Observationally, spectra of brown dwarfs indicate the presence of dust in their atmospheres while theoretically it is not clear what prevents the dust from settling and disappearing from the regions of spectrum formation. Consequently, standard models have to rely on ad hoc assumptions about the mechanism that keeps dust grains aloft in the atmosphere. We apply hydrodynamical simulations to develop an improved physical understanding of the mixing properties of macroscopic flows in M dwarf and brown dwarf atmospheres, in particular of the influence of the underlying convection zone. We performed 2D radiation hydrodynamics simulations including a description of dust grain formation and transport with the CO5BOLD code. The simulations cover the very top of the convection zone and the photosphere including the dust layers for effective temperatures between 900K and 2800K, all with logg=5 assuming solar chemical composition. Convective overshoot occurs in the form of exponentially declining velocities with small scale heights, so that it affects only the region immediately above the almost adiabatic convective layers. From there on, mixing is provided by gravity waves that are strong enough to maintain thin dust clouds in the hotter models. With decreasing effective temperature, the amplitudes of the waves become smaller but the clouds become thicker and develop internal convective flows that are more efficient in mixing material than gravity waves. The presence of clouds leads to a highly structured appearance of the stellar surface on short temporal and small spatial scales. We identify convectively excited gravity waves as an essential mixing process in M dwarf and brown dwarf atmospheres. Under conditions of strong cloud formation, dust convection is the dominant self-sustaining mixing component.
Variation of Stellar Envelope Convection and Overshoot with Metallicity
我们研究金属丰度如何影响对流和过冲的超绝热层的主序星。我们目前的结果，从一个网格的三维辐射流体动力学模拟与四个金属丰度（$Z=0.040$，0.020，0.010，0.001），并跨越一个范围内的有效温度（$4950 < \rm T_{\rm eff} < 6230$）。我们发现，改变金属丰度改变对流气体动力学的性质，超绝热层和大气的结构。我们的模拟网格显示，超绝热性的量（它跟踪从高效对流到低效对流的转变），\rev{is sensitive to changes in metallicity. We find that increasing the metallicity forces the location of the transition region to lower densities and pressures, and results in larger mean and turbulent velocities throughout the superadiabatic region.}我们还量化了大气中对流过冲的程度，并表明它也随着金属丰度的增加而增加。
Convective overshoot mixing is a critical ingredient of stellar structure models, but is treated in most cases by ad hoc extensions of the mixing-length theory for convection. Advanced theories which are both more physical and numerically treatable are needed. Convective flows in stellar interiors are highly turbulent. This poses a number of numerical challenges for the modelling of convection in stellar interiors. We include an effective turbulence model into a 1D stellar evolution code in order to treat non-local effects within the same theory. We use a turbulent convection model which relie…
We present results of a large, high resolution 3D hydrodynamical simulation of the surface layers of a DA white dwarf (WD) with $T_{\rm eff}=11800$ K and $\log(g)=8$ using the ANTARES code, the widest and deepest such simulation to date. Our simulations are in good agreement with previous calculations in the Schwarzschild-unstable region and in the overshooting region immediately beneath it. Farther below, in the wave-dominated region, we find that the rms horizontal velocities decay with depth more rapidly than the vertical ones. Since mixing requires both vertical and horizontal displacement…
Overshooting of turbulent motions from convective regions into adjacent stably stratified zones plays a significant role in stellar interior dynamics as this process may lead to mixing of chemical species, and contribute to the transport of angular momentum and magnetic fields. We present a series of fully non-linear, three-dimensional (3D) anelastic simulations of overshooting convection in a spherical shell which are focused on the dependence of the overshooting dynamics on the density stratification and the rotation, both key ingredients in stars which however have not been studied systemat…
The convective kissing instability in low-mass M-dwarf models: convective overshooting, semi-convection, luminosity functions, surface abundances and star cluster age dating
