scholarly journals Magnetic inhibition of convection in O-star envelopes

2019 ◽  
Vol 487 (3) ◽  
pp. 3904-3913 ◽  
Author(s):  
James MacDonald ◽  
Véronique Petit
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Main Sequence Stars ◽  
Lower Field ◽  
Dipole Strength ◽  
Evolved Stars ◽  
Hydrostatic Balance ◽  
Stellar Envelopes ◽  
Stars Evolution ◽  
Good Agreement

ABSTRACT It has been suggested that the absence of macroturbulence in the atmosphere of NGC 1624−2 is due its strong magnetic field (the strongest known for a massive O star) suppressing convection in its outer layers, removing the mechanism thought responsible for the observed macroturbulence in stars with lower field strengths. Here, we develop and apply a criterion for a uniform magnetic field to suppress convection in stellar envelopes in which radiation pressure is a significant contributor to hydrostatic balance. We find upper mass limits of ∼55 and ∼30 M⊙ for magnetic suppression to be possible in zero-age main-sequence and terminal-age main-sequence stars, respectively. For evolved stars, magnetic suppression of convection can significantly alter the stars’ evolution. For NGC 1624−2, we find that a polar dipole strength of 16.5 ± 5.9 kG is required to suppress convection, in good agreement with the value ∼20 kG measured by spectropolarimetry.

scholarly journals TESS Giants Transiting Giants. I.: A Noninflated Hot Jupiter Orbiting a Massive Subgiant

2022 ◽  
Vol 163 (2) ◽  
pp. 53
Author(s):  
Nicholas Saunders ◽  
Samuel K. Grunblatt ◽  
Daniel Huber ◽  
Karen A. Collins ◽  
Eric L. N. Jensen ◽  
...  
Keyword(s):  
Main Sequence ◽  
Planetary System ◽  
Scattered Light ◽  
Image Data ◽  
Velocity Measurements ◽  
Main Sequence Stars ◽  
Transiting Planets ◽  
Depth Difference ◽  
Evolved Stars ◽  
Hot Jupiter

Abstract While the population of confirmed exoplanets continues to grow, the sample of confirmed transiting planets around evolved stars is still limited. We present the discovery and confirmation of a hot Jupiter orbiting TOI-2184 (TIC 176956893), a massive evolved subgiant (M ⋆ = 1.53 ± 0.12 M ⊙, R ⋆ = 2.90 ± 0.14 R ⊙) in the Transiting Exoplanet Survey Satellite (TESS) Southern Continuous Viewing Zone. The planet was flagged as a false positive by the TESS Quick-Look Pipeline due to periodic systematics introducing a spurious depth difference between even and odd transits. Using a new pipeline to remove background scattered light in TESS Full Frame Image data, we combine space-based TESS photometry, ground-based photometry, and ground-based radial velocity measurements to report a planet radius of R p = 1.017 ± 0.051 R J and mass of M p = 0.65 ± 0.16 M J . For a planet so close to its star, the mass and radius of TOI-2184b are unusually well matched to those of Jupiter. We find that the radius of TOI-2184b is smaller than theoretically predicted based on its mass and incident flux, providing a valuable new constraint on the timescale of post-main-sequence planet inflation. The discovery of TOI-2184b demonstrates the feasibility of detecting planets around faint (TESS magnitude > 12) post-main-sequence stars and suggests that many more similar systems are waiting to be detected in the TESS FFIs, whose confirmation may elucidate the final stages of planetary system evolution.

scholarly journals An Improvement in the Visual Surface Brightness Scale for B5-F5 Main Sequence Stars

1985 ◽  
Vol 111 ◽  
pp. 523-524
Author(s):  
L. Pastori ◽  
G. Malaspina
Keyword(s):  
Surface Brightness ◽  
Color Index ◽  
Main Sequence ◽  
Surface Gravity ◽  
Main Sequence Stars ◽  
Interferometric Method ◽  
Gravity Effects ◽  
Good Agreement ◽  
Angular Diameters

