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.
delta Scuti variables are stars which exhibit periodic changes in their luminosity through radial and non-radial pulsations. Internally, these stars have relatively small convective cores, and convective overshoot can significantly affect the size. Recently, models of radial pulsation in delta Scuti stars found a strong correlation between the pulsation constant (Q) as a function of effective temperature and the amount of convective overshoot within the star. However, only models with metallicities of Z = 0.02 were examined, leaving the dependence of this relationship on chemical composition u…
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…
Recent hydrodynamical simulations of convection in a solar-like model suggest that penetrative convective flows at the boundary of the convective envelope modify the thermal background in the overshooting layer. Based on these results, we implement in one-dimensional stellar evolution codes a simple prescription to modify the temperature gradient below the convective boundary of a solar model. This simple prescription qualitatively reproduces the behaviour found in the hydrodynamical simulations, namely a local heating and smoothing of the temperature gradient below the convective boundary. We…
Core overshoot is a large source of uncertainty in constructing stellar models. Whether the amount of overshoot is constant or mass dependent is not completely known, even though models sometimes assume a mass-based trend. In this work we use asteroseismic data from stars observed by \textit{Kepler} to investigate the relationship between various stellar properties and the amount of overshoot needed to properly model a given star. We find a strong positive trend between stellar mass and overshoot amount for stars between 1.1 and 1.5 $M_\odot$, with a slope of 0.89. Additionally, we investigate…
We perform two-dimensional numerical simulations of core convection for zero-age-main-sequence stars covering a mass range from 3 $M_\odot$ to 20 $M_\odot$. The simulations are performed with the fully compressible time-implicit code MUSIC. We study the efficiency of overshooting, which describes the ballistic process of convective flows crossing a convective boundary, as a function of stellar mass and luminosity. We also study the impact of artificially increasing the stellar luminosity for 3 $M_\odot$ models. The simulations cover hundreds to thousands of convective turnover timescales. Appl…
Turbulent convection models are thought to be good tools to deal with the convective overshooting in the stellar interior. However, they are too complex to be applied in calculations of stellar structure and evolution. In order to understand the physical processes of the convective overshooting and to simplify the application of turbulent convection models, a semi-analytic solution is necessary. We obtain the approximate solution and asymptotic solution of the turbulent convection model in the overshooting region, and find some important properties of the convective overshooting: I. The oversh…
The dependence of convective core overshooting on stellar mass: semi-empirical determination using the diffusive approach with two different element mixtures
The convective kissing instability in low-mass M-dwarf models: convective overshooting, semi-convection, luminosity functions, surface abundances and star cluster age dating
We perform 2D, fully compressible, time-implicit simulations of convection in a solar-like model with the MUSIC code. Our main motivation is to explore the impact of a common tactic adopted in numerical simulations of convection that use realistic stellar conditions. This tactic is to artificially increase the luminosity and to modify the thermal diffusivity of the reference stellar model. This work focuses on the impact of these modifications on convective penetration (or overshooting) at the base of the convective envelope of a solar-like model. We explore a range of enhancement factors for …
The effects of the overshooting of the convective core on main-sequence turnoffs of young- and intermediate-age star clusters
最近的研究表明,扩展的主序星转折点(eMSTO)是麦哲伦星云中中等年龄星团的一个共同特征。eMSTO也出现在100年星团的色星等图(CMD)中。eMSTO的起源仍然是一个悬而未决的问题。此外,星震学表明,对流核的过冲参数$δ_{\rm ov}$的值对于质量近似相等的恒星是不固定的。因此,星团的MSTO可能会受到对流核心(OVCC)过冲的影响。我们计算了不同$δ_{\rm ov}$的OVCC对年轻和中等年龄星团MSTO的影响。\textbf{If $δ_{\rm ov}$ varies between stars in a cluster,}观测到的年轻和中等年龄星团的eMSTO可以很好地解释的影响。由OVCC引起的MSTO的等效年龄扩展与星团的年龄有关,并且与许多星团的观测结果符合得很好。此外,观测到的NGC 1856的eMSTO被具有不同$δ_{\rm ov}$的同时代人群所复制。星团的eMSTO可能与OVCC效应有关。OVCC的影响\textbf{are similar to that of rotation in some respects. But the effects cannot result in a significant split of main sequence of young star clusters at $m_{U}\lesssim 21$.}快速…
The extent of mixed regions around convective zones is one of the biggest uncertainties in stellar evolution. 1D overshooting descriptions introduce a free parameter ($f_{ov}$) that is in general not well constrained from observations. Especially in small central convective regions the value is highly uncertain due to its tight connection to the pressure scale height. Long-term multi-dimensional hydrodynamic simulations can be used to study the size of the overshooting region and the involved mixing processes. Here we show how one can calibrate an overshooting parameter by performing 2D Maestr…