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Current state-of-the-art computational modeling makes it possible to build realistic models of stellar convection zones and atmospheres that take into account chemical composition, radiative effects, ionization, and turbulence. The standard 1D mixing-length-based evolutionary models are not able to capture many physical processes of the stellar interior dynamics. Mixing-length models provide an initial approximation of stellar structure that can be used to initialize 3D radiative hydrodynamics simulations which include realistic modeling of turbulence, radiation, and other phenomena. In this paper, we present 3D radiative hydrodynamic simulations of an F-type main-sequence star with 1.47 solar mass. The computational domain includes the upper layers of the radiation zone, the entire convection zone, and the photosphere. The effects of stellar rotation is modeled in the f-plane approximation. These simulations provide new insight into the properties of the convective overshoot region, the dynamics of the near-surface, highly turbulent layer, and the structure and dynamics of granulation. They reveal anti-solar-type differential rotation and latitudinal dependence of the tachocline location., 8 pages, 6 figures; Proceeding of IAU Symposium #354: "Solar and Stellar Magnetic Fields: Origins and Manifestations", 30 June - 6 July 2019, Copiapo, Chile

Extreme convective events in the troposphere not only have immediate impacts on the surface, but they can also influence the dynamics and composition of the lower stratosphere (LS). One major impact is the moistening of the LS by overshooting convection. This effect plays a crucial role in climate feedback, as small changes of water vapor in the upper troposphere and lower stratosphere (UTLS) have a large impact on the radiative budget of the atmosphere. In this case study, we investigate water vapor injections into the LS by two consecutive convective events in the European mid-latitudes within the framework of the MOSES (Modular Observation Solutions for Earth Systems) measurement campaign during the early summer of 2019. Using balloon-borne instruments, measurements of convective water vapor injection into the stratosphere were performed. Such measurements with a high vertical resolution are rare. The magnitude of the stratospheric water vapor reached up to 12.1 ppmv (parts per million by volume), with an estimated background value of 5 ppmv. Hence, the water vapor enhancement reported here is of the same order of magnitude as earlier reports of water vapor injection by convective overshooting over North America. However, the overshooting took place in the extratropical stratosphere over Europe and has a stronger impact on long-term water vapor mixing ratios in the stratosphere compared to the monsoon-influenced region in North America. At the altitude of the measured injection, a sharp drop in a local ozone enhancement peak makes the observed composition of air very unique with high ozone up to 650 ppbv (parts per billion by volume) and high water vapor up to 12.1 ppmv. ERA-Interim does not show any signal of the convective overshoot, the water vapor values measured by the Microwave Limb Sounder (MLS) in the LS are lower than the in situ observations, and the ERA5 overestimated water vapor mixing ratios. Backward trajectories of the measured injected air masses reveal that the moistening of the LS took place several hours before the balloon launch. This is in good agreement with the reanalyses, which shows a strong change in the structure of isotherms and a sudden and short-lived increase in potential vorticity at the altitude and location of the trajectory. Similarly, satellite data show low cloud-top brightness temperatures during the overshooting event, which indicates an elevated cloud top height.

We report on the gravitational-wave signal computed in the context of a three-dimensional simulation of a core-collapse supernova explosion of a $15\text{ }{M}_{\ensuremath{\bigodot}}$ star. The simulation was performed with our neutrino hydrodynamics code chimera. We detail the gravitational wave strains as a function of time, for both polarizations, and discuss their physical origins. We also present the corresponding spectral signatures. Gravitational wave emission in our model has two key features: low-frequency emission (less than 200 Hz) emanates from the gain layer as a result of neutrino-driven convection and the standing accretion shock instability (SASI), and high-frequency emission (greater than 600 Hz) emanates from the proto--neutron star due to Ledoux convection within it. The high-frequency emission dominates the gravitational wave emission in our model and emanates largely from the convective layer itself, not from the convectively stable layer above it, due to convective overshoot. Moreover, the low-frequency emission emanates from the gain layer itself, not from the proto--neutron star, due to accretion onto it. We provide evidence of the SASI in our model and demonstrate that the peak of our low-frequency gravitational wave emission spectrum corresponds to it. Given its origin in the gain layer, we classify the SASI emission in our model as p-mode emission and assign a purely acoustic origin, not a vortical--acoustic origin, to it. We compare the results of our three-dimensional model analysis with those obtained from the model's two-dimensional counterpart and find a significant reduction in the strain amplitudes in the former case, as well as significant reductions in all related quantities. Our dominant proto--neutron star gravitational wave emission is not well characterized by emission from surface g modes, complicating the relationship between peak frequencies observed and the mass and radius of the proto--neutron star expressed by analytic estimates under the assumption of surface g-mode emission. We present our frequency normalized characteristic strain along with the sensitivity curves of current- and next-generation gravitational wave detectors. This simple analysis indicates that the spectrum of gravitational wave emission between approximately 20 Hz and approximately 1 kHz, stemming from neutrino-driven convection, the SASI, accretion onto the proto--neutron star, and proto--neutron star convection, will be accessible for a Galactic event.

We report distinctive core profiles in the strongest optical helium line, He I 5876, from high-resolution high-sensitivity observations of spectral type DB white dwarfs. By analyzing a sample of 40 stars from Keck/HIRES and VLT/UVES, we find the core appearance to be related to the degree of hydrogen and heavy element content in the atmosphere. New Ca K-line measurements or upper limits are reported for about half the sample stars. He I 5876 emission cores with a self-reversed central component are present for those stars with relatively low hydrogen abundance, as well as relatively low atmospheric heavy element pollution. This self-reversed structure disappears for stars with higher degrees of pollution and/or hydrogen abundance, giving way to a single absorption core. From our model atmospheres, we show that the self-reversed emission cores can be explained by temperature inversions in the upper atmosphere. We propose that the transition to a single absorption core is due to the additional opacity from hydrogen and heavy elements that inhibits the temperature inversions. Our current models do not exactly match the effective temperature range of the phenomenon nor the amplitude of the self-reversed structure, which is possibly a result of missing physics such as 3D treatment, convective overshoot, and/or non-LTE effects. The He I 5876 line structure may prove to be a useful new diagnostic for calibrating temperature profiles in DB atmosphere models., Accepted for publication in ApJ, 16 pages, 10 figures

International audience; A balloon-borne microlidar has been built at LATMOS to monitor cirrus optical properties and convective overshoot topography during long duration flights in the lower tropical stratosphere. Weighting less than 7 kg in a reduced volume and consuming less than 10 W, it will be involved in the CNES-Strateole2 campaign. This instrument paves the way to the use of microlidar technology for planetary missions.

