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KIC 3429637 (HD 178875) is a delta Sct star whose light-curve shows continuous pulsational amplitude growth in Kepler Mission photometry. Analysis of the three largest amplitude peaks in the Fourier transform indicates different growth rates for all three. We have ruled out instrumental causes, and determine the amplitude growth to be intrinsic to the star. We calculate time-dependent convection models and compare them with the observations. We confirm earlier characterisations that KIC 3429637 is a marginal Am star through the analysis of new spectroscopic data. With the data presently available, a plausible cause of the amplitude growth is increasing pulsational driving as evolutionary changes shift the He II driving zone deeper in this rho Puppis star. If this model is correct, then we are watching real-time stellar evolutionary changes.

Since the early days of helioseismology, adiabatic models have shown their limits for a precise fitting of individual oscillation frequencies. This discrepancy, which also exists for solar-type stars, is known to originate near the surface superadiabatic convective region where the interaction between oscillations and convection is likely to have a large effect on the frequencies. We present an asteroseismic study to address the adequacy of time-dependent convection (TDC) non-adiabatic models to better reproduce the observed individual frequencies. We select, for this purpose, three solar-like stars, in addition to the Sun, to which we fit the observed frequencies in a grid of TDC non-adiabatic models. The best model selection is done by applying a maximum likelihood method. The results are compared to pure adiabatic and near-surface corrected adiabatic models. We show that, first, TDC models give very good agreement for the mode frequencies and average lifetimes. In the solar case, the frequency discrepancy is reduced to

We analyse Kepler observations of the high-amplitude δ Scuti (HADS) star V2367 Cyg (KIC 9408694). The variations are dominated by a mode with frequency f1= 5.6611 d−1. Two other independent modes with f2= 7.1490 d−1 and f3= 7.7756 d−1 have amplitudes an order of magnitude smaller than f1. Nearly all the light variation is due to these three modes and their combination frequencies, but several hundred other frequencies of very low amplitude are also present. The amplitudes of the principal modes may vary slightly with time. The star has twice the projected rotational velocity of any other HADS star, which makes it unusual. We find a correlation between the phases of the combination frequencies and their pulsation frequencies, which is not understood. Since modes of highest amplitude in HADS stars are normally radial modes, we assumed that this would also be true in this star. However, attempts to model the observed frequencies as radial modes without mode interaction were not successful. For a star with such a relatively high rotational velocity, it is important to consider the effect of mode interaction. Indeed, when this was done, we were able to obtain a model in which a good match with f1 and f2 is obtained, with f1 being the fundamental radial mode.

We present an analysis of high-resolution spectra for two pulsating Am stars in the Kepler field. The stellar parameters derived in this way are important because parameters derived from narrow-band photometry may be affected by the strong metal lines in these stars. We analyse the Kepler time series of ten known Am stars and find that six of them clearly show δ Scuti pulsations. The other four appear to be non-pulsating. We derive fundamental parameters for all known pulsating Am stars from ground-based observations and also for the Kepler Am stars to investigate the location of the instability strip for pulsating Am stars. We find that there is not much difference between the Am-star instability strip and the δ Scuti instability strip. We find that the observed location of pulsating Am stars in the HR diagram does not agree with the location predicted from diffusion calculations.

