scholarly journals The open flux evolution of a solar-mass star on the main sequence

2017 ◽  
Vol 474 (1) ◽  
pp. 536-546 ◽  
Author(s):  
V. See ◽  
M. Jardine ◽  
A. A. Vidotto ◽  
J.-F. Donati ◽  
S. Boro Saikia ◽  
...  
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Source Surface ◽  
Stellar Rotation ◽  
Solar Mass ◽  
Field Geometry ◽  
Open Flux

Abstract Magnetic activity is known to be correlated to the rotation period for moderately active main-sequence solar-like stars. In turn, the stellar rotation period evolves as a result of magnetized stellar winds that carry away angular momentum. Understanding the interplay between magnetic activity and stellar rotation is therefore a central task for stellar astrophysics. Angular momentum evolution models typically employ spin-down torques that are formulated in terms of the surface magnetic field strength. However, these formulations fail to account for the magnetic field geometry, unlike those that are expressed in terms of the open flux, i.e. the magnetic flux along which stellar winds flow. In this work, we model the angular momentum evolution of main-sequence solar-mass stars using a torque law formulated in terms of the open flux. This is done using a potential field source surface model in conjunction with the Zeeman–Doppler magnetograms of a sample of roughly solar-mass stars. We explore how the open flux of these stars varies with stellar rotation and choice of source surface radii. We also explore the effect of field geometry by using two methods of determining the open flux. The first method only accounts for the dipole component while the second accounts for the full set of spherical harmonics available in the Zeeman–Doppler magnetogram. We find only a small difference between the two methods, demonstrating that the open flux, and indeed the spin-down, of main-sequence solar-mass stars is likely dominated by the dipolar component of the magnetic field.

scholarly journals On the Loss of Angular Momentum from Stars in the Pre-Main Sequence Phase

1970 ◽  
Vol 4 ◽  
pp. 73-81
Author(s):  
Isao Okamoto
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Main Sequence ◽  
Magnetic Energy ◽  
Rotational Velocity ◽  
The State ◽  
Rotational Instability ◽  
Stellar Rotation ◽  
Initial State ◽  
Initial Magnetic Field

AbstractThe braking of stellar rotation in the wholly convective phase in the pre-main sequence is numerically discussed. The structure of stars in that phase is expressed by a rotating polytrope with an index of 1.5 and the Schatzman-type mechanism is used as the means of loss of angular momentum. The magnetic energy is assumed to change with evolution as H02/8π(R/R0)s, where H0 and R0 are initial magnetic field and radius, and s is a free parameter. The changes of angular momentum, rotational velocity, etc. with contraction are calculated from the initial state, which is taken to be the state when the stars flared up to the Helmholtz-Kelvin contraction. It is shown that the exponent s must be in the range from – 1 to – 3 so that the stars with adequate strength of the initial magnetic field may lose almost all of their angular momenta in a suitable rate if they are initially in the state of rotational instability.Stellar rotation from the time of star formation to the main sequence stage is discussed. Also, the formation of the solar system and other planetary systems is discussed, with respect to the braking.

scholarly journals Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

2018 ◽  
Vol 618 ◽  
pp. A48 ◽  
Author(s):  
M. Mittag ◽  
J. H. M. M. Schmitt ◽  
K.-P. Schröder
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Magnetic Activity ◽  
Turnover Time ◽  
Rossby Number ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Rotation Periods ◽  
Period Distribution ◽  
Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

scholarly journals The Rotation of Low-Mass Pre-Main-Sequence Stars

2004 ◽  
Vol 215 ◽  
pp. 113-122 ◽  
Author(s):  
Robert D. Mathieu
Keyword(s):  
Angular Momentum ◽  
Main Sequence ◽  
Rotation Period ◽  
Rotating Stars ◽  
Stellar Rotation ◽  
Main Sequence Stars ◽  
Star Forming ◽  
Rotation Periods ◽  
Order Of Magnitude ◽  
Critical Velocities

Major photometric monitoring campaigns of star-forming regions in the past decade have provided rich rotation period distributions of pre-main-sequence stars. The rotation periods span more than an order of magnitude in period, with most falling between 1 and 10 days. Thus the broad rotation period distributions found in 100 Myr clusters are already established by an age of 1 Myr. The most rapidly rotating stars are within a factor of 2-3 of their critical velocities; if angular momentum is conserved as they evolve to the ZAMS, these stars may come to exceed their critical velocities. Extensive efforts have been made to find connections between stellar rotation and the presence of protostellar disks; at best only a weak correlation has been found in the largest samples. Magnetic disk-locking is a theoretically attractive mechanism for angular momentum evolution of young stars, but the links between theoretical predictions and observational evidence remain ambiguous. Detailed observational and theoretical studies of the magnetospheric environments will provide better insight into the processes of pre-main-sequence stellar angular momentum evolution.

