CoRoT Research Articles

Differential Rotation of CoRoT Stars and a Kepler Binary Star from Starspot Transit Mapping

Abstract While direct magnetic field measurements are rare for slowly rotating stars, spots observed during planetary transits provide a potential indicator of magnetic activity on stellar surfaces. Moreover, the rotation of the stellar surface can be probed by monitoring the spots’ position with time in subsequent transits. This study investigates the dynamic interplay of rotational shear and stellar rotation rate in six stars of spectral types F to M, all hosting exoplanets observed by the CoRoT space mission, except for one binary star from Kepler. The analysis, facilitated by the ECLIPSE code, unveils the physical properties of stellar spots, including radius, intensity, temperature, and position. The five CoRoT stars exhibit spot characteristics consistent with those observed in solar type stars. The determination of a spot longitude during different transits allows for the inference of the star’s differential rotation profile, revealing a decreasing trend of rotational shear with the mean stellar rotation period, given by Δ Ω ∝ P ¯ − 0.9 . This implies that, at least for slow rotators (mean rotation period >5 days), as stars age, their differential rotation decreases. Additionally, the six stars analyzed here seem to fall into two categories, according to their spot flux deficit: those with ΔF spot > 0.005 and those with ΔF spot < 0.002.

Pre-selection of solar analogues with differential rotation signature

The light curve of a star allows studying the variation of its brightness over time, being suitable for determining rotation periods, detecting planets, studying binary systems and differential rotation. The light curve analysis can be performed with the autocorrelation function (ACF), a robust mathematical tool in the detection of the stellar rotation period, by finding patterns in the photometric data. In this work, a sample of stars from the CoRoT mission is analyzed. The analysis is carried out with the ACF to carry out a pre-selection of stars that show signs of rotational modulation, a determining characteristic for the study of differential rotation. 17 stars were selected by visual inspection of the curves and by the ACF function. It was found that three stars have an excellent profile of autocorrelation function and rotational modulation, three have a good profile and the remaining eleven have inappropriate profiles for the study of differential rotation. The photometric time series analyzes show that the autocorrelation function method is very suitable as a previous criterion for the selection of candidate stars for the study of differential rotation, when they present exponential decay of the oscillation peaks of the light curve.

Testing angular momentum transport processes with asteroseismology of solar-type main-sequence stars

Context. Thanks to the so-called photometry revolution with the space-based missions CoRoT, Kepler, and TESS, asteroseismology has become a powerful tool to study the internal rotation of stars. The rotation rate depends on the efficiency of the angular momentum (AM) transport inside the star, and its study allows to constrain the internal AM transport processes, as well as improve our understanding of their physical nature. Aims. We compared the ratio of the rotation rate predicted by asteroseismology and starspot measurements of solar-type stars considering different AM transport prescriptions and investigated whether some of these prescriptions can be ruled out observationally. Methods. We conducted a two-step modelling procedure of four main-sequence stars from the Kepler LEGACY sample, which consists of an asteroseismic characterisation that serves as a guide for a modelling with rotating models, including a detailed and coherent treatment of the AM transport. The rotation profiles derived with this procedure were used to estimate the ratio of the mean asteroseismic rotation rate with the surface rotation rate from starspot measurements for each AM transport prescriptions. Comparisons between the models were then conducted. Results. In the hotter part of the Hertzsprung-Russell (HR) diagram (masses typically above ∼1.2 M⊙ at solar metallicity), models with only hydrodynamic transport processes and models with additional transport by magnetic instabilities are found to be consistent with previous measurements that observed a low degree (below 30%) of radial differential rotation between the radiative and convective zones. For these stars, which constitute a significant fraction of the Kepler LEGACY sample, a combination of asteroseismic constraints from the splitting of pressure modes and of the surface rotation rate does not allow us to conclude that an efficient AM transport is required in addition to transport by meridional circulation and shear instability alone. Even a model assuming local AM conservation cannot be ruled out. In the colder part of the HR diagram, the situation is different because of the efficient braking of the stellar surface by magnetised winds. We find a clear disagreement between the rotational properties of models that only include hydrodynamic processes and asteroseismic constraints, while models with magnetic fields correctly reproduce the observations, similarly to the solar case. Conclusions. This shows the existence of a mass regime corresponding to main-sequence F-type stars for which it is difficult to constrain the AM transport processes, unlike for hotter, Gamma Dor stars or colder, less massive solar analogues. The comparison between asteroseismic measurements and surface rotation rates enables us to easily rule out models with an inefficient transport of AM in the colder part of the HR diagram.