The novelty of the models of the turbulent dynamo of the solar cycle, which were proposed in recent years, is that the generation of the poloidal and toroidal components of the global magnetic field does not occur equally effectively in the entire solar convective zone (SCZ), as was previously thought; but this generation is concentrating in distributed areas of the SCZ. In this connection, the question of the localization of the Ω-effect, which excites the toroidal field, becomes a matter of urgency, since the amplitude of the solar cycle depends on the magnitude of the latter one. In this work, the role of deep layers near the bottom of the SCZ, covering the layer of permeable convection (convective overshoot layer) and the tachocline layer, in the generation of a powerful toroidal field is analyzed. The author's recent studies on the role of the deep layers of the SCZ in explaining the observed phenomenon of double peaks of the cycle of sunspots, are noted. In the convective overshoot there are created the necessary conditions for the formation of the layer of prolonged maintenance of magnetic field, whereas in the tachocline due to the sharp decrease in angular velocity in the presence of the weak poloidal field the powerful toroidal field is effectively generated. However, the further evolution of this field occurs in different regimes in the polar and equatorial domains of the SCZ. In the polar domain, two effects of antibuoyancy (macroscopic turbulent diamagnetism and ∇ρ-effect) block magnetic buoyancy of field. Whereas, in the equatorial domain in the lower part of the SCZ, one of the antibuoyancy effects (∇ρ-effect) changes its sign to the opposite, thereby helping Parker's magnetic buoyancy. As a result, the effect of Parker buoyancy is prevailing over the antibuoyancy effect of the macroscopic turbulent diamagnetism. Therefore, after some time, the toroidal field rises to the surface and forms magnetic bipolar groups of sunspots (the first directed upward wave of the toroidal field, which is responsible for the main maximum of the activity of spot formation). An important factor in the processes in the deep layers is the meridional flow directed towards equator, which ensures the migration of the toroidal field near the bottom of the SCZ from the high latitudes to the low ones. Therefore, after 1–2 years the blocked polar toroidal field reaches a site in the equatorial domain, where already there are favorable conditions for its lifting to the surface. Here, this delayed in time toroidal field rises to the solar surface (the second upward magnetic wave) and thus provides the repeating maximum of sunspots.

The time-mean and time-varying smoke and velocity structure of a wildfire convective plume is examined using a high-resolution scanning Doppler lidar. The mean plume is shown to exhibit the archetypal form of a bent-over plume in a crosswind, matching the well-established Briggs plume-rise equation. The plume cross section is approximately Gaussian and the plume radius increases linearly with height, consistent with plume-rise theory. The Briggs plume-rise equation is subsequently inverted to estimate the mean fire-generated sensible heat flux, which is found to be 87 kW m−2. The mean radial velocity structure of the plume indicates flow convergence into the plume base and regions of both convective overshoot and sinking flow in the upper plume. The updraft speed in the lower plume is estimated to be 13.5 m s−1 by tracking the leading edge of a convective element ascending through the plume. The lidar data also reveal aspects of entrainment processes during the plume rise. For example, the covariation of the radial velocity and smoke perturbations are shown to dilute the smoke concentration with height.

This chapter begins with the principles determining a star’s structure: hydrostatic and thermal balance, and energy generation and transport. These imply that some stars have stably stratified cores and convective envelopes, whereas other stars have convective cores and stably stratified envelopes. The convection in stars is predominantly low Mach number, but the density at the top of a convection zone can be orders of magnitude smaller than the density at the bottom. We derive the anelastic equations which can model efficient, low Mach number convection. The properties of stars can be inferred by studying the waves at their surface. Here we describe sound and internal gravity waves, both of which have been observed in the Sun or other stars. The second half of this chapter discusses two phenomena at the interface between the convective and stably stratified layers of stars. First we consider convective overshoot, the convective motions which can extend into an adjacent stably stratified fluid. This can lead to substantial mixing in the stably stratified part of stars. Then, we discuss internal gravity wave generation by convection, which can lead to wave-induced energy or momentum transport. These illustrate some important fluid dynamical problems in stellar astrophysics.

We present a theoretical description of macroscopic diffusion caused by convective overshoot in pure-hydrogen DA white dwarfs using three-dimensional (3D), closed-bottom, radiation hydrodynamics CO$^5$BOLD simulations. We rely on a new grid of deep 3D white dwarf models in the temperature range 11400 K $\leq T_{\mathrm{eff}} \leq$ 18000 K where tracer particles and a tracer density are used to derive macroscopic diffusion coefficients driven by convective overshoot. These diffusion coefficients are compared to microscopic diffusion coefficients from one-dimensional structures. We find that the mass of the fully mixed region is likely to increase by up to 2.5 orders of magnitude while inferred accretion rates increase by a more moderate order of magnitude. We present evidence that an increase in settling time of up to 2 orders of magnitude is to be expected which is of significance for time-variability studies of polluted white dwarfs. Our grid also provides the most robust constraint on the onset of convective instabilities in DA white dwarfs to be in the effective temperature range from 18000 to 18250 K., 21 pages, 24 figures, accepted for publication in MNRAS

The distribution of stars in the Hertzsprung Russell diagram (HRD) for a stellar conglomeration represents a snapshot of its evolving stellar population. Some of the supergiant stars may transit the HRD from blue to red and then again to blue during their late evolutionary stages, as exemplified by the progenitor of SN 1987A. Others may transit a given part of the HRD more than twice in a "blue loop" and end up as red supergiants before they explode. Since stars in blue loops spend a considerable part of their lives there, these stages may change the relative number of modeled supergiants in the HRD. Their lifetimes in turn depend upon the initial mass of the star, how convection in its interior is modeled, and how much mass loss takes place during its evolution. The observed ratio of the number of blue to red supergiants and yellow to red supergiants sensitively test the stellar evolution theory. We compare modeled number ratios of these supergiants with observed data from the Large Magellanic Cloud as it has a metallicity very similar to that of the environment of SN 2013ej. We successfully model these by taking into account moderate (exponential) convective overshooting. We explore its effect on the final radius and mass of the star prior to core collapse. The radius differs dramatically with overshoot. These factors controlling pre-supernova structure may affect the post-explosion optical/IR light curves and spectral development., Comment: 14 pages, 3 figures, 3 tables, accepted at the ApJ for publication

