CoRoT Targets Research Articles

Erratum: Fundamental properties of Kepler and CoRoT targets – IV. Masses and radii from frequencies of minimum Δν and their implications

Erratum: Fundamental properties of Kepler and CoRoT targets – IV. Masses and radii from frequencies of minimum Δν and their implications

Fundamental properties of Kepler and CoRoT targets – IV. Masses and radii from frequencies of minimum Δν and their implications

ABSTRACT Recently, by analysing the oscillation frequencies of 90 stars, Yıldız, Çelik Orhan & Kayhan have shown that the reference frequencies (νmin0, νmin1, and νmin2) derived from glitches due to He ii ionization zone have very strong diagnostic potential for the determination of their effective temperatures. In this study, we continue to analyse the same stars and compute their mass, radius, and age from different scaling relations including relations based on νmin0, νmin1, and νmin2. For most of the stars, the masses computed using νmin0 and νmin1 are very close to each other. For 38 stars, the difference between these masses is less than 0.024 M$\odot$. The radii of these stars from νmin0 and νmin1 are even closer, with differences of less than 0.007 R$\odot$. These stars may be the most well known solar-like oscillating stars and deserve to be studied in detail. The asteroseismic expressions we derive for mass and radius show slight dependence on metallicity. We therefore develop a new method for computing initial metallicity from this surface metallicity by taking into account the effect of microscopic diffusion. The time dependence of initial metallicity shows some very interesting features that may be important for our understanding of chemical enrichment of Galactic Disc. According to our findings, every epoch of the disc has its own lowest and highest values for metallicity. It seems that rotational velocity is inversely proportional to 1/2 power of age as given by the Skumanich relation.

The SpaceInn SISMA archive

The Spectroscopic Indicators in a SeisMic Archive (SISMA) has been built in the framework of the FP7 SpaceInn project to contain the 7013 HARPS spectra observed during the CoRoT asteroseismic groundbased program, along with their variability and asteroseismic indicators. The spectra pertain to 261 stars spread around the whole Herztsprung-Russell diagram: 72 of them were CoRoT targets while the others were observed in order to better characterize their variability classes. The Legacy Data lightcurves of the CoRoT targets are also stored in the archive.

SSI: The Stellar Seismic Indices Data Base

The Stellar Seismic Indices database (hereafter SSI) has been developed within the SPACEInn project funded under the UE Seventh Framework Programme (FP7). This data base aims at providing stellar seismic indices of solar-like oscillating stars as well as other stellar indices such as the main characteristics of stellar granulation. These indices are extracted using an automatic method, which is able to derive simultaneously the seismic and granulation parameters. This method was applied to almost all Kepler long-cadence light-curves and to almost all CoRoT targets observed in the faint fields. A total of about 320 000 targets have been analyzed, among which seismic indices and granulation parameters have been extracted for about 18 000 redgiants. In this article, we present the data base content, its interface, as well as some illustrative results.

Stellar classification of CoRoT targets

The CoRoT faint stars channel observed about 163 600 targets to detect transiting planetary companions. Because CoRoT targets are faint (11< r <16) and close to the galactic plane, only a small subsample has been observed spectroscopically. We describe the latest classification scheme used to derive the spectral type of CoRoT targets, which is based on broadband multi-colour photometry. We assess the accuracy of this spectral classification for the first time. We find that the classification method performs better for stars that were observed during the mission-dedicated photometric ground-based campaigns.The luminosity class is wrong for less than 7% of the targets. Generally, the effective temperature of stars classified as early type (O, B, and A) is overestimated. Conversely, the temperature of stars classified as later type tends to be underestimated. This is mainly due to the adverse effect of interstellar reddening. We find that the median error on the effective temperature is less than 5% for dwarf stars classified with a spectral later than F0, but it is worse for earlier type stars, with up to 20\% error for A and late-B dwarfs, and up to 70% for early-B and O-type dwarfs. Similar results are found for giants, with a median error that is lower than 7% for G- and later type giants, but greater than 25% for earlier types. Overall, we find an average median absolute temperature difference |Delta Teff| = 533+\-6 K for the whole sample of stars classified as dwarfs and |Delta Teff| = 280+\-3 K for the whole sample of giant stars. The corresponding standard deviation is of about 92+\-5 K for dwarfs and 304+\-4 K for giants. Typically for late-type stars, this means that the classification is accurate to about half a class.