Characterising the AGB bump and its potential to constrain mixing processes in stellar interiors

Context. In the 1990s, theoretical studies motivated the use of the asymptotic giant branch bump (AGBb) as a standard candle given the weak dependence between its luminosity and stellar metallicity. Because of the small size of observed asymptotic giant branch (AGB) samples, detecting the AGBb is not an easy task. However, this has now been made possible thanks to the wealth of data collected by the CoRoT, Kepler, and TESS space-borne missions. Aims. It is well-known that the AGB bump provides valuable information on the internal structure of low-mass stars, particularly on mixing processes such as core overshooting during the core He-burning phase. Here, we investigate the dependence of the AGBb position on stellar parameters such as the stellar mass and metallicity based on the calibration of stellar models to observations. Methods. In this context, we analysed ∼4000 evolved giants observed by Kepler and TESS, including red giant branch (RGB) stars and AGB stars, for which asteroseismic and spectrometric data are available. By using statistical mixture models, we detected the AGBb both in frequency at maximum oscillation power, νmax, and in effective temperature, Teff. Then, we used the Modules for Experiments in Stellar Astrophysics (MESA) stellar evolution code to model AGB stars and match the AGBb occurrence with observations. Results. From the observations, we were able to derive the AGBb location in 15 bins of mass and metallicity. We noted that the higher the mass, the later the AGBb occurs in the evolutionary track, which agrees with theoretical works. Moreover, we found a slight increase in the luminosity at the AGBb when the metallicity increases. By fitting those observations with stellar models, we noticed that low-mass stars (M ≤ 1.0 M⊙) require a small core overshooting region during the core He-burning phase. This core overshooting extent increases toward high mass; however, above M ≥ 1.5 M⊙, we found that the AGBb location cannot be reproduced with a realistic He-core overshooting alone. Thus, additional mixing processes have to be invoked instead. Conclusions. The observed dependence on metallicity complicates the application of the AGBb as a standard candle. Moreover, different mixing processes may occur according to stellar mass. At low mass (M ≤ 1.5 M⊙), the AGBb location can be used to constrain the He-core overshooting. At high mass (M ≥ 1.5 M⊙), an additional mixing induced, for instance, by rotation is needed to reproduce what is seen in observations.

Fundamental properties of red giants from Kepler and Gaia DR3 data: diagnostic potential of the mass–metallicity diagram

ABSTRACT For solar-like oscillators, the asteroseismic relations connect the stellar mean density and surface gravity to observable properties of the oscillations. Since the space missions CoRoT and Kepler, the asteroseismic relations have been massively employed out of their theoretical domain of validity, that is the main-sequence solar-type stars, in particular for red giant stars. However, despite a surprisingly good performance of the asteroseismic relation with red giant stars, they are biased and attempts to parametrize the departure of the relations for evolved stars have been the subject of many recent efforts. In this study, we propose a new parametrization of the asteroseismic relations for red giants of radii up to ${\sim } 50\, \mathrm{R}_\odot$, based on a calibration performed with Gaia parallax data. We find that the asteroseismic scaling relations depend on metallicity and effective temperature. For this solution, we obtain a parallax offset of about −0.026 ± 0.001 mas, and radius ranges from 3.8 to 53.2 R⊙. Most stars have masses that range from 0.9 to 1.7 M⊙, with the star count peaking around 1.2 M⊙. We also use data of the eclipsing binaries with solar-like oscillating components for calibration and discuss the diagnostic potential of the mass–metallicity diagram for our understanding of the evolution of red giants and the chemodynamics of the Galactic disc. Using this diagram, we identify stars that lost and gained mass and estimate the minimum change in mass.

