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Accurate photometric CoRoT space observations of a secondary seismological target, HD 174884, led to the discovery that this star is an astrophysically important double-lined eclipsing spectroscopic binary in an eccentric orbit ($e\sim0.3$), unusual for its short 3$\fd$65705 orbital period. The high eccentricity, coupled with the orientation of the binary orbit in space, explains the very unusual observed light curve with strongly unequal primary and secondary eclipses having the depth ratio of 1-to-100 in the CoRoT “seismo” passband. Without the high accuracy of the CoRoT photometry, the secondary eclipse, 1.5 mmag deep, would have gone unnoticed. A spectroscopic follow-up program provided 45 high dispersion spectra. The analysis of the CoRoT light curve was performed with an adapted version of PHOEBE that supports CoRoT passbands. The final solution was obtained by a simultaneous fitting of the light and the radial velocity curves. Individual star spectra were obtained by spectrum disentangling. The uncertainties of the fit were achieved by bootstrap resampling and the solution uniqueness was tested by heuristic scanning. The results provide a consistent picture of the system composed of two late B stars. The Fourier analysis of the light curve fit residuals yields two components, with orbital frequency multiples and an amplitude of ~0.1 mmag, which are tentatively interpreted as tidally induced pulsations. An extensive comparison with theoretical models is carried out by means of the Levenberg-Marquardt minimization technique, and the discrepancy between the models and the derived parameters is discussed. The best fitting models yield a young system age of 125 million years which is consistent with the eccentric orbit and synchronous component rotation at periastron.

Context. Solar-like oscillations have now been observed in several stars, thanks to ground-based spectroscopic observations and space-borne photometry. CoRoT, which has been in orbit since December 2006, has observed the star HD49933 twice. The oscillation spectrum of this star has proven difficult to interpret.

Aims. We report the discovery of very shallow ($\Delta F/F \approx 3.4\times 10^{-4}$), periodic dips in the light curve of an active $V = 11.7$G9V star observed by the CoRoT  satellite, which we interpret as caused by a transiting companion. We describe the 3-colour CoRoT data and complementary ground-based observations that support the planetary nature of the companion.

Context. The origin of the short-term variability in Be stars remains a matter of controversy. Pulsations and rotational modulation are the components of the favored hypothesis.

Context. The CoRoT mission provides asteroseismic data of very high quality allowing one to adopt new statistical approaches for mode detection in power spectra, especially with respect to testing the null hypothesis (H0, which assumes that what is observed is pure noise).

We present an analysis of the δ-Scuti star HD 174936(ID 7613) observed by CoRoT during the first short run SRc01 (27 days). A total number of 422 frequencies were extracted from the light curve using standard prewhitening techniques. This number of frequencies was obtained by considering a spectral significance limit of sig = 10 using the software package SigSpec. Our analysis of the oscillation frequency spectrum reveals a spacing periodicity around 52 μHz. Although modes considered here are not in the asymptotic regime, a comparison with stellar models confirms that this signature may stem from a quasi-periodic pattern similar to the so-called large separation in solar-like stars.

Context. The presence of pulsations in late-type Be stars is still a matter of controversy. It constitutes an important issue to establish the relationship between non-radial pulsations and the mass-loss mechanism in Be stars.

Context. HgMn chemically peculiar stars are among the quietest stars of the main-sequence. However, according to theoretical predictions, these stars could have pulsations related to the high overabundance of iron peak elements, which are produced by atomic diffusion in upper layers. Such pulsations have never been detected from ground-based observations.

Context. HD 181231is a B5IVe star, which has been observed with the CoRoT satellite during ~5 consecutive months and simultaneously from the ground in spectroscopy and spectropolarimetry.

Context. Solar-like oscillations have been observed in numerous red giants from ground and from space. An important question arises: could we expect to detect non-radial modes probing the internal structure of these stars?

Context. The CoRoT short runs give us the opportunity to observe a large variety of late-type stars through their solar-like oscillations. We report observations of the star HD 175726 that lasted for 27 days during the first short run of the mission. The time series reveals a high-activity signal and the power spectrum presents an excess due to solar-like oscillations with a low signal-to-noise ratio.

