scholarly journals 2. Manifestations of the Interaction between Convection and Rotation in Stellar Atmospheres (Lee Hartmann)

1985 ◽  
Vol 19 (1) ◽  
pp. 507-510
Keyword(s):  
Magnetic Fields ◽  
Internal Structure ◽  
Recent Progress ◽  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Stellar Rotation ◽  
Surface Rotation ◽  
Solar Type ◽  
Global Parameters

According to dynamo theories, the interaction between convection and rotation is responsible for the generation of magnetic fields in solar-type stars. Stellar observations can explore the ways the global parameters, like the surface rotation and internal structure, affect the production of magnetic fields. In this very brief review I wish to concentrate on recent progress in understanding the evolution of stellar rotation, and the dependence of stellar cycles and magnetic activity on rotation, internal structure, and age.

Solar Rotation, Irradiance Changes and Climate

1994 ◽  
Vol 143 ◽  
pp. 244-251
Author(s):  
Elizabeth Nesme-Ribes ◽  
Dmitry Sokoloff ◽  
Robert Sadourny
Keyword(s):  
Magnetic Fields ◽  
Internal Rotation ◽  
Solar Rotation ◽  
Total Solar Irradiance ◽  
Magnetic Activity ◽  
Convective Envelope ◽  
Surface Rotation ◽  
Solar Type ◽  
Activity Cycles ◽  
The Impact

Magnetic activity cycles for solar-type stars are believed to originate from non-uniform internal rotation. To determine this depthwise angular velocity distribution, helioseismology is a valuable source of information. Surface rotation, as traced by sunspot motion, is a well-observed parameter with data going back to the beginning of the telescopic era. This long sunspot series can be used in understanding the behaviour of the Sun’s surface rotation, the connection with its internal rotation, and thereby its magnetic activity. Apparent solar diameter is another important parameter. This is related to the structure of the convective envelope and how it reacts to the presence of magnetic fields. Both these parameters are related to the solar output, and can provide a surrogate for total solar irradiance, by way of a theoretical modeling of the response of the convective zone to the emergence of periodic magnetic fields. The impact of solar variability on the terrestrial climate is also addressed.

scholarly journals Dynamo Action in Evolved Stars

1991 ◽  
Vol 130 ◽  
pp. 336-341
Author(s):  
David F. Gray
Keyword(s):  
Angular Momentum ◽  
Magnetic Fields ◽  
Rotation Rate ◽  
Magnetic Activity ◽  
Dynamo Action ◽  
Rotation Rates ◽  
Evolved Stars ◽  
Solar Type ◽  
Limiting Value

AbstractEvolved stars tell us a great deal about dynamos. The granulation boundary shows us where solar-type convection begins. Since activity indicators also start at this boundary, it is a good bet that solar-type convection is an integral part of dynamo activity for all stars. The rotation boundary tells us where the magnetic fields of dynamos become effective in dissipating angular momentum, and rotation beyond the boundary tells us the limiting value needed for a dynamo to function. The observed uniqueness of rotation rates after the rotation boundary is crossed can be understood through the rotostat hypothesis. Quite apart from the reason for the unique rotation rate, its existence can be used to show that magnetic activity of giants is concentrated to the equatorial latitudes, as it is in the solar case. The coronal boundary in the H-R diagram is probably nothing more than a map of where rotation becomes too low to sustain dynamo activity.

scholarly journals Stellar magnetic activity and their influence on the habitability of exoplanets

2014 ◽  
Vol 10 (S305) ◽  
pp. 333-339
Author(s):  
T. Lüftinger ◽  
M. Güdel ◽  
C. Johnstone
Keyword(s):  
Magnetic Fields ◽  
International Research ◽  
Planetary Systems ◽  
Magnetic Activity ◽  
Doppler Imaging ◽  
Research Network ◽  
Stellar Rotation ◽  
Theoretical Understanding ◽  
Points Of View ◽  
Stellar Magnetic Fields