A probabilistic method for detecting solar-like oscillations using meaningful prior information

Context. Current and future space-based observatories such as the Transiting Exoplanet Survey Satellite (TESS) and PLATO are set to provide an enormous amount of new data on oscillating stars, and in particular stars that oscillate similar to the Sun. Solar-like oscillators constitute the majority of known oscillating stars and so automated analysis methods are becoming an ever increasing necessity to make as much use of these data as possible. Aims. Here we aim to construct an algorithm that can automatically determine if a given time series of photometric measurements shows evidence of solar-like oscillations. The algorithm is aimed at analyzing data from the TESS mission and the future PLATO mission, and in particular stars in the main-sequence and subgiant evolutionary stages. Methods. The algorithm first tests the range of observable frequencies in the power spectrum of a TESS light curve for an excess that is consistent with that expected from solar-like oscillations. In addition, the algorithm tests if a repeating pattern of oscillation frequencies is present in the time series, and whether it is consistent with the large separation seen in solar-like oscillators. Both methods use scaling relations and observations which were established and obtained during the CoRoT, Kepler, and K2 missions. Results. Using a set of test data consisting of visually confirmed solar-like oscillators and nonoscillators observed by TESS, we find that the proposed algorithm can attain a 94.7% true positive (TP) rate and a 8.2% false positive (FP) rate at peak accuracy. However, by applying stricter selection criteria, the FP rate can be reduced to ≈ 2%, while retaining an 80% TP rate.

Multi-cavity gravito-acoustic oscillation modes in stars

Context. Over recent decades, asteroseismology has proven to be a powerful method for probing stellar interiors. Analytical descriptions of the global oscillation modes, in combination with pulsation codes, have provided valuable help in processing and interpreting the large amount of seismic data collected, for instance, by space-borne missions CoRoT, Kepler, and TESS. These prior results have paved the way to more in-depth analyses of the oscillation spectra of stars in order to delve into subtle properties of their interiors. This purpose conversely requires innovative theoretical descriptions of stellar oscillations. Aims. In this paper, we aim to analytically express the resonance condition of the adiabatic oscillation modes of spherical stars in a very general way that is applicable at different evolutionary stages. Methods. In the present formulation, a star is represented as an acoustic interferometer composed of a multitude of resonant cavities where waves can propagate and the short-wavelength JWKB approximation is met. Each cavity is separated from the adjacent ones by barriers, which correspond to regions either where waves are evanescent or where the JWKB approximation fails. Each barrier is associated with a reflection and transmission coefficient. The stationary modes are then computed using two different physical representations: (1) studying the infinite-time reflections and transmissions of a wave energy ray through the ensemble of cavities or (2) solving the linear boundary value problem using the progressive matching of the wave function from one barrier to the adjacent one between the core and surface. Results. Both physical pictures provide the same resonance condition, which ultimately turns out to depend on a number of parameters: the reflection and transmission phase lags introduced by each barrier, the coupling factor associated with each barrier, and the wave number integral over each resonant cavity. Using such a formulation, we can retrieve, in a practical way, the usual forms derived in previous works in the case of mixed modes with two or three cavities coupled though evanescent barriers, low- and large-amplitude glitches, and the simultaneous presence of evanescent regions and glitches. Conclusions. The resonance condition obtained in this work provides a new tool that is useful in predicting the oscillation spectra of stars and interpreting seismic observations at different evolutionary stages in a simple way. Practical applications require more detailed analyses to make the link between the reflection-transmission parameters and the internal structure. These aspects will be the subject of a future paper.

Seismic signature of electron degeneracy in the core of red giants: Hints for mass transfer between close red-giant companions

