M. Chadid

Automated supervised classification of variable stars in the CoRoT programme. Method and application to the first four exoplanet fields

Context: Aims: In this work, we describe the pipeline for the fast supervised classification of light curves observed by the CoRoT exoplanet CCDs. We present the classification results obtained for the first four measured fields, which represent a one-year in-orbit operation. Methods: The basis of the adopted supervised classification methodology has been described in detail in a previous paper, as is its application to the OGLE database. Here, we present the modifications of the algorithms and of the training set to optimize the performance when applied to the CoRoT data. Results: Classification results are presented for the observed fields IRa01, SRc01, LRc01, and LRa01 of the CoRoT mission. Statistics on the number of variables and the number of objects per class are given and typical light curves of high-probability candidates are shown. We also report on new stellar variability types discovered in the CoRoT data. The full classification results are publicly available. The CoRoT space mission, launched on 27 December 2006, has been developed and is operated by the CNES, with the contribution of Austria, Belgium, Brazil , ESA, Germany, and Spain. The full classification results will be only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/506/519

First CoRoT light curves of RR Lyrae stars - Complex multiplet structure and non-radial pulsation detections in V1127 Aquilae

Context. The CoRoT-Convection Rotation and planetary Transits-space mission is a great opportunity for monitoring stars with excellent time-sampling and unprecedented photometric precision for up to 150 days. As an important benefit, high-quality RR Lyrae light curves are obtained with a quasi-uninterrupted coverage over several pulsation and Blazhko cycles. Aims. The Blazhko effect in RR Lyrae stars is an unsolved problem of astrophysics. We used the high-precision space data to contribute more precise knowledge to explain the possible physical processes behind the phenomenon. Methods. We applied different period-finding techniques including Period04, MuFrAn, PDM and SigSpec. Amplitude and phase modulation were investigated by an analytical function method as well as with the traditional O-C diagrams. Results. The Blazhko modulation frequency is directly detected in the spectrum, as well as its first and second harmonics. It shows the non-linear nature of the Blazhko modulation. Besides the triplets, further higher-order modulation side peaks appear around the pulsation frequency as quintuplet, septuplet, nonuplet, undecaplet, tredecaplet, quindecaplet and sepdecaplet structures. Additional frequencies, not belonging to the classical multiplet structures, are detected, as well as their linear combinations with the fundamental radial mode. We interpret these additional terms as non-radial modes. During the five consecutive Blazhko cycles, there is a shift of the maximum phase around 0.011 pulsation phase which is likely the consequence of a long term modulation.

CoRoT light curves of RR Lyrae stars - CoRoT 101128793: long–term changes in the Blazhko effect and excitation of additional modes

Context. The CoRoT (Convection, Rotation and planetary Transits) space mission provides a valuable opportunity to monitor stars with uninterrupted time sampling for up to 150 days at a time. The study of RR Lyrae stars, performed in the framework of the Additional Programmes belonging to the exoplanetary field, will particularly benefit from such dense, long-duration monitoring. Aims. The Blazhko effect in RR Lyrae stars is a long-standing, unsolved problem of stellar astrophysics. We used the CoRoT data of the new RR Lyrae variable CoRoT 101128793 (f0 = 2.119 d −1 , P = 0.4719296 d) to provide us with more detailed observational facts to understand the physical process behind the phenomenon. Methods. The CoRoT data were corrected for one jump and the long-term drift. We applied different period-finding techniques to the corrected timeseries to investigate amplitude and phase modulation. We detected 79 frequencies in the light curve of CoRoT 101128793. They have been identified as the main frequency f0 and its harmonics, two independent terms, the terms related to the Blazhko frequency fm, and to several combination terms. Results. A Blazhko frequency fm = 0.056 d −1 and a triplet structure around the fundamental radial mode and harmonics were detected, as were a long-term variability on the Blazhko modulation. Indeed, the amplitude of the main oscillation is decreasing along the CoRoT survey. The Blazhko modulation is one of the smallest observed in RR Lyrae stars. Moreover, the additional modes f1 = 3.630 and f2 = 3.159 d −1 are detected. Taking its ratio with the fundamental radial mode into account, the term f1 could be the identified as the second radial overtone. Detecting of these modes in horizontal branch stars is a new result obtained by CoRoT.

