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 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.

scholarly journals The rotational evolution of young low mass stars

2007 ◽  
Vol 3 (S243) ◽  
pp. 231-240 ◽  
Author(s):  
Jérôme Bouvier
Keyword(s):  
Angular Momentum ◽  
Accretion Disk ◽  
Main Sequence ◽  
Young Stars ◽  
Sequence Evolution ◽  
Main Sequence Stars ◽  
Low Mass Stars ◽  
Rotational Evolution ◽  
Low Mass

AbstractStar-disk interaction is thought to drive the angular momentum evolution of young stars. In this review, I present the latest results obtained on the rotational properties of low mass and very low mass pre-main sequence stars. I discuss the evidence for extremely efficient angular momentum removal over the first few Myr of pre-main sequence evolution and describe recent results that support an accretion-driven braking mechanism. Angular momentum evolution models are presented and their implication for accretion disk lifetimes discussed.

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 Measuring the Masses of Young Stars

2001 ◽  
Vol 200 ◽  
pp. 454-463
Author(s):  
Michal Simon
Keyword(s):  
Main Sequence ◽  
Circumstellar Disks ◽  
Theoretical Models ◽  
Young Stars ◽  
Sequence Evolution ◽  
The Masses

This paper presents the masses of young stars measured by the rotation of their circumstellar disks (Simon, Dutrey, and Guilloteau 2000). Their precision is good enough to enable meaningful tests of theoretical models of pre-main sequence evolution. The tests are however limited by the imprecision with which the distances to the stars are known. The astrometric instruments now being developed have the potential to remove this limitation.

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 Constraints on stellar evolution theory from precise eclipsing binary data

1984 ◽  
Vol 105 ◽  
pp. 391-394
Author(s):  
J. Andersen ◽  
J.V. Clausen ◽  
H.E. J⊘rgensen ◽  
B. Nordström
Keyword(s):  
Stellar Evolution ◽  
Binary Data ◽  
Main Sequence ◽  
Theoretical Models ◽  
Stellar Structure ◽  
Sequence Evolution ◽  
Evolution Theory ◽  
Eclipsing Binary ◽  
Standard Models ◽  
Structure Calculations

Previous attempts at a detailed confrontation of eclipsing binary data with theoretical models of main-sequence evolution were faced with the choice between data of inhomogeneous (mostly low) quality for many systems (Kriz, 1969; Lacy, 1979) or accurate values of mass, radius, and temperature (or luminosity) for very few systems only (Popper et al., 1970). In addition, more detailed and homogeneous stellar structure calculations for several compositions were needed. Since 1972, a coordinated photometric and spectroscopic programme at our institute contributes to building a sufficient observational basis for such a test. Among published standard models for the range 1–10 M⊙, Hejlesen's (1980) are the most extensive, agree well with other standard models, and are presented in a format suitable for comparison with binary data. Here we can only outline a few salient new results from this study.

scholarly journals Impact of Water-latent Heat on the Thermal Structure of Ultra-cool Objects: Brown Dwarfs and Free-floating Planets

2021 ◽  
Vol 922 (1) ◽  
pp. 26
Author(s):  
Shih-Yun Tang ◽  
Tyler D. Robinson ◽  
Mark S. Marley ◽  
Natasha E. Batalha ◽  
Roxana Lupu ◽  
...  
Keyword(s):  
Solar System ◽  
Latent Heat ◽  
Thermal Structure ◽  
Brown Dwarfs ◽  
Stellar Atmospheres ◽  
Strong Impact ◽  
Moist Convection ◽  
Atmospheric Conditions ◽  
Wide Range ◽  
Effective Temperatures

Abstract Brown dwarfs are essential targets for understanding planetary and sub-stellar atmospheres across a wide range of thermal and chemical conditions. As surveys continue to probe ever deeper and as observing capabilities continue to improve, the number of known Y dwarfs—the coldest class of sub-stellar objects, with effective temperatures below about 600 K—is rapidly growing. Critically, this class of ultra-cool objects has atmospheric conditions that overlap with solar-system worlds and, as a result, tools and ideas developed from studying Earth, Jupiter, Saturn, and other nearby worlds are well suited for application to sub-stellar atmospheres. To that end, we developed a one-dimensional (vertical) atmospheric structure model for ultra-cool objects that includes moist adiabatic convection, as this is an important process for many solar-system planets. Application of this model across a range of effective temperatures (350, 300, 250, 200 K), metallicities ([M/H] of 0.0, 0.5, 0.7, 1.5), and gravities (log g of 4.0, 4.5, 4.7, 5.0) demonstrates strong impact of water-latent heat release on simulated temperature-pressure profiles. At the highest metallicities, water-vapor mixing ratios reach an Earth-like 3% with associated major alterations to the thermal structure in the atmospheric regions where water condenses. Spectroscopic and photometric signatures of metallicity and moist convection should be readily detectable at near- and mid-infrared wavelengths, especially with James Webb Space Telescope observations, and can help indicate the formation history of an object.

scholarly journals Effect of accretion on the pre-main-sequence evolution of low-mass stars and brown dwarfs

2017 ◽  
Vol 605 ◽  
pp. A77 ◽  
Author(s):  
Eduard I. Vorobyov ◽  
Vardan Elbakyan ◽  
Takashi Hosokawa ◽  
Yuya Sakurai ◽  
Manuel Guedel ◽  
...  
Keyword(s):  
Main Sequence ◽  
Brown Dwarfs ◽  
Sequence Evolution ◽  
Low Mass Stars ◽  
Low Mass

scholarly journals The Effects of Rotation on the Atmospheres of Early-Type Main-Sequence Stars (Review Paper)

1970 ◽  
Vol 4 ◽  
pp. 85-109
Author(s):  
George W. Collins
Keyword(s):  
Angular Momentum ◽  
Total Angular Momentum ◽  
Main Sequence ◽  
Initial Conditions ◽  
Dominant Role ◽  
Axial Rotation ◽  
Stellar Atmospheres ◽  
Stellar Structure ◽  
Theoretical Development ◽  
Dominant Factor

In discussing the status of the theory of rotating stellar atmospheres, it is necessary to draw upon the contributions of many well established aspects of astrophysics and to interconnect them in a cohesive pattern structured so as to provide insight into a rather specific problem – namely, the structure and characteristics of a surface of the star undergoing axial rotation. Many different connections are possible having varying degrees of emphasis and, of necessity, those given here represent only one such presentation. The discussion could be much simplified if it were not necessary to test the efficacy of the theoretical development by referring to observations. Unfortunately, such a comparison is necessary and the results are at the moment somewhat inconclusive. This unhappy situation arises from the retrospectively obvious fact that axial rotation does not play a dominant role in determining the directly observable properties of stars. Indeed, if rotation were a dominant factor, earlier attempts at describing stellar structure and evolution would have met with little success. However, it is becoming increasingly clear that the structure and final evolution of highly evolved stars are greatly influenced by the total angular momentum which they retain from their earlier history. In order to understand the angular momentum distribution present in the final state, it is necessary to understand the effects of stellar evolution on the total angular momentum and its distribution. But even before this step can be taken, one must first successfully describe the rotational structure of the main-sequence phase as it is this state which provides the initial conditions necessary for any further study. It is further appropriate that we attempt to describe this period of a star’s life as the largest body of observational material with which we must test our results exists for these stars. In addition, we should expect a study of the atmospheric structure to be the most fruitful as this is the region of the star which provides the final modification of the radiation we observe.

Sign in / Sign up

Export Citation Format

Share Document