The detection of mixed modes in red giants with space missions COROT and Kepler has revealed their deep internal structure. These modes allow us to characterize the pattern of pressure modes (through the measurement of their asymptotic frequency separation Δν) and the pattern of gravity modes (through the determination of their asymptotic period spacing ΔΠ1). It has been shown that red giant branch (RGB) stars regroup on a well-defined sequence in the Δν − ΔΠ1 plane. Our first goal is to theoretically explain the features of this sequence and understand how it can be used to probe the interiors of red giants. Using a grid of red giant models computed with MESA, we demonstrate that red giants join the Δν − ΔΠ1 sequence whenever electron degeneracy becomes strong in the core. We argue that this can be used to estimate the central densities of these stars, and potentially to measure the amount of core overshooting during the main sequence part of the evolution. We also investigate a puzzling subsample of red giants that are located below the RGB sequence, in contradiction with stellar evolution models. After checking the measurements of the asymptotic period spacing for these stars, we show that they are mainly intermediate-mass red giants. This is doubly peculiar because these stars should have nondegenerate cores and they are expected to be located well above the RGB sequence. We show that these peculiarities are well accounted for if these stars result from the interaction between two low-mass (M ≲ 2 M⊙) close companions during the red giant branch phase. If the secondary component has already developed a degenerate core before mass transfer begins, it becomes an intermediate-mass giant with a degenerate core. The secondary star is then located below the degenerate sequence, which is in agreement with the observations.

Orbital Period Refinement of CoRoT Planets with TESS Observations

CoRoT was the first space mission dedicated to exoplanet detection. Operational between 2007 and 2012, this mission discovered 37 transiting planets, including CoRoT-7b, the first terrestrial exoplanet with a measured size. The precision of the published transit ephemeris of most of these planets has been limited by the relative short durations of the CoRoT pointings, which implied a danger that the transits will become unobservable within a few years due to the uncertainty of their future transit epochs. Ground-based follow-up observations of the majority of the CoRoT planets have been published in recent years. Between Dec. 2018 and Jan. 2021, the TESS mission in its sectors 6 and 33 re-observed those CoRoT fields that pointed towards the Galactic anti-center. These data permitted the identification of transits from nine of the CoRoT planets, and the derivation of precise new transit epochs. The main motivation of this study has been to derive precise new ephemerides of the CoRoT planets, in order to keep these planets’ transits observable for future generations of telescopes. The TESS data were analyzed for the presence of transits and the epochs of these re-observed transits were measured. The original CoRoT epochs, epochs from ground-based follow-up observations and those from TESS were collected. From these data, updated ephemerides are presented for nine transiting planets discovered by the CoRoT mission in its fields pointing towards the Galactic anti-center. In three cases (CoRoT-4b, 19b and 20b), transits that would have been lost for ground observations, due to the large uncertainty in the previous ephemeris, have been recovered. The updated ephemerides permit transit predictions with uncertainties of less than 30 min for observations at least until the year 2030. No significant transit timing variations were found in these systems.

Seismic constraints on the internal structure of evolved stars: From high-luminosity RGB to AGB stars

Context. The space-borne missions CoRoT and Kepler opened up a new opportunity for better understanding stellar evolution by probing stellar interiors with unrivalled high-precision photometric data. Kepler has observed stellar oscillation for four years, which gave access to excellent frequency resolution that enables deciphering the oscillation spectrum of evolved red giant branch and asymptotic giant branch stars. Aims. The internal structure of stars in the upper parts of the red and asymptotic giant branches is poorly constrained, which makes the distinction between red and asymptotic giants difficult. We perform a thorough seismic analysis to address the physical conditions inside these stars and to distinguish them. Methods. We took advantage of what we have learnt from less evolved stars. We studied the oscillation mode properties of ∼2.000 evolved giants in a model described by the asymptotic pressure-mode pattern of red giants, which includes the signature of the helium second-ionisation zone. Mode identification was performed with a maximum cross-correlation method. Then, the modes were fitted with Lorentzian functions following a maximum likelihood estimator technique. Results. We derive a large set of seismic parameters of evolved red and asymptotic giants. We extracted the mode properties up to the degree ℓ = 3 and investigated their dependence on stellar mass, metallicity, and evolutionary status. We identify a clear difference in the signature of the helium second-ionisation zone between red and asymptotic giants. We also detect a clear shortage of the energy of ℓ = 1 modes after the core-He-burning phase. Furthermore, we note that the mode damping observed on the asymptotic giant branch is similar to that observed on the red giant branch. Conclusions. We highlight that the signature of the helium second-ionisation zone varies with stellar evolution. This provides us with a physical basis for distinguishing red giant branch stars from asymptotic giants. Here, our investigation of stellar oscillations allows us to constrain the physical processes and the key events that occur during the advanced stages of stellar evolution, with emphasis on the ascent along the asymptotic giant branch, including the asymptotic giant branch bump.

