scholarly journals Testing the entropy calibration of the radii of cool stars: models of α Centauri A and B

2019 ◽  
Author(s):  
Federico Spada ◽  
Pierre Demarque
Keyword(s):  
Stellar Evolution ◽  
Ad Hoc ◽  
Calibration Method ◽  
Radiation Hydrodynamics ◽  
Mixing Length ◽  
Specific Entropy ◽  
Cool Stars ◽  
Host Stars ◽  
Comparable Quality ◽  
Temperature Surface

Abstract We present models of α Centauri A and B implementing an entropy calibration of the mixing-length parameter αMLT, recently developed and successfully applied to the Sun (Spada et al. 2018, ApJ, 869, 135). In this technique the value of αMLT in the 1D stellar evolution code is calibrated to match the adiabatic specific entropy derived from 3D radiation-hydrodynamics simulations of stellar convective envelopes, whose effective temperature, surface gravity, and metallicity are selected consistently along the evolutionary track. The customary treatment of convection in stellar evolution models relies on a constant, solar-calibrated αMLT. There is, however, mounting evidence that this procedure does not reproduce the observed radii of cool stars satisfactorily. For instance, modelling α Cen A and B requires an ad-hoc tuning of αMLT to distinct, non-solar values. The entropy-calibrated models of α Cen A and B reproduce their observed radii within $1\%$ (or better) without externally adjusted parameters. The fit is of comparable quality to that of models with freely adjusted αMLT for α Cen B (within 1 σ), while it is less satisfactory for α Cen A (within ≈2.5 σ). This level of accuracy is consistent with the intrinsic uncertainties of the method. Our results demonstrate the capability of the entropy calibration method to produce stellar models with radii accurate within $1\%$. This is especially relevant in characterising exoplanet-host stars and their planetary systems accurately.

scholarly journals Planetary nebula abundance determinations: A view from 1D-RHD simulations

2011 ◽  
Vol 7 (S283) ◽  
pp. 398-399
Author(s):  
Ralf Jacob ◽  
Christer Sandin ◽  
Detlef Schönberner ◽  
Matthias Steffen
Keyword(s):  
Chemical Composition ◽  
Stellar Evolution ◽  
Plasma Diagnostics ◽  
Planetary Nebula ◽  
Ad Hoc ◽  
Radiation Hydrodynamics ◽  
Self Consistent ◽  
Formation And Evolution ◽  
Determination Methods ◽  
Abundance Determination

AbstractIn the last years (metallicity-dependent) radiation-hydrodynamics simulations have become a powerful tool to understand the formation and evolution of PNe in terms of simple morphologies and kinematics. Contrary to photoionization models, with their ad-hoc assumptions on structure and physics, the RHD models are self-consistent with respect to their density distribution, velocity field, chemical composition, and stellar evolution. We use our models as simple proxies for real PNe and investigate the reliability of common abundance determination methods, which are based on either plasma diagnostics or static photoionization (PI) models.

scholarly journals The effects of stellar winds and magnetic fields on exoplanets

2013 ◽  
Vol 9 (S302) ◽  
pp. 228-236 ◽  
Author(s):  
A. A. Vidotto
Keyword(s):  
Solar System ◽  
Interplanetary Medium ◽  
Great Majority ◽  
Stellar Winds ◽  
Stellar Rotation ◽  
Low Mass Stars ◽  
Exoplanetary Systems ◽  
Cool Stars ◽  
Low Mass ◽  
Host Stars

AbstractThe great majority of exoplanets discovered so far are orbiting cool, low-mass stars whose properties are relatively similar to the Sun. However, the stellar magnetism of these stars can be significantly different from the solar one, both in topology and intensity. In addition, due to the present-day technology used in exoplanetary searches, most of the currently known exoplanets are found orbiting at extremely close distances to their host stars (< 0.1 au). The dramatic differences in stellar magnetism and orbital radius can make the interplanetary medium of exoplanetary systems remarkably distinct from that of the Solar System. To constrain interactions between exoplanets and their host-star's magnetised winds and to characterise the interplanetary medium that surrounds exoplanets, more realistic stellar wind models, which account for factors such as stellar rotation and the complex stellar magnetic field configurations of cool stars, must be employed. Here, I briefly review the latest progress made in data-driven modelling of magnetised stellar winds. I also show that the interaction of the stellar winds with exoplanets can lead to several observable signatures, some of which that are absent in our own Solar System.

scholarly journals An Improved Calibration of the Mixing-Length Based on Simulations of Solar-Type Convection

1998 ◽  
Vol 185 ◽  
pp. 115-116
Author(s):  
H.-G. Ludwig ◽  
B. Freytag ◽  
M. Steffen
Keyword(s):  
Thermal Structure ◽  
Time Dependent ◽  
Radiation Hydrodynamics ◽  
Mixing Length ◽  
Surface Layers ◽  
Stellar Envelope ◽  
Hydrodynamical Models ◽  
Solar Type ◽  
Effective Temperatures

Based on detailed 2D numerical radiation hydrodynamics (RHD) calculations of time-dependent compressible convection, we have studied the dynamics and thermal structure of the convective surface layers of stars in the range of effective temperatures and gravities between 4500 K ≤ Teff ≤ 7100 K and 2.54 ≤ log g ≤ 4.74. Although our hydrodynamical models describe only the shallow, strongly superadiabatic layers at the top of the convective stellar envelope, they provide information about the value of the entropy s∗ of the deeper, adiabatically stratified regions. E.g. in the solar case the helioseismically measured entropy jump is predicted within 9% of its actual value.