Stellar parameters for stars of the CoRoT exoplanet field

Aims:To support the computation and evolutionary interpretation of periods associated with the rotational modulation, oscillations, and variability of stars located in the CoRoT fields, we are conducting a spectroscopic survey for stars located in the fields already observed by the satellite. These observations allow us to compute physical and chemical parameters for our stellar sample. Method: Using spectroscopic observations obtained with UVES/VLT and Hydra/Blanco, and based on standard analysis techniques, we computed physical and chemical parameters ($T_{\rm{eff}}$, $\log \,(g)$, $\rm{[Fe/H]}$, $v_{\rm{mic}}$, $v_{\rm{rad}}$, $v \sin \,(i)$, and $A(\rm{Li})$) for a large sample of CoRoT targets. Results: We provide physical and chemical parameters for a sample comprised of 138 CoRoT targets. Our analysis shows the stars in our sample are located in different evolutionary stages, ranging from the main sequence to the red giant branch, and range in spectral type from F to K. The physical and chemical properties for the stellar sample are in agreement with typical values reported for FGK stars. However, we report three stars presenting abnormal lithium behavior in the CoRoT fields. These parameters allow us to properly characterize the intrinsic properties of the stars in these fields. Our results reveal important differences in the distributions of metallicity, $T_{\rm eff}$, and evolutionary status for stars belonging to different CoRoT fields, in agreement with results obtained independently from ground-based photometric surveys. Conclusions: Our spectroscopic catalog, by providing much-needed spectroscopic information for a large sample of CoRoT targets, will be of key importance for the successful accomplishment of several different programs related to the CoRoT mission, thus it will help further boost the scientific return associated with this space mission.

Stellar rotation, binarity, and lithium in the open cluster IC 4756

An important aspect in the evolutionary scenario of cool stars is their rotation and the rotationally induced magnetic activity and interior mixing. Stars in open clusters are particularly useful tracers for these aspects because of their known ages. We aim to characterize the open cluster IC4756 and measure stellar rotation periods and surface differential rotation for a sample of its member stars. Thirty-seven cluster stars were observed continuously with the CoRoT satellite for 78 days in 2010. Follow-up high-resolution spectroscopy of the CoRoT targets and deep Str\"omgren $uvby\beta$ and H$\alpha$ photometry of the entire cluster were obtained with our robotic STELLA facility and its echelle spectrograph and wide-field imager, respectively. We determined high-precision photometric periods for 27 of the 37 CoRoT targets and found values between 0.155 and 11.4 days. Twenty of these are rotation periods. Twelve targets are spectroscopic binaries of which 11 were previously unknown; orbits are given for six of them. Six targets were found that show evidence of differential rotation with $\Delta\Omega/\Omega$ in the range 0.04-0.15. Five stars are non-radially pulsating stars with fundamental periods of below 1d, two stars are semi-contact binaries, and one target is a micro-flaring star that also shows rotational modulation. Nine stars in total were not considered members because of much redder color(s) and deviant radial velocities with respect to the cluster mean. H$\alpha$ photometry indicates that the cluster ensemble does not contain magnetically over-active stars. The cluster average metallicity is -0.08$\pm$0.06 (rms) and its logarithmic lithium abundance for 12 G-dwarf stars is 2.39$\pm$0.17 (rms). [...]

The power of low‐resolution spectroscopy: On the spectral classification of planet candidates in the ground‐based CoRoT follow‐up