Through the observational study of stellar pulsations, the internal structure of stars can be probed and theoretical models can be tested. The main sequence -γ Doradus (Dor) and δ Scuti (Sct) stars with masses 1.2–2.5 M⊙ are particularly interesting for asteroseismic study. The -γ Dor stars pulsate in high-order gravity (g) modes, with pulsational periods of order of one day. The δ Sct stars, on the other hand, show low-order g and pressure (p) modes with periods of order of 2 hours. Theory predicts the existence of ‘hybrid’ stars, i.e. stars pulsating in both types of modes, in an overlap region between the instability strips of -γ Dor and δ Sct stars in the Hertzsprung-Russell diagram. Hybrid stars are particularly interesting as the two types of modes probe different regions of the stellar interior and hence provide complementary model constraints. Before the advent of Kepler, only a few hybrid stars had been confirmed. The Kepler satellite is providing a true revolution in the study of and search for hybrid stars. Analysis of the first 50 days of Kepler data of hundreds of -γ Dor and δ Sct candidates reveals extremely rich frequency spectra, with most stars showing frequencies in both the δ Sct and -γ Dor frequency range. As these results show that there are practically no pure δ Sct or -γ Dor pulsators, a new observational classification scheme is proposed by Grigahcene et al. (2010). We present their results and characterize 234 stars in terms of δ Sct, -γ Dor, δ Sct/-γ Dor or -γ Dor/δ Sct hybrids (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report spectroscopic observations for 19 $\delta$\,Sct candidates observed by the {\it Kepler} satellite both in long and short cadence mode. For all these stars, by using spectral synthesis, we derive the effective temperature, the surface gravity and the projected rotational velocity. An equivalent spectral type classification has been also performed for all stars in the sample. These determinations are fundamental for modelling the frequency spectra that will be extracted from the {\it Kepler} data for asteroseismic inference. For all the 19 stars, we present also periodograms obtained from {\it Kepler} data. We find that all stars show peaks in both low- ($\gamma$\,Dor; g mode) and high-frequency ($\delta$\,Sct; p mode) regions. Using the amplitudes and considering 5\,c/d as a boundary frequency, we classified 3 stars as pure $\gamma$\,Dor, 4 as $\gamma$\,Dor\,-\,$\delta$\ hybrid, Sct, 5 as $\delta$\,Sct\,-\,$\gamma$\,Dor hybrid, and 6 as pure $\delta$\,Sct. The only exception is the star KIC\,05296877 which we suggest could be a binary.

Mode identification is one of the first and main problems we encounter in trying to develop the complete potential of asteroseismology. In the particular case of g-mode pulsators, this is still an unsolved problem, from both the observational and theoretical points of view. Nevertheless, in recent years, some observational and theoretical efforts have been made to find a solution. In this work we use the latest theoretical and computational tools to understand asymptotic g-mode pulsators: 1) the Frequency Ratio Method, and 2) Time Dependent Convection. With these tools, a self-consistent procedure for mode identification and modelling of these g-mode pulsators can be constructed. This procedure is illustrated using observational information available for the γ Doradus star 9Aurigae. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The approximation known as Frozen Convection has been widely used in the context of stellar oscillations due to the lack of a Time Dependent Convection theory. This approximation supposes to neglect the interaction between convection and oscillations and there are different ways to do it. The choice can affect theoretical predictions concerning the non-adiabatic treatment of the stellar oscillations. In fact, fictitious excitation or damping of the pulsation modes can be produced depending on the approximation used.

International audience; Oscillations of gamma Dor-type were discovered in the star HD 218427 through simultaneous uvby photometric observations carried out in the year 2003. A few Hbeta-Crawford measurements were also collected for calibration purposes and they locate this star well inside the gamma Dor instability region. We find HD 218427 to be deficient in metals, similar to other well-defined gamma Dor stars, and discuss the possibility that it has a lambda Boo nature. We carried out frequency analysis for different filters, including the combined "vby" filter, and five frequencies were found as significant with periods ranging between 0.3 and 0.8 days. The recently-developed frequency ratio method is used in order to identify the excited modes. The results are consistent with an l=2 identification for all the modes and with high radial quantum numbers (n˜40) for the three main observed periodicities. The possibility of multiplet structures is also discussed. However, no consistency is found when using the time-dependent convection treatment to discriminate modes. This disagreement can be due to the large rotation velocity taking place in HD 218427 and, consequently, the significant coupling between the modes.