We collected a sample of ten hitherto unidentified SPB stars with the aim of describing their pulsational properties and identifying pulsational modes. Photometric time series data from various surveys were collected and analyzed using diverse frequency search algorithms. We calculated astrophysical parameters and investigated the location of our sample stars in the \logTeff\ versus \logl\ diagram. Current pulsational models were calculated and used for the identification of pulsational modes in our sample stars. An extensive grid of stellar models along with their g-mode eigenfrequencies was calculated and subsequently cross-matched with the observed pulsational frequencies. The best-fit models were then used in an attempt to constrain stellar parameters such as mass, age, metallicity, and convective overshoot. We present detected frequencies, corresponding g-mode identifications, and the masses and ages of the stellar models producing the best frequency cross-matches. We partially succeeded in constraining stellar parameters, in particular concerning mass and age. Where applicable, rotation periods have been derived from the spacing of triplet component frequencies. No evolved SPB stars are present in our sample. We identify two candidate high-metallicity objects (HD\,86424 and HD\,163285), one young SPB star (HD\,36999), and two candidate young SPB stars (HD\,61712 and HD\,61076). We demonstrate the feasibility of using ground-based observations to perform basic asteroseismological analyses of SPB stars. Our results significantly enlarge the sample of known SPB stars with reliable pulsational mode identifications, which provides important input parameters for modeling attempts aiming to investigate the internal processes at work in upper main-sequence stars., 11 pages, 5 figures, accepted for publication in Astronomy & Astrophysics

Recent detections of g-mode pulsations in subdwarf B (sdB) stars from the space missions CoRoT and Kepler have verified that nearly all of these pulsators have regular period spacings. This discovery has been extensively applied to mode identification for the observed g-mode sdB pulsators. However, the discovery of regular period spacings apparently contradicts the strong g-mode trapping structure expected in canonical sdB models, which is caused by steep chemical gradients in such evolved stars. Although the mode trapping efficiency could be reduced somewhat by taking into account the diffusion effect during the sdB phase, it is still difficult to explain the regular period spacings of g-mode sdB pulsators, especially in the short-period range. We suggest that the regular period spacings may arise earlier in the evolution of sdB stars rather than during the sdB stage itself. As for a canonical sdB star, it is evolved from a low-mass star and is considered to have gone through the He flash. The He flash causes extensive and aggressive convection that extends very close to the He/H transition zone. When considering a proper convective overshoot during the He-flash stage, the chemical profile in the He/H transition zone becomes smoother. Detailed model calculations show that the mode trapping efficiency could be reduced to approximately the level of observations throughout the observed period range by taking the He-flash overshoot into account.

V350 Sgr is a classical Cepheid suitable for mass determination. It has a hot companion which is prominent in the ultraviolet and which is not itself a binary. We have obtained two high resolution echelle spectra of the companion at orbital velocity maximum and minimum with the Hubble Space Telescope (HST) Space Telescope Imaging Spectrograph (STIS) in the 1320 to 1510 \AA\/ region. By cross-correlating these spectra we obtained the orbital velocity amplitude of the companion with an uncertainty in the companion amplitude of 1.9 km sec$^{-1}$. This provides a mass ratio of the Cepheid to the companion of 2.1. The ultraviolet energy distribution of the companion provides the mass of the companion, yielding a Cepheid mass of 5.2 $\pm$ 0.3 M$_\odot$. This mass requires some combination of moderate main sequence core convective overshoot and rotation to match evolutionary tracks., Comment: Accepted by ApJ

We present an attempt to reconcile the solar tachocline glitch, a thin layer immediately beneath the convection zone in which the seismically inferred sound speed in the Sun exceeds corresponding values in standard solar models, with a degree of partial material mixing which we presume to have resulted from a combination of convective overshoot, wave transport and tachocline circulation. We first summarize the effects of either modifying in the models the opacity in the radiative interior or of incorporating either slow or fast tachocline circulation. Neither alone is successful. We then consider, without physical justification, incomplete material redistribution immediately beneath the convection zone which is slow enough not to disturb radiative equilibrium. It is modelled simply as a diffusion process. We find that, in combination with an appropriate opacity modification, it is possible to find a density-dependent diffusion coefficient that removes the glitch almost entirely, with a radiative envelope that is consistent with seismology., Comment: Mon. Not. R. Astr. Soc., in the press

This paper continues the systematic investigation of diffusive shear instabilities initiated in Part I of this series. In this work, we primarily focus on quantifying the impact of non-local mixing, which is not taken into account in Zahn's mixing model \citep{Zahn92}. We present the results of direct numerical simulations in a new model setup designed to contain coexisting laminar and turbulent shear layers. As in Part I, we use the Low P\'eclet Number approximation of \citet{Lign1999} to model the evolution of the perturbations. Our main findings are twofold. First, turbulence is not necessarily generated whenever Zahn's nonlinear criterion \citep{Zahn1974} $J{\rm Pr} < (J{\rm Pr})_c$ is satisfied, where $J=N^2/S^2$ is the local gradient Richardson number, ${\rm Pr} = \nu/ \kappa_T$ is the Prandtl number, and $(J{\rm Pr})_c \simeq 0.007$. We have demonstrated that the presence or absence of turbulent mixing in this limit hysteretically depends on the history of the shear layer. Second, Zahn's nonlinear instability criterion only approximately locates the edge of the turbulent layer, and mixing beyond the region where $J{\rm Pr} < (J{\rm Pr})_c$ can also take place in a manner analogous to convective overshoot. We found that the turbulent kinetic energy decays roughly exponentially beyond the edge of the shear-unstable region, on a lengthscale $\delta$ that is directly proportional to the scale of the turbulent eddies, which are themselves of the order of the Zahn scale (see Part I). Our results suggest that mixing by diffusive shear instabilities should be modeled with more care than is currently standard in stellar evolution codes., Comment: Submitted to ApJ