AbstractStellar magnetism, explorable via polarimetry, is a crucial driver of activity, ionization, photodissociation, chemistry and winds in stellar environments. Thus it has an important impact on the atmospheres and magnetospheres of surrounding planets. Modeling of stellar magnetic fields and their winds is extremely challenging, both from the observational and the theoretical points of view, and only recent ground breaking advances in observational instrumentation - as were discussed during this Symposium - and a deeper theoretical understanding of magnetohydrodynamic processes in stars enable us to model stellar magnetic fields and winds and the resulting influence on surrounding planets in more and more detail. We have initiated a national and international research network (NFN): ‘Pathways to Habitability - From Disks to Active Stars, Planets to Life’, to address questions on the formation and habitability of environments in young, active stellar/planetary systems. In this contribution we discuss the work we are carrying out within this project and focus on how stellar magnetic fields, their winds and the relation to stellar rotation can be assessed observationally with relevant techniques such as Zeeman Doppler Imaging (ZDI), field extrapolation and wind simulations.

scholarly journals The Solar-Stellar Connection and Disconnection

2004 ◽  
Vol 219 ◽  
pp. 11-28 ◽  
Author(s):  
Klaus G. Strassmeier
Keyword(s):  
Magnetic Field ◽  
Meridional Circulation ◽  
Magnetic Activity ◽  
Driving Mechanism ◽  
The Sun ◽  
Dynamo Action ◽  
Convective Envelope ◽  
Surface Rotation ◽  
Single Process ◽  
Solar Type

The study of stellar activity is now an almost classical astronomical topic. The first Ca ii-H&K observations were made a hundred years ago by Eberhard & Schwarzschild1 and many thousand papers were published after its rediscovery some three decades ago by O. C. Wilson. The complexity of the atmospheric and interior magnetic activity as observed on the Sun is hard, if not impossible, to extrapolate to solar-type stars. So far there is no solar twin found, despite that it appears that just a single process acts as the driving mechanism for activity in all atmospheric layers and partially even in the convective envelope: the dynamodriven magnetic field. In this paper, I will try to give examples where the solar analogy holds and where it is clearly not appropriate, putting some emphases on differential surface rotation and meridional circulation. I stress the importance of mapping stellar surfaces as fingerprints of the underlying dynamo action and directly measure surface magnetic fields.

scholarly journals Magnetic Activity and Rotation in Brown Dwarfs and Low Mass Stars

2003 ◽  
Vol 211 ◽  
pp. 427-435 ◽  
Author(s):  
Gibor Basri ◽  
Subanjoy Mohanty
Keyword(s):  
Theoretical Work ◽  
Magnetic Activity ◽  
Stellar Rotation ◽  
Low Mass Stars ◽  
Strong Activity ◽  
Substellar Objects ◽  
Substantial Progress ◽  
Solar Type ◽  
Low Mass ◽  
M Stars

One of the triumphs of the last 2 decades has been the establishment of the relation between stellar rotation and magnetic activity in solar-type stars. Rapid rotation produces strong activity, which in turn provides magnetic braking to reduce rotation. A solar-type dynamo cannot operate in fully convective stars, so it is of interest to study mid and late M stars. Hints that a dramatic change occurs in very low-mass stars and substellar objects appeared in 1995. The past 7 years have seen substantial progress on this question, with the conclusion that the rotation-activity connection indeed breaks down. As one goes to the bottom of the main sequence and below, the amount of magnetic activity takes a sudden fall, with a concomitant increase in the spindown times of the objects. We summarize these results, and some theoretical work which helps explain them. We also present some remaining mysteries, such as why very young objects seem excessively active, and flaring in objects with no other signs of magnetic activity.

scholarly journals Forward modelling of Kepler-band variability due to faculae and spots

2021 ◽  
Author(s):  
Luke J Johnson ◽  
Charlotte M Norris ◽  
Yvonne C Unruh ◽  
Sami K Solanki ◽  
Natalie Krivova ◽  
...  
Keyword(s):  
Stellar Atmosphere ◽  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Area Coverage ◽  
Magnetic Flux Density ◽  
Model Parameters ◽  
Spot Area ◽  
Solar Type ◽  
Limb Darkening ◽  
Spot Temperature