We apply here the Time Dependent Convection (TDC) treatment presented in our earlier paper in this series to the study of δ Sct and γ Dor pulsating stars. Stabilization of the δ Sct p-modes at the red edge of the Instability Strip (IS) and the driving of the γ Dor g-modes are explained by our models. Theoretical IS obtained with different values of the Mixing Length (ML) parameter α are compared to observations and a good agreement is obtained for α between 1.8 and 2. The influence of each term of our TDC treatment (perturbation of convective flux, turbulent pressure, and turbulent kinetic energy dissipation) on the eigenfrequencies and on the driving and damping mechanisms is investigated. Finally, we show that our TDC models predict the likely existence of hybrid stars with both δ Sct p-modes and γ Dor g-modes oscillations.

We present in details a time-dependent convection treatment in the frame of the Mixing-Length Theory (MLT). Following the original ideas by Unno (1967, PASJ, 19, 140), this theory has been developed by Gabriel et al. (1974, Bull. Ac. Roy. Belgique, Classe des Sciences, 60, 866) and Gabriel (1996, Bull. Astron. Soc. India, 24, 233). In this paper, we present it in a united form, we detail the basic derivations and approximations and give final improvements. A new perturbation of the energy closure equation is proposed for the first time, making it possible to avoid the occurrence of short wavelength spatial oscillations of the thermal eigenfunctions. This theory accounts for the perturbation of the convective flux, the turbulent Reynolds stress and the turbulent kinetic energy dissipation. It has been numerically implemented in a non-radial non-adiabatic pulsation code and the first results published in a Letter by Dupret et al. (2004a, A&A, 414, L17) indicate that the theory predicts the observed red border of the lower end of the instability strip and the driving mechanism of the recently discovered γ Dor stars.

The Kepler spacecraft is providing time series of photometric data with micromagnitude precision for hundreds of A-F type stars. We present a first general characterization of the pulsational behaviour of A-F type stars as observed in the Kepler light curves of a sample of 750 candidate A-F type stars. We propose three main groups to describe the observed variety in pulsating A-F type stars: gamma Dor, delta Sct, and hybrid stars. We assign 63% of our sample to one of the three groups, and identify the remaining part as rotationally modulated/active stars, binaries, stars of different spectral type, or stars that show no clear periodic variability. 23% of the stars (171 stars) are hybrid stars, which is a much larger fraction than what has been observed before. We characterize for the first time a large number of A-F type stars (475 stars) in terms of number of detected frequencies, frequency range, and typical pulsation amplitudes. The majority of hybrid stars show frequencies with all kinds of periodicities within the gamma Dor and delta Sct range, also between 5 and 10 c/d, which is a challenge for the current models. We find indications for the existence of delta Sct and gamma Dor stars beyond the edges of the current observational instability strips. The hybrid stars occupy the entire region within the delta Sct and gamma Dor instability strips, and beyond. Non-variable stars seem to exist within the instability strips. The location of gamma Dor and delta Sct classes in the (Teff,logg)-diagram has been extended. We investigate two newly constructed variables 'efficiency' and 'energy' as a means to explore the relation between gamma Dor and delta Sct stars. Our results suggest a revision of the current observational instability strips, and imply an investigation of other pulsation mechanisms to supplement the kappa mechanism and convective blocking effect to drive hybrid pulsations., Accepted for publication in Astronomy and Astrophysics (language edited version). 70 pages, 4 tables, 13 figures. The tables can be found as an appendix to the paper. The quality of the figures has been downgraded compared to the original

Context. gamma Doradus (gamma Dor) are late A and F-type stars pulsating with high order gravity modes (g-modes). The existence of different evolutionary phases crossing the gamma Dor instability strip raises the question of the existence of pre-main sequence (PMS) gamma Dor stars. Aims. We intend to study the differences between the asteroseismic behaviour of PMS and main sequence (MS) gamma Dor pulsators as it is predicted by the current theory of stellar evolution and stability. Methods. We explore the adiabatic and non-adiabatic properties of high order g-modes in a grid of PMS and MS models covering the mass range 1.2 Msun < Mstar < 2.5 Msun. Results. We derive the theoretical instability strip (IS) for the PMS gamma Dor pulsators. This IS covers the same effective temperature range as the MS gamma Dor one. Nevertheless, the frequency domain of unstable modes in PMS models with a fully radiative core is larger than in MS models, even if they present the same number of unstable modes. Moreover, the differences between MS and PMS internal structures are reflected on the average values of the period spacing as well as on the dependence of the period spacing on the radial order of the modes, opening the window to the determination of the evolutionary phase of gamma Dor stars from their pulsation spectra., 9 pages, 17 figures, accepted for publication in A&A

