scholarly journals Calibration of the mixing-length theory for structures of helium-dominated atmosphere white dwarfs

2019 ◽  
Vol 490 (1) ◽  
pp. 1010-1025 ◽  
Author(s):  
E Cukanovaite ◽  
P-E Tremblay ◽  
B Freytag ◽  
H-G Ludwig ◽  
G Fontaine ◽  
...  
Keyword(s):  
White Dwarfs ◽  
Convection Zone ◽  
3D Models ◽  
Spectroscopic Technique ◽  
Atmospheric Parameter ◽  
Mixing Length ◽  
Instability Strip ◽  
Bottom Boundary ◽  
Pure Helium ◽  
Mixing Length Theory

ABSTRACT 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 radiation-hydrodynamics code, and a grid of 1D DB and DBA envelope structures. The 3D models span a parameter space of hydrogen-to-helium abundances in the range −10.0 ≤ log (H/He) ≤−2.0, surface gravities in the range 7.5 ≤ log g ≤ 9.0, and effective temperatures in the range 12 000 K ≲ Teff ≲ 34 000 K. The 1D envelopes cover a similar atmospheric parameter range, but are also calculated with different values of the mixing-length parameter, namely 0.4 ≤ ML2/α ≤ 1.4. The calibration is performed based on two definitions of the bottom boundary of the convection zone: the Schwarzschild and the zero convective flux boundaries. Thus, our calibration is relevant for applications involving the bulk properties of the convection zone including its total mass, which excludes the spectroscopic technique. Overall, the calibrated ML2/α is smaller than what is commonly used in evolutionary models and theoretical determinations of the blue edge of the instability strip for pulsating DB and DBA stars. With calibrated ML2/α we are able to deduce more accurate convection zone sizes needed for studies of planetary debris mixing and dredge-up of carbon from the core. We highlight this by calculating examples of metal-rich 3D DBAZ models and finding their convection zone masses. Mixing-length calibration represents the first step of in-depth investigations of convective overshoot in white dwarfs with helium-dominated atmospheres.

scholarly journals Fingering convection in accreting hydrogen white dwarfs

2019 ◽  
Vol 82 ◽  
pp. 183-187
Author(s):  
F.C. Wachlin ◽  
S. Vauclair ◽  
G. Vauclair ◽  
L.G. Althaus
Keyword(s):  
Numerical Simulations ◽  
Time Scale ◽  
White Dwarfs ◽  
Convection Zone ◽  
Mixing Length ◽  
Gravitational Settling ◽  
Accretion Rates ◽  
Debris Disk ◽  
Mixing Length Theory

The accretion of heavy material from debris disk on the surface of hydrogen-rich white dwarfs induces a double diffusivity instability known as the fingering convection. It leads to an efficient extra mixing which brings the accreted material deeper in the star than by considering only mixing in the surface dynamical convection zone, in a time scale much shorter than that of gravitational settling. We performed numerical simulations of a continuous accretion of heavy material having a bulk Earth composition on the two well studied DAZ and ZZ Ceti pulsators GD 133 and G 29-38. We find that the existence of fingering convection implies much larger accretion rates to explain the observed abundances than previous estimates based on the standard mixing length theory and gravitational settling only.

scholarly journals 3D spectroscopic analysis of helium-line white dwarfs

2020 ◽  
Author(s):  
Elena Cukanovaite ◽  
Pier-Emmanuel Tremblay ◽  
Pierre Bergeron ◽  
Bernd Freytag ◽  
Hans-Günter Ludwig ◽  
...  
Keyword(s):  
Effective Temperature ◽  
White Dwarfs ◽  
Energy Transport ◽  
3D Models ◽  
Spectroscopic Technique ◽  
Surface Gravity ◽  
Atmospheric Models ◽  
Physical Treatment ◽  
Spectroscopic Parameters ◽  
1D Model

