Do Magnetic Fields Actually Inflate Low-Mass Stars?

AbstractMagnetic fields have been hypothesized to inflate the radii of low-mass stars—defined as less than 0.8 M⊙–in detached eclipsing binaries (DEBs). We evaluate this hypothesis using the magnetic Dartmouth stellar evolution code. Results suggest that magnetic suppression of thermal convection can inflate low-mass stars that possess a radiative core and convective outer envelope. A scaling relation between X-ray luminosity and surface magnetic flux indicates that model surface magnetic field strength predictions are consistent with observations. This supports the notion that magnetic fields may be inflating these stars. However, magnetic models are unable to reproduce radii of fully convective stars in DEBs. Instead, we propose that model discrepancies below the fully convective boundary are related to metallicity.

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Rotational and Candidate-Eclipsing-Binary Light Curves for Pre-Main-Sequence Stars in the Chamaeleon I Star-Forming Cloud

Publications of the Astronomical Society of Australia ◽

10.1071/as08018 ◽

2009 ◽

Vol 26 (1) ◽

pp. 31-36 ◽

Cited By ~ 1

Author(s):

Warrick A. Lawson ◽

Lisa A. Crause

Keyword(s):

Angular Momentum ◽

Main Sequence ◽

Near Infrared ◽

Eclipsing Binaries ◽

Stellar Rotation ◽

Low Mass Stars ◽

X Ray ◽

Star Forming ◽

Star Forming Regions ◽

Low Mass

AbstractWe present the results of a photometric survey for variability in ten X-ray-emitting low-mass stars in the Chamaeleon region. Eight of the stars we observed are bona fide pre-main-sequence members of the ∼2 Myr-old Chamaeleon I star-forming cloud. The other two stars are young with high levels of relative X-ray emission, but with discordant proper motions they are probable non-members of the cloud. In six of the stars we monitored, periodic variations on timescales of 2.5–11.5 d were detected, that we ascribe to stellar rotation and the presence of cool starspots. Two other stars, CHXR 20 and CHXR 85, show large amplitude variations at visual and near-infrared wavelengths and are candidate eclipsing binaries. Compared to the rotational properties of low-mass stars in the ≈8 Myr-old η Chamaeleontis cluster, we find that the older η Chamaeleontis stars have several times higher surface specific angular momentum than the younger Chamaeleon I stars. The apparent increase in angular momentum between ∼2 and 8 Myr might be due to changes in stellar internal structure as the stars evolve, or evidence for a different rotational history between members of the two star-forming regions.

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Investigating the Magnetospheres of Rapidly Rotating B-type Stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921317002812 ◽

2016 ◽

Vol 12 (S329) ◽

pp. 369-372

Author(s):

C. L. Fletcher ◽

V. Petit ◽

Y. Nazé ◽

G. A. Wade ◽

R. H. Townsend ◽

...

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Centrifugal Force ◽

Point Of View ◽

Closed Field ◽

Theoretical Point ◽

X Rays ◽

Rapid Rotation ◽

X Ray ◽

Surface Magnetic Field

AbstractRecent spectropolarimetric surveys of bright, hot stars have found that ~10% of OB-type stars contain strong (mostly dipolar) surface magnetic fields (~kG). The prominent paradigm describing the interaction between the stellar winds and the surface magnetic field is the magnetically confined wind shock (MCWS) model. In this model, the stellar wind plasma is forced to move along the closed field loops of the magnetic field, colliding at the magnetic equator, and creating a shock. As the shocked material cools radiatively it will emit X-rays. Therefore, X-ray spectroscopy is a key tool in detecting and characterizing the hot wind material confined by the magnetic fields of these stars. Some B-type stars are found to have very short rotational periods. The effects of the rapid rotation on the X-ray production within the magnetosphere have yet to be explored in detail. The added centrifugal force due to rapid rotation is predicted to cause faster wind outflows along the field lines, leading to higher shock temperatures and harder X-rays. However, this is not observed in all rapidly rotating magnetic B-type stars. In order to address this from a theoretical point of view, we use the X-ray Analytical Dynamical Magnetosphere (XADM) model, originally developed for slow rotators, with an implementation of new rapid rotational physics. Using X-ray spectroscopy from ESA’s XMM-Newton space telescope, we observed 5 rapidly rotating B-types stars to add to the previous list of observations. Comparing the observed X-ray luminosity and hardness ratio to that predicted by the XADM allows us to determine the role the added centrifugal force plays in the magnetospheric X-ray emission of these stars.

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THE X-RAY LUMINOSITY FUNCTION OF M37 AND THE EVOLUTION OF CORONAL ACTIVITY IN LOW-MASS STARS

The Astrophysical Journal ◽

10.3847/0004-637x/830/1/44 ◽

2016 ◽

Vol 830 (1) ◽

pp. 44 ◽

Cited By ~ 7

Author(s):

Alejandro Núñez ◽

Marcel A. Agüeros

Keyword(s):

Luminosity Function ◽

Low Mass Stars ◽

X Ray ◽

Coronal Activity ◽

Low Mass

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Non-ideal magnetohydrodynamics versus turbulence – I. Which is the dominant process in protostellar disc formation?

