only surpassed by the four-parameter law. Predictions of the limb-darkening coefficients for the power-2 law are not widely available. We therefore compute them, using stellar atmosphere models generated by the ATLAS (plane-parallel) code. Limb-…
A close comparison of Kepler short- and long-cadence data released prior to 2011 Nov 1 has shown some subtle differences that make the short-cadence data superior to their long-cadence counterparts. The inevitable results of a faster sampling rate are present: the short-cadence data provide greater time resolution for short-lived events like flares, and have a much higher Nyquist frequency than the long-cadence data; however, they also contain fewer high-amplitude peaks at low frequency and allow a more precise determination of pulsation frequencies, amplitudes and phases. The latter observati…
While stellar rotation periods $P_\mathrm{rot}$ may be measured from broadband photometry, the photometric modulation becomes harder to detect for slower rotators, which could bias measurements of the long-period tail of the $P_\mathrm{rot}$ distribution. Alternatively, the $P_\mathrm{rot}$ distribution of stars can be inferred from their projected rotation velocities $v\sin i$ and radii $R$, without being biased against photometrically quiet stars. We solve this inference problem using a hierarchical Bayesian framework, which (i) is applicable to heteroscedastic measurements of $v\sin i$ and …
We describe the photometric calibration and stellar classification methods used to produce the Kepler Input Catalog (KIC). The KIC is a catalog containing photometric and physical data for sources in the Kepler Mission field of view; it is used by the mission to select optimal targets. We derived atmospheric extinction corrections from hourly observations of secondary standard fields within the Kepler field of view. Repeatability of absolute photometry for stars brighter than magnitude 15 is typically 2%. We estimated stellar parameters Teff, log(g), log (Z), E_{B-V} using Bayesian posterior p…
Photometry from the Kepler mission is optimized to detect small, short duration signals like planet transits at the expense of long-term trends. This long-term variability can be recovered in photometry from the Full Frame Images (FFIs), a set of calibration data collected approximately monthly during the Kepler mission. Here, we present f3, an open-source package to perform photometry on the Kepler FFIs in order to detect changes in the brightness of stars in the Kepler field of view over long time baselines. We apply this package to a sample of 4,000 Sun-like stars with measured rotation per…
Obtaining measurements of chromospheric and photometric activity of stars with near-solar fundamental parameters and rotation periods is important for a better understanding of solar-stellar connection. We select a sample of 2603 stars with near-solar fundamental parameters from the Large Sky Area Multi-Object Fiber Spectroscopic Telescope (LAMOST)-Kepler field and use LAMOST spectra to measure their chromospheric activity and Kepler light curves to measure their photospheric activity (i.e., the amplitude of the photometric variability). While the rotation periods of 1556 of these stars could …
We cross-match the GALEX and Kepler surveys to create a unique dataset with both ultraviolet (UV) measurements and highly precise photometric variability measurements in the visible light spectrum. As stellar activity is driven by magnetic field modulations, we have used UV emission from the magnetically heated gas in the stellar atmosphere to serve as our proxy for the more well-known stellar activity indicator, R'HK. The R'HK approximations were in turn used to estimate the level of astrophysical noise expected in radial velocity (RV) measurements and these were then searched for correlation…
We compare stellar photometric variability, as measured from Kepler light curves by Basri et al. (2011), with measurements of radial velocity (RV) root-mean-square (RMS) variations of all California Planet Search overlap stars. We newly derive rotation periods from the Kepler light curves for all of the stars in our study sample. The RV variations reported herein range from less than 4 m/s to 135 m/s, yet the stars all have amplitudes of photometric variability less than 3 mmag, reflecting the preference of the RV program for chromospherically "quiet" stars. Despite the small size of our sampl…
Mining the Kepler Field: Atmospheric Parameters, Bolometric Corrections, and Luminosities
Instituto de Astrofísica de Canarias (IAC)（加那利天体物理学研究所）；Universidad de La Laguna (ULL)（拉古纳大学）；Lowell Observatory（洛厄尔天文台）；Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto（葡萄牙大学空间天体物理学与科学研究所）；Departamento de Física e Astronomia, Universidade do Porto（波尔图大学物理与天文学系）；Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM（巴黎萨克雷大学、巴黎西岱大学、法国原子能与替代能源委员会、法国国家科学研究中心、天体物理学信息与方法实验室）
