Large-scale $\vec \alpha\mathsf{^2}$-dynamo in low-mass stars and brown dwarfs

Chabrier, G.; Kueker, M.

doi:10.1051/0004-6361:20042475

Cited by 212 publications

(237 citation statements)

References 35 publications

(51 reference statements)

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“…in the absence of shear, provided that rotation is rapid enough. Such turbulent dynamos probably operate in stars less massive than about 0.35 solar masses that are fully convective, and so cannot possess a transition region like the solar tachocline (Durney et al 1993;Küher & Rüdiger 1999;Chabrier & Küher 2006). A recent study of the magnetic activity-rotation for a large sample of field M dwarfs with known rotational periods (West et al 2012) show a trend of decreasing activity with increased rotation period for all M dwarf spectral types.…”

Section: Discussionmentioning

confidence: 99%

X-ray emission regimes and rotation sequences in M 35

Gondoin

2013

A&A

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Context. Late-type stars in young open clusters show two kinds of dependence of their X-ray emission on rotation. They also tend to group into two main sub-populations that lie on narrow sequences in diagrams where their rotation periods are plotted against their (B − V) colour indices. A correlation between these two regimes of X-ray emission and the rotation sequences has been recently observed in the M 34 open cluster. Sun-like M 34 stars also show a drop of their X-ray to bolometric luminosity ratio by about one order of magnitude at a Rossby number of about 0.3. Aims. The present study looks for similar connections between X-ray activity and rotation in an other open cluster. The aim is to consolidate a model of X-ray activity evolution on the main sequence and to provide observational constraints on dynamo processes in the interiors of late-type stars. Methods. The paper compares XMM-Newton measurements of X-ray stellar emission in M 35 with X-ray luminosity distributions derived from rotation period measurements assuming either an X-ray regime transition at a critical Rossby number or a correlation between X-ray emission regimes and rotation sequences. Results. This second hypothesis could account for the low number of M 35 stars detected in X-rays. A model of X-ray activity evolution is proposed based on the correlation. One major output is that the transition from saturated to non-saturated X-ray emission occurs at Rossby numbers between about 0.13 and 0.4 for each star depending on its mass and initial period of rotation on the ZAMS. This prediction agrees with observations of stellar X-ray emission in M 34. It explains the large range of X-ray luminosities observed among Sun-like stars in young open clusters. Conclusions. I conclude that the correlation between X-ray emission regimes and rotation sequences could be a fundamental property of the early evolution of stellar magnetic activity on the main sequence. I argue that the angular momentum redistribution mechanism(s) responsible for the transition between rotation sequences could result in a changing mixture of dynamo processes occurring side by side, possibly including a turbulent dynamo at high rotation rate and an interface-type dynamo whose efficiency decreases progressively at later stage of stellar evolution when rotation dies away.

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Section: Discussionmentioning

confidence: 99%

X-ray emission regimes and rotation sequences in M 35

Gondoin

2013

A&A

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“…However, it is not clear whether these mechanisms can give rise to a large-scale, longlived magnetic field. Recent magnetohydrodynamic simulations provide tantalizing evidence that a large-scale and stable field may in fact be generated (Chabrier & Küker 2006;Dobler et al 2006), contrary to earlier indications (Durney et al 1993), but initial observations suggest that the models are not complete (Donati et al 2006). It remains to be seen whether the same result holds for the much cooler L and T dwarfs, of which a large fraction lack hydrogen burning as a source of heat.…”

Section: Introductionmentioning

confidence: 85%

Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

Berger

2006

ApJ

202

237

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I present radio observations of 90 dwarf stars and brown dwarfs of spectral type M5YT8. Three sources exhibit radio activity, in addition to the six objects previously detected in quiescence and outburst, leading to an overall detection rate of $10% for objects later than M7. The inferred magnetic field strengths are $10 2 G in quiescence and nearly 1 kG during flares, while the majority of the nondetected objects have B P 50 G. Depending on the configuration and size of the magnetic loops, the surface fields may approach 1 kG even in quiescence, at most a factor of a few smaller than in early M dwarfs. With the larger sample of sources I find continued evidence for (1) a sharp transition around spectral type of M7 from a ratio of radio to X-ray luminosity of log (L R /L X ) $ À15:5 to kÀ12, (2) increased radio activity (L R /L bol ) with later spectral type, in contrast to H and X-ray observations, and (3) an overall drop in the fraction of active sources from $30% for M dwarfs to $5% for L dwarfs, consistent with H and X-ray observations. Taken together, these trends suggest that some late M and L dwarfs are capable of generating 0.1Y1 kG magnetic fields, but the overall drop in the fraction of such objects likely reflects changes in the structure of the chromospheres and coronae, possibly due to increasingly neutral atmospheres and /or a transition to a turbulent dynamo. These possibilities can best be tested through simultaneous observations, which can trace the effect of magnetic dissipation in a direct, rather than a statistical, manner. Still, a more extended radio survey currently holds the best promise for measuring the magnetic field properties of a large number of dwarf stars.

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“…This coupling, however, depends on the field generation mechanism (Kawaler 1988), and angular momentum loss becomes saturated at a certain level (Chaboyer et al 1995). According to Chabrier & Küker (2006), large-scale magnetic fields of equipartition strength (a few kG) can be generated by an α 2 dynamo. These authors predict that the field topology differs from an organized dipole field, which could explain our result that rotational braking does not grow with magnetic field strength.…”

Section: Discussionmentioning

confidence: 99%

Spectral-type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070

Reiners

Seifahrt

Käufl

et al. 2007

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We show individual high-resolution spectra of components A, B, and C of the nearby late-M type multiple system LHS 1070. Component A is a mid-M star, and B and C are known to have masses at the threshold to brown dwarfs. From our spectra we measure rotation velocities and the mean magnetic field for all three components individually. We find magnetic flux on the order of several kilo-Gauss in all components. The rotation velocities of the two late-M objects B and C are similar (v sin i = 16 km s −1 ), the earlier A component is spinning only at about half that rate. This suggest that net rotational braking at late-M spectral type is weakening, and that the lack of slowly rotating late-M and L dwarfs is real. Furthermore, we found that magnetic flux in the B component is about twice as strong as in component C at a similar rotation rate. This indicates that rotational braking is not proportional to magnetic field strength in fully convective objects and that a different field topology is the reason for the weak braking in low-mass objects.

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Large-scale $\vec \alpha\mathsf{^2}$-dynamo in low-mass stars and brown dwarfs

Cited by 212 publications

References 35 publications

X-ray emission regimes and rotation sequences in M 35

X-ray emission regimes and rotation sequences in M 35

Radio Observations of a Large Sample of Late M, L, and T Dwarfs: The Distribution of Magnetic Field Strengths

Spectral-type dependent rotational braking and strong magnetic flux in three components of the late-M multiple system LHS 1070

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