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Guillermo Torres - One of the best experts on this subject based on the ideXlab platform.
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the dependence of convective core Overshooting on stellar mass reality check and additional evidence
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: Antonio Claret, Guillermo TorresAbstract:Overshooting from the convective cores of stars more massive than about 1.2 M(Sun) has a profound impact on their subsequent evolution. And yet, the formulation of the Overshooting mechanism in current stellar evolution models has a free parameter (f[ov] in the diffusive approximation) that remains poorly constrained by observations, affecting the determination of astrophysically important quantities such as stellar ages. In an earlier series of papers we assembled a sample of 37 well-measured detached eclipsing binaries to calibrate the dependence of f[ov] on stellar mass, showing that it increases sharply up to a mass of roughly 2 M(Sun), and remains constant thereafter out to at least 4.4 M(Sun). Recent claims have challenged the utility of eclipsing binaries for this purpose, on the basis that the uncertainties in f[ov] from the model fits are typically too large to be useful, casting doubt on a dependence of Overshooting on mass. Here we reexamine those claims and show them to be too pessimistic, mainly because they did not account for all available constraints --- both observational and theoretical --- in assessing the true uncertainties. We also take the opportunity to add semi-empirical f[ov] determinations for 13 additional binaries to our previous sample, and to update the values for 9 others. All are consistent with, and strengthen our previous conclusions, supporting a dependence of f[ov] on mass that is now based on estimates for a total of 50 binary systems (100 stars).
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the dependence of convective core Overshooting on stellar mass additional binary systems and improved calibration
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Antonio Claret, Guillermo TorresAbstract:Many current stellar evolution models assume some dependence of the strength of convective core Overshooting on mass for stars more massive than 1.1-1.2 solar masses, but the adopted shapes for that relation have remained somewhat arbitrary for lack of strong observational constraints. In previous work we compared stellar evolution models to well-measured eclipsing binaries to show that, when Overshooting is implemented as a diffusive process, the fitted free parameter f(ov) rises sharply up to about 2 solar masses, and remains largely constant thereafter. Here we analyze a new sample of eight binaries selected to be in the critical mass range below 2 solar masses where f(ov) is changing the most, nearly doubling the number of individual stars in this regime. This interval is important because the precise way in which f(ov) changes determines the shape of isochrones in the turnoff region of 1-5 Gyr clusters, and can thus affect their inferred ages. It also has a significant influence on estimates of stellar properties for exoplanet hosts, on stellar population synthesis, and on the detailed modeling of interior stellar structures, including the calculation of oscillation frequencies that are observable with asteroseismic techniques. We find that the derived f(ov) values for our new sample are consistent with the trend defined by our earlier determinations, and strengthen the relation. This provides an opportunity for future series of models to test the new prescription, grounded on observations, against independent observations that may constrain Overshooting in a different way.
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the dependence of convective core Overshooting on stellar mass a semi empirical determination using the diffusive approach with two different element mixtures
The Astrophysical Journal, 2017Co-Authors: Antonio Claret, Guillermo TorresAbstract:Convective core Overshooting has a strong influence on the evolution of stars of moderate and high mass. Studies of double-lined eclipsing binaries and stellar oscillations have renewed interest in the possible dependence of Overshooting on stellar mass, which has been poorly constrained by observations so far. Here, we have used a sample of 29 well-studied double-lined eclipsing binaries in key locations of the H–R diagram to establish the explicit dependence of on mass, where is the free parameter in the diffusive approximation to Overshooting. Measurements of the masses, radii, and temperatures of the binary components were compared against stellar evolution calculations based on the MESA code to infer semi-empirical values of for each component. We find a clear mass-dependence such that rises sharply from zero in the range , and levels off thereafter up to the 4.4 limit of our sample. Tests with two different element mixtures indicate the trend is the same, and we find it to also be qualitatively similar to the one established in our previous study with the classical step-function implementation of Overshooting characterized by the free parameter . Based on these measurements, we infer an approximate relationship between the two Overshooting parameters of , with a possible dependence on stellar properties.