Context. The estimate of solar-like oscillation properties, such as their frequencies, amplitudes and lifetimes, is challenging because of their low amplitudes and will benefit from long and uninterrupted observing runs. The space telescope CoRoT allows us to obtain high-performance photometric data over a long and quasi continuous period. Among its main targets are stars for which we expect solar-like oscillations.

Context. We search for new variable B-type pulsators in the CoRoT data assembled primarily for planet detection, as part of CoRoT's additional programme.

Context.

Context. CoRoT is a space telescope dedicated to stellar seismology and the search for extrasolar planets. The mission is led by the CNES in association with French laboratories and has a large international participation. The European Space Agency (ESA), Austria, Belgium, and Germany contribute to the payload, and Spain and Brazil contribute to the ground segment. Development of the spacecraft, which is based on a PROTEUS low earth orbit (LEO) recurrent platform, commenced in October 2000, and the satellite was launched on December 27, 2006.

Context. Local-box hydrodynamical model atmospheres provide statistical information about a star's emergent radiation field which allows one to predict the level of its granulation-related micro-variability. Space-based photometry is now sufficiently accurate to test model predictions.

Context. Be stars undergo outbursts producing a circumstellar disk from the ejected material. The beating of non-radial pulsations has been put forward as a possible mechanism of ejection.

Context. The F8 star HD 181906 (effective temperature ${\sim}6300$K) was observed for 156 days by the CoRoT satellite during the first long run in the direction of the galactic centre. Analysis of the data reveals a spectrum of solar-like acoustic oscillations. However, the faintness of the target (mv= 7.65) means the signal-to-noise (S/N) in the acoustic modes is quite low, and this low S/N leads to complications in the analysis.

Context. We present an analysis of the observations of HD 51106 and HD 50747 by the satellite CoRoT, obtained during its initial run, and of the spectroscopic preparatory observations.

Context. Measuring amplitudes of solar-like oscillations and the granulation power spectral density constitute two promising sources of information to improve our understanding and description of the convection in outer layers of stars. However, different instruments, using different techniques and different bandpasses, give measurements that cannot be directly compared to each other or to theoretical values.

From different seismic observations we infer the energy supplied per unit of time by turbulent convection to the acoustic modes of αCentauri A (HD 128620), a star that is similar but not identical to the Sun. The inferred rates of energy supplied to the modes (i.e. mode excitation rates) are found to be significantly higher than in the Sun. They are compared with those computed with an excitation model that includes two sources of driving, the Reynolds stress contribution and the advection of entropy fluctuations. The model also uses a closure model, the Closure Model with Plumes (CMP hereafter), that takes the asymmetry between the up- and down-flows (i.e. the granules and plumes, respectively) into account. Different prescriptions for the eddy-time correlation function are also compared to observational data. Calculations based on a Gaussian eddy-time correlation underestimate excitation rates compared with the values derived from observations for αCentauri A. On the other hand, calculations based on a Lorentzian eddy-time correlation lie within the observational error bars. This confirms results in the solar case. Compared to the helioseismic data, those obtained for αCentauri A constitute an additional support for our model of excitation. We show that mode masses must be computed taking turbulent pressure into account. Finally, we emphasize the need for more accurate seismic measurements in order to distinguish between the CMP closure model and the quasi-normal approximation in the case of αCentauri A, as well as to confirm or not the need to include the excitation by the entropy fluctuations.

Context. The first asteroseismology results from CoRoT are presented, on a star showing Sun-like oscillations. We have analyzed a 60 day lightcurve of high-quality photometric data collected by CoRoT on the F5 V star HD 49933. The data reveal a rich spectrum of overtones of low-degree p modes.

Context. We conducted a 5-night observing run with the spectrometer harpsat the ESO 3.6-m telescope in August 2006, to continue exploring the asteroseismic properties of F-type stars. In fact, Doppler observations of F-type on the main sequence are demanding and remain currently limited to a single case (HD 49933). Comparison with photometric results obtained with the CoRoT space mission (Convection, Rotation and planetary Transits) on similar stars will be possible with an enhanced set of observations.

Context.Solar-like oscillations have been discovered in a few red giants, including ϵOph, through spectroscopy. Acoustic modes around 60 μHz were clearly seen in this star, but daily aliasing of the groundbased data made it impossible to unambiguously isolate the p-mode frequencies in the eigenspectrum, and hence the correct value of the large spacing, to asteroseismically constrain the mass of this pulsating star.

