Weak- and strong-field dynamos: from the Earth to the stars

Morin, J.; Dormy, Emmanuel; Schrinner, M.; Donati, J. F.

doi:10.1111/j.1745-3933.2011.01159.x

Cited by 73 publications

(76 citation statements)

References 40 publications

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“…This observational finding led Morin et al (2010) to propose two magnetic dynamo modes leading to differing magnetic topologies, with late M dwarfs having a bistable dynamo that may inhabit either mode. This concept is supported by recent results from geodynamo simulations that show bimodal dynamo outcomes in rapid rotators (Morin et al 2011;Gastine et al 2013).…”

Section: Discussionsupporting

confidence: 72%

Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement

Williams

Cook

Berger

2014

ApJ

152

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Although ultracool dwarfs (UCDs) are now known to generate and dissipate strong magnetic fields, a clear understanding of the underlying dynamo is still lacking. We have performed X-ray and radio observations of seven UCDs in a narrow range of spectral type (M6.5-M9.5) but spanning a wide range of projected rotational velocities (v sin i ≈ 3-40 km s −1 ). We have also analyzed unpublished archival Chandra observations of four additional objects. All of the newly observed targets are detected in the X-ray, while only one is detected in the radio, with the remainder having sensitive upper limits. We present a database of UCDs with both radio and X-ray measurements and consider the data in light of the so-called Güdel-Benz relation (GBR) between magnetic activity in these bands. Some UCDs have very bright radio emission and faint X-ray emission compared to what would be expected for rapid rotators, while others show the opposite behavior. We show that UCDs would still be radio-overluminous relative to the GBR even if their X-ray emission were at standard rapid-rotator "saturation" levels. Recent results from Zeeman-Doppler imaging and geodynamo simulations suggest that rapidly rotating UCDs may harbor a bistable dynamo that supports either a stronger, axisymmetric magnetic field or a weaker, nonaxisymmetric field. We suggest that the data can be explained in a scenario in which strong-field objects obey the GBR while weak-field objects are radio-overluminous and X-ray-underluminous, possibly because of a population of gyrosynchrotron-emitting coronal electrons that is continuously replenished by low-energy reconnection events.

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Section: Discussionsupporting

confidence: 72%

Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement

Williams

Cook

Berger

2014

ApJ

152

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“…Our observations suggest that dynamo bi-stability may indeed be present at different places of the M vs P rot diagram than previously identified by Morin et al (2011) and whose relation with theoretical predictions is yet to be checked in more details. More spectropolarimetric observations of M dwarfs in this range of mass and rotation periods are necessary to investigate this result in more details.…”

Section: Photosphere Occupancymentioning

confidence: 43%

“…In particular, we note that 2 stars of our sample feature different types of fields while sharing the same location in the M vs P rot plane. This is reminiscent of the bi-stable behavior of dynamo processes, as previously pointed out by, e.g., Morin et al (2011) in the case of active very low-mass dwarfs. The theoretical models (e.g., Gastine et al 2013) foresee a bistability around R o = 0.1, with a transition between fields with a simple dipolar topology (R o < 0.1) and fields with a complex topology…”

Section: Summary and Perspectivesmentioning

confidence: 73%

Modelling the RV jitter of early-M dwarfs using tomographic imaging

Hébrard

Donati

Delfosse

et al. 2016

Mon. Not. R. Astron. Soc.

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In this paper we show how tomographic imaging (Zeeman Doppler Imaging, ZDI) can be used to characterize stellar activity and magnetic field topologies, ultimately allowing to filter out the radial velocity (RV) activity jitter of M-dwarf moderate rotators. This work is based on spectropolarimetric observations of a sample of five weakly-active early M-dwarfs (GJ 205, GJ 358, GJ 410, GJ479, GJ 846) with HARPS-Pol and NARVAL. These stars have v sin i and RV jitters in the range 1-2 km s −1 and 2.7-10.0 m s −1 rms respectively.Using a modified version of ZDI applied to sets of phase-resolved Least-Squares-Deconvolved (LSD) profiles of unpolarized spectral lines, we are able to characterize the distribution of active regions at the stellar surfaces. We find that darks spots cover less than 2% of the total surface of the stars of our sample. Our technique is efficient at modeling the rotationally modulated component of the activity jitter, and succeeds at decreasing the amplitude of this component by typical factors of 2-3 and up to 6 in optimal cases. From the rotationally modulated time-series of circularly polarized spectra and with ZDI, we also reconstruct the large-scale magnetic field topology. These fields suggest that bi-stability of dynamo processes observed in active M dwarfs may also be at work for moderately active M dwarfs. Comparing spot distributions with field topologies suggest that dark spots causing activity jitter concentrate at the magnetic pole and/or equator, to be confirmed with future data on a larger sample.

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“…Indeed, the underlying magnetic dynamos of giant planets, brown dwarfs, and very low-mass stars might be very similar (Christensen et al 2009;Morin et al 2011); however, this idea is currently being tested (see Kao et al 2016). Nevertheless, the nature of the transition in magnetic activity across the brown dwarf regime is an open question, as is its dependence on physical properties such as mass and age.…”

Section: Brown Dwarfs: Between Stars and Planets?mentioning

confidence: 99%

A Panchromatic View of Brown Dwarf Aurorae

Pineda

Hallinan

Kao

2017

ApJ

118

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Stellar coronal activity has been shown to persist into the low-mass star regime, down to late M-dwarf spectral types. However, there is now an accumulation of evidence suggesting that at the end of the main sequence, there is a transition in the nature of the magnetic activity from chromospheric and coronal to planet-like and auroral, from local impulsive heating via flares and MHD wave dissipation to energy dissipation from strong large-scale magnetospheric current systems. We examine this transition and the prevalence of auroral activity in brown dwarfs through a compilation of multiwavelength surveys of magnetic activity, including radio, X-ray, and optical. We compile the results of those surveys and place their conclusions in the context of auroral emission as a consequence of large-scale magnetospheric current systems that accelerate energetic electron beams and drive the particles to impact the cool atmospheric gas. We explore the different manifestations of auroral phenomena, like Hα, in brown dwarf atmospheres and define their distinguishing characteristics. We conclude that large-amplitude photometric variability in the near-infrared is most likely a consequence of clouds in brown dwarf atmospheres, but that auroral activity may be responsible for long-lived stable surface features. We report a connection between auroral Hα emission and quiescent radio emission in electron cyclotron maser instability pulsing brown dwarfs, suggesting a potential underlying physical connection between quiescent and auroral emissions. We also discuss the electrodynamic engines powering brown dwarf aurorae and the possible role of satellites around these systems both to power the aurorae and seed the magnetosphere with plasma.

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Weak- and strong-field dynamos: from the Earth to the stars

Cited by 73 publications

References 40 publications

Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement

Trends in Ultracool Dwarf Magnetism. I. X-Ray Suppression and Radio Enhancement

Modelling the RV jitter of early-M dwarfs using tomographic imaging

A Panchromatic View of Brown Dwarf Aurorae

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