We have used high-resolution spectroscopy to observe the Kepler-16 eclipsing binary as a double-lined system, and measure precise radial velocities for both stellar components. These velocities yield a dynamical mass-ratio of q=0.2994+-0.0031. When combined with the inclination, i=90.3401+0.0016-0.0019 deg, measured from the Kepler photometric data by Doyle et al. 2011, we derive dynamical masses for the Kepler-16 components of M_A=0.654+-0.017 M_sun and M_B=0.1959+-0.0031 M_sun, a precision of 2.5% and 1.5% respectively. Our results confirm at the ~2% level the mass-ratio derived by Doyle et …
We present our analyses of 15 months of Kepler data on KIC 10139564. We detected 57 periodicities with a variety of properties not previously observed all together in one pulsating subdwarf B star. Ten of the periodicities were found in the low-frequency region, and we associate them with nonradial g-modes. The other periodicities were found in the high-frequency region, which are likely p-modes. We discovered that most of the periodicities are components of multiplets with a common spacing. Assuming that multiplets are caused by rotation, we derive a rotation period of 25.6(1.8) days. The mul…
The Kepler space telescope monitors over 156.000 stars with an unprecedented photometric precision. We are interested in stellar rotational periods which we find using Lomb-Scargle periodograms. This work focuses on the 306 exoplanet candidate host stars released on June 15, 2010. We present statistics on how many of them show periodic photometric variability, providing preliminary periods and estimates of stellar activity. In the future, our work will focus on spot evolution and differential rotation.
The Kepler mission provides an exciting opportunity to study the lightcurves of stars with unprecedented precision and continuity of coverage. This is the first look at a large sample of stars with photometric data of a quality that has heretofore been only available for our Sun. It provides the first opportunity to compare the irradiance variations of our Sun to a large cohort of stars ranging from vary similar to rather different stellar properties, at a wide variety of ages. Although Kepler data is in an early phase of maturity, and we only analyze the first month of coverage, it is suffici…
Brightness variations due to dark spots on the stellar surface encode information about stellar surface rotation and magnetic activity. In this work, we analyze the Kepler long-cadence data of 26,521 main-sequence stars of spectral types M and K in order to measure their surface rotation and photometric activity level. Rotation-period estimates are obtained by the combination of a wavelet analysis and autocorrelation function of the light curves. Reliable rotation estimates are determined by comparing the results from the different rotation diagnostics and four data sets. We also measure the p…
Stellar light curves are well known to encode physical stellar properties. Precise, automated and computationally inexpensive methods to derive physical parameters from light curves are needed to cope with the large influx of these data from space-based missions such as Kepler and TESS. Here we present a new methodology which we call The Swan, a fast, generalizable and effective approach for deriving stellar surface gravity ($\log g$) for main sequence, subgiant and red giant stars from Kepler light curves using local linear regression on the full frequency content of Kepler long cadence power…
The Kepler mission has been providing high-quality photometric data leading to many breakthroughs in the exoplanet search and in stellar physics. Stellar magnetic activity results from the interaction between rotation, convection, and magnetic field. Constraining these processes is important if we want to better understand stellar magnetic activity. Using the Sun, we want to test a magnetic activity index based on the analysis of the photo- metric response and then apply it to a sample of M dwarfs observed by Kepler. We estimate a global stellar magnetic activity index by measuring the standar…
We performed simple spot-model calculations for quasi-periodic brightness variations of solar-type stars showing superflares, by using Kepler photometric data. Most of superflare stars show quasi-periodic brightness modulations with the typical period of one to a few tens of days. Our results indicate that these brightness variations of superflare stars can be explained by the rotation of the star with fairly large starspots. Using the result of the period analysis, we investigated the relation between the energy and frequency of superflares and the rotation period. Stars with relatively slowe…
Light curve modulations in the sample of Kepler planet candidates allows the disentangling of the nature of the transiting object by photometrically measuring its mass. This is possible by detecting the effects of the gravitational pull of the companion (ellipsoidal modulations) and in some cases, the photometric imprints of the Doppler effect when observing in a broad band (Doppler beaming). We aim to photometrically unveil the nature of some transiting objects showing clear modulations in the phase-folded Kepler light curve. We selected a subsample among the large crop of Kepler objects of i…
