The Relation between Rotation and Magnetic Activity on Lower Main Sequence Stars as Derived from Chromospheric Indicators

The relation between magnetic activity and rotation in late-type stars provides fundamental information on stellar dynamos and angular momentum evolution. Rotation-activity studies found in the literature suffer from inhomogeneity in the measurement of activity indexes and rotation periods. We overcome this limitation with a study of the X-ray emitting, late-type main-sequence stars observed by XMM-Newton and Kepler. We measured rotation periods from photometric variability in Kepler light curves. As activity indicators, we adopted the X-ray luminosity, the number frequency of white-light flares, the amplitude of the rotational photometric modulation, and the standard deviation in the Kepler light curves. The search for X-ray flares in the light curves provided by the EXTraS (Exploring the X-ray Transient and variable Sky) FP-7 project allows us to identify simultaneous X-ray and white-light flares. A careful selection of the X-ray sources in the Kepler field yields 102 main-sequence stars with spectral types from A to M. We find rotation periods for 74 X-ray emitting main-sequence stars, 20 of which do not have period reported in the previous literature. In the X-ray activity-rotation relation, we see evidence for the traditional distinction of a saturated and a correlated part, the latter presenting a continuous decrease in activity towards slower rotators. For the optical activity indicators the transition is abrupt and located at a period of ~10 d but it can be probed only marginally with this sample, which is biased towards fast rotators due to the X-ray selection. We observe seven bona-fide X-ray flares with evidence for a white-light counterpart in simultaneous Kepler data. We derive an X-ray flare frequency of ~0.15 d−1, consistent with the optical flare frequency obtained from the much longer Kepler time-series.

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Differential Rotation and Magnetic Activity of the Lower Main Sequence Stars

International Astronomical Union Colloquium ◽

10.1017/s0252921100075539 ◽

1980 ◽

Vol 51 ◽

pp. 296-297

Author(s):

G. Belvedere ◽

L. Paterno ◽

M. Stix

Keyword(s):

Spherical Shell ◽

Differential Rotation ◽

Main Sequence ◽

Magnetic Activity ◽

The Sun ◽

Coupling Constant ◽

Main Sequence Stars ◽

Dynamo Theory ◽

Magnetic Cycles ◽

Surface Convection

AbstractWe extend to the lower main sequence stars the analysis of convection interacting with rotation in a compressible spherical shell, already applied to the solar case (Belvedere and Paterno, 1977; Belvedere et al. 1979a). We assume that the coupling constant ε between convection and rotation, does not depend on the spectral type. Therefore we take ε determined from the observed differential rotation of the Sun, and compute differential rotation and magnetic cycles for stars ranging from F5 to MO, namely for those stars which are supposed to possess surface convection zones (Belvedere et al. 1979b, c, d). The results show that the strength of differential rotation decreases from a maximum at F5 down to a minimum at G5 and then increases towards later spectral types. The computations of the magnetic cycles based on the αω-dynamo theory show that dynamo instability decreases from F5 to G5, and then increases towards the later spectral types reaching a maximum at MO. The period of the magnetic cycles increases from a few years at F5 to about 100 years at MO. Also the extension of the surface magnetic activity increases substantially towards the later spectral types. The results are discussed in the framework of Wilson’s (1978) observations.

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Modelling of an Eclipsing RS CVn Binary: V405 And

Proceedings of the International Astronomical Union ◽

10.1017/s1743921311027347 ◽

2011 ◽

Vol 7 (S282) ◽

pp. 199-200

Author(s):

Krisztián Vida ◽

Katalin Oláh ◽

Zsolt Kővári

Keyword(s):

Light Curve ◽

Main Sequence ◽

Magnetic Activity ◽

Light Curves ◽

Stellar Structure ◽

Main Sequence Stars ◽

M Dwarfs ◽

Primary Star ◽

Convective Envelope ◽

Eclipsing Binary

AbstractV405 And is an ultrafast-rotating (Prot ≈ 0.46 days) eclipsing binary. The system consists of a primary star with radiative core and convective envelope, and a fully convective secondary. Theories have shown that stellar structure can depend on magnetic activity, i.e., magnetically active M-dwarfs should have larger radii. Earlier light curve modelling of V405 And indeed showed this behaviour: we found that the radius of the primary is significantly larger than the theoretically predicted value for inactive main sequence stars (the discrepancy is the largest of all known objects), while the secondary fits well to the mass-radius relation. By modelling our recently obtained light curves, which show significant changes of the spotted surface of the primary, we can find further proof for this phenomenon.