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the dependence of convective core Overshooting on stellar mass
Astronomy and Astrophysics, 2016Co-Authors: Antonio Claret, Guillermo TorresAbstract:The Spanish MEC (AYA2015-71718-R) is gratefully acknowledged for its support during the development of this work. G.T. acknowledges partial support from the NSF through grant AST-1509375
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the dependence of convective core Overshooting on stellar mass
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: Antonio Claret, Guillermo TorresAbstract:Convective core Overshooting extends the main-sequence lifetime of a star. Evolutionary tracks computed with Overshooting are quite different from those that use the classical Schwarzschild criterion, which leads to rather different predictions for the stellar properties. Attempts over the last two decades to calibrate the degree of Overshooting with stellar mass using detached double-lined eclipsing binaries have been largely inconclusive, mainly due to a lack of suitable observational data. Here we revisit the question of a possible mass dependence of Overshooting with a more complete sample of binaries, and examine any additional relation there might be with evolutionary state or metal abundance Z. We use a carefully selected sample of 33 double-lined eclipsing binaries strategically positioned in the H-R diagram, with accurate absolute dimensions and component masses ranging from 1.2 to 4.4 solar masses. We compare their measured properties with stellar evolution calculations to infer semi-empirical values of the Overshooting parameter alpha(ov) for each star. Our models use the common prescription for the overshoot distance d(ov) = alpha(ov) Hp, where Hp is the pressure scale height at the edge of the convective core as given by the Schwarzschild criterion, and alpha(ov) is a free parameter. We find a relation between alpha(ov) and mass that is defined much more clearly than in previous work, and indicates a significant rise up to about 2 solar masses followed by little or no change beyond this mass. No appreciable dependence is seen with evolutionary state at a given mass, or with metallicity at a given mass despite the fact that the stars in our sample span a range of a factor of ten in [Fe/H], from -1.01 to +0.01.
Cameron R Homeyer - One of the best experts on this subject based on the ideXlab platform.
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tropopause penetrating convection from three dimensional gridded nexrad data
Journal of Applied Meteorology and Climatology, 2016Co-Authors: David L Solomon, Kenneth P Bowman, Cameron R HomeyerAbstract:AbstractA new method that combines radar reflectivities from individual Next Generation Weather Radars (NEXRAD) into a three-dimensional composite with high horizontal and vertical resolution is used to estimate storm-top altitudes for the continental United States east of the Rocky Mountains. Echo-top altitudes are compared with the altitude of the lapse-rate tropopause calculated from the ERA-Interim reanalysis and radiosondes. To sample the diurnal and annual cycles, tropopause-penetrating convection is analyzed at 3-h intervals throughout 2004. Overshooting convection is most common in the north-central part of the United States (the high plains). There is a pronounced seasonal cycle; the majority of Overshooting systems occur during the warm season (March–August). There is also a strong diurnal cycle, with maximum Overshooting occurring near 0000 UTC. The Overshooting volume decreases rapidly with height above the tropopause. Radiosonde observations are used to evaluate the quality of the reanalysis ...
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microphysical characteristics of Overshooting convection from polarimetric radar observations
Journal of the Atmospheric Sciences, 2015Co-Authors: Cameron R Homeyer, Matthew R KumjianAbstract:The authors present observations of the microphysical characteristics of deep convection that overshoots the altitude of the extratropical tropopause from analysis of the polarimetric radar variables of radar reflectivity factor at horizontal polarization ZH, differential reflectivity ZDR, and specific differential phase KDP. Identified Overshooting convective storms are separated by their organization and intensity into three classifications: organized convection, discrete ordinary convection, and discrete supercell convection. Composite analysisof identifiedstorms foreach classification reveals microphysical featuressimilar tothose found inpreviousstudiesofdeepconvection,withdeepcolumnsofhighlypositiveZDRandKDPrepresentinglofting of liquid hydrometeors within the convective updraft and above the melting level. In addition, organized and discrete supercell classifications show distinct near-zero ZDR minima aligned horizontally with and at altitudes higher than the updraft column features, likely indicative of the frequent presence of large hail in each case. Composites for organized convective systems show a similar ZDR minimum throughout the portion of the convective core that is Overshooting the tropopause, corresponding to ZH in the range of 15‐30dBZ and negative KDP observations, in agreement with the scattering properties of small hail and/or lump or conical graupel. Additional analyses of the evolution of Overshooting storms reveals that the ZDR minima indicative of hail in the middle and upper troposphere and graupel in the Overshooting top are associated with the mature and decaying stages of Overshooting, respectively, supporting their inferred contributions to the observed polarimetric fields.