We create isochrones of M67 using the Yale Rotating Stellar Evolution Code. In addition to metallicity, parameters that are traditionally held fixed, such as the mixing length parameter and initial helium abundance, also vary. The amount of convective overshoot is also changed in different sets of isochrones. Models are constructed both with and without diffusion. From the resulting isochrones that fit the cluster, the age range is between 3.6 and 4.8 Gyr and the distance is between 755 and 868 pc. We also confirm Michaud et al. (2004) claim that M67 can be fit without overshoot if diffusion is included., Comment: 4 pages, 3 figures, to appear in the proceedings of the joint TASC2/KASC9/SPACEINN/HELAS8 conference "Seismology of the Sun and the Distant Stars 2016"

For applying the k − ω model proposed by Li to the general stellar environment, it is necessary to study the physical meanings of the parameters in this model, in order to set a limit for the range of their values. It is indicated by the study that the variation of the parameter clμ impacts all the Peclet number, kinetic energy, characteristic timescale and characteristic length of turbulent flows. Besides, as the model parameter clμ increases, the damping rate of turbulent kinetic energy in the bottom convective overshoot region can be accelerated evidently. Both the model parameter clμ and the equivalent mixing length parameter α are proportional to the effciency of convective heat transfer in the convection zone, and their logarithms have a linear relation. There is also a linear relation between the logarithms of the model parameter clμ and the damping index θ of turbulent kinetic energy in the convective overshoot region. For the Sun, a group of appropriate model parameters are obtained to be: clμ = 0.004, and α = 1.7.

We present the evolution of massive star progenitors of supernovae of type IIP. We take the example of the nearby and well-studied SN 2013ej. We explore how convective overshoot affects the stellar structure, surface abundances, and effective temperature of massive stars, using the Modules for Experiments in Stellar Astrophysics (MESA). In particular models with moderate overshoot ($f$ = 0.02 to 0.031) show the presence of blue loops in the Hertzsprung-Russell diagram with a red to blue [$log_{10}(T_{eff}/\rm K)$ from $< 3.6$ to $> 4.0$] excursion and transition back to red, during core helium burning phase. Models with overshoot outside this range of $f$ values kept the star in the red supergiant state throughout the post helium ignition phases. The surface CNO abundance shows enrichment post-main-sequence and again around the time when helium is exhausted in core. These evolutionary changes in surface CNO abundance are indistinguishable in the currently available observations due to large observational uncertainties. However, these observations may distinguish between the ratios of surface nitrogen to oxygen at different evolutionary stages of the star. We also compare the effects of convective overshoot on various parameters related to likelihood of explosion of a star as opposed to collapse to a black hole. These parameters are the compactness parameter, M$_4$, and $��_4$. Combination $��_4 \times $M$_4$, and $��_4$ have similar variation with $f$ and both peak out at $f$ = 0.032. We find that all of our 13 M$_{\odot}$ models are likely to explode., 17 pages, 8 Figures, 2 Tables, Accepted for publication in The Astrophysical Journal

Helioseismic observations have revealed many properties of the Sun: the depth and the helium abundance of the convection zone, the sound-speed and the density profiles in the solar interior. Those constraints have been used to judge the stellar evolution theory. With the old solar composition (e.g., GS98), the solar standard model is in reasonable agreement with the helioseismic constraints. However, a solar model with revised composition (e.g., AGSS09) with low abundance $Z$ of heavy elements cannot be consistent with those constraints. This is the so-called "solar abundance problem", standing for more than ten years even with the recent upward revised Ne abundance. Many mechanisms have been proposed to mitigate the problem. However, there is still not a low-$Z$ solar model satisfying all helioseismic constraints. In this paper, we report a possible solution to the solar abundance problem. With some extra physical processes that are not included in the standard model, solar models can be significantly improved. Our new solar models with convective overshoot, the solar wind, and an early mass accretion show consistency with helioseismic constraints, the solar Li abundance, and observations of solar neutrino fluxes., Comment: 25 pages, 20 figures, 5 tables; accepted to ApJ

The standard model of stellar structure is unable to account for various observational facts, such as anomalies in the surface composition, and there is now a broad consensus that some extra mixing must occur in the radiation zones, in addition to the always present convective overshoot or penetration. The search for the causes of this extra mixing started in the late seventies, and it was quickly realized – in particular by Sylvie Vauclair and her co-workers – that some mild turbulence must be present to counteract the effect of gravitational settling and radiative levitation. What could be responsible for this turbulence? One suggestion was the internal gravity waves emitted at the boundary of convection zones, but it is still not established whether these waves will lead to true mixing. However they transport angular momentum, and therefore they generate differential rotation, which may be shear-unstable and thus lead to turbulence. Another way to transport angular momentum and produce an unstable rotation profile is through the large-scale circulation which is induced by the structural adjustments as the star evolves, or by the torques applied to it (due to stellar wind, accretion, tides). These processes participate in what is called the “rotational mixing”; their implementation in stellar evolution codes – again under Sylvie's impulse – has given birth to a new generation of stellar models, which agree much better with the observational constraints, although there is still room for improvement.

The classical 'ballistic' overshoot models show some contradictions and are not consistence with numerical simulations and asteroseismic studies. Asteroseismic studies imply that overshoot is a weak mixing process. Diffusion model is suitable to deal with it. The form of diffusion coefficient in a diffusion model is crucial. Because the overshoot mixing is related to the convective heat transport (i.e., entropy mixing), there should be a similarity between them. A recent overshoot mixing model shows consistence between composition mixing and entropy mixing in overshoot region. A prerequisite to apply the model is to know the dissipation rate of turbulent kinetic energy. The dissipation rate can be worked out by solving turbulent convection models (TCMs). But it is difficult to apply TCMs because of some numerical problems and the enormous time cost. In order to find a convenient way, we have used the asymptotical solution and simplified the TCM to be a single linear equation for turbulent kinetic energy. This linear model is easy to be implemented in the calculations of stellar evolution with ignorable extra time cost. We have tested the linear model in stellar evolution, and have found that the linear model can well reproduce the turbulent kinetic energy profile of full TCM, as well as the diffusion coefficient, abundance profile and the stellar evolutionary tracks. We have also studied the effects of different values of the model parameters and have found that the effect due to the modification of temperature gradient in the overshoot region is slight., 20 pages, 10 figures, accepted for publication in ApJ

We present here, for the first time, a 2D study of the overshoot convective mechanism in nova outbursts for a wide range of possible compositions of the layer underlying the accreted envelope. Previous surveys studied this mechanism only for solar composition matter accreted on top of carbon–oxygen (C–O) white dwarfs. Since, during the runaway, mixing with carbon enhances the hydrogen burning rates dramatically, one should question whether significant enrichment of the ejecta is possible also for other underlying compositions (He, O, Ne, Mg), predicted by stellar evolution models. We simulated several non-carbon cases and found significant amounts of those underlying materials in the ejected hydrogen layer. Despite large differences in rates, time-scales and energetics between the cases, our results show that the convective dredge-up mechanism predicts significant enrichment in all our non-carbon cases, including helium enrichment in recurrent novae. The results are consistent with observations.