Abstract Variability observed in photometric lightcurves of late-type stars (on timescales longer than a day) is a dominant noise source in exoplanet surveys and results predominantly from surface manifestations of stellar magnetic activity, namely faculae and spots. The implementation of faculae in lightcurve models is an open problem, with scaling typically based on spectra equivalent to hot stellar atmospheres or assuming a solar-derived facular contrast. We modelled rotational (single period) lightcurves of active G2, K0, M0 and M2 stars, with Sun-like surface distributions and realistic limb-dependent contrasts for faculae and spots. The sensitivity of lightcurve variability to changes in model parameters such as stellar inclination, feature area coverage, spot temperature, facular region magnetic flux density and active band latitudes is explored. For our lightcurve modelling approach we used actress, a geometrically accurate model for stellar variability. actress generates 2-sphere maps representing stellar surfaces and populates them with user-prescribed spot and facular region distributions. From this, lightcurves can be calculated at any inclination. Quiet star limb darkening and limb-dependent facular contrasts were derived from MURaM 3D magnetoconvection simulations using ATLAS9. 1D stellar atmosphere models were used for the spot contrasts. We applied actress in Monte Carlo simulations, calculating lightcurve variability amplitudes in the Kepler band. We found that, for a given spectral type and stellar inclination, spot temperature and spot area coverage have the largest effect on variability of all simulation parameters. For a spot coverage of $1{{\ \rm per\ cent}}$, the typical variability of a solar-type star is around 2 parts-per-thousand. The presence of faculae clearly affects the mean brightness and lightcurve shape, but has relatively little influence on the variability.

scholarly journals Activity Tracers

1993 ◽  
Vol 137 ◽  
pp. 605-619
Author(s):  
Klaus G. Strassmeier
Keyword(s):  
Solar Activity ◽  
Magnetic Fields ◽  
Binary Stars ◽  
Main Sequence ◽  
Stellar Atmospheres ◽  
Late Type ◽  
Stellar Rotation ◽  
Strong Magnetic Fields ◽  
Rapid Rotation ◽  
Spatially Resolved

AbstractThe vast variety of solar-like phenomena on other late-type stars, so-called activity tracers, provide an important tool for studying the structure of active stellar atmospheres and their connection to the stellar interior via strong magnetic fields. These “chromospherically active” stars include single and binary stars as well as pre- and post-main sequence objects and have rapid rotation and deep convective layers in common. They serve as astrophysical laboratories to study the vast phenomenology of activity tracers: starspots, plages, flares, prominences, which might be seen as enhanced analogs of solar activity and could be spatially resolved due to rotationally modulated indicators. In this paper we review the current observational material and discuss its impact on our knowledge of “active” atmospheres, especially in the context of stellar rotation.

scholarly journals Surface Magnetic Fields in Early-Type Stars

2000 ◽  
Vol 175 ◽  
pp. 334-336 ◽  
Author(s):  
V. S. Airapetian
Keyword(s):  
Magnetic Fields ◽  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Early Type ◽  
Inertial Oscillations ◽  
Early Type Stars ◽  
Stressed Surface

AbstractRecent observations imply magnetic activity in atmospheres of early-type stars. We explore the possibility that stressed surface magnetic fields can be driven by inertial oscillations, such as r-modes which are vorticity waves. We show that vorticIAL MOTIOns are able to supply helicity to drive magnetic activity in stellar atmospheres.

scholarly journals Effects of rotation and magnetic fields on the structure and surface abundances of solar-type stars

2009 ◽  
Vol 5 (S268) ◽  
pp. 381-386 ◽  
Author(s):  
P. Eggenberger ◽  
A. Maeder ◽  
G. Meynet
Keyword(s):  
Magnetic Fields ◽  
Internal Structure ◽  
Solid Body ◽  
Body Rotation ◽  
Solid Body Rotation ◽  
History Of ◽  
Solar Type ◽  
Effects Of Rotation ◽  
Host Stars ◽  
Hr Diagram

AbstractThe effects of shellular rotation on the modelling of solar-type stars (in particular internal structure, evolutionary tracks in the HR diagram, lifetimes and surface abundances) are first examined. Then the effects of a dynamo possibly occuring in the internal stellar radiative zone by imposing nearly solid body rotation are studied. These results are finally discussed in the context of the rotational history of exoplanet host stars and the link between lithium depletion and the presence of exoplanets.

scholarly journals Evolution of Stellar Magnetic Fields

2012 ◽  
Vol 10 (H16) ◽  
pp. 90-91
Author(s):  
Manuel Güdel
Keyword(s):  
Magnetic Fields ◽  
Magnetic Activity ◽  
Stellar Atmospheres ◽  
Dynamic Processes ◽  
Radio Luminosity ◽  
Physical Processes ◽  
X Ray ◽  
Stellar Magnetic Fields

AbstractStellar magnetic fields can reliably be characterized by several magnetic activity indicators, such as X-ray or radio luminosity. Physical processes leading to such emission provide important information on dynamic processes in stellar atmospheres and magnetic structuring.

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