scholarly journals The Sun Through Time

2020 ◽  
Vol 216 (8) ◽  
Author(s):  
Manuel Güdel
Keyword(s):  
Stellar Wind ◽  
Rotation Period ◽  
High Energy ◽  
Magnetic Activity ◽  
Energy Output ◽  
The Sun ◽  
Stellar Rotation ◽  
Solar Mass ◽  
Rotation Periods ◽  
Rotational Evolution

AbstractMagnetic activity of stars like the Sun evolves in time because of spin-down owing to angular momentum removal by a magnetized stellar wind. These magnetic fields are generated by an internal dynamo driven by convection and differential rotation. Spin-down therefore converges at an age of about 700 Myr for solar-mass stars to values uniquely determined by the stellar mass and age. Before that time, however, rotation periods and their evolution depend on the initial rotation period of a star after it has lost its protostellar/protoplanetary disk. This non-unique rotational evolution implies similar non-unique evolutions for stellar winds and for the stellar high-energy output. I present a summary of evolutionary trends for stellar rotation, stellar wind mass loss and stellar high-energy output based on observations and models.

scholarly journals Magnetospheric accretion & outflows in stars & brown dwarfs: theories and observational constraints

2009 ◽  
Vol 5 (H15) ◽  
pp. 753-753
Author(s):  
S. Mohanty
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Hot Spot ◽  
Brown Dwarfs ◽  
Field Measurements ◽  
Spot Size ◽  
Stellar Rotation ◽  
Magnetic Field Generation ◽  
T Tauri ◽  
Field Geometry

The manner in which young classical T Tauri stars (cTTs) and brown dwarfs accrete gas from their surrounding disks and simultaneously drive jets and outflows is central to star and planet formation and angular momentum evolution, but remains an ill-understood and hotly debated subject. One of the central concerns is the stellar field geometry: while analytic theories assume an idealized stellar dipole, T Tauri fields are observed to be complex multipolar beasts. I present an analytic generalization of the X-wind theory to include such fields. Independent of the precise field geometry, the generalized model makes a unique prediction about the relationship between various cTTs observables. I show that this prediction is supported by observations of accretion rate, hot spot size, stellar rotation and field strength from stellar to brown dwarf masses, including recent detailed spectropolarimetric measurements. I also discuss the unique insights offered by recent magnetic field measurements on accreting brown dwarfs: while they agree with the accretion theory above, they also pose a puzzle for magnetic field generation theory. Resolving this conundrum promises to illuminate our general picture of accretion and angular momentum transport in fully convective objects.

scholarly journals Search for Magnetic Stars at Early Stages of Evolution

1993 ◽  
Vol 137 ◽  
pp. 669-671
Author(s):  
Yu. V. Glagolevskij
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Rotation Velocity ◽  
Electric Vector ◽  
Magnetic Star ◽  
Stellar Rotation ◽  
Magnetic Stars ◽  
Hydrogen Lines ◽  
Direction Of Polarization ◽  
Gaseous Envelope

Young stars, as a rule, are too faint for measurements of magnetic field either by photographic method with the use of Zeeman analizer, or photoelectrically from hydrogen lines. That is why it is necessary to look for indirect ways of magnetic field detection, for example, by measurement of polarization. Ae/Be Herbig stars without a magnetic field are surrounded by a gaseous envelope in the form of a globe or a spheroid, flattened along the rotational axes (as dependent on stellar rotation velocity), and also by a gaseous-dust accretion disc in the plane of equator. There are powerful flows in gaseous envelopes of stars, connected with mass loss and accretion. If a star is a magnetic oblique rotator (as a magnetic star of the Main Sequence), then the gaseous envelope may acquire the shape of alon-gated ellipsoid with the major axes coincident with that of dipole (Dolginov et al., 1979). From the poles there arises a jet flow controlled by a magnetic field, as in He-r and He-w stars, having already reached the Main Sequence (Barker et al., 1982). Calculations show (Dolginov et al., 1979), that maximum polarization in the extended envelope p ≈ 4% arises when the ratio of ellipsoid axes is ≈ 2.5b. The electric vector of the dominating oscillation of the light wave is perpendicular to the plane through the axis of symmetry of the ellipsoid and the line of sight. Naturally, the magnetosphere rotates together with the star, involving the gaseous envelope, resulting in the variation of the degree and direction of polarization. Additional polarization is created by the polar jets, where the direction of the dominating oscillations of the electric vector is perpendicular to the axis of the polar stream, and value of maximal polarization may reach 5% along the beam.

