Learning about Stellar Dynamos from Long–term Photometry of Starspots

Hall, D. S.

doi:10.1017/s0252921100079914

Cited by 6 publications

(4 citation statements)

References 28 publications

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“…Significant surface toroidal fields are detected even in DS Leo and GJ 49, that is the two slowest rotators with masses larger than 0.5 M ; it suggests that the transition between mainly poloidal and mainly toroidal fields in M > 0.5 M stars occurs at Ro 0.5 − 1.0, with the Sun located on the other side of this boundary (at Ro 1.5−2.0). Note that this boundary coincides with the sharp onset of photometric variability in convective stars (occurring below Ro 0.7, Hall 1991). With a poloidal field concentrating 70-80 per cent of the reconstructed magnetic energy, OT Ser is off this trend; we suspect that this is due to its proximity with the 0.5 M sharp threshold below which magnetic topologies become dominantly poloidal.…”

Section: S U M M a Ry A N D Discussionmentioning

confidence: 58%

Large-scale magnetic topologies of early M dwarfs^★

Morin

Donati

Petit

et al. 2008

Monthly Notices of the Royal Astronomical Society

280

328

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We present here additional results of a spectropolarimetric survey of a small sample of stars ranging from spectral type M0 to M8 aimed at investigating observationally how dynamo processes operate in stars on both sides of the full convection threshold (spectral type M4). The present paper focuses on early M stars (M0–M3), that is above the full convection threshold. Applying tomographic imaging techniques to time series of rotationally modulated circularly polarized profiles collected with the NARVAL spectropolarimeter, we determine the rotation period and reconstruct the large‐scale magnetic topologies of six early M dwarfs. We find that early‐M stars preferentially host large‐scale fields with dominantly toroidal and non‐axisymmetric poloidal configurations, along with significant differential rotation (and long‐term variability); only the lowest‐mass star of our subsample is found to host an almost fully poloidal, mainly axisymmetric large‐scale field resembling those found in mid‐M dwarfs. This abrupt change in the large‐scale magnetic topologies of M dwarfs (occurring at spectral type M3) has no related signature on X‐ray luminosities (measuring the total amount of magnetic flux); it thus suggests that underlying dynamo processes become more efficient at producing large‐scale fields (despite producing the same flux) at spectral types later than M3. We suspect that this change relates to the rapid decrease in the radiative cores of low‐mass stars and to the simultaneous sharp increase of the convective turnover times (with decreasing stellar mass) that models predict to occur at M3; it may also be (at least partly) responsible for the reduced magnetic braking reported for fully convective stars.

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Section: S U M M a Ry A N D Discussionmentioning

confidence: 58%

Large-scale magnetic topologies of early M dwarfs^★

Morin

Donati

Petit

et al. 2008

Monthly Notices of the Royal Astronomical Society

280

328

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“…• With its deep convection and rapid rotation, the K-type subgiant in SW Cyg lies well within the domain of "activedynamo stars," those apt to manifest pronounced magnetic phenomena. According to Hall (1991a), the active dynamo stars are those with a Rossby number (ratio of rotation period to convective turnover time) less than 2/3. With its rotation period known (4?573) and its convective turnover time derivable (Hall 1991a) from its temperature (spectral type) and radius (luminosity class), its Rossby number must be around 0.05 , smaller than 2/3 by a wide margin.…”

Section: The Cycle Lengthmentioning

confidence: 99%

“…For background on the occurrence of magnetic cycles in stars of various different types, see Hall (1990). For the relation between rotation, convection, and dynamo action, see Hall (1991a). For the connection between magnetic cycles and changes in orbital period, when the active star happens to be in a binary, see Hall (1991b) and Applegate (1992).…”

Section: Introductionmentioning

confidence: 99%

The cyclic period variability in SW Cygni

Berrington¹,

Hall²

1994

The Astronomical Journal

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Timings of primary eclipse up through 1992 are collected, plus a few overlooked in earlier studies. Analysis shows that, after decreasing until =1919 and increasing since then, the period has now started decreasing again, with the inflection point of the O-C curve being =1972. The shortest period was 4?5727, the longest 4?5732, with a full cycle length of 96 yr. Explanation by the time-delay effect, due to orbital motion around a third body, would require an unseen black hole companion of ;a, 10.,,,/6'0. A magnetic cycle operating in the K-type subgiant, an active dynamo star because of its convection and rapid rotation, is the best explanation.

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“…In simple flows the α-effect is related to the kinetic helicity. In rapidly rotating stars, differential rotation appears to play only a minor role (Hall 1991, Reiners and Schmitt 2003a, Reiners and Schmitt 2003b, so the α-effect must regenerate both poloidal and toroidal fields. These systems are α 2 -dynamos.…”

Section: Introductionmentioning

confidence: 99%

Homogeneous planar and two-dimensional mean-field antidynamo theorems with zero mean flow

Ivers¹,

Phillips²

2014

Geophysical & Astrophysical Fluid Dynamics

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In an electrically conducting fluid, two types of turbulence with a preferred direction are distinguished: planar turbulence, in which every velocity in the turbulent ensemble of flows has no component in the given direction; and two-dimensional turbulence, in which every velocity in the turbulent ensemble is invariant under translation in the preferred direction. Under the additional assumptions of two-scale and homogeneous turbulence with zero mean flow, the associated magnetohydrodynamic alpha-and beta-effects are derived in the second-order correlation approximation (SOCA) when the electrically conducting fluid occupies all space. Limitations of the SOCAare well known, but alpha-and beta-effects of a turbulent flow are useful in interpreting the dynamo effects of the turbulence. Two antidynamo theorems, which establish necessary conditions for dynamo action, are shown to follow from the special structures of these alpha-and beta-effects. The theorems, which are analogues of the laminar planar velocity and two-dimensional antidynamo theorems, apply to all turbulent ensembles with the prescribed alpha-and beta-effects, not just the planar and two-dimensional ensembles. The mean magnetic field is general in the planar theorem but only two-dimensional in the two-dimensional theorem. The two theorems relax the previous restriction to turbulence which is both two-dimensional and planar. The laminar theorems imply decay of the total magnetic field for any velocity of the associated turbulent ensemble. However, the mean-field theorems are not fully consistent with the laminar theorems because further conditions beyond those arising from the turbulence must be imposed on the beta-effect to establish decay of the mean magnetic field. In particular, negative turbulent magnetic diffusivities must be restricted. It is interesting that there is no inconsistency in the alphaeffects. The failure of the SOCA with the two-scale approximation to simply preserve the laminar antidynamo theorems at the beta-effect level is a further demonstration of the restricted validity of the theory and shows that negative diffusivity effects derived by approximation methods must be treated cautiously.

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Learning about Stellar Dynamos from Long–term Photometry of Starspots

Cited by 6 publications

References 28 publications

Large-scale magnetic topologies of early M dwarfs^★

Large-scale magnetic topologies of early M dwarfs^★

The cyclic period variability in SW Cygni

Homogeneous planar and two-dimensional mean-field antidynamo theorems with zero mean flow

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Learning about Stellar Dynamos from Long–term Photometry of Starspots

Cited by 6 publications

References 28 publications

Large-scale magnetic topologies of early M dwarfs★

Large-scale magnetic topologies of early M dwarfs★

The cyclic period variability in SW Cygni

Homogeneous planar and two-dimensional mean-field antidynamo theorems with zero mean flow

Contact Info

Product

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Large-scale magnetic topologies of early M dwarfs^★

Large-scale magnetic topologies of early M dwarfs^★