The stratification near the base of the Sun's convective envelope is governed by processes of convective overshooting and element diffusion, and the region is widely believed to play a key role in the solar dynamo. The stratification in that region gives rise to a characteristic signal in the frequencies of solar p modes, which has been used to determine the depth of the solar convection zone and to investigate the extent of convective overshoot. Previous helioseismic investigations have shown that the Sun's spherically symmetric stratification in this region is smoother than that in a standard solar model without overshooting, and have ruled out simple models incorporating overshooting, which extend the region of adiabatic stratification and have a more-or-less abrupt transition to subadiabatic stratification at the edge of the overshoot region. In this paper we consider physically motivated models which have a smooth transition in stratification bridging the region from the lower convection zone to the radiative interior beneath. We find that such a model is in better agreement with the helioseismic data than a standard solar model.

The long-standing puzzle of reversed-C bisectors of spectral lines is addressed with the tools of the third-signature plot and bisector mapping. The F8 supergiant γ Cyg, which shows strongly reversed bisectors, is analyzed and found to have a monotonic decline in velocities with height in its photosphere, consistent with a convective overshoot region similar to what is seen for stars on the cool side of the granulation boundary. Further, a flux deficit is derived that is not unlike those for stars on the cool side of the granulation boundary, although the γ Cyg flux deficit is wider and peaks at a higher velocity. The reversed-C bisector shape is caused by a combination of a steep decline in velocities with height, as shown in the third-signature plot, with a flux deficit spanning only a fraction of the red wing of the line profiles, in contrast to cooler stars where the deficit extends over most of the red wing. Apparently no unusual velocity fields or other bizarre behaviors are needed to explain the granulation boundary; it stems more simply from the continuous changes of normal convective overshoot across the HR diagram.

Deep moist convection generates turbulence in the clear air above and around developing clouds, penetrating convective updrafts and mature thunderstorms. This turbulence can be due to shearing instabilities caused by strong flow deformations near the cloud top, and also to breaking gravity waves generated by cloud–environment interactions. Turbulence above and around deep convection is an important safety issue for aviation, and improved understanding of the conditions that lead to out-of-cloud turbulence formation may result in better turbulence avoidance guidelines or forecasting capabilities. In this study, a series of high-resolution two- and three-dimensional model simulations of a severe thunderstorm are conducted to examine the sensitivity of above-cloud turbulence to a variety of background flow conditions—in particular, the above-cloud wind shear and static stability. Shortly after the initial convective overshoot, the above-cloud turbulence and mixing are caused by local instabilities in the vicinity of the cloud interfacial boundary. At later times, when the convection is more mature, gravity wave breaking farther aloft dominates the turbulence generation. This wave breaking is caused by critical-level interactions, where the height of the critical level is controlled by the above-cloud wind shear. The strength of the above-cloud wind shear has a strong influence on the occurrence and intensity of above-cloud turbulence, with intermediate shears generating more extensive regions of turbulence, and strong shear conditions producing the most intense turbulence. Also, more stable above-cloud environments are less prone to turbulence than less stable situations. Among other things, these results highlight deficiencies in current turbulence avoidance guidelines in use by the aviation industry.

Although it presently includes convective overshoot, microscopic diffusion, gravitational settling and radiative acceleration, the standard model of stellar structure is still unable to account for various observational facts, and there is now a large consensus that some extra mixing must occur in the radiation zones. To account for such mixing, the minimalist approach consists in introducing a parametrized turbulent diffusivity, and to adjust it so as to match the observations. A better way is to strive to implement the physical processes that may be responsible for this mixing, in particular shear-induced turbulence and large-scale meridional circulation, which both are linked with the differential rotation of the star. To describe that rotational mixing, as we call it, one has thus to follow the evolution of the internal rotation profile. By making some plausible assumptions, its is possible to reduce the advection of angular momentum through the 2-D circulation to a 1-D process, and that of the chemical elements to a vertical diffusion. It is then possible to implement these transports in a 1-D stellar evolution code. In massive stars, angular momentum is transported mainly by the meridional circulation, and there is good agreement between the observations and the predictions based on the rotational mixing. This not so for solar-type stars where another, more efficient mechanism is required to transport angular momentum. A fossil magnetic field has been invoked, but recently it has been shown that such a field would connect with the convection zone, and imprint its differential rotation on the radiation zone, which is not observed. The other candidate is the transport of angular momentum by the internal gravity waves that are emitted at the base of the convection zone; although their treatment is still somewhat crude, it appears that they can explain both the quasi-uniform rotation of the solar interior, and the depletion of lithium observed in such stars.

Convective overshoot is a physical mechanism for which no recipe based on first principles is available; hence the necessity of assuming some parametrisations, which undermine the predictive power of the current generation of stellar models. We evidentiate the non local aspects of the convective phenomenon particularly in the proximities of the convective borders, and identify some cases when the use of a diffusive rather than the classic instantaneous approach to describe overshooting is highly recommended.