Abstract In this paper, we present corrections to the spectroscopic parameters of DB and DBA white dwarfs with −10.0 ≤ log (H/He) ≤−2.0, 7.5 ≤ log g ≤9.0 and 12 000 K ≲ Teff ≲ 34 000 K, based on 282 3D atmospheric models calculated with the CO5BOLD radiation-hydrodynamics code. These corrections arise due to a better physical treatment of convective energy transport in 3D models when compared to the previously available 1D model atmospheres. By applying the corrections to an existing SDSS sample of DB and DBA white dwarfs, we find significant corrections both for effective temperature and surface gravity. The 3D log g corrections are most significant for Teff ≲ 18, 000 K, reaching up to −0.20 dex at log g = 8.0. However, in this low effective temperature range, the surface gravity determined from the spectroscopic technique, can also be significantly affected by the treatment of the neutral van der Waals line broadening of helium and by non-ideal effects due to the perturbation of helium by neutral atoms. Thus, by removing uncertainties due to 1D convection, our work showcases the need for improved description of microphysics for DB and DBA model atmospheres. Overall, we find that our 3D spectroscopic parameters for the SDSS sample are generally in agreement with Gaia DR2 absolute fluxes within 1-3σ for individual white dwarfs. By comparing our results to DA white dwarfs, we determine that the precision and accuracy of DB/DBA atmospheric models are similar. For ease of user application of the correction functions, we provide an example Python code.

scholarly journals New insights on pulsating white dwarfs from 3D radiation-hydrodynamical simulations

2015 ◽  
Vol 11 (A29B) ◽  
pp. 667-672
Author(s):  
Pier-Emmanuel Tremblay ◽  
Gilles Fontaine ◽  
Hans-Günter Ludwig ◽  
Alexandros Gianninas ◽  
Mukremin Kilic
Keyword(s):  
White Dwarfs ◽  
3D Model ◽  
Atmospheric Parameter ◽  
Pure Hydrogen ◽  
Internal Layers ◽  
Instability Strip ◽  
3D Effects ◽  
Free Parameters ◽  
Hydrodynamical Simulations ◽  
Definition Of

AbstractWe have recently computed a grid of 3D radiation-hydrodynamical simulations for the atmosphere of pure-hydrogen DA white dwarfs in the range 5.0 < log g < 9.0. Our grid covers the full ZZ Ceti instability strip where pulsating DA white dwarfs are located. We have significantly improved the theoretical framework to study these objects by removing the free parameters of 1D convection, which were previously a major modeling hurdle. We present improved atmospheric parameter determinations based on spectroscopic fits with 3D model spectra, allowing for an updated definition of the empirical edges of the ZZ Ceti instability strip. Our 3D simulations also precisely predict the depth of the convection zones, narrowing down the internal layers where pulsation are being driven. We hope that these 3D effects will be included in asteroseismic models in the future to predict the region of the HR diagram where white dwarfs are expected to pulsate.

Pulsating white dwarfs and convection

2019 ◽  
Vol 15 (S357) ◽  
pp. 127-130
Author(s):  
J. L. Provencal ◽  
M. H. Montgomery ◽  
H. L. Shipman ◽  
Keyword(s):  
Local Time ◽  
White Dwarfs ◽  
Recent Progress ◽  
Three Dimensional ◽  
Theoretical Uncertainty ◽  
Mixing Length ◽  
Mixing Length Theory

AbstractConvection is a highly turbulent, three dimensional process that is traditionally treated using a simple, local, time independent description. Convection is one of the largest sources of theoretical uncertainty in stellar modeling. We outline recent progress in studies using pulsating white dwarfs to constrain convection and calibrate mixing length theory.