Aims.We extend semi-analytical computations of excitation rates for solar oscillation modes to those of other solar-like oscillating stars to compare them with recent observations

Context.Oscillations of stellar pmodes, excited by turbulent convection, are investigated. In the uppermost part of the solar convection zone, radiative cooling is responsible for the formation of turbulent plumes, hence the medium is modelled with downdrafts and updrafts.

We compute the rates Pat which acoustic energy is injected into the solar radial pmodes for several solar models. The solar models are computed with two different local treatments of convection: the classical mixing-length theory (MLT) and the formulation by Canuto et al. (1996, ApJ, 473, 550, CGM). Among the models investigated here, our best models reproduce both (i) the solar radius and the solar luminosity at solar age and (ii) the observed Balmer line profiles. For the MLT treatment, the rates Pdo significantly depend on the properties of the atmosphere, whereas for the CGM treatment, the dependence of Pon the properties of the atmosphere is found to be smaller than the error bars attached to the seismic measurements. The excitation rates Pfor modes associated with the MLT models are significantly underestimated compared with the solar seismic constraints. The CGM models yield values for Pcloser to the seismic data than do the MLT models. We conclude that the solar p-mode excitation rates provide valuable constraints and, according to the present investigation, clearly favor the CGM treatment with respect to the MLT, although neither of them yields values of Pas close to the observations as recently found for 3D numerical simulations.

The observation of a few mixed modes on solar-like oscillating stars would enable their rotation profile to be inverted with success. Here simulated data are used to show that it is possible to find models for solar-like stars that present stochastically excited mixed modes with detectable amplitudes. We take special care to build the mode set by computing the mode amplitudes and selecting those modes with amplitudes compatible with the performance of the forthcoming seismic space experiment, COROT. The frequency set is inverted for various cases where input and trial stellar models differ and where random noise is added to the splittings. We show it is possible to localize a rotation gradient and assess its magnitude. Moreover the use of inverse and forward procedures in parallel gives access to a large part of the profile. We provide several constraints to help the selection of such stars. One looks for a relatively evolved star (still on the main sequence) of $\simeq $$ 1.5 ~ M_\odot$and the rotation rate on the surface should not be too small.

Acoustic rates of excitation of solar pmodes can be estimated from observations in order to place constraints on the modelling of the excitation process and the layers where it occurs in the star. For several reasons (including a poor signal to noise ratio and mode overlap), this estimation is difficult. In this work, we use three completely independent datasets to obtain robust estimates in the solar case for $\ell=1$modes. We also show that the height in the solar atmosphere where the modes are observed must be taken into account. Our three sets of results are shown to be consistent, particularly in the lower part of the p-mode spectrum (from 1.8 mHz to 2.8 mHz). At higher frequencies, the agreement is not as good, because of a larger dispersion of the measurements and also because of some systematic differences which might be due to observation height estimation or to a systematic influence of the noise.

A 10-night asteroseismic observation programme has been conducted in January 2004 with the spectrometer harpsat the ES0 3.6-m telescope. The selected target, the 6th magnitude F5V star HD 49933, was chosen among the prime candidates of corot, the European space mission dedicated to characterize stellar oscillations mode with high precision photometry measurements. This star shows important line profiles variations, indicating a surprisingly high activity with respect to its low rotation rate. However, with the help of tools developed for disentangling the signatures of activity and oscillations, we are able to observe its oscillation spectrum in the frequency range [1.2, 2.2 mHz]. We measure the large separation ($88.7\pm0.4$μHz) and the maximum amplitude (around $0.4 \pm 0.1$m s-1rms), respectively in agreement and marginal agreement with the predicted values.

