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تسجيل مستخدم جديدThe dependence of convective core overshooting on stellar mass: semi-empirical determination using the diffusive approach with two different element mixtures
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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 f(ov) on mass, where f(ov) 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 f(ov) for each component. We find a clear mass dependence such that f(ov) rises sharply from zero in the range 1.2--2.0 solar masses, and levels off thereafter up to the 4.4 solar mass limit of our sample. Tests with two different element mixtures indicate the trend is the same, and we find it is also qualitatively similar to the one established in our previous study with the classical step-function implementation of overshooting characterized by the free parameter alpha(ov). Based on these measurements we infer an approximate relationship between the two overshooting parameters of alpha(ov)/f(ov) = 11.36 +/- 0.22, with a possible dependence on stellar properties.
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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 t
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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).
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
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