Hypersonic shock wave evidence in RR Lyrae stars - First detection of neutral line disappearance in S Arae

Context. We carried out a new high resolution spectroscopic analysis of S Arae, one of the largest amplitude RR Lyrae star, to constrain the dynamical structure of the outermost layers of RR Lyrae stars. Aims. New high precision spectroscopic observations of S Arae were obtained using UVES at VLT–UT2 and FEROS at ESO–1.52 telescopes, during the rise of the S Arae light curve, over an entire pulsation period and covering a large wavelength range, from 376 to 1000 nm. Methods. We used the first line moment method to measure the radial velocity and to establish the radial velocity curves. The atmosphere stratification was done by the line identification method derived from the Kurucz atmosphere models. Results. For the first time, we show the existence of hypersonic shock waves in the photosphere of RR Lyrae stars. Our spectroscopic results reveal a new mechanism: the neutral line disappearance phenomenon. All neutral metallic absorption lines, over the whole spectrum, undergo a total line disappearance during the hump, while the ionized lines show line doubling connected by the two-step Schwarzschild’s mechanism. Thus, a hypersonic shock wave propagating through the photosphere around the maximum light phase results in the line disappearance. The shock energy is high enough to ionize the atoms, forming the layer crossed by the shock wave. The shock at first recedes, then becomes stationary and finally advances with a very high Mach number crossing the high atmosphere of the star. We stress the need for additional theoretical efforts to come to a better understanding of hypersonic shock wave and dynamic pulsational motion relationship.

First Antarctica light curve - PAIX monitoring of the Blazhko RR Lyrae star: S Arae

Context. In the context of long and continuous time-series photometry and after the MOST, CoRoT, KEPLER space missions and large geographic longitude ground-based networks, a new method is offered by the polar location helping to cope with the problem associated with the Earth’s day-night cycle. Aims. We present the first long time -series photometry from Dome Charlie in the heart of Antarctica and analyze ∼4400 photometric measurements of RR Lyrae star S Arae obtained in July and August 2007. Methods. The 40-cm telescope and Photometer AntarctIca eXtinction named PAIX were used for long photometric measurements with high time resolution and a duty cycle of 65%. The Fourier technique was used for the frequency analysis. New high-precision spectroscopic observations of S Arae were also collected using HARPS over one pulsation cycle of S Arae. Results. We confirm the known radial fundamental period of 0.452 day. The high order of its harmonics shows the nonlinear nature of the radial mode. The Blazhko modulation is directly detected in the spectrum and shows a value around 48 d. Together with the Blazhko frequency we also detected triplet structure components up to 15th order showing a nonlinear behavior of the Blazhko modulation. Thus, we conclude that S Arae is a bona fide Blazhko star. The data reveal that there are irregular changes in the S Arae atmosphere during the Blazhko cycle. A residual scatter phenomenon is detected and is more intense where the nonlinear effects are strongest. Therefore, the connection between temporal hydrodynamic phenomena and such modulation has to be taken into account in Blazhko models. Conclusions. High-precision CCD photometry with a very good time resolution can be undertaken at Dome Charlie in Antarctica, and it helps to complete astrophysical measurement time-series.

First Single Star Scidar measurements at Dome C, Antarctica

Aims. First operational running of the Single Star Scidar (SSS) under harsh weather conditions at Dome C in Antarctica. Continuous monitoring of the optical turbulence and wind speed profiles throughout the whole atmosphere. Methods. SSS is mainly composed of “off the shelf” light-weight components, a 16 inches telescope installed on an equatorial mount. Scintillation patterns are computed (auto and cross-corre lations) in real time and analyzed off line to retrieve continuously vertical profiles of optical turbulence C 2 (h) and wind speed V(h), from ground up to 20 km . Results. Using a simulated annealing method, we have analyzed about 6.5 hours of observations, putting into evidence the strong s urface layer contribution to seeing degradation. SSS results show a good agreement with simultaneous measurements with a Differential Image Motion Monitor as well as weather archive from NOAA. Conclusions. SSS has shown its usefullness for site characterization sin ce it measures simultaneously C 2 N and V profiles, from which most of adaptative optic parameters are deduced, such as isoplanatic angle and coherence time of the wavefront. Due to its small size, it is well adapted to site characterization, even when low infrastructure is available.

CoRoT light curves of Blazhko RR Lyrae stars - Evidence of a strong correlation between phase and amplitude modulations of CoRoT ID 0105288363