Angular diameters of 593 B5-F5 main sequence stars listed in the “Catalogue of apparent diameters and absolute radii of stars” (CADARS; Fracassini et al. 1981) have been analysed in order to improve the precision of the visual surface brightness Sv. The new relations between this quantity and the color index (B-V)o turn out to be in good agreement with those found with the interferometric method (Barnes et al. 1978). Moreover, the results suggest that surface gravity effects may bias the Sv-(B-V)o relations.

scholarly journals Quadratic and Cubic Couplings of Oscillation Modes of Stars

1995 ◽  
Vol 155 ◽  
pp. 287-288
Author(s):  
T. Van Hoolst
Keyword(s):  
Main Sequence ◽  
Oscillation Mode ◽  
The Self ◽  
Nonlinear Interactions ◽  
Nonlinear Coupling ◽  
Coupling Coefficients ◽  
Main Sequence Stars ◽  
Evolved Stars ◽  
Oscillation Modes ◽  
Nonradial Oscillation

The strength of nonlinear interactions of oscillation modes of stars is determined by the amplitudes as well as by the eigenfunctions of the oscillation modes. The intrinsic couplings of modes through their eigenfunctions can be described by coupling coefficients. Here, we concentrate on quadratic and cubic coupling coefficients that describe the nonlinear coupling of modes with itself and are called self-coupling coefficients.We considered radial and nonradial oscillation modes of polytropic models with degrees of central condensation that correspond to central condensations of main sequence stars to highly condensed evolved stars. We study the influence of the radial order and the degree of the oscillation mode on the self- coupling coefficients.

scholarly journals Dynamo and Fossil Magnetic Field in Young Stars

1993 ◽  
Vol 157 ◽  
pp. 171-175
Author(s):  
A.E. Dudorov
Keyword(s):  
Magnetic Field ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Young Stars ◽  
Small Scale ◽  
Scaling Relations ◽  
Main Sequence Stars ◽  
Nonlinear Stabilization ◽  
Turbulent Dynamo ◽  
Solar Type

The theory of fossil magnetic fields shows that new born stars may have internal magnetic fields of more than 1 million gauss. Convection inside young solar type stars will tangle any strong fossil magnetic field. The small scale magnetic field rises to the surface and determines the young stars activity attenuating with their age. When a fossil field is diminished a turbulent dynamo may begin to work in the condition of nonlinear stabilization. The scaling relations for the turbulent αω dynamo show that the strength of the generated “fossil” magnetic field inside the main sequence stars is stabilized on the level one tenth — 10 millions gauss, depending on the mass of the stars.

Fossil magnetic fields and rotation of early-type stars

1994 ◽  
Vol 162 ◽  
pp. 184-185
Author(s):  
A.E. Dudorov
Keyword(s):  
Magnetic Field ◽  
Star Formation ◽  
Magnetic Fields ◽  
Molecular Clouds ◽  
Main Sequence ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars ◽  
Quadrupole Magnetic Field ◽  
Interstellar Molecular Clouds

Observational data of the last 10 years allow two main conclusions:a) Main sequence stars can be separated in two classes: - magnetic (Bp) stars with surface strengths of a dipole or quadrupole magnetic field of Bs ≈ n · (102 − 103) G, n = 2,3,4…7, and - normal main sequence stars (F-O) with magnetic fields Bs ≈ 1 − 100 G (< 300 G);b) Typical star formation takes place in interstellar molecular clouds with magnetic field strengths B ≈ 10-5 G (See Dudorov 1990).

scholarly journals The Remarkable Rotational Braking of G5 Giants

1983 ◽  
Vol 102 ◽  
pp. 461-466
Author(s):  
David F. Gray
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Driving Forces ◽  
Stellar Winds ◽  
Main Sequence Stars ◽  
Dynamo Mechanism ◽  
Slow Progress ◽  
Time Variations ◽  
The Magnetic Field ◽  
Stellar Dynamo

