scholarly journals Sphericity Effects of Extended Atmospheres of Late-Type Giants in the HR Diagram

1978 ◽  
Vol 80 ◽  
pp. 387-390
Author(s):  
Keiichi Kodaira

In the late phases of stellar evolution, evolutionary tracks of stars with different masses come together along the Hayashi line in the HR diagram. The theoretical HR diagram (log L, log Teff) is accordingly partially degenerate in the domain of late-type giants and supergiants, with respect to the third parameter, the stellar mass M. The stellar radius, R, being determined by log L and log Teff, the mass determines the surface gravity log g at the radius R. These parameters enable us to transform a point in the theoretical HR diagram to the corresponding point in the empirical HR diagram MV, (R-I) or spectral type. This transformation is conventionally carried out within the framework of the plane-parallel approximation in stellar atmospheres, and the parameters for the abscissa of the empirical HR diagram are dependant upon Teffand log g alone, irrespective of the mass itself. In this case, the parameter M indirectly affects the observable quantities through log g, but the effects of a variation by Δlog g=±0.5, corresponding to Δlog M=±0.5, are almost insignificant (cf. Tsuji 1976). The transformation between the theoretical and the empirical HR diagram is, therefore, almost one-to-one, within the framework of the plane-parallel approximation. Late-type giants and supergiants, however, have moderately extended atmospheres in general (cf. Schmid-Burgk and Scholz 1975), and their photometric colors and spectra are expected to be influenced by the sphericity of the atmospheric structure. Consequently, in comparing empirical HR diagrams with theoretical ones, it is important to know how atmospheric sphericity affects the transformation in the degenerate domains of the theoretical diagram.

2007 ◽  
Vol 3 (S242) ◽  
pp. 236-245
Author(s):  
Athol J. Kemball

AbstractThis paper reviews recent advances in the study or circumstellar masers and masers found toward supernova remnants. The review is organized by science focus area, including the astrophysics of extended stellar atmospheres, stellar mass-loss processes and outflows, late-type evolved stellar evolution, stellar maser excitation and chemistry, and the use of stellar masers as independent distance estimators. Masers toward supernova remnants are covered separately. Recent advances and open future questions in this field are explored.


1987 ◽  
Vol 122 ◽  
pp. 307-318
Author(s):  
Dieter Reimers

It is shown that the binary technique - a B star companion is used as a light source which probes the wind of the red giant primary - has yielded accurate mass-loss rates and wind velocities for 8 G to M (super) giants and (in some cases) estimates of wind temperature.Eclipsing binary systems have in addition revealed that G and K supergiants possess extended chromospheres which could be detected outwards to ∼ 1 R* stellar radius) above the photospheres. Electron temperatures Te and hydrogen ionization ne/nH seem to increase with height up to at least 0.5 R* (ne/nH= 10−2, Te = 104 K at 0.5 R*), and the winds start to be accelerated at heights above ∼ 0.5 R*.Mass-loss rates appear to increase steeper than linearly with L/g · R. It is shown that the observed mass-loss rates are consistent with stellar evolution constraints for both Pop. II and Pop I stars.


2013 ◽  
Vol 22 (2) ◽  
Author(s):  
A. Sapar ◽  
R. Poolamäe ◽  
L. Sapar

AbstractIn the present study we had three main aims. First to study the possibility of reducing the initial model atmosphere data to short analytical polynomials. The second was to use as the depth variable the logarithm of the local gas pressure instead the Rosseland mean. The third aim was to check the applicability of the derived formulae and proposed computation methods to obtain high precision self-consistent results in modeling hot plane-parallel stellar atmospheres. Introducing the dimensionless (reduced) local quantities


1998 ◽  
Vol 11 (1) ◽  
pp. 571-571
Author(s):  
M. Haywood ◽  
J. Palasi ◽  
A. Gómez ◽  
L. Meillon Dasgal

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.


1998 ◽  
Vol 11 (2) ◽  
pp. 671-673
Author(s):  
G. Alecian

We present a brief review about recent progresses concerning the study of diffusion processes in CP stars. The most spectacular of them concerns the calculation of radiative accelerations in stellar envelopes for which an accuracy better than 30% can now be reached for a large number of ions. This improvement is mainly due to huge and accurate atomic and opacity data bases available since the beginning of the 90’s. Developments of efficient computational methods have been carried out to take advantage of these new data. These progresses have, in turn, led to a better understanding of how the element stratification is building up with time. A computation of self-consistent stellar evolution models, including time-dependent diffusion, can now be within the scope of the next few years. However, the progresses previously mentioned do not apply for stellar atmospheres and upper layers of envelopes.