scholarly journals Magnetic braking of supermassive stars through winds

2019 ◽  
Vol 623 ◽  
pp. L7 ◽  
Author(s):  
L. Haemmerlé ◽  
G. Meynet
Keyword(s):  
Magnetic Field ◽  
Angular Momentum ◽  
Formation Process ◽  
Magnetic Coupling ◽  
Rotation Velocity ◽  
Stellar Surface ◽  
Stellar Winds ◽  
Stellar Rotation ◽  
Magnetic Braking ◽  
The Magnetic Field

Context. Supermassive stars (SMSs) are candidates for being progenitors of supermassive quasars at high redshifts. However, their formation process requires strong mechanisms that would be able to extract the angular momentum of the gas that the SMSs accrete. Aims. We investigate under which conditions the magnetic coupling between an accreting SMS and its winds can remove enough angular momentum for accretion to proceed from a Keplerian disc. Methods. We numerically computed the rotational properties of accreting SMSs that rotate at the ΩΓ-limit and estimated the magnetic field that is required to maintain the rotation velocity at this limit using prescriptions from magnetohydrodynamical simulations of stellar winds. Results. We find that a magnetic field of 10 kG at the stellar surface is required to satisfy the constraints on stellar rotation from the ΩΓ-limit. Conclusions. Magnetic coupling between the envelope of SMSs and their winds could allow for SMS formation by accretion from a Keplerian disc, provided the magnetic field is at the upper end of present-day observed stellar fields. Such fields are consistent with primordial origins.

scholarly journals 3D Modeling of the Structure and Dynamics of a Main-Sequence F-type Star

2019 ◽  
Vol 15 (S354) ◽  
pp. 86-93
Author(s):  
Irina N. Kitiashvili ◽  
Alan A. Wray
Keyword(s):  
Main Sequence ◽  
Initial Approximation ◽  
Stellar Structure ◽  
Computational Domain ◽  
Mixing Length ◽  
Stellar Rotation ◽  
Solar Mass ◽  
Turbulent Layer ◽  
Near Surface ◽  
Structure And Dynamics

AbstractCurrent state-of-the-art computational modeling makes it possible to build realistic models of stellar convection zones and atmospheres that take into account chemical composition, radiative effects, ionization, and turbulence. The standard 1D mixing-length-based evolutionary models are not able to capture many physical processes of the stellar interior dynamics. Mixing-length models provide an initial approximation of stellar structure that can be used to initialize 3D radiative hydrodynamics simulations which include realistic modeling of turbulence, radiation, and other phenomena.In this paper, we present 3D radiative hydrodynamic simulations of an F-type main-sequence star with 1.47 solar mass. The computational domain includes the upper layers of the radiation zone, the entire convection zone, and the photosphere. The effects of stellar rotation is modeled in the f-plane approximation. These simulations provide new insight into the properties of the convective overshoot region, the dynamics of the near-surface, highly turbulent layer, and the structure and dynamics of granulation. They reveal solar-type differential rotation and latitudinal dependence of the tachocline location.

scholarly journals Age-related observations of low mass pre-main and young main sequence stars

2008 ◽  
Vol 4 (S258) ◽  
pp. 81-94 ◽  
Author(s):  
Lynne A. Hillenbrand
Keyword(s):  
Main Sequence ◽  
Rotation Period ◽  
Age Dating ◽  
Main Sequence Stars ◽  
Solar Mass ◽  
Age Related ◽  
Stellar Ages ◽  
Dating Methods ◽  
Low Mass ◽  
Lithium Depletion

AbstractThis overview summarizes the age dating methods available for young sub-solar mass stars. Pre-main sequence age diagnostics include the Hertzsprung-Russell (HR) diagram, spectroscopic surface gravity indicators, and lithium depletion; asteroseismology is also showing recent promise. Near and beyond the zero-age main sequence, rotation period or vsiniand activity (coronal and chromospheric) diagnostics along with lithium depletion serve as age proxies. Other authors in this volume present more detail in each of the aforementioned areas. Herein, I focus on pre-main sequence HR diagrams and address the questions: Do empirical young cluster isochrones match theoretical isochrones? Do isochrones predict stellar ages consistent with those derived via other independent techniques? Do the observed apparent luminosity spreads at constant effective temperature correspond to true age spreads? While definitive answers to these questions are not provided, some methods of progression are outlined.

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