Turbulent convection is certainly one of the most important and thorny issues in stellar physics. Our deficient knowledge of this crucial physical process introduces a fairly large uncertainty concerning the internal structure and evolution of stars. A striking example is overshoot at the edge of convective cores. Indeed, nearly all stellar evolutionary codes treat the overshooting zones in a very approximative way that considers both its extent and the profile of the temperature gradient as free parameters. There are only a few sophisticated theories of stellar convection such as Reynolds str…
In this paper, we investigate the upward overshooting by three-dimensional numerical simulations. We find that the above convectively stable zone can be partitioned into three layers: the thermal adjustment layer (mixing both entropy and material), the turbulent dissipation layer (mixing material but not entropy), and the thermal dissipation layer (mixing neither entropy nor material). The turbulent dissipation layer is separated from the thermal adjustment layer and the thermal dissipation layer by the first and second zero points of the vertical velocity correlation. The simulation results a…
Convective-core overshoot mixing is a significant uncertainty in stellar evolution. Because numerical simulations and turbulent convection models predict exponentially decreasing radial rms turbulent velocity, a popular treatment of the overshoot mixing is to apply a diffusion process with exponentially decreasing diffusion coefficient. It is important to investigate the parameters of the diffusion coefficient because they determine the efficiency of the mixing in the overshoot region. In this paper, we have investigated the effects of the core overshoot mixing on the properties of the core in…
Thermal convection is one of the main mechanisms of heat transport and mixing in stars in general and also in the photospheric layers which emit the radiation that we observe with astronomical instruments. The present lecture notes first introduce the role of convection in astrophysics and explain the basic physics of convection. This is followed by an overview on the modelling of convection. Challenges and pitfalls in numerical simulation based modelling are discussed subsequently. Finally, a particular application for the previously introduced concepts is described in more detail: the study …
(abridged) Context: The ratio of kinematic viscosity to thermal diffusivity, the Prandtl number, is much smaller than unity in stellar convection zones. Aims: To study the statistics of convective flows and energy transport as functions of the Prandtl number. Methods: Three-dimensional numerical simulations convection in Cartesian geometry are used. The convection zone (CZ) is embedded between two stably stratified layers. Statistics and transport properties of up- and downflows are studied separately. Results: The rms velocity increases with decreasing Prandtl number. At the same time the fil…
Convection in the cores of massive stars becomes anisotropic when they rotate. This anisotropy leads to a misalignment of the thermal gradient and the thermal flux, which in turn results in baroclinicity and circulation currents in the upper radiative zone. We show that this induces a much stronger meridional flow in the radiative zone than previously thought. This drives significantly enhanced mixing, though this mixing does not necessarily reach the surface. The extra mixing takes on a similar form to convective overshooting, and is relatively insensitive to the rotation rate above a thresho…
Excitation of gravity waves by fingering convection, and the formation of compositional staircases in stellar interiors
指状对流（英语：Fingering convection）是一种微弱但重要的混合现象，发生在稳定分层的恒星辐射带中，且存在反平均分子量梯度。Brown等人（2013）最近提出了一种新的指状对流混合模型，该模型不包含自由参数，并且发现几乎在所有情况下都符合直接数值模拟的结果。然而，值得注意的是，他们发现，在少数情况下，大规模的重力波，其次是热成分分层，从指状对流自发增长的混合大大增强了他们的预测值以上。这种效应在海洋学方面是众所周知的，并归因于所谓的“集体不稳定性”的激发。在这项工作中，我们建立在Brown et al.（2013）和Traxler et al.（2011 b）的结果基础上，以确定集体不稳定性可能发生的条件。我们发现，它只在恒星区域有一个相对较大的普朗特数（运动粘度的热扩散率的比率），$O(10^{-3})$或更大。这意味着集体不稳定性不会发生在主序星中，那里的普朗特数总是比这小得多（除了表面对流区的外层，那里的指进是无关紧要的）。原则上，它可以在高电子简并的区域，在He核心闪光期间，或在白矮星的内部被激发。我们讨论了我们的研究结果对这些对象的影响，无论是从理论上还是从观测的角度来看。
Twists in the flow: revisiting convective mixing in rotating stellar models. I. Effect on the stellar structure
Dartmouth College（达特茅斯学院）；Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM（巴黎萨克雷大学，巴黎西岱大学，法国原子能和替代能源委员会，法国国家科学研究中心，天体物理学与分子间介质实验室）；KU Leuven（荷语鲁汶大学）；Radboud University Nijmegen（奈梅亨拉德堡德大学）；Max Planck Institute for Astronomy（马克斯·普朗克天文学研究所）
