Intermediate-band photometry of late-type stars. VI - Main-sequence stars near the sun. VII - The HR 1614 group of overabundant stars

1978 ◽  
Vol 222 ◽  
pp. 191 ◽  
Author(s):  
O. J. Eggen
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Late Type ◽  
Main Sequence Stars ◽  
Intermediate Band

Global Oscillations in Late-Type Stars (Invited Paper)

1987 ◽  
Vol 7 (1) ◽  
pp. 2-5
Author(s):  
L. E. Cram
Keyword(s):  
Main Sequence ◽  
The Sun ◽  
Late Type ◽  
Main Sequence Stars ◽  
Oscillation Spectrum ◽  
New Techniques ◽  
Solar Structure ◽  
Stellar Oscillations ◽  
Global Oscillation ◽  
Instrumental Development

AbstractThe advent of new techniques to measure the global oscillation spectrum of the Sun has provided a new and powerful tool to investigate solar structure. One of the most challenging and potentially rewarding problems in contemporary astronomy is to devise techniques which will allow similar studies of other stars. This paper outlines the theory of global oscillations of late-type main-sequence stars, and summarises some of the achievements of global oscillation studies of the Sun. It then reviews the very few successful attempts that have been made to study stellar oscillations, and briefly discusses several apparently promising lines for further instrumental development.

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 A preliminary color-magnitude diagram for late-type stars in the solar neighborhood

1959 ◽  
Vol 10 ◽  
pp. 39-40
Author(s):  
O. C. Wilson
Keyword(s):  
Main Sequence ◽  
Star Clusters ◽  
Solar Neighborhood ◽  
Late Type ◽  
Main Sequence Stars ◽  
Individual Values ◽  
Magnitude Diagram ◽  
Trigonometric Parallaxes ◽  
Nearby Stars ◽  
The Mean

Modern photoelectric techniques yield magnitudes and colors of stars with accuracies of the order of a few thousandths and a few hundredths of a magnitude respectively. Hence for star clusters it is possible to derive highly accurate color-magnitude arrays since all of the members of a cluster may be considered to be at the same distance from the observer. It is much more difficult to do this for the nearby stars where all of the objects concerned are at different, and often poorly determined, distances. If one depends upon trigonometric parallaxes, the bulk of the reliable individual values will refer to main sequence stars, and while the mean luminosities of brighter stars are given reasonably well by this method, the scatter introduced into a color-magnitude array by using individual trigonometrically determined luminosities could obscure important features. Somewhat similar objections could be raised against the use of the usual spectroscopic parallaxes which also should be quite good for the main sequence but undoubtedly exhibit appreciable scatter for some, at least, of the brighter stars.

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.

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 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.

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