AbstractPlanetary transits detected by the CoRoT mission can be mimicked by a low‐mass star in orbit around a giant star. Spectral classification helps to identify the giant stars and also early‐type stars which are often excluded from further follow‐up. We study the potential and the limitations of low‐resolution spectroscopy to improve the photometric spectral types of CoRoT candidates. In particular, we want to study the influence of the signal‐to‐noise ratio (SNR) of the target spectrum in a quantitative way. We built an own template library and investigate whether a template library from the literature is able to reproduce the classifications. Including previous photometric estimates, we show how the additional spectroscopic information improves the constraints on spectral type. Low‐resolution spectroscopy (R ≈ 1000) of 42 CoRoT targets covering a wide range in SNR (1–437) and of 149 templates was obtained in 2012–2013 with the Nasmyth spectrograph at the Tautenburg 2 m telescope. Spectral types have been derived automatically by comparing with the observed template spectra. The classification has been repeated with the external CFLIB library. The spectral class obtained with the external library agrees within a few sub‐classes when the target spectrum has a SNR of about 100 at least. While the photometric spectral type can deviate by an entire spectral class, the photometric luminosity classification is as close as a spectroscopic classification with the external library. A low SNR of the target spectrum limits the attainable accuracy of classification more strongly than the use of external templates or photometry. Furthermore we found that low‐resolution reconnaissance spectroscopy ensures that good planet candidates are kept that would otherwise be discarded based on photometric spectral type alone. (© 2015 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Abundance study of the two solar-analogue CoRoT targets HD 42618 and HD 43587 from HARPS spectroscopy

We present a detailed abundance study based on spectroscopic data obtained with HARPS of two solar-analogue main targets for the asteroseismology programme of the CoRoT satellite: HD 42618 and HD 43587. The atmospheric parameters and chemical composition are accurately determined through a fully differential analysis with respect to the Sun observed with the same instrumental set-up. Several sources of systematic errors largely cancel out with this approach, which allows us to narrow down the 1-sigma error bars to typically 20 K in effective temperature, 0.04 dex in surface gravity, and less than 0.05 dex in the elemental abundances. Although HD 42618 fulfils many requirements for being classified as a solar twin, its slight deficiency in metals and its possibly younger age indicate that, strictly speaking, it does not belong to this class of objects. On the other hand, HD 43587 is slightly more massive and evolved. In addition, marked differences are found in the amount of lithium present in the photospheres of these two stars, which might reveal different mixing properties in their interiors. These results will put tight constraints on the forthcoming theoretical modelling of their solar-like oscillations and contribute to increase our knowledge of the fundamental parameters and internal structure of stars similar to our Sun.

Improved variability classification of CoRoT targets with Giraffe spectra

Aims. We present an improved method for automated stellar variability classification, using fundamental parameters derived from high resolution spectra, with the goal to improve the variability classification obtained using information derived from CoRoT light curves only. Although we focus on Giraffe spectra and CoRoT light curves in this work, the methods are much more widely applicable.Methods. In order to improve the variability classification obtained from the photometric time series, only rough estimates of the stellar physical parameters (T eff and log (g )) are needed because most variability types that overlap in the space of time series parameters, are well separated in the space of physical parameters (e.g. γ Dor/SPB or δ Sct/β Cep). In this work, several state-of-the-art machine learning techniques are combined to estimate these fundamental parameters from high resolution Giraffe spectra. Next, these parameters are used in a multi-stage Gaussian-Mixture classifier to perform an improved supervised variability classification of CoRoT light curves. The variability classifier can be used independently of the regression module that estimates the physical parameters, so that non-spectroscopic estimates derived e.g. from photometric colour indices can be used instead.Results. T eff and log (g ) are derived from Giraffe spectra, for 6832 CoRoT targets. The use of those parameters in addition to information extracted from the CoRoT light curves, significantly improves the results of our previous automated stellar variability classification. Several new pulsating stars are identified with high confidence levels, including hot pulsators such as SPB and β Cep, and several γ Dor-δ Sct hybrids. From our samples of new γ Dor and δ Sct stars, we find strong indications that the instability domains for both types of pulsators are larger than previously thought.

