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 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.

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 Photospheric and “Chromospheric” CIV Lines in B-Main-Sequence Stars

1986 ◽  
Vol 116 ◽  
pp. 113-116
Author(s):  
Fiorella Castelli ◽  
Carlo Morossi ◽  
Roberto Stalio
Keyword(s):  
High Temperature ◽  
Stellar Wind ◽  
High Velocity ◽  
Main Sequence ◽  
Stellar Atmosphere ◽  
Uv Spectra ◽  
Spectral Lines ◽  
Early Type ◽  
Main Sequence Stars ◽  
Early Type Stars

The presence in the far-UV spectra of early-type stars of spectral lines of superionized atoms is argument of controversial debate among astronomers. Presently there is agreement on the non-radiative origin of these ions but not on the proposed mechanisms for their production nor on the proposed locations in the stellar atmosphere where they are abundant. Cassinelli et al. (1978) suggest that the Auger mechanism is operative in a cool wind blowing above a narrow corona to produce these ions; Lucy and White (1980) introduce radiative instabilities growing into hot blobs distributed across the stellar wind; Doazan and Thomas (1982) make these ions to be formed in both pre- and post-coronal, high temperature regions at low and high velocity respectively.

scholarly journals The Rotation of Pre-Main Sequence Stars

1980 ◽  
Vol 5 ◽  
pp. 835-837
Author(s):  
Leonard V. Kuhi ◽  
Stuart Vogel
Keyword(s):  
Angular Momentum ◽  
Spectral Type ◽  
Main Sequence ◽  
Rotational Velocity ◽  
The Sun ◽  
Main Sequence Stars ◽  
Lower Mass ◽  
B Stars ◽  
Large Numbers ◽  
Rapid Deceleration

Kraft (1970) obtained the rotational velocities for large numbers of stars located in the field and in clusters of different ages. He noted that (a) among the field stars those stars with strong Call K emission had larger rotational velocities than those without; (b) stars in the Hyades and Pleiades (which are much younger than the field) had both larger rotational velocities and stronger Call K emission than field stars; (c) there was a pronounced break at spectral type early F in v sini as a function of spectral type and (d) the distribution of angular momentum per unit, mass J(M⊚) was proportional to M0.57 for main sequence stars with mass M > 1.5 Mʘ. This distribution predicted a v sini of ˜75 km/sec for stars of lower mass (e.g. G type) but such high velocities were not seen in the Pleiades nor in the sun. This implied a more rapid deceleration of v sini for lower mass stars and led to estimates of the e-folding time of ˜4×l08 years for stars of 1.2 M⊚ to reduce their v sini from that of the Pleiades to that of the Hyades and ˜4×l09 years to go from the Hyades to the sun’s v sini. We note also that the age of the Pleiades is approximately equal to the pre-main sequence lifetime of a 1.0 M0 star so that the zero-age main sequence cannot have J(M) α M0.57 for ˜1 M0 stars. Skumanich (1972) showed that both the Call k emission and the rotational velocity decayed as the (age)-½ for main-sequence stars.

scholarly journals The Existence of Moving Groups and the Disk Heating Problem

1996 ◽  
Vol 169 ◽  
pp. 513-514
Author(s):  
B. Chen ◽  
F. Figueras ◽  
J. Torra ◽  
R. Asiain ◽  
C. Jordi
Keyword(s):  
Main Sequence ◽  
Velocity Dispersion ◽  
A Priori ◽  
The Sun ◽  
Main Sequence Stars ◽  
Large Sample ◽  
Objective Evidence ◽  
Moving Groups ◽  
Input Catalogue ◽  
Local Disk

A powerful moving group-finding algorithm has been developed and applied to a large sample of B, A and F main sequence stars from the Hipparcos Input Catalogue. Four moving groups near the Sun (Pleiades, Sirius, IC2391 and Hyades) have been identified without assuming any a priori knowledge of the properties of moving groups. This is the most convincing objective evidence for their existence. After the members of the moving groups are removed from the sample, we investigate the disk heating problem. The results show that the velocity dispersion of the local disk stars increases with age, roughly as ≃ ∞ τ1/5.

scholarly journals Open issues in probing interiors of solar-like oscillating main sequence stars 1. From the Sun to nearly suns

2011 ◽  
Vol 271 ◽  
pp. 012031 ◽  
Author(s):  
M J Goupil ◽  
Y Lebreton ◽  
J P Marques ◽  
R Samadi ◽  
F Baudin
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Main Sequence Stars ◽  
Open Issues

scholarly journals Emission of Gravitational Waves from a Magnetohydrodynamic Dynamo in the Center of the Sun

2015 ◽  
Vol 70 (7) ◽  
pp. 545-551
Author(s):  
Friedwardt Winterberg
Keyword(s):  
Magnetic Field ◽  
Gravitational Waves ◽  
Main Sequence ◽  
Fusion Reaction ◽  
Large Mass ◽  
The Sun ◽  
The Moon ◽  
Convection Pattern ◽  
Main Sequence Stars ◽  
The Waves

AbstractBy Birkhoff’s theorem a spherical symmetric convection pattern, as it is assumed to exist for main sequence stars as our sun, cannot lead to the emission of gravitational waves, but all stars that have a magnetic field generated by a magnetohydrodynamic dynamo must by a theorem of Cowling have a non-spherical symmetric convections pattern and for this reason have to emit gravitational waves. The intensity of the thusly emitted gravitational waves depends on the efficiency of this dynamo, expressed by the departure from a spherical convection pattern. The magnitude of the asymmetry is determined by a solution of Elsaesser’s dynamo equations which only recently has become possible with supercomputers. The waves are emitted through large mass motions in the center of the sun by a thermonuclear fusion reaction-driven magnetohydrodynamic dynamo, with thermomagnetic currents in the tachocline shielding the strong magnetic field in the solar core. Using the moon as a large Weber bar, the gravitational waves are focused into the lunar shadow by Poisson diffraction where their effect might become observable during a total solar eclipse.

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