The interaction of rotation and magnetic field in the Solar System

During the early phases of star formation, torsional Alfven waves propagating along the locally distorted galactic magnetic field can transport away the bulk of the initial angular momentum of a condensation, so enabling it to contract to solar nebula dimensions. Enough primeval magnetic flux may be retained for magnetic redistribution of angular momentum to continue until the proto-Sun reaches the pre-Main Sequence Hayashi phase. A rotating star with a convective envelope has a dynamomaintained field which brakes the star through coupling to the stellar wind. Observational evidence from young star clusters and from T Tauri stars suggests that the zero-age Main Sequence Sun had about ten times its present angular momentum. The same braking process, scaled up because of the much more powerful T Tauri winds, can explain why the zero-age Sun had lost at least nine-tenths of the centrifugal upper limit, and is more acceptable than the suggestion that the ‘missing’ solar angular momentum has been magnetically fed into the planetary system.

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

International Astronomical Union Colloquium ◽

10.1017/s025292110002707x ◽

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.

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HII regions and young star clusters in the Magellanic Clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900200387 ◽

1991 ◽

Vol 148 ◽

pp. 207-208

Author(s):

Myung Gyoon Lee

Keyword(s):

Main Sequence ◽

Star Clusters ◽

Hii Regions ◽

Young Star ◽

Magellanic Clouds ◽

Stellar Content ◽

Ccd Photometry ◽

Luminosity Functions ◽

Young Star Clusters ◽

Mass Functions

Using U BV CCD photometry, the stellar content of HII regions and young star clusters in the Magellanic Clouds has been studied: (1) the reddenings have been determined, and ages of OB associations and young star clusters have been measured; (2) the stellar initial mass functions have been determined by using the main-sequence luminosity functions; and (3) U BV CCD surface photometry of nine young star clusters has been obtained and their structural properties investigated.

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Shape of a slowly rotating star measured by asteroseismology

Science Advances ◽

10.1126/sciadv.1601777 ◽

2016 ◽

Vol 2 (11) ◽

pp. e1601777 ◽

Cited By ~ 8

Author(s):

Laurent Gizon ◽

Takashi Sekii ◽

Masao Takata ◽

Donald W. Kurtz ◽

Hiromoto Shibahashi ◽

...

Keyword(s):

Magnetic Field ◽

Rotation Period ◽

Solar Radius ◽

Spherically Symmetric ◽

Convective Envelope ◽

Stellar Radius ◽

Nearby Stars ◽

The Difference ◽

Rotating Star ◽

The Magnetic Field

Stars are not perfectly spherically symmetric. They are deformed by rotation and magnetic fields. Until now, the study of stellar shapes has only been possible with optical interferometry for a few of the fastest-rotating nearby stars. We report an asteroseismic measurement, with much better precision than interferometry, of the asphericity of an A-type star with a rotation period of 100 days. Using the fact that different modes of oscillation probe different stellar latitudes, we infer a tiny but significant flattening of the star’s shape of ΔR/R = (1.8 ± 0.6) × 10−6. For a stellar radius R that is 2.24 times the solar radius, the difference in radius between the equator and the poles is ΔR = 3 ± 1 km. Because the observed ΔR/R is only one-third of the expected rotational oblateness, we conjecture the presence of a weak magnetic field on a star that does not have an extended convective envelope. This calls to question the origin of the magnetic field.

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Measuring T Tauri star magnetic fields

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309030713 ◽

2008 ◽

Vol 4 (S259) ◽

pp. 345-356 ◽

Cited By ~ 3

Author(s):

Christopher M. Johns–Krull

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Scaling Laws ◽

Main Sequence ◽

Critical Role ◽

X Rays ◽

Stellar Rotation ◽

T Tauri ◽

Dipole Component ◽

Stellar Magnetic Fields

AbstractStellar magnetic fields including a strong dipole component are believed to play a critical role in the early evolution of newly formed stars and their circumstellar accretion disks. It is currently believed that the stellar magnetic field truncates the accretion disk several stellar radii above the star. This action forces accreting material to flow along the field lines and accrete onto the star preferentially at high stellar latitudes. It is also thought that the stellar rotation rate becomes locked to the Keplerian velocity near the radius where the disk is truncated. This paper reviews recent efforts to measure the magnetic field properties of low mass pre-main sequence stars, focussing on how the observations compare with the theoretical expectations. A picture is emerging indicating that quite strong fields do indeed cover the majority of the surface on these stars; however, the dipole component of the field appears to be alarmingly small. The current measurements also suggest that given their strong magnetic fields, T Tauri stars are somewhat faint in X-rays relative to what is expected from simple main sequence star scaling laws.