We explore the detailed and broad properties of carbon burning in Super Asymptotic Giant Branch (SAGB) stars with 2755 MESA stellar evolution models. The location of first carbon ignition, quenching location of the carbon burning flames and flashes, angular frequency of the carbon core, and carbon core mass are studied as a function of the ZAMS mass, initial rotation rate, and mixing parameters such as convective overshoot, semiconvection, thermohaline and angular momentum transport. In general terms, we find these properties of carbon burning in SAGB models are not a strong function of the initial rotation profile, but are a sensitive function of the overshoot parameter. We quasi-analytically derive an approximate ignition density, $\rho_{ign} \approx 2.1 \times 10^6$ g cm$^{-3}$, to predict the location of first carbon ignition in models that ignite carbon off-center. We also find that overshoot moves the ZAMS mass boundaries where off-center carbon ignition occurs at a nearly uniform rate of $\Delta M_{\rm ZAMS}$/$\Delta f_{\rm{ov}}\approx$ 1.6 $M_{\odot}$. For zero overshoot, $f_{\rm{ov}}$=0.0, our models in the ZAMS mass range $\approx$ 8.9 to 11 $M_{\odot}$ show off-center carbon ignition. For canonical amounts of overshooting, $f_{\rm{ov}}$=0.016, the off-center carbon ignition range shifts to $\approx$ 7.2 to 8.8 $M_{\odot}$. Only systems with $f_{\rm{ov}}$ $\geq 0.01$ and ZAMS mass $\approx$ 7.2-8.0 $M_{\odot}$ show carbon burning is quenched a significant distance from the center. These results suggest a careful assessment of overshoot modeling approximations on claims that carbon burning quenches an appreciable distance from the center of the carbon core., Comment: Accepted ApJ; 23 pages, 21 figures, 5 tables

The study is devoted to radial pulsations in Population I Cepheids with masses from 5.4 to 6 Msun. Solution of the equations of radiation hydrodynamics and turbulent convection for nonlinear stellar pulsations was obtained with initial conditions as the core helium burning models of computed evolutionary sequences. For each value of the initial mass we considered stellar evolution from the zero age main sequence to central helium exhaustion with three values of the convective overshoot parameter: aov=0.1, 0.15 and 0.2. Models for the Cepheid SZ Tau with pulsation period 3.149 day can be constructed only for aov=0.1 and aov=0.15. The star is at the evolutionary stage of the second crossing of the instability strip and pulsates in the first overtone just near the boundary between the fundamental mode and the first overtone. The mass, radius and age of the star are in the ranges 5.46, 13 pages, 4 figures. Accepted for publication in Astronomy Letters

Continued progress in observational stellar astrophysics requires a deep understanding of the underlying convection dynamics. We present results of realistic 3D radiative hydrodynamic simulations of the outer layers of a moderate mass star (1.47 Msun), including the full convection zone, the overshoot region, and the top layers of the radiative zone. The simulation results show that the surface granulation has a broad range of scales, from 2 to 12 Mm, and that large granules are organized in well-defined clusters, consisting of several granules. Comparison of the mean structure profiles from 3D simulations with the corresponding 1D standard stellar model shows an increase of the stellar radius by ~800 km, as well as significant changes in the thermodynamic structure and turbulent properties of the ionization zones. Convective downdrafts in the intergranular lanes between granulation clusters reach speeds of more than 20 km/s, penetrate through the whole convection zone, hit the radiative zone, and form a 8 Mm thick overshoot layer. Contrary to semi-empirical overshooting models, our results show that the 3D dynamic overshoot region consists of two layers: a nearly adiabatic extension of the convection zone and a deeper layer of enhanced subadiabatic stratification. This layer is formed because of heating caused by the braking of the overshooting convective plumes. This effect has to be taken into account in stellar modeling and the interpretation of asteroseismology data. In particular, we demonstrate that the 3D model can qualitatively explain deviations from the standard solar model found by helioseismology., Comment: 14 pages, 5 figures, submitted to ApJ Letters

We present numerical simulations of convective overshoot in a two-dimensional model of the solar equatorial plane. The simulated domain extends from 0.001 R_sun to 0.93 R_sun, spanning both convective and radiative regions. We show that convective penetration leads to a slightly extended, mildly subadiabatic temperature gradient beneath the convection zone below which there is a rapid transition to a strongly subadiabatic region. A slightly higher temperature is maintained in the overshoot region by adiabatic heating from overshooting plumes. This enhanced temperature may partially account for the sound speed discrepancy between the standard solar model and helioseismology. Simulations conducted with tracer particles suggest that a fully mixed region exists down to at least 0.687 R_sun., Comment: 24 pages, 8 figures, submitted to ApJ

Observations of rapidly rotating solar-like stars show a significant mixture of opposite-polarity magnetic fields within their polar regions. To explain these observations, models describing the surface transport of magnetic flux demand the presence of fast meridional flows. Here, we link sub-surface and surface magnetic flux transport simulations to investigate (i) the impact of meridional circulations with peak velocities of

Based on a series of spectrograms taken with the German Vacuum Tower Telescope (VTT) at the Observatorio del Teide (Tenerife), we study the temporal evolution of granular dynamics and energy transport in the photospheric layers. We consider the ensemble of the granules cut by the spectrograph slit, modulated by wave motion, as a complex system. We describe this ensemble by the rms of the fluctuations of the observables along the slit: continuum intensity I, gas velocity v measured from line center Doppler shifts with respect to the mean profile, and line width w. The history of the rms of the observables v and w reflects the dynamical change of the system over the 20 min observation time. We find a burst-like change for both observables. However, the cross-correlation between I and v remains virtually constant, with the exception of two gaps. Using six lines of different strength we measure the rms of v in the deep photospheric layers. On the basis of this v variation we derive an upper limit of the kinetic energy flux as a function of height in the photosphere for different times during the observation. The shape of the variation with height is constant over time. A limit for the convective enthalpy flux is calculated using the temperature variations of our earlier models. Its shape remains the same over time. Taken together, these results quantify the different roles that the lower and higher photospheric layers play in the energetics of convective overshoot.

Regions of rapid variation in stellar interiors introduce characteristic periodic signatures into the frequencies of their p-mode oscillations. These signatures have previously been isolated in solar data and have been used to estimate the level of convective overshoot and the envelope helium abundance in the Sun. Precise asteroseismic data are now becoming available for a number of distant Sun-like stars, and the techniques developed for low-degree solar data can be used in the stellar case. In this paper, we address the observational duration required to isolate and examine these rapid-variation signatures through the use of low-degree solar data as a Sun-like stellar analog. We also outline the difficulties involved and bias introduced when using a limited number of poorly constrained mode frequencies. The signature resulting from the region of the second ionization of helium is found to be detectable in spectra of shorter duration than those required to isolate the smaller amplitude signature arising from the base of the convective envelope. Observations of just 84 days may be sufficient to extract reliable information on the acoustic depth and extent of the He II ionization zone, although longer observations are required to obtain sufficient precision to estimate the envelope helium abundance. The acoustic radius of the base of the convective envelope and the amplitude of the signature are reliably isolated in spectra of at least 182 days.