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convective transport of water vapor into the lower stratosphere observed during double tropopause events
Journal of Geophysical Research, 2014Co-Authors: Cameron R Homeyer, Laura L Pan, Samuel W Dorsi, L M Avallone, A J Weinheimer, Anthony S Obrien, J P Digangi, Mark A Zondlo, Thomas B RyersonAbstract:We present in situ observations of convectively injected water vapor in the lower stratosphere from instruments aboard two aircraft operated during the Deep Convective Clouds and Chemistry experiment. Water vapor mixing ratios in the injected air are observed to be 60–225 ppmv at altitudes 1–2 km above the tropopause (350–370 K potential temperature), well above observed background mixing ratios of 5–10 ppmv in the lower stratosphere. Radar observations of the responsible convective systems show deep Overshooting at altitudes up to 4 km above the lapse rate tropopause and above the flight ceilings of the aircraft. Backward trajectories from the in situ observations show that convectively injected water vapor is observed from three distinct types of systems: isolated convection, a convective line, and a leading line-trailing stratiform mesoscale convective system. Significant transport of additional tropospheric or boundary layer trace gases is observed only for the leading line-trailing stratiform case. In addition, all observations of convective injection are found to occur within large-scale double-tropopause events from poleward Rossby wave breaking. Based on this relationship, we present a hypothesis on the role of the large-scale lower stratosphere during convective Overshooting. In particular, the reduced lower stratosphere stability associated with double-tropopause environments may facilitate deeper levels of Overshooting and convective injection.
F R N Schneider - One of the best experts on this subject based on the ideXlab platform.
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confronting uncertainties in stellar physics calibrating convective Overshooting with eclipsing binaries
Astronomy and Astrophysics, 2015Co-Authors: Richard J Stancliffe, L Fossati, Jeanclaude Passy, F R N SchneiderAbstract:As part of a larger program aimed at better quantifying the uncertainties in stellar computations, we attempt to calibrate the extent of convective Overshooting in low to intermediate mass stars by means of eclipsing binary systems. We model 12 such systems, with component masses between 1.3 and 6.2 M� , using the detailed binary stellar evolution code STARS, producing grids of models in both metallicity and Overshooting parameter. From these, we determine the best fit parameters for each of our systems. For three systems, none of our models produce a satisfactory fit. For the remaining systems, no single value for the convective Overshooting parameter fits all the systems, but most of our systems can be well described with an Overshooting parameter between 0.09 and 0.15, corresponding to an extension of the mixed region above the core of about 0.1−0.3 pressure scale heights. Of the nine systems where we are able to obtain a good fit, seven can be reasonably well fit with a single parameter of 0.15. We find no evidence for a trend of the extent of Overshooting with either mass or metallicity, though the data set is of limited size. We repeat our calculations with a second evolution code, MESA, and we find general agreement between the two codes. The extension of the mixed region above the convective core required by the MESA models is about 0.15−0.4 pressure scale heights. For the system EI Cep, we find that MESA gives an Overshooting region that is larger than the STARS one by about 0.1 pressure scale heights for the primary, while for the secondary the difference is only 0.05 pressure scale heights.
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confronting uncertainties in stellar physics calibrating convective Overshooting with eclipsing binaries
arXiv: Solar and Stellar Astrophysics, 2015Co-Authors: Richard J Stancliffe, L Fossati, Jeanclaude Passy, F R N SchneiderAbstract:As part of a larger program aimed at better quantifying the uncertainties in stellar computations, we attempt to calibrate the extent of convective Overshooting in low to intermediate mass stars by means of eclipsing binary systems. We model 12 such systems, with component masses between 1.3 and 6.2 solar masses, using the detailed binary stellar evolution code STARS, producing grids of models in both metallicity and Overshooting parameter. From these, we determine the best fit parameters for each of our systems. For three systems, none of our models produce a satisfactory fit. For the remaining systems, no single value for the convective Overshooting parameter fits all the systems, but most of our systems can be well described with an Overshooting parameter between 0.09 and 0.15, corresponding to an extension of the mixed region above the core of about 0.1-0.3 pressure scale heights. Of the nine systems where we are able to obtain a good fit, seven can be reasonably well fit with a single parameter of 0.15. We find no evidence for a trend of the extent of Overshooting with either mass or metallicity, though the data set is of limited size. We repeat our calculations with a second evolution code, MESA, and we find general agreement between the two codes. For the extension of the mixed region above the convective core required by the MESA models is about 0.15-0.4 pressure scale heights. For the system EI Cep, we find that MESA gives an Overshooting region that is larger than the STARS one by about 0.1 pressure scale heights for the primary, while for the secondary the difference is only 0.05 pressure scale heights.
P J Kapyla - One of the best experts on this subject based on the ideXlab platform.