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Comparisons between observational color-magnitude diagrams and synthetic cluster diagrams for young star clusters in the Magellanic Clouds

Symposium - International Astronomical Union ◽

10.1017/s0074180900030588 ◽

1984 ◽

Vol 105 ◽

pp. 83-87

Author(s):

Stephen A. Becker ◽

Grant J. Mathews ◽

Wendee M. Brunish

Keyword(s):

Stellar Evolution ◽

Main Sequence ◽

Star Clusters ◽

Theoretical Models ◽

Young Star ◽

Asymptotic Giant Branch ◽

Current Status ◽

Large Numbers ◽

Young Star Clusters ◽

Convective Overshoot

Young star clusters (<3 × 108 yr) in the Maqellanic Clouds (MC) can be used to test the current status of the theory of stellar evolution as applied to intermediate and massive stars. The color-magnitude diagram of many young clusters in the MC shows, unlike the case of clusters in our Galaxy, large numbers of stars in both the main sequence and post main sequence evolutionary phases. Usina a arid of stellar evolution models, synthetic cluster H-R diagrams are constructed and compared to observed color-magnitude diagrams to determine the age, age spread, and composition for any given cluster. In addition, for those cases where the data is of high quality, detailed comparisons between theory and observation can provide a diagnostic of the accuracy of the stellar evolution models. Initial indications of these comparisons suggest that the theoretical models should be altered to include: a larger value for the mixing length parameter (α), a larger rate of mass loss during the asymptotic giant branch (AGB) phase, and possibly convective overshoot during the core burning phases.

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Study of the equilibrium and motions of the medium in a rotating star with a magnetic field using tools from angular momentum theory

Technical Physics ◽

10.1134/1.1259460 ◽

1999 ◽

Vol 44 (9) ◽

pp. 1014-1017

Author(s):

Yu. V. Vandakurov

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Momentum Theory ◽

Angular Momentum Theory ◽

Rotating Star

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Magnetospheric accretion in the intermediate-mass T Tauri star HQ Tauri

Astronomy and Astrophysics ◽

10.1051/0004-6361/202038086 ◽

2020 ◽

Vol 642 ◽

pp. A99 ◽

Cited By ~ 1

Author(s):

K. Pouilly ◽

J. Bouvier ◽

E. Alecian ◽

S. H. P. Alencar ◽

A.-M. Cody ◽

...

Keyword(s):

Magnetic Field ◽

Strong Magnetic Field ◽

Main Sequence ◽

T Tauri Stars ◽

Line Profiles ◽

Main Sequence Stars ◽

Intermediate Mass ◽

Accretion Process ◽

T Tauri ◽

Magnetospheric Accretion

Context. Classical T Tauri stars are pre-main sequence stars surrounded by an accretion disk. They host a strong magnetic field, and both magnetospheric accretion and ejection processes develop as the young magnetic star interacts with its disk. Studying this interaction is a major goal toward understanding the properties of young stars and their evolution. Aims. The goal of this study is to investigate the accretion process in the young stellar system HQ Tau, an intermediate-mass T Tauri star (1.9 M⊙). Methods. The time variability of the system is investigated both photometrically, using Kepler-K2 and complementary light curves, and from a high-resolution spectropolarimetric time series obtained with ESPaDOnS at CFHT. Results. The quasi-sinusoidal Kepler-K2 light curve exhibits a period of 2.424 d, which we ascribe to the rotational period of the star. The radial velocity of the system shows the same periodicity, as expected from the modulation of the photospheric line profiles by surface spots. A similar period is found in the red wing of several emission lines (e.g., HI, CaII, NaI), due to the appearance of inverse P Cygni components, indicative of accretion funnel flows. Signatures of outflows are also seen in the line profiles, some being periodic, others transient. The polarimetric analysis indicates a complex, moderately strong magnetic field which is possibly sufficient to truncate the inner disk close to the corotation radius, rcor ∼ 3.5 R⋆. Additionally, we report HQ Tau to be a spectroscopic binary candidate whose orbit remains to be determined. Conclusions. The results of this study expand upon those previously reported for low-mass T Tauri stars, as they indicate that the magnetospheric accretion process may still operate in intermediate-mass pre-main sequence stars, such as HQ Tauri.

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The influence of the magnetic field orientation on the angular momentum loss in the pre-main sequence phase: The case of very slowly rotating magnetic Ap stars

Astronomy and Astrophysics ◽

10.1051/0004-6361:20020053 ◽

2002 ◽

Vol 384 (2) ◽

pp. 554-561 ◽

Cited By ~ 12

Author(s):

K. Stępień ◽

J. D. Landstreet

Keyword(s):

Magnetic Field ◽

Angular Momentum ◽

Main Sequence ◽

Field Orientation ◽

Magnetic Field Orientation ◽

Momentum Loss ◽

Angular Momentum Loss ◽

Ap Stars ◽

The Magnetic Field

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Magnetic Fields of T Tauri Stars

Symposium - International Astronomical Union ◽

10.1017/s0074180900061866 ◽

1997 ◽

Vol 182 ◽

pp. 465-474

Author(s):

Eike W. Guenther

Keyword(s):

Magnetic Field ◽

Main Sequence ◽

T Tauri Stars ◽

Main Sequence Stars ◽

Emission Component ◽

Upper Limit ◽

T Tauri ◽

Late Type Star ◽

T Tau ◽

The Magnetic Field

The magnetic field strengths of several T Tauri stars are derived by measuring the width of unblended Fe I lines of high and low values of geff·λ2 using the autocorrelation function. The T Tauri stars were selected for their low values of v · sin i, and large strengths of the Ca II emission component. The derived magnetic field strength are 2.0 ± 0.6 kG and 2.6 ± 0.8 kG for the classical T Tauri stars Lk Ca 15 and T Tau, respectively. An upper limit of 0.6 ± 0.8 kG is found for the weak-line T Tauri star Lk Ca 16. The method is tested by analysing two non-magnetic main sequence stars, and a late-type star that is known to have a strong magnetic field.

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