Fundamental properties of stars from Kepler and Gaia data: parallax offset and revised scaling relations

ABSTRACT Data from the space missions Gaia, Kepler, CoRoT and TESS, make it possible to compare parallax and asteroseismic distances. From the ratio of two densities ρsca/ρπ, we obtain an empirical relation fΔν between the asteroseismic large frequency separation and mean density, which is important for more accurate stellar mass and radius. This expression for main-sequence (MS) and subgiant stars with K-band magnitude is very close to the one obtained from interior MS models by Yıldız, Çelik & Kayhan. We also discuss the effects of effective temperature and parallax offset as the source of the difference between asteroseismic and non-asteroseismic stellar parameters. We have obtained our best results for about 3500 red giants (RGs) by using 2MASS data and model values for fΔν from Sharma et al. Another unknown scaling parameter $f_{\nu _{\rm max}}$ comes from the relationship between the frequency of maximum amplitude and gravity. Using different combinations of $f_{\nu _{\rm max}}$ and the parallax offset, we find that the parallax offset is generally a function of distance. The situation where this slope disappears is accepted as the most reasonable solution. By a very careful comparison of asteroseismic and non-asteroseismic parameters, we obtain very precise values for the parallax offset and $f_{\nu _{\rm max}}$ for RGs of –0.0463 ± 0.0007 mas and 1.003 ± 0.001, respectively. Our results for mass and radius are in perfect agreement with those of APOKASC-2: the mass and radius of ∼3500 RGs are in the range of about 0.8–1.8 M⊙ (96 per cent) and 3.8–38 R⊙, respectively.

The K2 Galactic Archaeology Program Data Release 2: Asteroseismic Results from Campaigns 4, 6, and 7

Abstract Studies of Galactic structure and evolution have benefited enormously from Gaia kinematic information, though additional, intrinsic stellar parameters like age are required to best constrain Galactic models. Asteroseismology is the most precise method of providing such information for field star populations en masse, but existing samples for the most part have been limited to a few narrow fields of view by the CoRoT and Kepler missions. In an effort to provide well-characterized stellar parameters across a wide range in Galactic position, we present the second data release of red giant asteroseismic parameters for the K2 Galactic Archaeology Program (GAP). We provide ν max and Δ ν based on six independent pipeline analyses; first-ascent red giant branch (RGB) and red clump (RC) evolutionary state classifications from machine learning; and ready-to-use radius and mass coefficients, κ R and κ M , which, when appropriately multiplied by a solar-scaled effective temperature factor, yield physical stellar radii and masses. In total, we report 4395 radius and mass coefficients, with typical uncertainties of 3.3% (stat.) ± 1% (syst.) for κ R and 7.7% (stat.) ± 2% (syst.) for κ M among RGB stars, and 5.0% (stat.) ± 1% (syst.) for κ R and 10.5% (stat.) ± 2% (syst.) for κ M among RC stars. We verify that the sample is nearly complete—except for a dearth of stars with ν max ≲ 10 – 20 μ Hz —by comparing to Galactic models and visual inspection. Our asteroseismic radii agree with radii derived from Gaia Data Release 2 parallaxes to within 2.2% ± 0.3% for RGB stars and 2.0% ± 0.6% for RC stars.

Asteroseismology of High-Mass Stars: New Insights of Stellar Interiors With Space Telescopes

