scholarly journals Convection Theory and Relevant Problems in Stellar Structure, Evolution, and Pulsational Stability I Convection Theory and Structure of Convection Zone and Stellar Evolution

2021 ◽  
Vol 7 ◽  
Author(s):  
Da-run Xiong
Keyword(s):  
Main Sequence ◽  
Massive Stars ◽  
Thermal Structure ◽  
Stellar Structure ◽  
Temperature Fields ◽  
Structure Evolution ◽  
The Sun ◽  
Convective Envelope ◽  
Sequence Band ◽  
Lithium Depletion

A non-local and time-dependent theory of convection was briefly described. This theory was used to calculate the structure of solar convection zones, the evolution of massive stars, lithium depletion in the atmosphere of the Sun and late-type dwarfs, and stellar oscillations (in Part Ⅱ). The results show that: 1) the theoretical turbulent velocity and temperature fields in the atmosphere and the thermal structure of the convective envelope of the Sun agree with the observations and inferences from helioseismic inversion very well. 2) The so-called semi-convection contradiction in the evolutionary calculations of massive stars was removed automatically, as predicted by us. The theoretical evolution tracks of massive stars run at higher luminosity and the main sequence band becomes noticeably wider in comparison with those calculated using the local mixing-length theory (MLT). This means that the evolutionary mass for a given luminosity was overestimated and the width of the main sequence band was underestimated by the local MLT, which may be part of the reason for the contradiction between the evolutionary and pulsational masses of Cepheid variables and the contradiction between theoretical and observed distributions of luminous stars in the H-R diagram. 3) The predicted lithium depletion, in general, agrees well with the observation of the Sun and Galactic open clusters of different ages. 4) Our theoretical results for non-adiabatic oscillations are in good agreement with the observed mode instability from classic variables of high-luminosity red giants. Almost all the instability strips of the classical pulsating variables (including the Cepheid, δ Scuti, γ Doradus, βCephei, and SPB strips) were reproduced (Part Ⅱ).

scholarly journals Modelling of an Eclipsing RS CVn Binary: V405 And

2011 ◽  
Vol 7 (S282) ◽  
pp. 199-200
Author(s):  
Krisztián Vida ◽  
Katalin Oláh ◽  
Zsolt Kővári
Keyword(s):  
Light Curve ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Light Curves ◽  
Stellar Structure ◽  
Main Sequence Stars ◽  
M Dwarfs ◽  
Primary Star ◽  
Convective Envelope ◽  
Eclipsing Binary

AbstractV405 And is an ultrafast-rotating (Prot ≈ 0.46 days) eclipsing binary. The system consists of a primary star with radiative core and convective envelope, and a fully convective secondary. Theories have shown that stellar structure can depend on magnetic activity, i.e., magnetically active M-dwarfs should have larger radii. Earlier light curve modelling of V405 And indeed showed this behaviour: we found that the radius of the primary is significantly larger than the theoretically predicted value for inactive main sequence stars (the discrepancy is the largest of all known objects), while the secondary fits well to the mass-radius relation. By modelling our recently obtained light curves, which show significant changes of the spotted surface of the primary, we can find further proof for this phenomenon.

scholarly journals Effects of a stochastic initial mass function on the upper main sequence band

1981 ◽  
Vol 59 ◽  
pp. 293-296
Author(s):  
C. Chiosi ◽  
L. Greggio
Keyword(s):  
Mass Loss ◽  
Effective Temperature ◽  
Main Sequence ◽  
Massive Stars ◽  
Mass Function ◽  
Initial Mass Function ◽  
High Mass ◽  
Initial Mass ◽  
Sequence Band ◽  
High Mass Loss

The theoretical (Mb versus Log Te) HR diagram for the brightest galactic OB stars shows an upper boundary for the luminosity, which is characterized by a decreasing luminosity with decreasing effective temperature (Humphreys and Davidson, 1979). The existence of this limit was interpreted by Chiosi et al. (1978) as due to the effect of mass loss by stellar wind on the evolution of most massive stars in core H-burning phase. In fact, evolutionary models calculated at constant mass cover a wider and wider range in effective temperature as the initial mass increases during the main sequence phase. On the contrary, sufficiently high mass-loss rates make the evolutionary sequences of most massive stars (M 60⩾Mʘ) shrink toward the zero age main sequence whenever, due to mass loss, CNO processed material is brought to the surface (Chiosi et al., 1978; de Loore et al., 1978; Maeder, 1980).