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Overshooting in simulations of compressible convection
Astronomy and Astrophysics, 2019Co-Authors: P J KapylaAbstract:(abridged) Context: Convective motions Overshooting to regions that are formally convectively stable cause extended mixing. Aims: To determine the scaling of Overshooting depth ($d_{\rm os}$) at the base of the convection zone as a function of imposed energy flux ($\mathscr{F}_{\rm n}$) and to estimate the extent of Overshooting at the base of the solar convection zone. Methods: Three-dimensional Cartesian simulations of compressible non-rotating convection with unstable and stable layers are used. The simulations use either a fixed heat conduction profile or a temperature and density dependent formulation based on Kramers opacity law. The simulations cover a range of almost four orders of magnitude in the imposed flux. Results: A smooth heat conduction profile (either fixed or through Kramers opacity law) leads to a relatively shallow power law with $d_{\rm os}\propto \mathscr{F}_{\rm n}^{0.08}$ for low $\mathscr{F}_{\rm n}$. A fixed step-profile of the heat conductivity at the bottom of the convection zone leads to a somewhat steeper dependency with $d_{\rm os}\propto \mathscr{F}_{\rm n}^{0.12}$. Experiments with and without subgrid-scale entropy diffusion revealed a strong dependence on the effective Prandtl number which is likely to explain the steep power laws as a function of $\mathscr{F}_{\rm n}$ reported in the literature. Furthermore, changing the heat conductivity artificially below the convection zone is shown to lead to substantial underestimation of Overshooting depth. Conclusions: Extrapolating from the results obtained with smooth heat conductivity profiles suggest that the Overshooting depth for the solar flux is of the order of $0.2$ pressure scale heights at the base of the convection zone which is two to four times higher than estimates from helioseismology.
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extended subadiabatic layer in simulations of Overshooting convection
The Astrophysical Journal, 2017Co-Authors: A Lagg, P J Kapyla, Matthias Rheinhardt, Axel Brandenburg, R Arlt, Maarit J Kapyla, N OlspertAbstract:We present numerical simulations of hydrodynamic Overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stra ...
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extended subadiabatic layer in simulations of Overshooting convection
arXiv: Solar and Stellar Astrophysics, 2017Co-Authors: A Lagg, P J Kapyla, Matthias Rheinhardt, Axel Brandenburg, R Arlt, Maarit J Kapyla, N OlspertAbstract:We present numerical simulations of hydrodynamic Overshooting convection in local Cartesian domains. We find that a substantial fraction of the lower part of the convection zone (CZ) is stably stratified according to the Schwarzschild criterion while the enthalpy flux is outward directed. This occurs when the heat conduction profile at the bottom of the CZ is smoothly varying, based either on a Kramers-like opacity prescription as a function of temperature and density or a static profile of a similar shape. We show that the subadiabatic layer arises due to nonlocal energy transport by buoyantly driven downflows in the upper parts of the CZ. Analysis of the force balance of the upflows and downflows confirms that convection is driven by cooling at the surface. We find that the commonly used prescription for the convective enthalpy flux being proportional to the negative entropy gradient does not hold in the stably stratified layers where the flux is positive. We demonstrate the existence of a non-gradient contribution to the enthalpy flux, which is estimated to be important throughout the convective layer. A quantitative analysis of downflows indicates a transition from a tree-like structure where smaller downdrafts merge into larger ones in the upper parts to a structure in the deeper parts where a height-independent number of strong downdrafts persist. This change of flow topology occurs when a substantial subadiabatic layer is present in the lower part of the CZ.
Antonio Claret - One of the best experts on this subject based on the ideXlab platform.
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the dependence of convective core Overshooting on stellar mass reality check and additional evidence
arXiv: Solar and Stellar Astrophysics, 2019Co-Authors: Antonio Claret, Guillermo TorresAbstract:Overshooting from the convective cores of stars more massive than about 1.2 M(Sun) has a profound impact on their subsequent evolution. And yet, the formulation of the Overshooting mechanism in current stellar evolution models has a free parameter (f[ov] in the diffusive approximation) that remains poorly constrained by observations, affecting the determination of astrophysically important quantities such as stellar ages. In an earlier series of papers we assembled a sample of 37 well-measured detached eclipsing binaries to calibrate the dependence of f[ov] on stellar mass, showing that it increases sharply up to a mass of roughly 2 M(Sun), and remains constant thereafter out to at least 4.4 M(Sun). Recent claims have challenged the utility of eclipsing binaries for this purpose, on the basis that the uncertainties in f[ov] from the model fits are typically too large to be useful, casting doubt on a dependence of Overshooting on mass. Here we reexamine those claims and show them to be too pessimistic, mainly because they did not account for all available constraints --- both observational and theoretical --- in assessing the true uncertainties. We also take the opportunity to add semi-empirical f[ov] determinations for 13 additional binaries to our previous sample, and to update the values for 9 others. All are consistent with, and strengthen our previous conclusions, supporting a dependence of f[ov] on mass that is now based on estimates for a total of 50 binary systems (100 stars).