Massive stars are important metal factories in the Universe. They have short and energetic lives, and many of them inevitably explode as a supernova and become a neutron star or black hole. In turn, the formation, evolution and explosive deaths of massive stars impact the surrounding interstellar medium and shape the evolution of their host galaxies. Yet the chemical and dynamical evolution of a massive star, including the chemical yield of the ultimate supernova and the remnant mass of the compact object, strongly depend on the interior physics of the progenitor star. We currently lack empirically calibrated prescriptions for various physical processes at work within massive stars, but this is now being remedied by asteroseismology. The study of stellar structure and evolution using stellar oscillations -- asteroseismology -- has undergone a revolution in the last two decades thanks to high-precision time series photometry from space telescopes. In particular, the long-term light curves provided by the MOST, CoRoT, BRITE, Kepler/K2 and TESS missions provided invaluable data sets in terms of photometric precision, duration and frequency resolution to successfully apply asteroseismology to massive stars and probe their interior physics. The observation and subsequent modelling of stellar pulsations in massive stars has revealed key missing ingredients in stellar structure and evolution models of these stars. Thus asteroseismology has opened a new window into calibrating stellar physics within a highly degenerate part of the Hertzsprung--Russell diagram. In this review, I provide a historical overview of the progress made using ground-based and early space missions, and discuss more recent advances and breakthroughs in our understanding of massive star interiors by means of asteroseismology with modern space telescopes.

On using dipolar modes to constrain the helium glitch in red giant stars

ABSTRACT The space-borne missions CoRoT and Kepler have revealed numerous mixed modes in red giant stars. These modes carry a wealth of information about red giant cores, but are of limited use when constraining rapid structural variations in their envelopes. This limitation can be circumvented if we have access to the frequencies of the pure acoustic dipolar modes in red giants, i.e. the dipole modes that would exist in the absence of coupling between gravity and acoustic waves. We present a pilot study aimed at evaluating the implications of using these pure acoustic mode frequencies in seismic studies of the helium structural variation in red giants. The study is based on artificial seismic data for a red giant branch stellar model, bracketing seven acoustic dipole radial orders around νmax. The pure acoustic dipole-mode frequencies are derived from a fit to the mixed-mode period spacings and then used to compute the pure acoustic dipole-mode second differences. The pure acoustic dipole-mode second differences inferred through this procedure follow the same oscillatory function as the radial-mode second differences. The additional constraints brought by the dipolar modes allow us to adopt a more complete description of the glitch signature when performing the fit to the second differences. The amplitude of the glitch retrieved from this fit is 15${{\ \rm per\ cent}}$ smaller than that from the fit based on the radial modes alone. Also, we find that thanks to the additional constraints, a bias in the inferred glitch location, found when adopting the simpler description of the glitch, is avoided.

Optimized cutting off transit algorithm to study stellar rotation from PLATO mission light curves

Measuring the stellar rotation of one of the components in eclipsing binaries (EBs) or planetary systems is a challenging task. The difficulty is mainly due to the complexity of analyzing, in the same light curve, the signal from the stellar rotation mixed with the transit signal of a stellar or sub-stellar companion, like a brown dwarf or planet. There are many methods to correct the long-term trend of the light curve. However, the correction often erases the signal of the stellar rotation from spots crossing the visible stellar disk and other weaker signals like planets. In this work, we present the DiffeRencial flUx Method of cuTting Off biNariES (DRUM TONES) to identify the signal of the binary transits and disentangle it from stellar rotation planet signals. We present our technique with applications to EBs from CoRoT, Kepler, Kepler K2 and TESS missions. We also applied our method to simulated synthetic EB from the PLATO mission. Our method shows good agreement in the determination of stellar rotation periods for few observed targets from last space missions, as well it is naturally useful for future European missions, such as PLAnetary Transits and Oscillations of stars (PLATO).