An Improved Vitense Model of the Solar Convection Zone

1971 ◽  
Vol 2 (1) ◽  
pp. 46-47 ◽  
Author(s):  
R. van der Borght
Keyword(s):  
Ad Hoc ◽  
Convection Zone ◽  
Scale Height ◽  
Mixing Length ◽  
Variational Technique ◽  
Height Ratio ◽  
Solar Convection Zone ◽  
Solar Convection ◽  
Mixing Length Theory

Recently, Unno has proposed a new method for constructing models of convective non-grey atmospheres, making two substantial improvements to the mixing-length theory of Böhm-Vitense. His method, based on the variational technique of Glansdorff-Prigogine, enables the determination of the eddy-size and is no longer dependent on an ad hoc assumption of a constant mixing-length to scale height ratio. It also takes into account the effects of non-grey radiative transfer.

scholarly journals Concentration of magnetic fields in the deep convection zone

1968 ◽  
Vol 35 ◽  
pp. 108-111
Author(s):  
G. W. Simon ◽  
N. O. Weiss
Keyword(s):  
Magnetic Fields ◽  
Convection Zone ◽  
Deep Convection ◽  
Giant Cells ◽  
Scale Height ◽  
Characteristic Scale ◽  
Mixing Length ◽  
Strong Magnetic Fields ◽  
Mixing Length Theory ◽  
Density Scale

The strong magnetic fields observed between supergranules indicate that there must be subphotospheric convection in cells with a preferred diameter of about 30000 km. Orthodox mixing length theory assumes that the dimensions of cells are limited by the density scale-height. This is adequate for explaining granules but cannot account for supergranulation. A model is therefore proposed in which cellular motions extend over several scale-heights. In addition to granules and supergranules this model predicts a third characteristic scale of motion, with giant cells around 300000 km in diameter. These cells may produce a pattern of magnetic fields like that suggested by Bumba and Howard for complexes of activity.

scholarly journals The Effects of Time Dependant Convection on White Dwarf Radial Pulsations

1989 ◽  
Vol 114 ◽  
pp. 115-118
Author(s):  
S. Starrfield ◽  
A. N. Cox
Keyword(s):  
White Dwarf ◽  
Time Scale ◽  
White Dwarfs ◽  
Pure Hydrogen ◽  
Helium Surface ◽  
Surface Layers ◽  
Instability Strip ◽  
Blue Edge ◽  
Pure Helium ◽  
Radial Pulsations

AbstractWe have investigated the effects of relaxing the normal assumption of frozen in convection on studies of radial instabilities in 0.6M⊙ carbon-oxygen white dwarfs with either pure hydrogen layers overlying pure helium layers or 0.6M⊙ carbon-oxygen white dwarfs with pure helium surface layers. In this paper we assume that convection can adjust to the pulsation at a rate determined by the time scale of a convective eddy. Using this assumption in our analysis stabilizes most of the modes in both the DA and DB radial instability strips. We also find that the blue edge of the DA radial instability strip, assuming frozen in convection, is between 12,0O0K and 13,000K. The blue edge for the DB radial instability strip (frozen in convection) is between 32,000K and 33,000K.

scholarly journals Non-Adiabatic Seismic Study of The Thin Convective Envelope of δ Scuti Stars

2004 ◽  
Vol 193 ◽  
pp. 470-473
Author(s):  
M.-A. Dupret ◽  
A. Grigahcène ◽  
R. Garrido ◽  
J. Montalban ◽  
M. Gabriel ◽  
...  
Keyword(s):  
Instability Strip ◽  
Full Spectrum ◽  
Convective Envelope ◽  
Theoretical Predictions ◽  
Mode Stability ◽  
Different Color ◽  
Mixing Length Theory ◽  
Scuti Stars ◽  
Δ Scuti Stars ◽  
Phase Differences

AbstractFor δ Sct stars, the theoretical predictions of a non-adiabatic pulsation code are very dependent on the characteristics of the thin convective envelope of the models (Balona & Evers 1999). The treatment of the non-adiabatic interaction between convection and pulsation also has a significant impact on the results, particularly near the red edge of the instability strip. The non-adiabatic theoretical predictions can be tested upon observations by comparing them to the amplitude ratios and phase differences as observed in different color passbands (Dupret et al. 2003). In the first part of this paper, we compare the results obtained by adopting different treatments of convection in the interior and atmosphere models: mixing-length theory (MLT) and full spectrum of turbulence (FST) (Canuto et al. 1996, CGM). In the second part, we examine the problem of the interaction between convection and pulsation and compare the mode stability obtained with and without including time-dependent convection in our non-adiabatic code.

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