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1339 ◽

2020 ◽

Vol 495 (4) ◽

pp. 3795-3806 ◽

Cited By ~ 2

Author(s):

James Wurster ◽

Benjamin T Lewis

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Rotation Axis ◽

Low Mass Stars ◽

Rotation Rates ◽

Ideal Mhd ◽

Low Mass ◽

Dominant Process ◽

Ideal Magnetohydrodynamics ◽

Disc Formation

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of circumstellar discs that form nearly simultaneously with the formation of the protostar. This is done by modelling the gravitational collapse of a 1 M⊙ gas cloud that is threaded with a magnetic field and imposed with both rotational and turbulent velocities. We investigate magnetic fields that are parallel/antiparallel and perpendicular to the rotation axis, two rotation rates, and four Mach numbers. Disc formation occurs preferentially in the models that include non-ideal MHD where the magnetic field is antiparallel or perpendicular to the rotation axis. This is independent of the initial rotation rate and level of turbulence, suggesting that subsonic turbulence plays a minimal role in influencing the formation of discs. Aside from first core outflows that are influenced by the initial level of turbulence, non-ideal MHD processes are more important than turbulent processes during the formation of discs around low-mass stars.

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The Intermediate Polars

International Astronomical Union Colloquium ◽

10.1017/s0252921100090291 ◽

1983 ◽

Vol 72 ◽

pp. 155-172

Author(s):

Brian Warner

Keyword(s):

Magnetic Fields ◽

White Dwarf ◽

Variable Star ◽

Theoretical Models ◽

Cataclysmic Variable ◽

General Role ◽

X Ray ◽

Low Mass ◽

Intermediate Polars ◽

X Ray Binaries

Until 1976, cataclysmic variable star research proceeded with few requirements for the inclusion of magnetic fields in theoretical models. Although models for low-mass X-ray binaries stressed the importance of magnetic fields (Lamb et at. 1973) and there was an increasing number of known magnetic single white dwarfs (Angel 1977), and a magnetised white dwarf had been one of the models proposed to explain the rapid oscillations in DQ Her (Herbst et al. 1974, Katz 1975), there was no anticipation of the more general role that magnetic fields now seem destined to play. The two major reviews of the time (Robinson 1976, Warner 1976) scarcely considered the presence of magnetic fields.

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X-Ray Detection of Brown Dwarfs with Chandra

Symposium - International Astronomical Union ◽

10.1017/s0074180900211108 ◽

2003 ◽

Vol 211 ◽

pp. 447-450 ◽

Cited By ~ 1

Author(s):

Scott J. Wolk

Keyword(s):

Brown Dwarfs ◽

Stellar Clusters ◽

Brown Dwarf ◽

Low Mass Stars ◽

X Ray ◽

Low Mass ◽

Vast Range

I review recent observations of brown dwarfs by the Chandra X-ray Observatory. These observations fall in 2 categories, young stellar clusters which contain brown dwarfs and brown dwarf candidates and directed pointings at brown dwarfs and very low mass stars. Surprisingly, there are already over 60 published detections of brown dwarfs by Chandra. A review of the X–ray characteristics shows these objects are subject to flaring and their temperatures and luminosities have a vast range which is related to age.

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The X-Ray View of the Low-Mass Stars in the Solar Neighborhood

The Astrophysical Journal ◽

10.1086/176149 ◽

1995 ◽

Vol 450 ◽

pp. 392 ◽

Cited By ~ 179

Author(s):

Juergen H. M. M. Schmitt ◽

Thomas A. Fleming ◽

Mark S. Giampapa

Keyword(s):

Solar Neighborhood ◽

Low Mass Stars ◽

X Ray ◽

Low Mass

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Non-ideal magnetohydrodynamics versus turbulence II: Which is the dominant process in stellar core formation?

Monthly Notices of the Royal Astronomical Society ◽

10.1093/mnras/staa1340 ◽

2020 ◽

Vol 495 (4) ◽

pp. 3807-3818 ◽

Cited By ~ 3

Author(s):

James Wurster ◽

Benjamin T Lewis

Keyword(s):

Magnetic Field ◽

Magnetic Fields ◽

Rotation Axis ◽

Core Formation ◽

Low Mass Stars ◽

Stellar Core ◽

Ideal Mhd ◽

Low Mass ◽

Dominant Process ◽

Ideal Magnetohydrodynamics

ABSTRACT Non-ideal magnetohydrodynamics (MHD) is the dominant process. We investigate the effect of magnetic fields (ideal and non-ideal) and turbulence (sub- and transsonic) on the formation of protostars by following the gravitational collapse of 1 M⊙ gas clouds through the first hydrostatic core to stellar densities. The clouds are imposed with both rotational and turbulent velocities, and are threaded with a magnetic field that is parallel/antiparallel or perpendicular to the rotation axis; we investigate two rotation rates and four Mach numbers. The initial radius and mass of the stellar core are only weakly dependent on the initial parameters. In the models that include ideal MHD, the magnetic field strength implanted in the protostar at birth is much higher than observed, independent of the initial level of turbulence; only non-ideal MHD can reduce this strength to near or below the observed levels. This suggests that not only is ideal MHD an incomplete picture of star formation, but that the magnetic fields in low mass stars are implanted later in life by a dynamo process. Non-ideal MHD suppresses magnetically launched stellar core outflows, but turbulence permits thermally launched outflows to form a few years after stellar core formation.

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