A 3D simulation of the upper part of the solar convective zone is used to obtain information on the frequency component, $\chi_k$, of the correlation product of the turbulent velocity field. This component plays an important role in the stochastic excitation of acoustic oscillations. A time analysis of the solar simulation shows that a Gaussian function does not correctly reproduce the ν-dependency of $\chi_k$inferred from the 3D simuation in the frequency range where the acoustic energy injected into the solar pmodes is important ($\nu \simeq 2 {-} 4$mHz). The ν-dependency of $\chi_k$is fitted with different analytical functions which can then conveniently be used to compute the acoustic energy supply rate Pinjected into the solar radial oscillations. With constraints from a 3D simulation, adjustment of free parameters to solar data is no longer necessary and is not performed here. The result is compared with solar seismic data. Computed values of Pobtained with the analytical function which fits best $\chi_k$are found ~2.7 times larger than those obtained with the Gaussian model and reproduce better the solar seismic observations. This non-Gaussian description also leads to a Reynolds stress contribution of the same order as the one arising from the advection of the turbulent fluctuations of entropy by the turbulent motions. Some discrepancy between observed and computed Pvalues still exist at high frequency and possible causes for this discrepancy are discussed.

Analyses of a 3D simulation of the upper layers of a solar convective envelope provide constraints on the physical quantities which enter the theoretical formulation of a stochastic excitation model of solar pmodes, for instance the convective velocities and the turbulent kinetic energy spectrum. These constraints are then used to compute the acoustic excitation rate for solar pmodes, P. The resulting values are found ~5 times larger than the values resulting from a computation in which convective velocities and entropy fluctuations are obtained with a 1D solar envelope model built with the time-dependent, nonlocal Gough ([CITE]) extension of the mixing length formulation for convection (GMLT). This difference is mainly due to the assumed mean anisotropy properties of the velocity field in the excitation region. The 3D simulation suggests much larger horizontal velocities compared to vertical ones than in the 1D GMLT solar model. The values of Pobtained with the 3D simulation constraints however are still too small compared with the values inferred from solar observations. Improvements in the description of the turbulent kinetic energy spectrum and its depth dependence yield further increased theoretical values of Pwhich bring them closer to the observations. It is also found that the source of excitation arising from the advection of the turbulent fluctuations of entropy by the turbulent movements contributes ~$65{-}75 \%$to the excitation and therefore remains dominant over the Reynolds stress contribution. The derived theoretical values of Pobtained with the 3D simulation constraints remain smaller by a factor ~3 compared with the solar observations. This shows that the stochastic excitation model still needs to be improved.

Model computations of δScuti stars, located in the vicinity of the red edge of the classical instability strip, suggest amplitudes of solar-like oscillations larger than in cooler models located outside the instability strip. Solar-like amplitudes in our δScuti models are found to be large enough to be detectable with ground-based instruments provided they can be distinguished from the opacity-driven large-amplitude pulsations. There would be advantages in observing simultaneously opacity-driven and stochastically excited modes in the same star. We anticipate their possible detection in the context of the planned asteroseismic space missions, such as the French mission COROT (COnvection ROtation and planetary Transits). We propose known δScuti stars as potential candidates for the target selection of these upcoming space missions.

Acoustic power and oscillation amplitudes of radial oscillations computed for a solar model are compared with solar seismic observations. The oscillations are assumed to be stochastically excited by turbulence. The numerical computations are based upon a theoretical formulation of the power going into solar-like oscillation modes, as proposed by Samadi & Goupil (2001) in a companion paper. This formulation allows us to investigate several assumptions concerning properties of the stellar turbulence. We find that the entropy source plays a dominant role in the stochastic excitation compared with the Reynold stress source, in agreement with [CITE]. We consider several turbulent kinetic energy spectra suggested by different observations of the solar granulation. Differences between turbulent spectra manifest in large differences in the computed oscillation powers at high oscillation frequency. Two free parameters which are introduced in the description of the turbulence enter the expression for the acoustic power. These parameters are adjusted in order to fit to the solar observations of the surface velocity oscillations. The best fit is obtained with the kinetic energy spectrum deduced from the observations of the solar granulation by [CITE]; the corresponding adjusted parameters are found to be compatible with the theoretical upper limit which can be set on these parameters. The adopted theoretical approach improves the agreement between solar seismic observations and numerical results.

Context. We present an analysis of the observations of HD?51106 and HD?50747 by the satellite CoRoT, obtained during its initial run, and of the spectroscopic preparatory observations.Aims. We complete an analysis of the light curve, extract the main frequencies observed, and discuss some preliminary interpretations about the stars.Methods. We used standard Fourier transform and pre-whitening methods to extract information about the periodicities of the stars.Results. HD 51106 is an ellipsoidal binary, the light curve of which can be completely explained by the tidal deformation of the star and smaller secondary effects. HD?50747 is a triple system containing a variable star, which exhibits many modes of oscillation with periods in the range of a few hours. On the basis of this period range and the analysis of the physical parameters of the star, we conclude that HD?50747 is a ?-Doradus star.

