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Register a new userOverview of semi-sinusoidal stellar variability with the CoRoT satellite
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To date, the CoRoT space mission has produced more than 124,471 light curves. Classifying these curves in terms of unambiguous variability behavior is mandatory for obtaining an unbiased statistical view on their controlling root-causes. The present study provides an overview of semi-sinusoidal light curves observed by the CoRoT exo-field CCDs. We selected a sample of 4,206 light curves presenting well-defined semi-sinusoidal signatures. The variability periods were computed based on Lomb-Scargle periodograms, harmonic fits, and visual inspection. Color-period diagrams for the present sample show the trend of an increase of the variability periods as long as the stars evolve. This evolutionary behavior is also noticed when comparing the period distribution in the Galactic center and anti-center directions. These aspects indicate a compatibility with stellar rotation, although more information is needed to confirm their root-causes. Considering this possibility, we identified a subset of three Sun-like candidates by their photometric period. Finally, the variability period versus color diagram behavior was found to be highly dependent on the reddening correction.
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For 6 years the Convection, Rotation, and Planetary Transits (CoRoT) space mission has acquired photometric data from more than one hundred thousand point sources towards and directly opposite from the inner and outer regions of the Galaxy. The high temporal resolution of the CoRoT data combined with the wide time span of the observations has enabled the study of short and long time variations in unprecedented detail. From the initial sample of 2534 stars classified as M-giants in the CoRoT databasis, we selected 1428 targets that exhibit well defined variability, using visual inspection. The variability period and amplitude of C1 stars (stars having Teff < 4200 K) were computed using Lomb-Scargle and harmonic fit methods. The trends found in the V-I vs J-K color-color diagram are in agreement with standard empirical calibrations for M-giants. The sources located towards the inner regions of the Galaxy are distributed throughout the diagram while the majority of the stars towards the outer regions of the Galaxy are spread between the calibrations of M-giants and the predicted position for Carbon stars. The stars classified as supergiants follow a different sequence from the one found for giant stars. We also performed a KS test of the period and amplitude of stars towards the inner and outer regions of the Galaxy. We obtained a low probability that the two samples come from the same parent distribution. The observed behavior of the period-amplitude and period-Teff diagrams are, in general, in agreement with those found for Kepler sources and ground based photometry, with pulsation being the dominant cause responsible for the observed modulation. We also conclude that short-time variations on M-Giant stars do not exist orare very rare and the few cases we found are possibly related to biases or background stars.
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.
Six O-type stars were observed continuously by the CoRoT satellite during a 34.3-day run. The unprecedented quality of the data allows us to detect even low-amplitude stellar pulsations in some of these stars (HD 46202 and the binaries HD 46149 and Plasketts star). These cover both opacity-driven modes and solar-like stochastic oscillations, both of importance to the asteroseismological modelling of O stars. Additional effects can be seen in the CoRoT light curves, such as binarity and rotational modulation. Some of the hottest O-type stars (HD 46223, HD 46150 and HD 46966) are dominated by the presence of red-noise: we speculate that this is related to a sub-surface convection zone.
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.
A precise characterisation of the red giants in the seismology fields of the CoRoT satellite is a prerequisite for further in-depth seismic modelling. The optical spectra obtained for 19 targets have been used to accurately estimate their fundamental parameters and chemical composition. The extent of internal mixing is also investigated through the abundances of Li, CNO and Na (as well as 12C/13C in a few cases).
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