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Eclipsing binary stars as tests of stellar evolutionary models and stellar ages

Proceedings of the International Astronomical Union ◽

10.1017/s1743921309031810 ◽

2008 ◽

Vol 4 (S258) ◽

pp. 161-170 ◽

Cited By ~ 4

Author(s):

Keivan G. Stassun ◽

Leslie Hebb ◽

Mercedes López-Morales ◽

Andrej Prša

Keyword(s):

Binary Stars ◽

Main Sequence ◽

Magnetic Activity ◽

Eclipsing Binaries ◽

Evolutionary Models ◽

Main Sequence Stars ◽

Eclipsing Binary ◽

Eclipsing Binary Stars ◽

Sequence Components ◽

Low Mass

AbstractEclipsing binary stars provide highly accurate measurements of the fundamental physical properties of stars. They therefore serve as stringent tests of the predictions of evolutionary models upon which most stellar age determinations are based. Models generally perform very well in predicting coeval ages for eclipsing binaries with main-sequence components more massive than ≈1.2 M⊙; relative ages are good to ~5% or better in this mass regime. Low-mass main-sequence stars (M < 0.8 M⊙) reveal large discrepancies in the model predicted ages, primarily due to magnetic activity in the observed stars that appears to inhibit convection and likely causes the radii to be 10–20% larger than predicted. In mass-radius diagrams these stars thus appear 50–90% older or younger than they really are. Aside from these activity-related effects, low-mass pre–main-sequence stars at ages ~1 Myr can also show non-coevality of ~30% due to star formation effects, however these effects are largely erased after ~10 Myr.

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Non thermal emission from T Tauri stars

Proceedings of the International Astronomical Union ◽

10.1017/s1743921310016492 ◽

2010 ◽

Vol 6 (S275) ◽

pp. 404-405

Author(s):

María V. del Valle ◽

Gustavo E. Romero

Keyword(s):

Main Sequence ◽

Thermal Emission ◽

High Energy ◽

Magnetic Activity ◽

T Tauri Stars ◽

Main Sequence Stars ◽

High Energy Radiation ◽

X Ray ◽

T Tauri ◽

Low Mass

AbstractT Tauri stars are low mass, pre-main sequence stars. These objects are surrounded by an accretion disk and present strong magnetic activity. T Tauri stars are copious emitters of X-ray emission which belong to powerful magnetic reconnection events. Strong magnetospheric shocks are likely outcome of massive reconnection. Such shocks can accelerate particles up to relativistic energies through Fermi mechanism. We present a model for the high-energy radiation produced in the environment of T Tauri stars. We aim at determining whether this emission is detectable. If so, the T Tauri stars should be very nearby.

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Revisiting the connection between magnetic activity, rotation period, and convective turnover time for main-sequence stars

Astronomy and Astrophysics ◽

10.1051/0004-6361/201833498 ◽

2018 ◽

Vol 618 ◽

pp. A48 ◽

Cited By ~ 12

Author(s):

M. Mittag ◽

J. H. M. M. Schmitt ◽

K.-P. Schröder

Keyword(s):

Main Sequence ◽

Rotation Period ◽

Magnetic Activity ◽

Turnover Time ◽

Rossby Number ◽

Stellar Rotation ◽

Main Sequence Stars ◽

Rotation Periods ◽

Period Distribution ◽

Cool Stars

The connection between stellar rotation, stellar activity, and convective turnover time is revisited with a focus on the sole contribution of magnetic activity to the Ca II H&K emission, the so-called excess flux, and its dimensionless indicator R+HK in relation to other stellar parameters and activity indicators. Our study is based on a sample of 169 main-sequence stars with directly measured Mount Wilson S-indices and rotation periods. The R+HK values are derived from the respective S-indices and related to the rotation periods in various B–V-colour intervals. First, we show that stars with vanishing magnetic activity, i.e. stars whose excess flux index R+HK approaches zero, have a well-defined, colour-dependent rotation period distribution; we also show that this rotation period distribution applies to large samples of cool stars for which rotation periods have recently become available. Second, we use empirical arguments to equate this rotation period distribution with the global convective turnover time, which is an approach that allows us to obtain clear relations between the magnetic activity related excess flux index R+HK, rotation periods, and Rossby numbers. Third, we show that the activity versus Rossby number relations are very similar in the different activity indicators. As a consequence of our study, we emphasize that our Rossby number based on the global convective turnover time approaches but does not exceed unity even for entirely inactive stars. Furthermore, the rotation-activity relations might be universal for different activity indicators once the proper scalings are used.

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