Variability induced motion (VIM) is an observable effect in simultaneous astrometric and photometric measurements caused by brightness variation in one of the components of a double source or blended image, which manifests itself as a strongly correlated shift of the optical photocenter. We have processed the entire collection of the Kepler long-cadence light curve data looking for correlated signals in astrometry and photometry on the time basis of a quarter year. Limiting the VIM correlation coefficient to 0.3, VIM events are detected for 129,525 Kepler stars at least in one quarter. Of 7305…
High-precision space-based photometry obtained by the \emph{Kepler} and \emph{TESS} missions has revealed evidence of rotational modulation associated with main sequence (MS) A and late-B type stars. Generally, such variability in these objects is attributed to inhomogeneous surface structures (e.g. chemical spots), which are typically linked to strong magnetic fields ($B\gtrsim100\,{\rm G}$) visible at the surface. It has been reported that $\approx44$~per~cent of all A-type stars observed during the \emph{Kepler} mission exhibit rotationally modulated light curves. This is surprising conside…
High-precision time series photometry with the Kepler satellite has been crucial to our understanding both of exoplanets, and via asteroseismology, of stellar physics. After the failure of two reaction wheels, the Kepler satellite has been repurposed as Kepler-2 (K2), observing fields close to the ecliptic plane. As these fields contain many more bright stars than the original Kepler field, K2 provides an unprecedented opportunity to study nearby objects amenable to detailed follow-up with ground-based instruments. Due to bandwidth constraints, only a small fraction of pixels can be downloaded…
Stellar flares are an important aspect of magnetic activity -- both for stellar evolution and circumstellar habitability viewpoints - but automatically and accurately finding them is still a challenge to researchers in the Big Data era of astronomy. We present an experiment to detect flares in space-borne photometric data using deep neural networks. Using a set of artificial data and real photometric data we trained a set of neural networks, and found that the best performing architectures were the recurrent neural networks (RNNs) using Long Short-Term Memory (LSTM) layers. The best trained ne…
We examine the photometric variability of stars in the M 67 field using Kepler/K2-Campaign-5 light curves. Variabilities and periods were determined for 639 stars. The mean photometric period of 28 single Sun-like members stars in M 67 is 23.4 +/- 1.2 d. This corresponds to a gyro-age of 3.7 +/- 0.3 Gyr, assuming the periods can be associated with rotation. The intrinsic variabilities of the solar analogs are greater than the Sun's variability, as measure from VIRGO fluxes. We also find evidence that the single cluster members have a different distribution of variability than the binary member…
The mutual orbital alignment in multiple planetary systems is an important parameter for understanding their formation. There are a number of elaborate techniques to determine the alignment parameters using photometric or spectroscopic data. Planet--planet occultations (PPOs), which can occur in multiple transiting systems, are one intuitive example. While the presence of PPOs constrains the orbital alignment, the absence at first glance does not. Planetary systems, for which the measurement of orbital obliquities with conventional techniques remains elusive, call for new methods whereby at le…
We present the first data release of the Kepler Smear Campaign, using collateral 'smear' data obtained in the Kepler four-year mission to reconstruct light curves of 102 stars too bright to have been otherwise targeted. We describe the pipeline developed to extract and calibrate these light curves, and show that we attain photometric precision comparable to stars analyzed by the standard pipeline in the nominal Kepler mission. In this paper, aside from publishing the light curves of these stars, we focus on 66 red giants for which we detect solar-like oscillations, characterizing 33 of these i…
We present the initial results of a 40 night contiguous ground-based campaign of time series photometric observations of a 1.39 sq. deg field located within the NASA Kepler mission field of view. The goal of this pre-launch survey was to search for transiting extrasolar planets and to provide independent variability information of stellar sources. We have gathered a data set containing light curves of 54,687 stars from which we have created a statistical sub-sample of 13,786 stars between 14< r <18.5 and have statistically examined each light curve to test for variability. We present a summary…
