Semiconvection and Overshooting: Schwarzschild and Ledoux Criteria Revisited

Canuto, V. M.

doi:10.1086/312654

Cited by 14 publications

(13 citation statements)

References 9 publications

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“…Salasnich et al 1999;Stothers & Chin 1994;Canuto 2000). However the stability criterion has a marginal effect on stellar models ranging from ≈3 M to ≈12 M , as the ones taken into account in our work Stothers & Chin (1993).…”

Section: Modeling the Blue Loop Morphologymentioning

confidence: 57%

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

Valle

Marconi²,

Degl’Innocenti

et al. 2009

A&A

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Context. With their period-luminosity relation, "classical Cepheids" (CC) are the most common primary distance indicators within the Local Group, also providing an absolute calibration of important secondary distance indicators. However, the predicted position of these pulsators in the HR diagram, along the so called blue loop, that is the expected distribution of Cepheids within the instability strip is affected by several model inputs, reflecting upon the predicted PL relation. Aims. The aim of this work is to quantitatively evaluate the effects on the theoretical PL relation of current uncertainties on the chemical abundances of Cepheids in the Large Magellanic Cloud (LMC) and on several physical assumptions adopted in the evolutionary models. We will separately analyse how the different factors influence the evolutionary and pulsational observables and the resulting PL relation. Methods. To achieve this goal we computed new sets of updated evolutionary and pulsational models. Results. As a result, we find that present uncertainties on the most relevant H and He burning reaction rates do not influence in a relevant way the loop extension in temperature. On the contrary, current uncertainties on the LMC chemical composition significantly affect the loop extension and also reflect in the morphology of the instability strip; however their influence on the predicted pulsational parameters is negligible. We also discussed how overshooting and mass loss, sometimes suggested as possible solutions for the long-standing problem of the Cepheid mass discrepancy, influence the ML relation and the pulsational parameters. Conclusions. In summary, the present uncertainties on the physical inputs adopted in the evolutionary codes and in the LMC chemical composition are negligible for the prediction of the main pulsational properties. On the other hand, the inclusion of overshooting in the previous hydrogen burning phase and/or of mass loss is expected to significantly change the resulting theoretical pulsational scenario for Cepheids, as well as the calibration of their distance scale. These systematic effects are expected to influence the theoretical Cepheid calibration of the secondary distance indicators and in turn the resulting evaluation of the Hubble constant.

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Section: Modeling the Blue Loop Morphologymentioning

confidence: 57%

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

Valle

Marconi²,

Degl’Innocenti

et al. 2009

A&A

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“…Whether the Schwarzschild or Ledoux criterion is satisfied in regions where composition gradients exist in massive stars is still an open question in astrophysics. Semiconvection, a relatively slow mixing caused by composition gradients, is poorly understood, and models that examine the Schwarzschild versus Ledoux gradients yield conflicting results (Merryfield 1995;Canuto 2000). Moreover, Canuto (2000) demonstrates the Schwarzschild criterion necessarily implies convective overshooting, and the Ledoux criterion also necessarily implies overshooting if convection is nonlocal.…”

Section: Discussionmentioning

confidence: 99%

“…Semiconvection, a relatively slow mixing caused by composition gradients, is poorly understood, and models that examine the Schwarzschild versus Ledoux gradients yield conflicting results (Merryfield 1995;Canuto 2000). Moreover, Canuto (2000) demonstrates the Schwarzschild criterion necessarily implies convective overshooting, and the Ledoux criterion also necessarily implies overshooting if convection is nonlocal. The current model includes all the key elements of convective and semiconvective processes, and it provides an opportunity to further examine this important issue.…”

Section: Discussionmentioning

confidence: 99%

Low Mach Number Modeling of Type I X‐Ray Burst Deflagrations

Lin

Bayliss

Taam

2006

ApJ

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The low Mach number approximation (LMNA) is applied to two-dimensional hydrodynamic modeling of type I X-ray bursts on a rectangular patch on the surface of a nonrotating neutron star. Because such phenomena involve decidedly subsonic flows, the time step increase offered by the LMNA makes routine simulations of these deflagrations feasible in an environment where strong gravity produces significant stratification, while allowing for potentially significant lateral differences in temperature and density. The model is employed to simulate the heating, peak, and initial cooling stages in the deep envelope layers of a burst. During the deflagration, Bénard-like cells naturally fill up a vertically expanding convective layer. The Mach number is always less than 0.15 throughout the simulation, thus justifying the low Mach number approximation. While the convective layer is superadiabatic on average, significant fluctuations in adiabaticity occur within it on subconvective timescales. Due to convective layer expansion, significant compositional mixing naturally occurs, but tracer particle penetration through the convective layer boundaries on convective timescales is temporary and spatially limited. Thus, mixing occurs on the relatively slow burst timescale through thermal expansion of the convective layer rather than from mass penetration of the convective layer boundary through particle convection. At the convective layer boundaries where mixing is less efficient, the actual temperature gradient more closely follows the Ledoux criterion.

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“…Canuto has been working on his own theory since 1992 (Canuto 1999b(Canuto , 2000(Canuto , 2011a, but so far no code has included his formalism. In order to progress it would be highly desirable that people developing evolution codes introduce some better theory than the LMLT; however, we have to note that this has not been done so far and worse, that A&A 569, A63 (2014) some codes use a wrong numerical scheme to locate the boundaries of convective zones.…”

Section: Introductionmentioning

confidence: 99%

Proper use of Schwarzschild Ledoux criteria in stellar evolution computations

Gabriel

Noels

Montalbán

et al. 2014

A&A

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The era of detailed asteroseismic analyses opened by space missions such as CoRoT and Kepler has highlighted the need for stellar models devoid of numerical inaccuracies, in order to be able to diagnose which physical aspects are being ignored or poorly treated in standard stellar modeling. We tackle here the important problem of fixing convective zone boundaries in the frame of the local mixing length theory. First we show that the only correct way to locate a convective zone boundary is to find, at each iteration step, through interpolations or extrapolations from points within the convective zone, the mass where the radiative luminosity is equal to the total luminosity. We then discuss two misuses of the boundary condition and the ways they affect stellar modeling and stellar evolution. The first consists in applying the neutrality condition for convective instability on the radiative side of the convective boundary. The second way of misusing the boundary condition comes from the process of fixing the convective boundary through the search for a change of sign of a possibly discontinuous function. We show that these misuses can lead to completely wrong estimates of convective core sizes with important consequences for the following evolutionary phases. We point out the advantages of using a double mesh point at each convective zone boundary. The specific problem of a convective shell is discussed and some remarks concerning overshooting are given.

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Semiconvection and Overshooting: Schwarzschild and Ledoux Criteria Revisited

Cited by 14 publications

References 9 publications

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

Uncertainties on the theoretical predictions for classical Cepheid pulsational quantities

Low Mach Number Modeling of Type I X‐Ray Burst Deflagrations

Proper use of Schwarzschild Ledoux criteria in stellar evolution computations

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