Connecting measurements of solar and stellar brightness variations

Context. A comparison of solar and stellar brightness variations is hampered by the difference in spectral passbands that are used in observations, and also by the possible difference in the inclination of the solar and stellar rotation axes from the line of sight. Aims. We calculate the rotational variability of the Sun as it would be measured in passbands used for stellar observations. In particular, we consider the filter systems used by the CoRoT, Kepler, TESS, and Gaia space missions. We also quantify the effect of the inclination of the rotation axis on the solar rotational variability. Methods. We employed the spectral and total irradiance reconstruction (SATIRE) model to calculate solar brightness variations in different filter systems as observed from the ecliptic plane. We then combined the simulations of the surface distribution of the magnetic features at different inclinations using a surface flux transport model with the SATIRE calculations to compute the dependence of the variability on the inclination. Results. For an ecliptic-bound observer, the amplitude of the solar rotational variability, as observed in the total solar irradiance (TSI), is 0.68 mmag (averaged over solar cycles 21–24). We obtained corresponding amplitudes in the Kepler (0.74 mmag), CoRoT (0.73 mmag), TESS (0.62 mmag), Gaia G (0.74 mmag), Gaia GRP (0.62 mmag), and Gaia GBP (0.86 mmag) passbands. Decreasing the inclination of the rotation axis decreases the rotational variability. For a sample of randomly inclined stars, the variability is on average 15% lower in all filter systems we considered. This almost compensates for the difference in amplitudes of the variability in TSI and Kepler passbands, making the amplitudes derived from the TSI records an ideal representation of the solar rotational variability for comparison to Kepler stars with unknown inclinations. Conclusions. The TSI appears to be a relatively good measure of solar variability for comparisons with stellar measurements in the CoRoT, Kepler, TESS Gaia G, and Gaia GRP filters. Whereas the correction factors can be used to convert the variability amplitude from solar measurements into the values expected for stellar missions, the inclination affects the shapes of the light curves so that a much more sophisticated correction than simple scaling is needed to obtain light curves out of the ecliptic for the Sun.

Convective excitation and damping of solar-like oscillations

ABSTRACT The last decade has seen a rapid development in asteroseismology thanks to the CoRoT and Kepler missions. With more detailed asteroseismic observations available, it is becoming possible to infer exactly how oscillations are driven and dissipated in solar-type stars. We have carried out three-dimensional (3D) stellar atmosphere simulations together with one-dimensional (1D) stellar structural models of key benchmark turn-off and subgiant stars to study this problem from a theoretical perspective. Mode excitation and damping rates are extracted from 3D and 1D stellar models based on analytical expressions. Mode velocity amplitudes are determined by the balance between stochastic excitation and linear damping, which then allows the estimation of the frequency of maximum oscillation power, νmax, for the first time based on ab initio and parameter-free modelling. We have made detailed comparisons between our numerical results and observational data and achieved very encouraging agreement for all of our target stars. This opens the exciting prospect of using such realistic 3D hydrodynamical stellar models to predict solar-like oscillations across the Hertzsprung–Russell diagram, thereby enabling accurate estimates of stellar properties such as mass, radius, and age.

Transiting exoplanets from the CoRoT space mission

Aims. We report the discovery as well as the orbital and physical characterizations of two new transiting giant exoplanets, CoRoT-30 b and CoRoT-31 b, with the CoRoT space telescope. Methods. We analyzed two complementary data sets: photometric transit light curves measured by CoRoT, and radial velocity curves measured by the HARPS spectrometer. To derive the absolute masses and radii of the planets, we modeled the stars from available magnitudes and spectra. Results. We find that CoRoT-30 b is a warm Jupiter on a close-to-circular 9.06-day orbit around a G3V star with a semi-major axis of about 0.08 AU. It has a radius of 1.01 ± 0.08 RJ, a mass of 2.90 ± 0.22 MJ, and therefore a mean density of 3.45 ± 0.65 g cm−3. The hot Jupiter CoRoT-31 b is on a close-to-circular 4.63-day orbit around a G2 IV star with a semi-major axis of about 0.05 AU. It has a radius of 1.46 ± 0.30 RJ, a mass of 0.84 ± 0.34 MJ, and therefore a mean density of 0.33 ± 0.18 g cm−3. Conclusions. Neither system seems to support the claim that stars hosting planets are more depleted in lithium. The radii of both planets are close to that of Jupiter, but they differ in mass; CoRoT-30 b is ten times denser than CoRoT-31 b. The core of CoRoT-30 b would weigh between 15 and 75 Earth masses, whereas relatively weak constraints favor no core for CoRoT-31 b. In terms of evolution, the characteristics of CoRoT-31 b appear to be compatible with the high-eccentricity migration scenario, which is not the case for CoRoT-30 b. The angular momentum of CoRoT-31 b is currently too low for the planet to evolve toward synchronization of its orbital revolution with stellar rotation, and the planet will slowly spiral-in while its host star becomes a red giant. CoRoT-30 b is not synchronized either: it looses angular momentum owing to stellar winds and is expected reach steady state in about 2 Gyr. CoRoT-30 and 31, as a pair, are a truly remarkable example of diversity in systems with hot Jupiters.