We present detailed evolutionary calculations focused on the evolution of intermediate mass stars with 3 Msun < M < 9 Msun of metallicity typical of the Large Magellanic Cloud (LMC), i.e. Z=0.008. We compare carefully the models calculated by adopting a diffusive scheme for chemical mixing, in which nuclear burning and mixing are self-consistently coupled, while the eddy velocities beyond the formal convective core boundary are treated to decay exponentially, and those calculated with the traditional instantaneous mixing approximation. We find that: i) the physical and chemical behaviour of the models during the H-burning phase is independent of the scheme used for the treatment of mixing inside the CNO burning core; ii) the duration of the He-burning phase relative to the MS phase is systematically longer in the diffusive models, due to a slower redistribution of helium to the core from the outer layers; iii) the fraction of time spent in the blue part of the clump, compared to the stay in the red, is larger in the diffusive models. The differences described in points ii) and iii) tend to vanish for M > Msun. In terms of the theoretical interpretation of an open cluster stellar population, the differences introduced by the use of a self-consistent scheme for mixing in the core with adjacent exponential decay are relevant for ages in the range 80 Myr < t < 200 Myr. These results are robust, since they are insensitive to the choice of the free-parameters regulating the extension of the extra-mixing region., 14 pages, 14 figure, accepted for publication on Astronomy & Astrophysics

We present a comprehensive set of stellar evolution models for Procyon A in an effort to guide future measurements of both traditional stellar parameters and seismic frequencies towards constraining the amount of core overshoot in Procyon A and possibly other stars. Current observational measurements of Procyon A when combined with traditional stellar modeling only place a large upper limit on overshoot of alphaOV < 1.1. By carrying out a detailed pulsation analysis, we further demonstrate, how p- and g-mode averaged spacings can be used to gain better estimates of the core size. For both p- and g-modes, the frequency spacings for models without overshoot are clearly separated from the models with overshoot. In addition, measurements of the l=0 averaged small p-mode spacings could be used to establish Procyon A's evolutionary stage. For a fixed implementation of overshoot and under favorable circumstances, the g-mode spacings can be used to determine the overshoot extent to an accuracy of +-0.05 Hp. However, we stress that considerable confusion is added due to the unknown treatment of the overshoot region. This ambiguity might be removed by analyzing many different stars. A simple non-local convection theory developed by Kuhfuss is implemented in our stellar evolution code and contrasted with the traditional approaches. We show that this theory supports a moderate increase of the amount of convective overshoot with stellar mass of Delta(alphaOV) = +0.10 between 1.5 Msun and 15 Msun. This theory places an upper limit on Procyon A's core overshoot extent of ~0.4 Hp which matches the limit imposed by Roxburgh's integral criterion., 45 pages, 26 figures, accepted in ApJ

We have undertaken a LTE analysis of the high-excitation 7774 A O I triplet in high-resolution, moderate signal-to-noise spectra of 15 Pleiades (HET/HRS) and 8 M34 (Keck/HIRES) open cluster dwarfs. Effective temperatures range from 5048 - 6172 K for the Pleiades sample and from 5290 - 6130 K for the M34 sample. Relative O abundances have been derived using model atmospheres interpolated from four different sets of ATLAS9 grids: with convective overshoot, without convective overshoot, with the mixing length parameter set to 0.5, and with the convective treatment of Canuto, Goldman, & Mazzitelli. In contrast to existing NLTE predictions, a dramatic increase in O I triplet abundance with decreasing temperature is seen for both clusters, regardless of atmospheric model. S I abundances of three Pleiads derived from the high-excitation λ 6052.67 feature mimic the O I abundance behavior. O abundances have also been derived from the 6300 A [OI] feature in three Pleiads; the abundances exhibit a much lower mean value than the cool dwarf triplet results. These data suggest LTE abundances derived from the O I triplet for cool dwarfs ( T eff ⩽ 5800 K ) should be viewed with caution.

We examine the convective core helium burning phase of intermediate mass stars, and investigate the role of coupling of nuclear burning and mixing on the extension and duration of the blue loop phase. We compare the theoretical scenario with the distribution of stars in the colour-magnitude (CM) diagram of the Large Magellanic Cloud (LMC) cluster NGC1866, whose largely populated clump of He-burning stars is equally populated in the blue and red side. We compare the distributions expected by adopting either a diffusive scheme within the instability regions, in which nuclear burning and mixing are self-consistently coupled, or the traditional instantaneous mixing approximation. We analyze with particular care the sensitivity of the results to: a) the e-folding distance with which the velocity of convective eddies decays outside the formal border of the convective regions fixed by the Schwarzschild criterion; b) the convective model adopted to evaluate the temperature gradient; c) the rate of reaction C12+alpha -> O16. Models not including convective overshoot are also commented., 14 pages,16 figures. Accepted for publication on Astronomy & Astrophysics

In this paper, first solutions of the dust moment equations developed in (Woitke & Helling 2003) for the description of dust formation and precipitation in brown dwarf and giant gas planet atmospheres are presented. We consider the special case of a static brown dwarf atmosphere, where dust particles continuously nucleate from the gas phase, grow by the accre- tion of molecules, settle gravitationally and re-evaporate thermally. Mixing by convective overshoot is assumed to replenish the atmosphere with condensable elements, which is necessary to counterbalance the loss of condensable elements by dust formation and gravitational settling (no dust without mixing). Applying a kinetic description of the relevant microphysical and chemical processes for TiO2-grains, the model makes predictions about the large-scale stratification of dust in the atmosphere, the depletion of molecules from the gas phase, the supersaturation of the gas in the atmosphere as well as the mean size and the mass fraction of dust grains as function of depth. Our results suggest that the presence of relevant amounts of dust is re- stricted to a layer, where the upper boundary (cloud deck) is related to the requirement of a minimum mixing activity (mixing time-scale τmix ≈ 10 6 s) and the lower boundary (cloud base) is determined by the thermodynamical stability of the grains. The nucleation occurs around the cloud deck where the gas is cool, strongly depleted, but nevertheless highly supersaturated (S � 1). These particles settle gravitationally and populate the warmer layers below, where the in situ formation (nucleation) is ineffective or even not possible. During their descent, the particles grow and reach mean radii of ≈30 µm ... 400 µm at the cloud base, but the majority of the particles in the cloud layer remains much smaller. Finally, the dust grains sink into layers which are sufficiently hot to cause their thermal evaporation. Hence, an effective transport mechanism for condensable elements exists in brown dwarfs, which depletes the gas above and enriches the gas below the cloud base of a considered solid/liquid material. The dust-to-gas mass fraction in the cloud layer results to be approximately given by the mass fraction of condensable elements in the gas being mixed up. Only for artificially reduced mixing we find a self-regulation mechanism that approximately installs phase equilibrium (S ≈ 1) in a limited region around the cloud base.