scholarly journals A Dialogue on Dynamical Pre-Main Sequence Tracks

2001 ◽  
Vol 200 ◽  
pp. 492-495 ◽  
Author(s):  
Günther Wuchterl
Keyword(s):  
Main Sequence ◽  
Thermal Structure ◽  
Brown Dwarfs ◽  
Young Star ◽  
Hydrostatic Equilibrium ◽  
Young Stars ◽  
Stellar Atmospheres ◽  
Stellar Structure ◽  
Sequence Evolution ◽  
Stellar Matter

Based on the theory of stellar structure and evolution combined with the theory of stellar atmospheres theoretical properties of young stars can be calculated. These calculations of pre-main sequence evolution have been refined over the last decades and do now provide theoretical spectra and colours even for very cool objects like young stars brown dwarfs and planets. Two of their key assumptions must become invalid towards the formation phases: (1) the hydrostatic equilibrium of pressure forces and gravity that assumes stellar matter to be at rest and (2) the non-dependence on the initial thermal structure. The former (1) is violated by accretion- and collapse flows, the latter (2) because a new born young star is observed with the specific thermal structure produced by the cloud collapse. I discuss changes in the theoretical properties of young stars that follow from calculating the pre-main sequence evolution as the consequence of the collapse of Bonnor-Ebert spheres.

scholarly journals Detailed characterization of stellar high energy (FUV/EUV/X-ray) radiation fields during protoplanetary system formation

2009 ◽  
Vol 5 (S264) ◽  
pp. 395-400 ◽  
Author(s):  
Alexander Brown
Keyword(s):  
Main Sequence ◽  
Thermal Structure ◽  
High Energy ◽  
Young Stars ◽  
The Sun ◽  
Nearby Star ◽  
Stellar Activity ◽  
Local Association ◽  
X Ray ◽  
Radiation Fields

AbstractYoung stars undergoing the conversion of pre-main-sequence circumstellar disks into protoplanetary systems are strong sources of high energy (FUV/EUV/X-ray) radiation that controls the physical and chemical processes in their circumstellar environment out to hundreds of AU from the star. The high energy radiation resulting from magnetic activity and accretion onto the central star controls the thermal structure of disks, the formation process of planetesimals, and the photoexcitation and photoionization of protoplanets and young planetary atmospheres. Modeling of the dust and gas evolution requires an accurate understanding of the local radiation field throughout the ultraviolet (UV) and X-ray spectral regions, even those parts of the spectrum that are impossible to observe from Earth.Our current research efforts are directed towards developing a better understanding of UV (using HST and FUSE) and X-ray (using Chandra, XMM-Newton, and Swift) stellar activity and the resulting radiation fields during pre-main-sequence evolution from ages of a few to several hundred million years. These studies include extensive UV and X-ray spectral sampling of individual stars in nearby star formation regions and the various moving groups of the Local Association, including our HST Cycle 17 Large Project (GO-11616), which is using 111 HST orbits to observe 32 T Tauri stars with the COS UV spectrograph. Most young stars are well over 100 pc from the Sun and are consequently hard to observe in the UV and X-ray regions at even moderate spectral resolution. However, members of the Local Association, whose ages range from 7 Myr to a few hundred Myr, surround the Sun at distances of 50 pc or less and permit the detailed study of the later stages of the early evolution of stellar activity when gas giant and terrestrial protoplanets are forming. We illustrate our methodology using the 12 Myr old early-M dwarf AU Mic, which possesses a striking dust debris disk, as an example.

scholarly journals Observational tests for overshoot and opacity enhancement based on massive stars

1984 ◽  
Vol 105 ◽  
pp. 333-336
Author(s):  
G. Bertelli ◽  
A. Bressan ◽  
C. Chiosi
Keyword(s):  
Mass Loss ◽  
Stellar Wind ◽  
Classical Limit ◽  
Main Sequence ◽  
Massive Stars ◽  
Heavy Elements ◽  
Moderate Increase ◽  
Theoretical Expectation ◽  
Sequence Band ◽  
Convective Overshoot