Probing the mid-layer structure of red giants

Context. The space-borne missions CoRoT and Kepler have already brought stringent constraints on the internal structure of low-mass evolved stars, a large part of which results from the detection of mixed modes. However, all the potential of these oscillation modes as a diagnosis of the stellar interior has not been fully exploited yet. In particular, the coupling factor or the gravity-offset of mixed modes, q and εg, are expected to provide additional constraints on the mid-layers of red giants, which are located between the hydrogen-burning shell and the neighborhood of the base of the convective zone. The link between these parameters and the properties of this region, nevertheless, still remains to be precisely established. Aims. In the present paper, we investigate the potential of the coupling factor in probing the mid-layer structure of evolved stars. Methods. Guided by typical stellar models and general physical considerations, we modeled the coupling region along with evolution. We subsequently obtained an analytical expression of q based on the asymptotic theory of mixed modes and compared it to observations. Results. We show that the value of q is degenerate with respect to the thickness of the coupling evanescent region and the local density scale height. On the subgiant branch and the beginning of the red giant branch (RGB), the model predicts that the peak in the observed value of q is necessarily associated with the important shrinking and the subsequent thickening of the coupling region, which is located in the radiative zone at these stages. The large spread in the measurement is interpreted as the result of the high sensitivity of q to the structure properties when the coupling region becomes very thin. Nevertheless, the important degeneracy of q in this regime prevents us from unambiguously concluding on the precise structural origin of the observed values. In later stages, the progressive migration of the coupling region toward the convective zone is expected to result in a slight and smooth decrease in q, which is in agreement with observations. At one point just before the end of the first-dredge up and the luminosity bump, the coupling region becomes entirely located in the convective region and its continuous thickening is shown to be responsible for the observed decrease in q. We demonstrate that q has the promising potential to probe the migration of the base of the convective region as well as convective extra-mixing during this stage. We also show that the frequency-dependence of q cannot be neglected in the oscillation spectra of such evolved RGB stars, which is in contrast with what is assumed in the current measurement methods. This fact can have an influence on the physical interpretation of the observed values. In red clump stars, in which the coupling regions are very thin and located in the radiative zone, the small variations and spread observed in q suggest that their mid-layer structure is very stable. Conclusions. A structural interpretation of the global observed variations in q was obtained and the potential of this parameter in probing the dynamics of the mid-layer properties of red giants is highlighted. This analytical study paves the way for a more quantitative exploration of the link of q with the internal properties of evolved stars using stellar models for a proper interpretation of the observations. This will be undertaken in the following papers of this series.

Generation of nonlinear magnetic flux tube wave energies in Procyon A

Abstract The aim of the current study is the computation of the magnetic flux tube wave energies and fluxes generated in the convection zone of Procyon A. This is a subgiant of spectral type F5 IV-V showing chromospheric and coronal activities. The mechanisms responsible for the generation of different wave modes include the interaction of the thin and vertically oriented magnetic flux tube embedded in magnetic-free regions with turbulence in the convection zone of Procyon A. We are considering longitudinal, transverse and acoustic wave modes. Turbulence in the convection zone is modeled by the extended Kolmogorov turbulent energy spectrum and the modified Gaussian frequency factor. Different magnetic flux tube models with different degrees of magnetic activities were considered. The current approach takes the nonlinear effects into consideration. The results show that there is enough wave energy in the three different forms to heat the outer layers of the star. The obtained acoustic wave energies are larger than those of the longitudinal tube wave energies compared to the solar case. This can be explained by the relatively low magnetic field strength. On the other side, our computations show the importance of the transverse wave energies compared to the energies carried by the longitudinal waves. The former waves carry energy several (between 2 and 14) times higher than the latter. The obtained wave energies are essential for constructing time-dependent model chromospheres and for the predictions of atmospheric oscillations to be compared e.g. with the data collected by the CoRoT and Kepler missions.