scholarly journals Differential Rotation and Magnetic Activity of the Lower Main Sequence Stars

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.

scholarly journals Stellar Dynamo Characteristics

1986 ◽  
Vol 7 ◽  
pp. 385-392 ◽  
Author(s):  
N.O. Weiss
Keyword(s):  
Magnetic Activity ◽  
The Sun ◽  
Rotating Stars ◽  
Dynamo Theory ◽  
Simplified Models ◽  
Current State ◽  
Cool Stars ◽  
Hydromagnetic Dynamo ◽  
Magnetic Cycles ◽  
Stellar Dynamo

AbstractRecent discoveries have shown that magnetic activity is typical of cool stars with deep convective zones and magnetic cycles are found in slowly rotating stars like the sun. The current state of hydromagnetic dynamo theory is reviewed, and simplified models are used in an attempt to isolate the dominant nonlinear processes in stellar dynamos.

scholarly journals Possibilities and Attempts to Determine the Differential Rotation on F-Type Main-Sequence Stars

1983 ◽  
Vol 102 ◽  
pp. 155-159
Author(s):  
Hubertus Wöhl
Keyword(s):  
Differential Rotation ◽  
Fourier Transformation ◽  
Preliminary Analysis ◽  
Main Sequence ◽  
Spectral Lines ◽  
The Sun ◽  
Main Sequence Stars ◽  
Upper Limits

A preliminary analysis of spectral lines by Fourier transformation was used to determine upper limits of differential rotation for 11 F-type stars, mainly from the main-sequence. No evidence for a much steeper differential rotation on these stars than on the Sun could be found.

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

1983 ◽  
Vol 102 ◽  
pp. 133-147 ◽  
Author(s):  
R.W. Noyes
Keyword(s):  
Magnetic Fields ◽  
Main Sequence ◽  
Large Fraction ◽  
Magnetic Activity ◽  
The Sun ◽  
Emission Flux ◽  
Main Sequence Stars ◽  
Coronal Emission ◽  
Strong Magnetic Fields ◽  
Chromospheric Emission

It is now firmly established that lower main sequence (LMS) stars show a qualitative correlation between rotation rate and chromospheric and coronal emission. By analogy with the Sun, the emission is believed to be intimately associated with surface magnetic fields. This association is especially close on the Sun for the Ca II H and K lines, for which the spatial correlation between chromospheric emission and photospheric fields is essentially one-to-one down to scales at least as fine as a few arcseconds and for which the emission flux from an area on the Sun is approximately proportional to the total magnetic flux passing through the same area in the underlying photosphere (Leighton 1959; Skumanich, Smythe, and Frazier 1975; Frazier 1971). The extension of the association to other LMS stars, while based on appeal to analogy, has been strengthened by recent detections of strong magnetic fields covering large fraction of the surface area of chromospherically active stars (see review by Marcy in this symposium).

3D Perspectives of Stellar Activity: Observation and Modelling

2012 ◽  
Vol 10 (H16) ◽  
pp. 109-110
Author(s):  
Moira Jardine
Keyword(s):  
Magnetic Field ◽  
Main Sequence ◽  
Solar Magnetic Field ◽  
3D Structure ◽  
Magnetic Activity ◽  
The Sun ◽  
Main Sequence Stars ◽  
Rotation Rates ◽  
Low Activity ◽  
Coronal Structures

AbstractAs the Sun emerges from a period of unprecedented low activity, the nature of the Sun's magnetic field compared to that of other stars is a particularly timely question. Just as observations of the full 3D structure of the solar magnetic field are becoming available through STEREO and SDO, advances in spectropolarimetric techniques now allow us to map the surface magnetic fields of other stars, revealing the great diversity of magnetic geometries that stars of different masses and rotation rates can display. This has now been possible for over 60 main sequence stars, with a smaller number of younger, pre-main sequence stars also mapped. Modelling of coronal structures based on these observations is revealing the full nature of stellar magnetic activity and its possible impact on orbiting planets.

Chromospheric activity as a function of age in main-sequence stars

1966 ◽  
Vol 24 ◽  
pp. 40-43
Author(s):  
O. C. Wilson ◽  
A. Skumanich
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Main Sequence Stars ◽  
Tentative Conclusion ◽  
Chromospheric Activity ◽  
General Field ◽  
Large Clusters

Evidence previously presented by one of the authors (1) suggests strongly that chromospheric activity decreases with age in main sequence stars. This tentative conclusion rests principally upon a comparison of the members of large clusters (Hyades, Praesepe, Pleiades) with non-cluster objects in the general field, including the Sun. It is at least conceivable, however, that cluster and non-cluster stars might differ in some fundamental fashion which could influence the degree of chromospheric activity, and that the observed differences in chromospheric activity would then be attributable to the circumstances of stellar origin rather than to age.

scholarly journals Stellar 5 min Oscillations

1983 ◽  
Vol 66 ◽  
pp. 469-486
Author(s):  
Jørgen Christensen-Dalsgaard ◽  
Søren Frandsen
Keyword(s):  
Spectral Type ◽  
Main Sequence ◽  
Total Flux ◽  
The Sun ◽  
Major Fraction ◽  
Main Sequence Stars ◽  
Red Supergiants

AbstractEstimates are given for the amplitudes of stochastically excited oscillations in Main Sequence stars and cool giants; these were obtained using the equipartition between convective and pulsational energy which was originally proposed by Goldreich and Keeley. The amplitudes of both velocity and luminosity perturbation generally increase with increasing mass along the Main Sequence as long as convection transports a major fraction of the total flux, and the amplitudes also increase with the age of the model. The 1.5 Mʘ ZAMS model, of spectral type F0, has velocity amplitudes ten times larger than those found in the Sun. For very luminous red supergiants luminosity amplitudes of up to about 0ṃ.1 are predicted, in rough agreement with observations presented by Maeder.

scholarly journals A comparison of the density gradients of main sequence stars obtained by two different methods in different galactic latitudes

1970 ◽  
Vol 38 ◽  
pp. 232-235
Author(s):  
W. Becker ◽  
R. Fenkart
Keyword(s):  
Main Sequence ◽  
Milky Way ◽  
Absolute Magnitude ◽  
Galactic Centre ◽  
Density Gradients ◽  
The Sun ◽  
Main Sequence Stars

The Basel Observatory program of the determination of disc- and halo-density gradients for different intervals of absolute magnitude comprises in addition to Milky Way fields several directions, all pointing to Selected Areas near a plane perpendicular to the galactic equator and passing through the sun and the galactic centre. It was started with SA 51 (Becker, 1965) and continued with Sa 57, 54 and 141 (Fenkart, 1967, 1968, 1969).

scholarly journals Activity and rotation of the X-ray emitting Kepler stars

2019 ◽  
Vol 628 ◽  
pp. A41 ◽  
Author(s):  
D. Pizzocaro ◽  
B. Stelzer ◽  
E. Poretti ◽  
S. Raetz ◽  
G. Micela ◽  
...  
Keyword(s):  
White Light ◽  
Main Sequence ◽  
Magnetic Activity ◽  
Light Curves ◽  
Late Type ◽  
Main Sequence Stars ◽  
Photometric Variability ◽  
X Ray ◽  
Rotation Periods ◽  
Flare Frequency

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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