Power-2 limb-darkening coefficients for the $uvby$, $UBVRIJHK$, SDSS $ugriz$, Gaia, Kepler, TESS, and CHEOPS photometric systems II. PHOENIX spherically symmetric stellar atmosphere models
Dark magnetic spots crossing the stellar disc lead to quasi-periodic brightness variations, which allow us to constrain stellar surface rotation and photometric activity. The current work is the second of this series (Santos et al. 2019; Paper I), where we analyze the Kepler long-cadence data of 132,921 main-sequence F and G stars and late subgiant stars. Rotation-period candidates are obtained by combining wavelet analysis with autocorrelation function. Reliable rotation periods are then selected via a machine learning (ML) algorithm (Breton et al. 2021), automatic selection, and complementar…
Kepler and K2 data analysis reported in the literature is mostly based on aperture photometry. Because of Kepler's large, undersampled pixels and the presence of nearby sources, aperture photometry is not always the ideal way to obtain high-precision photometry and, because of this, the data set has not been fully exploited so far. We present a new method that builds on our experience with undersampled HST images. The method involves a point-spread function (PSF) neighbour-subtraction and was specifically developed to exploit the huge potential offered by the K2 "super-stamps" covering the cor…
We make use of the high photometric precision of Kepler to search for periodic modulations among 14 normal (DA- and DB-type, likely non-magnetic) hot white dwarfs (WDs). In five, and possibly up to seven of the WDs, we detect periodic, ~2 hr to 10 d, variations, with semi-amplitudes of 60 - 2000 ppm, lower than ever seen in WDs. We consider various explanations: WD rotation combined with magnetic cool spots; rotation combined with magnetic dichroism; rotation combined with hot spots from an interstellar-medium accretion flow; transits by size ~50 - 200 km objects; relativistic beaming due to r…
By combining ground-based spectrographic observations of variability in the chromospheric emission from Sun-like stars with the variability seen in their eigenmode frequencies, it is possible to relate the changes observed at the surfaces of these stars to the changes taking place in the interior. By further comparing this variability to changes in the relative flux from the stars, one can obtain an expression for how these activity indicators relate to the energy output from the stars. Such studies become very pertinent when the variability can be related to stellar cycles as they can then be…
We provide an overview of stellar variability in the first quarter of data from the Kepler mission. The intent of this paper is to examine the entire sample of over 150,000 target stars for periodic behavior in their lightcurves, and relate this to stellar characteristics. These data constitute an unprecedented study of stellar variability given its great precision and complete time coverage (with a half hour cadence). Because the full Kepler pipeline is not currently suitable for a study of stellar variability of this sort, we describe our procedures for treating the "raw" pipeline data. Abou…
Space photometric missions have been steadily accumulating observations of Cepheids in recent years, leading to a flow of new discoveries. In this short review we summarize the findings provided by the early missions such as WIRE, MOST, and CoRoT, and the recent results of the Kepler and K2 missions. The surprising and fascinating results from the high-precision, quasi-continuous data include the detection of the amplitude increase of Polaris, and exquisite details about V1154 Cyg within the original Kepler field of view. We also briefly discuss the current opportunities with the K2 mission, a…
Spectroscopic iron-to-hydrogen ratios, radial velocities, atmospheric parameters, and new photometric analyses are presented for 41 RR Lyrae stars (and one probable high-amplitude delta Scuti star) located in the field-of-view of the Kepler space telescope. Thirty-seven of the RR Lyrae stars are fundamental-mode pulsators (i.e., RRab stars) of which 16 exhibit the Blazhko effect. Four of the stars are multiperiodic RRc pulsators oscillating primarily in the first-overtone mode. Spectroscopic [Fe/H] values for the 34 stars for which we were able to derive estimates range from -2.54 +/- 0.13 (NR…
Context: [abbreviated] Aims: Kepler data of three known Be stars are re-visited to establish their pulsational nature and assess the properties of additional, non-pulsational variations. The three program stars turned out to be one inactive Be star, one active, continuously outbursting Be star, and one Be star transiting from a non-outbursting into an outbursting phase, thus forming an excellent sample to distill properties of Be stars in the various phases of their life-cycle. Methods: [abbreviated] Results: The short-term photometric variability of Be stars must be disentangled into a stella…