Context. The CoRoT- Convection Rotation and planetary Transits - space mission provides a unique opportunity to monitor RR Lyrae stars with excellent time-sampling, unprecedented photometric precision, and a long time base of 150 days. Aims. The pulsation characteristics of RR Lyrae stars rely on robust physics, but we still lack a firm quantitative understanding of the physical mechanisms driving the Blazhko modulation and the long-term changes in their pulsation behavior. We use the high-precision space data of an unknown RR Lyrae star CoRoT ID 0105288363 observed during a second long run centered on the Galaxy - LRc02 -, to improve our understanding of the pulsation properties of RR Lyrae stars. Methods. The CoRoT data were corrected using a jump and trend filtering code. We applied different period-finding techniques including Period04, MuFrAn, PDM, and SigSpec. Amplitude and phase modulation were investigated using an analytical function method as well as traditional O-C diagrams. Results. For the first time, we detect significant cycle-to-cycle changes in the Blazhko modulation, which appear to be analogous to those predicted by Stothers - owing to the suppression of turbulent convection - to explain this phenomenon. We discuss the clear correlations between the phase and the amplitude of the bump, and the skewness and acuteness of the light curve during different Blazhko cycles. We find that these quantities are strongly anticorrelated with the fundamental pulsation period. This provides a strong support to the slow convective cycle model suggested by Stothers. We also detect a long-term modulation period in the maximum brightness spectrum. A more extended coverage of the long-term modulation is required to constrain its period. Seventh-order side peaks of the pulsation multiplet structure are also visible with the left-side peak amplitudes being higher than those of the right. This has never previously been detected. Future theoretical investigations are required to understand on a quantitative basis the complex behavior of the Blazhko effect. In particular, we still lack firm constraints of the physical mechanisms driving both phase and amplitude modulations during consecutive Blazhko cycles and their correlation, if any, with the long-term modulation.

Periodicity search as a tool for disentangling the contaminated colour light curve of CoRoT 102781750

Context. The CoRoT space mission (COnvection, ROtation and planetary Transits) launched in December 2006, aims at finding transiting exoplanets and investigating stellar oscillation in adjacent stellar fields, called exo- and seismofields, respectively. Besides the seismofields, CoRoT has a strong potential for seismological research on the exofields. Up to now, only a limited number of RRLyrae stars have been classified among the CoRoT targets. Knowing the astrophysical importance of the RRLyrae stars, we attempted to get useful information even from a contaminated light curve of a possible RR Lyrae pulsator. Aims. The star CoRoT 102781750 reveals a puzzle, showing a very complex and altering variation in different CoRoT colours. We established without doubt that more than a single star was situated within the CoRoT mask. Using a search for periodicity as a tool, our aim is to disentangle the composite light curve and identify the type of sources behind the variability. Methods. Both flux and magnitude light curves were used. Conversion was applied after a jump- and trend-filtering algorithm. We applied different types of period-finding techniques including MuFrAn and Period04. Results. The amplitude and phase peculiarities obtained from the independent analysis of CoRoT r, g, and b colours and ground-based follow-up photometric observations ruled out the possibility of either a background monoperiodic or a Blazhko type RR Lyrae star being in the mask. The main target, an active star, shows at least two spotted areas that reveal a P rot = 8.8 h (f 0 = 2.735 c d ―1 ) mean rotation period. The evolution of the active regions helped to derive a period change of dP/dt = 1.6 x 10- 6 (18 s over the run) and a differential rotation of α = ΔΩ/Ω = 0.0074. The 0 m . 015 linear decrease and a local 0 m . 005 increase in the dominant periods amplitude are interpreted as a decay of the old spotted region and an appearance of a new one, respectively. A star that is detected only in the CoRoT b domain shows a f 1 = 7.172 c d ―1 pulsation connected to a 14 d . 83 periodicity via an equidistant triplet structure. The best explanation for our observation is a β Cep star with a corotating dust disk.

Amplitude and phase modulation in CoRoT RR lyrae stars

We analyzed the light curves of RR Lyrae stars in the the first long run field (LRc01) of the CoRoT mission. A high incidence rate of the Blazhko phenomenon was found in the RRab sample. Two of these stars are blended and exhibit low amplitude pulsation. Utilizing the outstanding photometric accuracy of CoRoT we derived accurate instantaneous amplitudes and periods and compared the amplitude and phase modulation properties of the CoRoT Blazhko stars.

First RR Lyrae Light Curve from CoRoT Big Challenge and Constraint to the Theoretical Models

In this paper we report on new RR Lyrae stars discovered in the first long run fields (LRc01 and LRa01) of the CoRoT mission. CoRoT frequency analysis of the high quality light curve with an unprecedented photometric accuracy and uninterrupted coverage over long period (∼150 days) are shown. The CoRoT results reveal four of the seven stars affected with the Blazhko effect and showing a very high fraction of the Blazhko stars ever obtained. The CoRoT RRLyrae stars show pulsation periods strongly nonlinear and complex multiplet structures, such as the triplet, quintuplet, septuplet, nonuplet, undecaplet, tredecaplet, quindecaplet and sepdecaplet frequency structures. Additional frequencies are detected as well with a linear combination with the fundamental radial mode, showing a strong evidence for direct detections of nonradial pulsations. We put into evidence new classes of RR Lyrae stars showing the Blazhko double mode RR Lyrae stars (fundamental and second overtone), nonradial and radial (multimodes) pu...