scholarly journals Spotted Stars in the Perspective of HR Diagram

2021 ◽  
Author(s):  
Ingila Rahim
Keyword(s):  
Spotted Stars ◽  
Hr Diagram

.

Meridional Circulation, Turbulence, and Diffusion Processes in Stars

1976 ◽  
Vol 32 ◽  
pp. 109-116 ◽  
Author(s):  
S. Vauclair
Keyword(s):  
Convection Zone ◽  
Diffusion Processes ◽  
Meridional Circulation ◽  
Diffusion Time ◽  
Work In Progress ◽  
First Results ◽  
Stellar Envelopes ◽  
And Diffusion ◽  
Turbulence And Diffusion ◽  
Hr Diagram

This paper gives the first results of a work in progress, in collaboration with G. Michaud and G. Vauclair. It is a first attempt to compute the effects of meridional circulation and turbulence on diffusion processes in stellar envelopes. Computations have been made for a 2 Mʘstar, which lies in the Am - δ Scuti region of the HR diagram.Let us recall that in Am stars diffusion cannot occur between the two outer convection zones, contrary to what was assumed by Watson (1970, 1971) and Smith (1971), since they are linked by overshooting (Latour, 1972; Toomre et al., 1975). But diffusion may occur at the bottom of the second convection zone. According to Vauclair et al. (1974), the second convection zone, due to He II ionization, disappears after a time equal to the helium diffusion time, and then diffusion may happen at the bottom of the first convection zone, so that the arguments by Watson and Smith are preserved.

scholarly journals Sakurai’s Object: Constraints from its Variability?

1998 ◽  
Vol 11 (1) ◽  
pp. 360-360
Author(s):  
T. Gautschy ◽  
H.W. Duerbeck ◽  
A.M. Van Genderen ◽  
S. Benetti
Keyword(s):  
Cooling Rate ◽  
Nuclear Energy ◽  
Stability Analyses ◽  
The Past ◽  
Stellar Pulsations ◽  
First Results ◽  
Effective Temperatures ◽  
Stellar Envelopes ◽  
Light Variability ◽  
Hr Diagram

The peculiar outburst of the star baptized Sakurai’s Object (SO) is a conceivable example of a late He shell flash in a post-AGB object. The new source of nuclear energy forces such objects toward high luminosities and eventually low effective temperatures; they cross the HR diagram in a comparable fashion as FG Sge did in the past - i.e., they move noticeably on the HR diagram on human timescales. From monitoring campaigns of SO during the last year, first estimates of its cooling rate were derived and in particular cyclic light variability was established. We present first results from attempts to model stellar envelopes appropriate for SO. As we hypothesize the light variability to be attributable to stellar pulsations, we aim at constraining the basic stellar parameters based on stability analyses of our envelope models. Radial, nonadiabatic stability computations provided predictions of the modal content which should be observable as SO evolves. The particular components in such mode spectra of SO as they are to appear in the coming years should indeed help to constrain basic stellar parameters such as mass and luminosity.

scholarly journals The Metallicity Distribution of Late Type Dwarfs

1998 ◽  
Vol 11 (1) ◽  
pp. 571-571
Author(s):  
M. Haywood ◽  
J. Palasi ◽  
A. Gómez ◽  
L. Meillon Dasgal
Keyword(s):  
Star Formation Rate ◽  
Formation Rate ◽  
Angular Separation ◽  
Stellar Populations ◽  
Late Type ◽  
Limited Sample ◽  
Hipparcos Catalogue ◽  
Galactic Stellar Populations ◽  
Metallicity Distribution ◽  
Hr Diagram

The Hipparcos catalogue provides an accurate and extensive sampling of the solar neighbourhood HR diagram. The morphology of this diagram depends on selection criteria of the catalogue such as the limiting magnitude, angular separation and on the characteristics of the stellar populations near the sun (space density, metallicity, star formation rate, etc). Since the Hipparcos data are so accurate, one needs to model precisely the different selection bias and, at the same time, parametrize models of the galactic stellar populations with sufficient flexibility that as much information as possible can be grasped from the catalogue. Comparisons between our model and the Hipparcos catalogue will be presented elsewhere. Since the quantity of information contained in the Hipparcoscatalogue is so important, models ought to be complex, and external contraints, obtained prior to any general comparison with the model, are welcome. A major factor that influences the distribution of the stars in the HR diagram is the metallicity. For the late type stars, the metallicity distribution can be best studied by re-analysing a volume-limited sample of stars from the catalogue.