Characterisation of the Galactic thin disc with CoRoT targets

We use kinematical and chemical properties of 754 Corot stars to characterise the stellar populations of the Milky Way disc in three beams close the Galactic plane. From the atmospheric parameters derived in Gazzano et al. (2010) with the Matisse algorithm, we derived stellar distances using isochrones. Combining these data with proper motions, we provide the complete kinematical description of stars in three Corot fields. Finally, we used kinematical criteria to identify the Galactic populations in our sample and study their characteristics, particularly their chemistry. Comparing our kinematics with the Besancon Galactic model, we show that, within 3-sigma, simulated and observed kinematical distributions are in good agreement. We study the characteristics of the thin disc, finding a correlation that is significant at a value of 2-sigma between the V-velocity component and the metallicity for two different radial distance bins (8-9kpc and 9-10kpc; but not for the most inner bin 7-8kpc, probably because of the uncertainties in the abundances) which could be interpreted as radial migration evidence. We also measured a radial metallicity gradient value of -0.097+/-0.015dex/kpc with giant stars, and -0.053+/-0.015dex/kpc with dwarfs. Finally, we identified metal-rich stars with peculiar high [alpha/Fe] values in the directions pointing to the inner part of the Galaxy. Applying the same methodology to the planet-hosting stars detected by Corot shows that they mainly belong to the thin disc population with normal chemical and kinematical properties.

Pulsational content and abundance analysis of some δ Scuti stars observed by CoRoT

AbstractSeveral δ Sct stars were photometrically monitored with the satellite CoRoT and observed in high‐resolution spectroscopy from ground. We present here the preliminary results of the abundance analysis of several potential CoRoT targets. Moreover, new insights are given about the problem of the huge number of detected frequencies by comparing δ Sct stars with normal A‐type stars (© 2010 WILEY‐VCH Verlag GmbH &amp; Co. KGaA, Weinheim)

Accurate fundamental parameters of CoRoT asteroseismic targets

The CoRoT satellite has provided high-quality light curves of several solar-like stars. Analysis of the light curves provides oscillation frequencies that make it possible to probe the interior of the stars. However, additional constraints on the fundamental parameters of the stars are important for the theoretical modelling to be successful. We will estimate the fundamental parameters (mass, radius and luminosity) of the first four solar-like targets to be observed in the asteroseismic field. In addition, we will determine their effective temperature, metallicity and detailed abundance pattern. To constrain the stellar mass, radius and age we use the SHOTGUN software which compares the location of the stars in the Hertzsprung-Russell diagram with theoretical evolution models. This method takes into account the uncertainties of the observed parameters including the large separation determined from the solar-like oscillations. We determine the effective temperatures and abundance patterns in the stars from the analysis of high-resolution spectra. We have determined the mass, radius and luminosity of the four CoRoT targets to within 5-10 percent, 2-4 percent and 5-13 percent, respectively. The quality of the stellar spectra determines how well we can constrain the effective temperature. For the two best spectra we get 1-sigma uncertainties below 60 K and for the other two 100-150 K. The uncertainty on the surface gravity is less than 0.08 dex for three stars while for HD 181906 it is 0.15 dex. The reason for the larger uncertainty is that the spectrum has two components with a luminosity ratio of Lp/Ls = 0.50+-0.15. While Hipparcos astrometric data strongly suggest it is a binary star we find evidence that the fainter star may be a background star, since it is less luminous but hotter.

Planet formation by nucleated-instability: comparison with the two first CoRoT runs

<i>Aims. <i/>We use extended planet formation models to analyse the results the two first runs of CoRoT, namely the initial one (IRa01) and the first long one (LRc01). We compare our calculations to the actual observations and discuss the possible origins of the differences between the two, not only in term of detection rate, but also regarding the magnitude of transiting planets parent stars.<i>Methods. <i/>We use a galactic population synthesis model in order to calculate the characteristics of stars in the field-of-view of CoRoT. In a second step, we calculate planet formation models to derive the population of planets expected around these stars. In a third step, using planet evolution models, we calculate the radius of planets predicted by the model and estimate the probability of transit detection and confirmation by radial-velocity follow-up.<i>Results. <i/>Our calculations show that the present day CoRoT detection rate, based only on IRa01 and LRc01, is a factor 5 lower than the one predicted assuming all target are dwarf stars. On the other hand, the two detection rates become similar if one assumes some contamination of CoRoT targets by giant stars, or some less optimistic determinations of the noise level, the limiting signal-to-noise ratio and radial velocity follow-up performances. However, in all the cases we have considered, the majority of transits are found around faint stars, in conflict with the actual CoRoT detections.