Convective overshooting in super asymptotic giant branch stars has been suggested to lead to the formation of hybrid white dwarfs with carbon-oxygen cores and oxygen-neon mantles. As the white dwarf cools, this core-mantle configuration becomes convectively unstable and should mix. This mixing has been previously studied using stellar evolution calculations, but these made the approximation that convection did not affect the temperature profile of the mixed region. In this work, we perform direct numerical simulations of an idealized problem representing the core-mantle interface of the hybrid…
Fingering convection is a turbulent mixing process that can occur in stellar radiative regions whenever the mean molecular weight increases with radius. In some cases, it can have a significant observable impact on stellar structure and evolution. The efficiency of mixing by fingering convection as a standalone process has been studied by Brown et al. (2013), but other processes such as rotation, magnetic fields and shear can affect it. In this paper, we present a first study of the effect of shear on fingering (thermohaline) convection in astrophysics. Using Direct Numerical Simulations we fi…
Numerous observational evidence has suggested the presence of active meteorology in the atmospheres of brown dwarfs. A near-infrared brightness variability has been observed. Clouds have a major role in shaping the thermal structure and spectral properties of these atmospheres. The mechanism of such variability is still unclear and both 1D and global circulation model cannot fully study this topics due to resolution. In this study, a convective resolving model is coupled to grey-band radiative transfer in order to study the coupling between the convective atmosphere and the variability of clou…
We perform a calibration of the mixing length parameter at the bottom boundary of the convection zone for helium-dominated atmospheres of white dwarfs. This calibration is based on a grid of 3D DB (pure-helium) and DBA (helium-dominated with traces of hydrogen) model atmospheres computed with the CO5BOLD code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances between -10.0 and -2.0, surface gravities between 7.5 and 9.0 and effective temperatures between 12000 K and 34000 K. The 1D envelopes cover a similar atmospheric parame…
Ground-based and spacecraft telescopic observations, combined with an intensive modeling effort, have greatly enhanced our understanding of hot giant planets and brown dwarfs over the past ten years. Although these objects are all fluid, hydrogen worlds with stratified atmospheres overlying convective interiors, they exhibit an impressive diversity of atmospheric behavior. Hot Jupiters are strongly irradiated, and a wealth of observations constrain the day-night temperature differences, circulation, and cloudiness. The intense stellar irradiation, presumed tidal locking and modest rotation lea…
As brown dwarfs and young giant planets cool down, they are known to experience various chemical transitions --- for example from CO rich L-dwarfs to methane rich T-dwarfs. Those chemical transitions are accompanied by spectral transitions whose sharpness cannot be explained by chemistry alone. In a series of articles, Tremblin et al. proposed that some of the yet unexplained features associated to these transitions could be explained by a reduction of the thermal gradient near the photosphere. To explain, in turn, this more isothermal profile, they invoke the presence of an instability analog…
Brown dwarfs are essential targets for understanding planetary and sub-stellar atmospheres across a wide range of thermal and chemical conditions. As surveys continue to probe ever deeper, and as observing capabilities continue to improve, the number of known Y dwarfs -- the coldest class of sub-stellar objects, with effective temperatures below about 600 K -- is rapidly growing. Critically, this class of ultra-cool objects has atmospheric conditions that overlap with Solar System worlds and, as a result, tools and ideas developed from studying Earth, Jupiter, Saturn and other nearby worlds ar…
The rotational spectral modulation (spectro-photometric variability) of brown dwarfs is usually interpreted as a sign of the presence of inhomogeneous cloud covers in the atmosphere. This paper aims at exploring the role of temperature fluctuations in these spectral modulations. These fluctuations could naturally arise in a convective atmosphere impacted by diabatic processes such as complex chemistry, i.e. the recently proposed mechanism to explain the L/T transition: CO/CH4 radiative convection. We use the 1D radiative/convective code ATMO with ad-hoc modifications of the temperature gradien…