The question of the existence of pre-main sequence (PMS) $\gamma$~Doradus ($\gamma$~Dor) has been raised by the observations of young clusters such as NGC~884 hosting $\gamma$~Dor members. We have explored the properties of $\gamma$~Dor type pulsations in a grid of PMS models covering the mass range $1.2 M_\odot < M_* < 2.5 M_\odot$ and we derive the theoretical instability strip (IS) for the PMS $\gamma$~Dor pulsators. We explore the possibility of distinguishing between PMS and MS $\gamma$~Dor by the behaviour of the period spacing of their high order $gravity$-modes ($g$-modes)., Comment: 5 pages, 6 figures, Proc. HELAS IV Conference, Lanzarote, February 2010. Eds T. Roca Cort\'es, P. Pall\'e and S. Jim\'enez Reyes. Accepted in Astron. Nach

Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main sequence $\gamma$ Doradus (Dor) and $\delta$ Scuti (Sct) stars with masses 1.2-2.5 $M_{\sun}$ are particularly useful for these studies. The $\gamma$ Dor stars pulsate in high-order $g$ modes with periods of order 1 day, driven by convective blocking at the base of their envelope convection zone. The $\delta$ Sct stars pulsate in low-order $g$ and $p$ modes with periods of order 2 hours, driven by the $\kappa$ mechanism operating in the Heii ionization zone. Theory predicts an overlap region in the Hertzsprung-Russell diagram between instability regions, where 'hybrid' stars pulsating in both types of modes should exist. The two types of modes with properties governed by different portions of the stellar interior provide complementary model constraints. Among the known $\gamma$ Dor and $\delta$ Sct stars, only four have been confirmed as hybrids. Now, analysis of combined Quarter 0 and Quarter 1 Kepler data for hundreds of variable stars shows that the frequency spectra are so rich that there are practically no pure $\delta$ Sct or $\gamma$ Dor pulsators, i.e. essentially all of the stars show frequencies in both the $\delta$ Sct and $\gamma$ Dor frequency range. A new observational classification scheme is proposed that takes into account the amplitude as well as the frequency, and is applied to categorize 234 stars as $\delta$ Sct, $\gamma$ Dor, $\delta$ Sct/$\gamma$ Dor or $\gamma$ Dor/$\delta$ Sct hybrids., Comment: 6 pages with 4 figures and 1 table - submitted to ApJL

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? We investigate under what physical circumstances non-radial modes could be observable in red giants; what would be their amplitudes, lifetimes and heights in the power spectrum (PS)? Using a non-radial non-adiabatic pulsation code including a non-local time-dependent treatment of convection, we compute the theoretical lifetimes of radial and non-radial modes in several red giant models. Next, using a stochastic excitation model, we compute the amplitudes of these modes and their heights in the PS. Very distinct results are found depending on the evolutionary status of the star., Source tex file : 1713.tex Bibliography file : 1713.bbl 24 figures : 1713f1.eps, ..., 1713f24.eps

International audience; The variability of HD 239276 was suspected photometrically nearly twenty years ago, but was confirmed with new observations obtained in 2001 during a two-site photometric campaign carried out from Spain, in uvbybeta Strömgren-Crawford photometry, and China, using the Johnson V filter. Two low-dispersion spectra were also collected. The results establish this star as a new multiperiodic gamma Dor-type pulsator with deficiency in metallicity. Its possible lambda Boo nature is discussed. The frequency analysis shows three pulsational frequencies as significant, but some more are probably present among the residuals. The method based on phase shifts and amplitude ratios in multicolour photometry is used to identify the excited modes with non-adiabatic time-dependent convection models. A very good agreement between the theoretical and observed amplitude ratios is obtained and the two main modes are identified as l=1 modes. Nevertheless, our results do not allow us to discriminate between a solar abundance and a metal deficient nature for this star. The frequency ratio method is further used for the identification of the modes. The results suggest low metallicity for this star, but a lambda Boo nature may be not ruled out.