It is shown that a moderate increase in the opacity due to heavy elements in models of massive stars, incorporating convective overshoot and mass loss by stellar wind, can remove the well known discrepancy between theoretical expectation and observed frequencies of luminous stars in the HR diagram. These models in fact extend their main sequence band far beyond the classical limit.

scholarly journals Revisiting the pre-main-sequence evolution of stars

2018 ◽  
Vol 618 ◽  
pp. A132 ◽  
Author(s):  
Masanobu Kunitomo ◽  
Tristan Guillot ◽  
Shigeru Ida ◽  
Taku Takeuchi
Keyword(s):  
Solar System ◽  
Stellar Evolution ◽  
Main Sequence ◽  
Planet Formation ◽  
Surface Composition ◽  
Convective Zone ◽  
Stellar Surface ◽  
Stellar Structure ◽  
High Mass ◽  
The Sun

Aims. We want to investigate how planet formation is imprinted on stellar surface composition using up-to-date stellar evolution models. Methods. We simulate the evolution of pre-main-sequence stars as a function of the efficiency of heat injection during accretion, the deuterium mass fraction, and the stellar mass, M⋆. For simplicity, we assume that planet formation leads to the late accretion of zero-metallicity gas, diluting the surface stellar composition as a function of the mass of the stellar outer convective zone. We estimate that in the solar system, between 97 and 168 M⊕ of condensates formed planets or were ejected from the system. We adopt 150 M⊕(M⋆/M⊙)(Z/Z⊙) as an uncertain but plausible estimate of the mass of heavy elements that is not accreted by stars with giant planets, including our Sun. By combining our stellar evolution models to these estimates, we evaluate the consequences of planet formation on stellar surface composition. Results. We show that after the first ~0.1 Myr during which stellar structure can differ widely from the usually assumed fully convective structure, the evolution of the convective zone follows classical pre-main-sequence evolutionary tracks within a factor of two in age. We find that planet formation should lead to a scatter in stellar surface composition that is larger for high-mass stars than for low-mass stars. We predict a spread in [Fe/H] of approximately 0.05 dex for stars with a temperature of Teff ~ 6500 K, to 0.02 dex for stars with Teff ~ 5500 K, marginally compatible with differences in metallicities observed in some binary stars with planets. Stars with Teff ≤ 7000 K may show much larger [Fe/H] deficits, by 0.6 dex or more, in the presence of efficient planet formation, compatible with the existence of refractory-poor λ Boo stars. We also find that planet formation may explain the lack of refractory elements seen in the Sun as compared to solar twins, but only if the ice-to-rock ratio in the solar-system planets is less than ≈0.4 and planet formation began less than ≈1.3 Myr after the beginning of the formation of the Sun.

scholarly journals Probing the shape of the mixing profile and of the thermal structure at the convective core boundary through asteroseismology

2019 ◽  
Vol 628 ◽  
pp. A76 ◽  
Author(s):  
M. Michielsen ◽  
M. G. Pedersen ◽  
K. C. Augustson ◽  
S. Mathis ◽  
C. Aerts
Keyword(s):  
Temperature Gradient ◽  
Main Sequence ◽  
Thermal Structure ◽  
Theoretical Perspective ◽  
Core Region ◽  
Stellar Structure ◽  
Evolutionary Stage ◽  
Sequence Evolution ◽  
Boundary Mixing ◽  
Core Boundary

Aims. We investigate from a theoretical perspective if space asteroseismology can be used to distinguish between different thermal structures and shapes of the near-core mixing profiles for different types of coherent oscillation modes in massive stars with convective cores; we also examine whether this capacity depends on the evolutionary stage of the models along the main sequence. Methods. We computed 1D stellar structure and evolution models for four different prescriptions of the mixing and temperature gradient in the near-core region. We investigated their effect on the frequencies of dipole prograde gravity modes in slowly pulsating B stars and in β Cep stars as well as pressure modes in β Cep stars. Results. A comparison between the mode frequencies of the different models at various stages during the main sequence evolution reveals that they are more sensitive to a change in temperature gradient than to the exact shape of the mixing profile in the near-core region. Depending on the duration of the observed light curve, we can distinguish between either just the temperature gradient, or also between the shapes of the mixing coefficient. The relative frequency differences are in general larger for more evolved models and are largest for the higher frequency pressure modes in β Cep stars. Conclusions. In order to unravel the core boundary mixing and thermal structure of the near-core region, we must have asteroseismic masses and radii with ∼1% relative precision for hundreds of stars.