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the dependence of convective core Overshooting on stellar mass additional binary systems and improved calibration
arXiv: Solar and Stellar Astrophysics, 2018Co-Authors: Antonio Claret, Guillermo TorresAbstract:Many current stellar evolution models assume some dependence of the strength of convective core Overshooting on mass for stars more massive than 1.1-1.2 solar masses, but the adopted shapes for that relation have remained somewhat arbitrary for lack of strong observational constraints. In previous work we compared stellar evolution models to well-measured eclipsing binaries to show that, when Overshooting is implemented as a diffusive process, the fitted free parameter f(ov) rises sharply up to about 2 solar masses, and remains largely constant thereafter. Here we analyze a new sample of eight binaries selected to be in the critical mass range below 2 solar masses where f(ov) is changing the most, nearly doubling the number of individual stars in this regime. This interval is important because the precise way in which f(ov) changes determines the shape of isochrones in the turnoff region of 1-5 Gyr clusters, and can thus affect their inferred ages. It also has a significant influence on estimates of stellar properties for exoplanet hosts, on stellar population synthesis, and on the detailed modeling of interior stellar structures, including the calculation of oscillation frequencies that are observable with asteroseismic techniques. We find that the derived f(ov) values for our new sample are consistent with the trend defined by our earlier determinations, and strengthen the relation. This provides an opportunity for future series of models to test the new prescription, grounded on observations, against independent observations that may constrain Overshooting in a different way.
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the dependence of convective core Overshooting on stellar mass a semi empirical determination using the diffusive approach with two different element mixtures
The Astrophysical Journal, 2017Co-Authors: Antonio Claret, Guillermo TorresAbstract:Convective core Overshooting has a strong influence on the evolution of stars of moderate and high mass. Studies of double-lined eclipsing binaries and stellar oscillations have renewed interest in the possible dependence of Overshooting on stellar mass, which has been poorly constrained by observations so far. Here, we have used a sample of 29 well-studied double-lined eclipsing binaries in key locations of the H–R diagram to establish the explicit dependence of on mass, where is the free parameter in the diffusive approximation to Overshooting. Measurements of the masses, radii, and temperatures of the binary components were compared against stellar evolution calculations based on the MESA code to infer semi-empirical values of for each component. We find a clear mass-dependence such that rises sharply from zero in the range , and levels off thereafter up to the 4.4 limit of our sample. Tests with two different element mixtures indicate the trend is the same, and we find it to also be qualitatively similar to the one established in our previous study with the classical step-function implementation of Overshooting characterized by the free parameter . Based on these measurements, we infer an approximate relationship between the two Overshooting parameters of , with a possible dependence on stellar properties.
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the dependence of convective core Overshooting on stellar mass
Astronomy and Astrophysics, 2016Co-Authors: Antonio Claret, Guillermo TorresAbstract:The Spanish MEC (AYA2015-71718-R) is gratefully acknowledged for its support during the development of this work. G.T. acknowledges partial support from the NSF through grant AST-1509375
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the dependence of convective core Overshooting on stellar mass
arXiv: Solar and Stellar Astrophysics, 2016Co-Authors: Antonio Claret, Guillermo TorresAbstract:Convective core Overshooting extends the main-sequence lifetime of a star. Evolutionary tracks computed with Overshooting are quite different from those that use the classical Schwarzschild criterion, which leads to rather different predictions for the stellar properties. Attempts over the last two decades to calibrate the degree of Overshooting with stellar mass using detached double-lined eclipsing binaries have been largely inconclusive, mainly due to a lack of suitable observational data. Here we revisit the question of a possible mass dependence of Overshooting with a more complete sample of binaries, and examine any additional relation there might be with evolutionary state or metal abundance Z. We use a carefully selected sample of 33 double-lined eclipsing binaries strategically positioned in the H-R diagram, with accurate absolute dimensions and component masses ranging from 1.2 to 4.4 solar masses. We compare their measured properties with stellar evolution calculations to infer semi-empirical values of the Overshooting parameter alpha(ov) for each star. Our models use the common prescription for the overshoot distance d(ov) = alpha(ov) Hp, where Hp is the pressure scale height at the edge of the convective core as given by the Schwarzschild criterion, and alpha(ov) is a free parameter. We find a relation between alpha(ov) and mass that is defined much more clearly than in previous work, and indicates a significant rise up to about 2 solar masses followed by little or no change beyond this mass. No appreciable dependence is seen with evolutionary state at a given mass, or with metallicity at a given mass despite the fact that the stars in our sample span a range of a factor of ten in [Fe/H], from -1.01 to +0.01.