Context. Aims. This work presents the results obtained by CoRoT on HD?50844, the only ? Sct star observed in the CoRoT initial run (57.6?d). The aim of these CoRoT observations was to investigate and characterize for the first time the pulsational behaviour of a ? Sct star, when observed at a level of precision and with a much better duty cycle than from the ground.Methods. The 140?016 datapoints were analysed using independent approaches (SigSpec software and different iterative sine-wave fittings) and several checks performed (splitting of the timeseries in different subsets, investigation of the residual light curves and spectra). A level of 10-5?mag was reached in the amplitude spectra of the CoRoT timeseries. The space monitoring was complemented by ground-based high-resolution spectroscopy, which allowed the mode identification of 30?terms. Results. The frequency analysis of the CoRoT timeseries revealed hundreds of terms in the frequency range 0?30?d-1. All the cross-checks confirmed this new result. The initial guess that ? Sct stars have a very rich frequency content is confirmed. The spectroscopic mode identification gives theoretical support since very high-degree modes (up to ?=14) are identified. We also prove that cancellation effects are not sufficient in removing the flux variations associated to these modes at the noise level of the CoRoT measurements. The ground-based observations indicate that HD?50844 is an evolved star that is slightly underabundant in heavy elements, located on the Terminal Age Main Sequence. Probably due to this unfavourable evolutionary status, no clear regular distribution is observed in the frequency set. The predominant term (f1=6.92?d-1) has been identified as the fundamental radial mode combining ground-based photometric and spectroscopic data.?Conclusions.

Context. The CoRoT short runs give us the opportunity to observe a large variety of late-type stars through their solar-like oscillations. We report observations of the star HD?175726 that lasted for 27?days during the first short run of the mission. The time series reveals a high-activity signal and the power spectrum presents an excess due to solar-like oscillations with a low signal-to-noise ratio.Aims. Our aim is to identify the most efficient tools to extract as much information as possible from the power density spectrum.Methods. The most productive method appears to be the autocorrelation of the time series, calculated as the spectrum of the filtered spectrum. This method is efficient, very rapid computationally, and will be useful for the analysis of other targets, observed with CoRoT or with forthcoming missions such as Kepler and Plato.Results. The mean large separation has been measured to be 97.2?0.5???Hz, slightly below the expected value determined from solar scaling laws. We also show strong evidence for variation of the large separation with frequency. The bolometric mode amplitude is only 1.7?0.25?ppm for radial modes, which is 1.7?times less than expected. Due to the low signal-to-noise ratio, mode identification is not possible for the available data set of HD?175726.

We focus on the influence of the Coriolis acceleration on the stochastic excitation of oscillation modes in convective regions of rotating stars. Our aim is to estimate the asymmetry between excitation rates of prograde and retrograde modes. We extend the formalism derived for obtaining stellar p- and g-mode amplitudes (Samadi & Goupil 2001, A&A, 370, 136; Belkacem et al. 2008, A&A, 478, 163) to include the effect of the Coriolis acceleration. We then study the special case of uniform rotation for slowly rotating stars by performing a perturbative analysis. This allows us to consider the cases of the Sun and the CoRoT target HD?49933. We find that, in the subsonic regime, the influence of rotation as a direct contribution to mode driving is negligible compared to the Reynolds stress contribution. In slow rotators, the indirect effect of the modification of the eigenfunctions on mode excitation is investigated by performing a perturbative analysis of the excitation rates. The excitation of solar p?modes is found to be affected by rotation with excitation-rate asymmetries between prograde and retrograde modes of the order of several percent. Solar low-order g?modes are also affected by uniform rotation and their excitation-rate asymmetries are found to reach 10%. The CoRoT target HD?49933 is rotating more rapidly than the Sun (?/ ???8), and we show that the resulting excitation-rate asymmetry is about 10% for the excitation rates of p?modes. We demonstrate that p?and g?mode excitation rates are modified by uniform rotation through the Coriolis acceleration. A study of the effect of differential rotation will be presented in a forthcoming paper.