Chromospheric activity as a function of age in main-sequence stars

1966 ◽  
Vol 24 ◽  
pp. 40-43
Author(s):  
O. C. Wilson ◽  
A. Skumanich
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Main Sequence Stars ◽  
Tentative Conclusion ◽  
Chromospheric Activity ◽  
General Field ◽  
Large Clusters

Evidence previously presented by one of the authors (1) suggests strongly that chromospheric activity decreases with age in main sequence stars. This tentative conclusion rests principally upon a comparison of the members of large clusters (Hyades, Praesepe, Pleiades) with non-cluster objects in the general field, including the Sun. It is at least conceivable, however, that cluster and non-cluster stars might differ in some fundamental fashion which could influence the degree of chromospheric activity, and that the observed differences in chromospheric activity would then be attributable to the circumstances of stellar origin rather than to age.

scholarly journals Mass-loss varying luminosity and its implication to the solar evolution

2019 ◽  
Vol 15 (S356) ◽  
pp. 403-404
Author(s):  
Negessa Tilahun Shukure ◽  
Solomon Belay Tessema ◽  
Endalkachew Mengistu
Keyword(s):  
Standard Model ◽  
Mass Loss ◽  
Main Sequence ◽  
The Sun ◽  
Solar Mass ◽  
Solar Luminosity ◽  
Solar Evolution

AbstractSeveral models of the solar luminosity, , in the evolutionary timescale, have been computed as a function of time. However, the solar mass-loss, , is one of the drivers of variation in this timescale. The purpose of this study is to model mass-loss varying solar luminosity, , and to predict the luminosity variation before it leaves the main sequence. We numerically computed the up to 4.9 Gyrs from now. We used the solution to compute the modeled . We then validated our model with the current solar standard model (SSM). The shows consistency up to 8 Gyrs. At about 8.85 Gyrs, the Sun loses 28% of its mass and its luminosity increased to 2.2. The model suggests that the total main sequence lifetime is nearly 9 Gyrs. The model explains well the stage at which the Sun exhausts its central supply of hydrogen and when it will be ready to leave the main sequence. It may also explain the fate of the Sun by making some improvements in comparison to previous models.

scholarly journals Solar structure and evolution

2021 ◽  
Vol 18 (1) ◽  
Author(s):  
Jørgen Christensen-Dalsgaard
Keyword(s):  
Solar Surface ◽  
Solar Neutrinos ◽  
Solar Interior ◽  
Stellar Structure ◽  
The Sun ◽  
Physical Processes ◽  
Solar Evolution ◽  
Internal Properties ◽  
Atmospheric Phenomena

AbstractThe Sun provides a critical benchmark for the general study of stellar structure and evolution. Also, knowledge about the internal properties of the Sun is important for the understanding of solar atmospheric phenomena, including the solar magnetic cycle. Here I provide a brief overview of the theory of stellar structure and evolution, including the physical processes and parameters that are involved. This is followed by a discussion of solar evolution, extending from the birth to the latest stages. As a background for the interpretation of observations related to the solar interior I provide a rather extensive analysis of the sensitivity of solar models to the assumptions underlying their calculation. I then discuss the detailed information about the solar interior that has become available through helioseismic investigations and the detection of solar neutrinos, with further constraints provided by the observed abundances of the lightest elements. Revisions in the determination of the solar surface abundances have led to increased discrepancies, discussed in some detail, between the observational inferences and solar models. I finally briefly address the relation of the Sun to other similar stars and the prospects for asteroseismic investigations of stellar structure and evolution.

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