scholarly journals Towards an Improved Model of the Galaxy

1998 ◽  
Vol 11 (1) ◽  
pp. 570-570
Author(s):  
Johan Holmberg ◽  
Lennart Lindegren ◽  
Chris Flynn
Keyword(s):  
Three Dimensional ◽  
Direct Determination ◽  
Galactic Structure ◽  
Open Clusters ◽  
Selection Effects ◽  
Observational Errors ◽  
Red Giant ◽  
Improved Model ◽  
First Time ◽  
Hr Diagram

We use the Hipparcos survey to derive an improved model of the local galactic structure. The availability of parallaxes for all the stars permits direct determination of stellar distributions, eliminating the basic indeterminacy of classical methods based on star counts. Hipparcos gives for the first time a truly three-dimensional view of the solar vicinity, and a complete, homogeneous and highly accurate set of magnitudes and colours. This means that new techniques can be applied in the treatment of the data which place strong constraints on a model that tries to describe the local Galactic structure. Here we investigate how well a static model of low complexitycan describe the Hipparcos observations. The interpretation of the Hipparcos data is complicated by various observational errors and selection effects that are hard to treat correctly. We do not try to correct the data, but instead use a model and subject this model to the same observational errors and selection effects. A model catalogue is created that can be compared with the observed catalogue directly in the observational domain, thereby eliminating the effects from various biases. Many features in the HR diagram are for the first time seen in field stars thanks to Hipparcos, such as the slanted red giant clump, previously seen in rich old open clusters such as Berkeley 18. This and other features ofthe observed HR diagram are well reproduced by the model thanks to the rather detailed modelling of the joint Mv/B — V distribution. Actually, separate distributions were derived for the three different components, disk, thick disk and halo, using the kinematic characteristics of the components to discriminate between them.

scholarly journals Luminosity Calibration of Giants and Supergiants, G0–M5

1973 ◽  
Vol 54 ◽  
pp. 68-77
Author(s):  
Ph. C. Keenan
Keyword(s):  
Calibration Curves ◽  
Hr Diagram

Calibration curves giving Mv for stars of luminosity classes III, II, Ib, Iab and 0 are derived and shown graphically in the HR diagram. There are serious gaps in which the calibration needs to be improved.

scholarly journals Relating turbulent pressure and macroturbulence across the HR diagram with a possible link toγDoradus stars

2015 ◽  
Vol 584 ◽  
pp. L2 ◽  
Author(s):  
L. Grassitelli ◽  
L. Fossati ◽  
N. Langer ◽  
A. Miglio ◽  
A. G. Istrate ◽  
...  
Keyword(s):  
Turbulent Pressure ◽  
Hr Diagram

scholarly journals Tests of Evolution Models Using Eclipsing Binaries

1995 ◽  
Vol 10 ◽  
pp. 419-422
Author(s):  
J. Andersen
Keyword(s):  
Chemical Composition ◽  
Star Clusters ◽  
Stellar Model ◽  
Eclipsing Binaries ◽  
Fundamental Parameter ◽  
Fundamental Parameters ◽  
Stellar Models ◽  
Absolute Age ◽  
Cluster Sequence ◽  
Hr Diagram

Stellar models are the means by which we describe and understand the distribution of stars in the HR diagram. A stellar model is, in principle, completely specified by the three fundamental parameters mass, chemical composition, and age. Comparing the properties of models and real stars with the same parameters will tell us if our recipe for constructing stellar models is realistic. Unfortunately, the only star for which all three are known independently of stellar models is the Sun. For stars of other masses and ages we must devise observational tests in which at least one fundamental parameter is unknown. Two such popular test objects are double-lined eclipsing binaries and star clusters.In suitable eclipsing binaries we can determine both masses and chemical composition; the absolute age is unknown, but the same for both stars. Since evolution depends most sensitively on the mass, eclipsing binaries provide a very direct test of the models, but only for two points on a single isochrone. In star clusters, neither ages nor individual masses are known, but the detailed shape and population of a well-observed cluster sequence in the HR diagram provide a number of additional probes into the models.

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