Close-up of primary and secondary asteroseismic CoRoT targets and the ground-based follow-up observations

To optimise the science results of the asteroseismic part of the CoRoT satellite mission a complementary simultaneous ground-based observational campaign is organised for selected CoRoT targets. The observations include both high-resolution spectroscopic and multicolour photometric data. We present the preliminary results of the analysis of the ground-based observations of three targets. A line-profile analysis of 216 high-resolution FEROS spectra of the δ Sct star HD 50844 reveals more than ten pulsation frequencies in the frequency range 5-18 d-1, including possibly one radial fundamental mode (6.92 d-1). Based on more than 600 multi-colour photometric datapoints of the β Cep star HD 180642, spanning about three years and obtained with different telescopes and different instruments, we confirm the presence of a dominant radial mode ν1 = 5.48695 d-1, and detect also its first two harmonics. We find evidence for a second mode ν2 = 0.3017 d-1, possibly a g-mode, and indications for two more frequencies in the 7-8 d-1 domain. From Stromgren photometry we find evidence for the hybrid 5 Sct/γ Dor character of the F0 star HD 44195, as frequencies near 3 d-1 and 21 d-1 are detected simultaneously in the different filters.

An algorithm for the detection of transits of planets around eclipsing binaries in CoRoT

AbstractWe present a matched filter algorithm to detect transits of planets that orbit both components of close eclipsing binaries in CoRoT targets. The formation of binary systems surrounded by disks is one of the most common outcomes of stellar formation; their detection would therefore constitute an important discovery. In an eclipsing binary system, the binary-planet alignment gives raised transit probabilities and the special transit shapes from circumbinary planets provide a unique identifier for their planetary nature; the problems of false alarms are largely avoided. CoRoT data have unprecedented time coverage and photometric precision that make them especially suitable for the search of transits of planets across eclipsing binaries. No reliable detections of circumbinary planets have been reported yet, and their discovery would constitute a new class of planets.

High precision determination of the atmospheric parameters and abundances of the COROT main targets

One of the main goals of the COROT mission is to get precise photometric observations of selected bright stars in order to allow the modelling of their interior through asteroseismology. However, in order to interpret the asteroseismological data, the effective temperature, surface gravity, and chemical composition of the stars must be known with sufficient accuracy. To carry out this task, we have developed a spectroscopic method called APASS (Atmospheric Parameters and Abundances from Synthetic Spectra) which allows precise analysis of stars with a moderate to high rotational velocity, which is the case for most primary COROT targets. Our method is based on synthetic spectra and works differentially with respect to the Sun. Using high signal-to-noise spectra and the APASS method, we determined the atmospheric parameters and chemical abundances of 13 primary COROT targets. Our results agree well with those obtained by Bruntt using his software VWA and with those obtained with the software TEMPLOGG. However, in both cases, our error bars are significantly smaller than those of other methods. Our effective temperatures are also in excellent agreement with those obtained with the IR photometry method. For five stars with relatively low rotational velocity, we also performed an analysis with a classical equivalent-width method to test agreement with APASS results. We show that equivalent-width measurements by Gaussian or Voigt profile-fitting are sensitive to the rotational broadening, leading to systematic errors whenever the projected rotation velocity is non-negligible. The APASS method appears superior in all cases and should thus be preferred.

The double-mode nature of the HADS star GSC 00144-03031 and the Petersen diagram of the class

The double-mode pulsation of GSC 00144-03031 has been detected when searching for COROT targets. A very large dataset composed of 4722 photometric measurements was collected at six observatories in Europe and America. There is no hint of the excitation of additional modes (down to 0.6 mmag) and therefore GSC 00144-03031 seems to be a pure double-mode pulsator, with a very short fundamental radial mode (P = 84 min). From uvbyβ photometry and evolutionary tracks it appears to be a Pop. I star with M = 1.75 , located in the middle of the instability strip, close to the Zero-Age Main Sequence. We also discovered other new double-mode pulsators in the databases of large-scale projects: OGLE BW2_V142, OGLE BW1_V207, ASAS3 094303-1707.3, ASAS3 000116-6037.0, NSVS 3234596 and NSVS 3324715. An observational Petersen diagram is presented and explained by means of new models. A common sequence connecting Pop. I stars from the shortest to the longest periods is proposed and the spreads in the period ratios are ascribed to different metallicities (at the shortest periods) and to different masses (at the longest ones).