International audience; We apply for the first time the time-dependent convection (TDC) treatment of Gabriel and Grigahcène et al. to the photometric mode identification in gamma Doradus (gamma Dor) stars. We consider the influence of this treatment on the theoretical amplitude ratios and phase differences. Comparison with the observed amplitudes and phases of the stars gamma Dor, 9 Aurigae, HD 207223 = HR 8330, HD 12901 and 48501 is presented and enables us to identify the degree l of the pulsation modes for four of them. We also determine the mode stability for different models of these stars. We show that our TDC models agree better with observations than with frozen convection models. Finally, we compare the results obtained with different values of the mixing-length parameter alpha.

International audience; The effect of rotation on the Frequency Ratio Method (Moya et al. 2005, A&A, 432, 189) is examined. Its applicability to observed frequencies of rotating gamma Doradus stars is discussed taking into account the following aspects: the use of a perturbative approach to compute adiabatic oscillation frequencies; the effect of rotation on the observational Brunt-Väisälä integral determination and finally, the problem of disentangling multiplet-like structures from frequency patterns due to the period spacing expected for high-order gravity modes in asymptotic regime. This analysis reveals that the FRM produces reliable results for objects with rotational velocities up to 70 {km s-1}, for which the FRM intrinsic error increases one order of magnitude with respect to the typical FRM errors given in Moya et al. (2005, A&A, 432, 189). Our computations suggest that, given the spherical degree l identification, the FRM may be discriminating for m=0 modes, in the sense that the method avoids any misinterpretation induced by the presence of rotationally split multiplet-like structures, which reinforces the robustness of the method. However, if l is unknown, such discrimination is not ensured. In order to check the FRM in presence of slow-moderate rotation, we have applied it to the three observed frequencies of the slowly rotating ({v sin i}=29 {km s-1}) gamma Doradus star HD 48501.

Aims. The determination of the location of the theoretical ZZ Ceti instability strip in the log g −Teff diagram has remained a challenge over the years due to the lack of a suitable treatment for convection in these stars. For the first time, a full nonadiabatic approach including time-dependent convection is applied to ZZ Ceti pulsators, and we provide the appropriate details related to the inner workings of the driving mechanism. Methods. We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 MDA white dwarf. This sequence is made of state-of-the-art models that include a detailed modeling of the feedback of convection on the atmospheric structure. The assumed convective efficiency in these models is the so-called ML2/α = 1.0 version. We also carried out, for comparison purposes, nonadiabatic computations within the frozen convection approximation, as well as calculations based on models with standard grey atmospheres. Results. We find that pulsational driving in ZZ Ceti stars is concentrated at the base of the superficial H convection zone, but at depths, near the blue edge of the instability strip, somewhat larger than those obtained with the frozen convection approach. Despite the fact that this approach is formally invalid in such stars, particularly near the blue edge of the instability strip, the predicted boundaries are not dramatically different in both cases. The revised blue edge for a 0.6 Mmodel is found to be around Teff = 11 970 K, some 240 K hotter than the value predicted within the frozen convection approximation, in rather good agreement with the empirical value. On the other hand, our predicted red edge temperature for the same stellar mass is only about 5600 K (80 K hotter than with the frozen convection approach), much lower than the observed value. Conclusions. We correctly understand the development of pulsational instabilities of a white dwarf as it cools at the blue edge of the ZZ Ceti instability strip. Our current implementation of time-dependent convection however still lacks important ingredients to fully account for the observed red edge of the strip. We will explore a number of possibilities in the future papers of this series.