scholarly journals New models for the convective flux in stellar atmospheres

1996 ◽  
Vol 176 ◽  
pp. 557-564 ◽  
Author(s):  
F. Kupka
Keyword(s):  
Stellar Evolution ◽  
Stellar Atmosphere ◽  
Stellar Atmospheres ◽  
Turbulence Theory ◽  
Mixing Length ◽  
Convective Flux ◽  
The Past ◽  
New Models ◽  
F Stars ◽  
Mixing Length Theory

Over the past decades various forms of the mixing length theory (MLT) have been used to describe convection in stellar atmospheres. Recent advances in turbulence theory now allow for major improvements in modelling thermal convection. We review several models for convection which have been derived from turbulence theory, and describe one of them, the “CM model”, in detail. The CM model has been used in several stellar evolution and helioseismology codes during the last four years and has now been applied to model atmospheres. An overwiew comparing stellar atmosphere models based on the CM formulation with its MLT predecessors indicates improvements on model atmospheres for A and F stars.

scholarly journals Planets spinning up their host stars: a twist on the age-activity relationship

2013 ◽  
Vol 9 (S302) ◽  
pp. 239-242
Author(s):  
K. Poppenhaeger ◽  
S. J. Wolk
Keyword(s):  
Magnetic Activity ◽  
Negative Control ◽  
Detection Methods ◽  
Close Binaries ◽  
Activity Levels ◽  
Planet Detection ◽  
Hot Jupiters ◽  
Cool Stars ◽  
Host Stars ◽  
Hot Jupiter

AbstractIt is a long-standing question in exoplanet research if Hot Jupiters can influence the magnetic activity of their host stars. While cool stars usually spin down with age and become inactive, an input of angular momentum through tidal interaction, as seen for example in close binaries, can preserve high activity levels over time. This may also be the case for cool stars hosting a Hot Jupiter. However, selection effects from planet detection methods often dominate the activity levels seen in samples of exoplanet host stars, and planet-induced, systematically enhanced stellar activity has not been detected unambiguously so far. We have developed an approach to identify planet-induced stellar spin-up avoiding the selection biases from planet detection, by using visual proper motion binaries in which only one of the stars possesses a Hot Jupiter. This approach immediately rids one of the ambiguities of detection biases: with two co-eval stars, the second star acts as a negative control. We present results from our ongoing observational campaign at X-ray wavelengths and in the optical, and present several outstanding systems which display significant age/activity discrepancies presumably caused by their Hot Jupiters.

scholarly journals Magnetic fields of Sun-like stars

2013 ◽  
Vol 9 (S302) ◽  
pp. 180-189 ◽  
Author(s):  
Rim Fares
Keyword(s):  
Solar Cycle ◽  
Magnetic Fields ◽  
Stellar Evolution ◽  
Large Scale ◽  
Observational Constraints ◽  
Field Generation ◽  
Magnetic Cycles ◽  
Host Stars

AbstractMagnetic fields play an important role at all stages of stellar evolution. In Sun-like stars, they are generated in the outer convective layers. Studying the large-scale magnetic fields of these stars enlightens our understanding of the field properties and gives us observational constraints for the field generation models. In this review, I summarise the current observational picture of the large-scale magnetic fields of Sun-like stars, in particular solar-twins and planet-host stars. I will discuss the observations of large-scale magnetic cycles, and compare these to the solar cycle.

scholarly journals Atmospheric motions and granulation in cool stars - observations and theory

1997 ◽  
Vol 189 ◽  
pp. 239-244
Author(s):  
P.L. Cottrell
Keyword(s):  
Stellar Atmospheres ◽  
Fundamental Parameters ◽  
Cool Star ◽  
Cool Stars ◽  
Modelling Techniques ◽  
Atmospheric Motion ◽  
Simulated Images ◽  
Detailed Imaging ◽  
Temperature Surface

This review presents a number of aspects in our quest for an understanding of the detailed structure of cool star stellar atmospheres through the study of atmospheric motion. It includes the observational tools (e.g. direct imaging, detailed analyses of line profiles) used to determine the type and amount of motion, as well as the modelling techniques (e.g. simulated images, velocity and pressure fields) used to interpret and describe these motions.Linkages will be made through the solar-stellar connection to provide insight into the structure of stars where detailed imaging is not possible, and to the determination of the fundamental parameters of stars, such as effective temperature, surface gravity and elemental abundances.

Prediction of Turbulent Source Flow Between Corotating Disks With an Anisotropic Two-Equation Turbulence Model

1988 ◽  
Vol 110 (2) ◽  
pp. 187-194 ◽  
Author(s):  
S. A. Shirazi ◽  
C. R. Truman
Keyword(s):  
Reynolds Number ◽  
Kinetic Energy ◽  
Turbulent Kinetic Energy ◽  
Turbulence Model ◽  
Ad Hoc ◽  
Low Reynolds Number ◽  
Wall Region ◽  
Mixing Length ◽  
Mean Flow ◽  
Low Reynolds

An anisotropic form of a low-Reynolds-number two-equation turbulence model has been implemented in a numerical solution for incompressible turbulent flow between corotating parallel disks. Transport equations for turbulent kinetic energy and dissipation rate were solved simultaneously with the governing equations for the mean-flow variables. Comparisons with earlier mixing-length predictions and with measurements are presented. Good agreement between the present predictions and the measurements of velocity components and turbulent kinetic energy was obtained. The low-Reynolds-number two-equation model was found to model adequately the near-wall region as well as the effects of rotation and streamline divergence, which required ad hoc assumptions in the mixing-length model.

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