The strong rotational braking seen in the G5 III stage of evolution may be the key to understanding how stellar dynamos work.We are all familiar with the leisurely spin-down seen in cool main-sequence stars like our Sun. The time scales here are ∼ 109 years and the accepted cause is the loss of high angular momentum mass in the form of stellar winds interacting with the stellar magnetic field. The magnetic field is believed to result from the interaction of envelope convection with the rotation of the star through a dynamo mechanism. Our understanding of how a dynamo actually operates, how that operation depends on the driving forces of rotation and convection, what kind of stochastic and secular time variations are to be expected, remains fragmentary even though many inventive minds have contributed. One reason for slow progress is simply that the Sun is almost the only example of a stellar dynamo we have had. But nature has given us another, much more powerful dynamo in the G5 giants, it just took us a little longer to discover it.

scholarly journals Emission of Gravitational Waves from a Magnetohydrodynamic Dynamo in the Center of the Sun

2015 ◽  
Vol 70 (7) ◽  
pp. 545-551
Author(s):  
Friedwardt Winterberg
Keyword(s):  
Magnetic Field ◽  
Gravitational Waves ◽  
Main Sequence ◽  
Fusion Reaction ◽  
Large Mass ◽  
The Sun ◽  
The Moon ◽  
Convection Pattern ◽  
Main Sequence Stars ◽  
The Waves

AbstractBy Birkhoff’s theorem a spherical symmetric convection pattern, as it is assumed to exist for main sequence stars as our sun, cannot lead to the emission of gravitational waves, but all stars that have a magnetic field generated by a magnetohydrodynamic dynamo must by a theorem of Cowling have a non-spherical symmetric convections pattern and for this reason have to emit gravitational waves. The intensity of the thusly emitted gravitational waves depends on the efficiency of this dynamo, expressed by the departure from a spherical convection pattern. The magnitude of the asymmetry is determined by a solution of Elsaesser’s dynamo equations which only recently has become possible with supercomputers. The waves are emitted through large mass motions in the center of the sun by a thermonuclear fusion reaction-driven magnetohydrodynamic dynamo, with thermomagnetic currents in the tachocline shielding the strong magnetic field in the solar core. Using the moon as a large Weber bar, the gravitational waves are focused into the lunar shadow by Poisson diffraction where their effect might become observable during a total solar eclipse.

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

2020 ◽  
Vol 642 ◽  
pp. A99 ◽  
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.

scholarly journals Abundance and Magnetic Field Geometries of Helium-Strong and Helium-Weak Stars

1988 ◽  
Vol 132 ◽  
pp. 309-312
Author(s):  
David A. Bohlender ◽  
J. D. Landstreet
Keyword(s):  
Magnetic Field ◽  
High Temperature ◽  
Magnetic Fields ◽  
Main Sequence ◽  
Line Profiles ◽  
Main Sequence Stars ◽  
Hot Stars ◽  
Surface Magnetic Field ◽  
Peculiar Stars ◽  
Ap Stars

The helium-weak and helium-strong stars are main sequence stars with anomalously weak and strong helium lines for their spectral types respectively. Many members of the two classes have strong, globally ordered magnetic fields (Thompson and Landstreet 1985; Bohlender et al. 1987) and are currently thought to represent high temperature extensions of the Ap stars. In collaboration with C. T. Bolton (U. of Toronto), we have obtained high S/N phase resolved spectra of several stars using the coudé reticon detector at CFHT. One of the principle goals of this work is to determine abundance and surface magnetic field geometries of several helium peculiar stars with large, well-determined effective fields. We employ a line synthesis program (Landstreet 1987) that incorporates the effects of surface magnetic fields and non-uniform abundances on the observed line profiles of a star. Since these stars are rapid rotators the surface magnetic field strength must be inferred from differential magnetic intensification of lines with different magnetic sensitivities. Of the few lines with suitable strengths in these hot stars we have decided that the Si III multiplet 2 lines are best suited for this aspect of our investigation. We have also modelled the unblended He I line λ4437, ignoring magnetic effects for the time being. Individual results are discussed below.

scholarly journals Magnetic Fields of T Tauri Stars

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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