The structure and evolution of central stars of planetary nebulae (CSPNe) is reviewed. CSPNe represent the rapid transitional stage between the Asymptotic Giant Branch (AGB) and the white-dwarf domain. It is shown that the whole evolution off the AGB through the central-star regime depends on the evolutionary history. The detailed evolution into a white dwarf is controlled by the internal stellar structure which, in turn, is determined by the duration of the preceding AGB evolution and therefore by the AGB mass-loss history. The evolution of hydrogen-deficient central stars has been a matter of debate since many years. Convective overshoot appears to be a key ingredient to model these objects. Various thermal-pulse scenarios with inclusion of overshoot are discussed, leading to surface abundances in general agreement with those observed for Wolf-Rayet central stars., 8 pages, 4 postscript figures, also available from http://www.mpifr-bonn.mpg.de/div/ir-interferometry; invited review at IAU Symp. 209 "Planetary Nebulae", ASP, in press

We constructed hydrodynamical model atmospheres for mid M-type main-, as well as pre-main-sequence objects. Despite the complex chemistry encountered in such cool atmospheres a reasonably accurate representation of the radiative transfer is possible. The detailed treatment of the interplay between radiation and convection in the hydrodynamical models allows to study processes usually not accessible within the framework of conventional model atmospheres. In particular, we determined the efficiency of the convective energy transport, and the efficiency of mixing by convective overshoot. The convective transport efficiency expressed in terms of an equivalent mixing-length parameter amounts to values around ≈ 2 in the optically thick, and ≈ 2.8 in the optically thin regime. The thermal structure of the formally convectively stable layers is little affected by convective overshoot and wave heating, i.e. stays close to radiative equilibrium. Mixing by convective overshoot shows an exponential decline with geometrical distance from the Schwarzschild stability boundary. The scale height of the decline varies with gravitational acceleration roughly as g–1/2, with 0.5 pressure scale heights at log g=5.0.

We investigate the pulsation properties of stellar models that are representative of δ Scuti and γ Doradus variables. We have calculated a grid of stellar models from 1.2 to 2.2 M ⊙, including the effects of both rotation and convective overshoot using MESA, and we investigate the pulsation properties of these models using GYRE. We discuss the observable patterns in the frequency spacing for p modes and the period spacings for g modes. Using the observable patterns in the g mode period spacings, it may be possible to observationally constrain the convective overshoot and rotation of a model. We also calculate the pulsation constant (Q) for all models in our grid and investigate the variation with convective overshoot and rotation. The variation in the Q values of the radial modes can be used to place constraints on the convective overshoot and rotation of stars in this region. As a test case, we apply this method to a sample of 22 High-Amplitude δ Scuti stars (HADS) and provide estimates for the convective overshoot of the sample.

In spite of the great effort made in the last decades to improve our understanding of stellar evolution, significant uncertainties remain due to our poor knowledge of some complex physical processes that require an empirical calibration, such as the efficiency of the interior mixing related to convective overshoot. Here we review the impact of convective overshoot on the evolution of stars during the main Hydrogen and Helium burning phases.

We carried out a series of linear stability analyses of the radial and low-degree non-radial p modes for stellar models with initial masses of . The stellar models were computed by using convective overshoot distance , 0.25 and 0.40 HP. Our numerical results show that the β Cephei instability strip forms a horn-shaped region pointing upwards near the main sequence on the Hertzsprung–Russell diagram (HRD). The lower part of the instability strip for the radial modes join the zero-age main-sequence (ZAMS) at , while the top of the instability strip extends up to . The instability strip for the non-radial modes is even wider. The overall instability strip is dominated by the radial and non-radial fundamental modes. The first overtone (the radial-order index is also pulsationally unstable. We have shown that the β Cephei stability is almost independent of the overshoot parameter dover used for the stellar models, while it depends critically on the metal abundance. With decreasing metal abundance, the instability region shrinks and eventually disappears for .

We present multicolour CCD photometry for two poorly studied open clusters (King 5 and Be 20). Photometry for a field near King 5 was also carried out to estimate the contamination by field stars. The colour magnitude diagrams (CMD) of the clusters show a well defined main sequence extending to the limit of the photometry, $V \approx20$ mag. The reddening for King 5, estimated from the colour-colour diagram, is ~0.82, whereas that for Be 20 as estimated by comparing theoretical main-sequence (MS) with the observed MS is 0.10. The morphology of the CMDs indicates that these clusters are old. The CMD of Be 20 shows a globular cluster-like horizontal branch. In case of King 5 the comparison of observational CMDs with the standard isochrones of VandenBerg ([CITE]) indicates an apparent discrepancy between the shape of the turnoff and isochrones. The CMDs of King 5 seem to be better understood in terms of stellar models with convective overshoot. The comparison of the CMDs with the stellar models by Bertelli et al. ([CITE]) with convective overshoot produces a good fit for a metallicity $Z = 0.008$ and an age = 1 Gyr for King 5 and 5 Gyr for Be 20. An apparent distance modulus $(m-M) = 14.0$ and 15.1 has been estimated for King 5 and Be 20 respectively. They correspond to a distance of $1900 \pm100$ pc and 9026 ± 480 pc, respectively. The radial density distribution in King 5 indicates that there is an excess of low mass stars in the outer region of the cluster, whereas the density distribution in Be 20 shows a good fit with the empirical King ([CITE]) model. For both clusters, observations have also been carried out to search for variable stars.

High-precision data on absolute dimensions of evolved eclipsing binaries have been used for a quantitative evaluation of the significance of convective overshoot in the stellar core. Eight detached double-lined eclipsing binaries with components close to or beyond the terminal age main sequence (TAMS) and masses between 2 and 12 M⊙ have been compared with evolutionary models using different overshooting parameters. The results are robust and indicate a systematic increase of the amount of convective overshoot with the stellar mass. Such determination constitutes a fundamental point for stellar structure and evolution theory.