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Non-adiabatic study of the Kepler subgiant KIC 6442183 Grosjean, Mathieu ; Dupret, Marc-Antoine ; et al in The Space Photometry Revolution CoRoT Symposium 3, Kepler KASC-7 joint meeting (2015, September) Thanks to the precision of Kepler observations, [3] were able to measure the linewidth and amplitude of individual modes (including mixed modes) in several sub- giant power spectra. We perform a forward ... [more ▼] Thanks to the precision of Kepler observations, [3] were able to measure the linewidth and amplitude of individual modes (including mixed modes) in several sub- giant power spectra. We perform a forward modelling of a Kepler subgiant based on sur- face properties and observed frequencies. Non-adiabatic computations including a time- dependent treatment of convection give the lifetimes of radial and non-radial modes. Next, combining the lifetimes and inertias with a stochastic excitation model gives the ampli- tudes of the modes. We can now directly compare theoretical and observed linewidths and amplitudes of mixed-modes to obtain new constraints on our theoretical models. [less ▲] Detailed reference viewed: 23 (6 ULg)The instability strip of ZZ Ceti white dwarfs I. Introduction of time-dependent convection Van Grootel, Valérie ; Dupret, Marc-Antoine ; et al in Astronomy and Astrophysics (2012), 539 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 ... [more ▼] 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 work- ings of the driving mechanism at work. Methods. We used the nonadiabatic pulsation code MAD with a representative evolutionary sequence of a 0.6 M⊙ DA 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 M⊙ model 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. [less ▲] Detailed reference viewed: 29 (4 ULg)Hybrid gamma Doradus-delta Scuti Pulsators: New Insights into the Physics of the Oscillations from Kepler Observations ; ; et al in Astrophysical Journal (2010), 713 Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main-sequence Î³ Doradus (Dor) and Î´ Scuti (Sct) stars with masses 1.2-2.5 M [SUB ... [more ▼] Observations of the pulsations of stars can be used to infer their interior structure and test theoretical models. The main-sequence Î³ Doradus (Dor) and Î´ Scuti (Sct) stars with masses 1.2-2.5 M [SUB]sun[/SUB] are particularly useful for these studies. The Î³ 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 Î´ Sct stars pulsate in low-order g- and p-modes with periods of order 2 hr, driven by the Îº mechanism operating in the He II 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 Î³ Dor and Î´ 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 Î´ Sct or Î³ Dor pulsators, i.e., essentially all of the stars show frequencies in both the Î´ Sct and the Î³ 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 Î´ Sct, Î³ Dor, Î´ Sct/Î³ Dor or Î³ Dor/Î´ Sct hybrids. [less ▲] Detailed reference viewed: 28 (2 ULg) |
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