Comunicar ◽  
2005 ◽  
Vol 13 (25) ◽  
Author(s):  
Ana-Luisa Fernandes-Gonçalves

The present article is about the presence of the tv in the society, in general, and its potential as an important agent, such as the family and the school, on the children’s and young people’s development and education. Nowadays we live in a complex world of images with sound, the most of them transmitted by the tv, the cinema and the internet. The infantile universe is full of those images. This way we structured this article in four parts, refering some of the contents related with this subject. The first part describes the presence of the television in the children's daylife and their relationship with the magic box, even the identification that they make with its favourites characters, in order to understand the space that the TV occupies in their lives. The second part presents the different messages transmitted by the television. It is important to analyze the main risks and advantages of television, that is to say, in spite of some programs show bad values, foment the excessive consumption, expose the children to an atmosphere of violence and pornography, also entertains, accompanies, educates, informs, favours the access to the culture and allows to unite the family around a program. At last the third part, this one outlines the necessity to put the tele as a curricular resource in the school, to teaches the students how to analyze, how to criticize and to think about the texts and the news of the world today. The television, considered a powerful tool, is decisive in the communication, such in the formation of values and ideas as in the culture transmission. The function of the school is not to remain far from this factor, but to try to meditate, to understand and to decide its impact, in a better way, on the development of the children and youngs. As conclusion we can add that we’ve tried to aim some strategies that educators and parents will be able to work, united to the children and the youngs, so they could have a new look of the Tv’s world and that they could know how to make their own selection and how to have a critical attitude before the watched product.La presente comunicación se refiere a la presencia de la tele en la sociedad en general y a su potencial como agente importante de formación y educación, junto con la escuela y la familia, en la vida de los niños y de los jóvenes. Actualmente vivimos inmersos en un mundo complejo de imágenes con sonido, que recibimos muy especialmente a través de la tele, del cine y de Internet, los cuales hacen parte del cotidiano del universo infantil. Así, a fin de organizar algunos de los contenidos desde los que es posible enfocar el tema, estructuramos este artículo en cuatro partes. La primera parte describe la presencia de la tele a diario de los niños y su relación con la caja mágica, incluso la identificación que hacen de sus personajes favoritos, a fin de comprender el espacio que ocupa la tele en su cotidianidad. La segunda parte presenta los distintos mensajes transmitidos por la tele. Es importante analizar las principales riesgos y ventajes de la televisión, o sea, a pesar de algunos programas muestran antivalores de la tele: fomentar excesivamente el consumo, exponer a los niños a un ambiente de violencia y pornografía también entretiene, acompaña, educa, informa, favorece el acceso a la cultura y permite unir a la familia en torno a un programa. Cuanto a la tercera parte, plantea la necesidad de incorporar la tele como recurso curricular en la escuela, para que enseñe a los alumnos a analizar crítica y reflexivamente los textos mediáticos. La televisión es considerada como una herramienta poderosa, determinante en la comunicación, tanto en la formación de valores e ideas, como en la transmisión de cultura; la función de la escuela no es permanecer ajena a este factor, pero procurar reflexionar para comprender y decidir mejor su impacto en el desarrollo de los niños y jóvenes. Como conclusión podemos añadir que hemos tratado de apuntar algunas estrategias que educadores y padres podrán trabajar junto con los niños y los jóvenes, para que éstes tengan una nueva mirada del mundo de la tele y para que sepan hacer su propia selección y tener una actitud crítica ante el producto visionado.


1997 ◽  
Vol 189 ◽  
pp. 209-216
Author(s):  
D. J. Hillier

The modeling of hot star atmospheres falls into two broad classes: those where the plane parallel approximation can be used, and those where the effects of spherical extension and stellar winds are important. In both cases non-LTE modeling is a necessity for reliable spectroscopic analyses.While simple ions (e.g., H, He I, and He II) have been treated routinely in non-LTE for many years it is only recently that advances in computing power, computational techniques, and the availability of atomic data have made it feasible to perform non-LTE line blanketing calculations. Present models, with varying degrees of approximation and sophistication, are now capable of treating the effects of tens of thousands of lines. We review the latest efforts in incorporating non-LTE line blanketing, highlighting recent advances in the modeling of 0 stars, hot sub-dwarfs, Wolf-Rayet stars, novae, and supernovae.


1987 ◽  
Vol 122 ◽  
pp. 449-450
Author(s):  
Raman K. Prinja ◽  
Ian D. Howarth

The most sensitive indicators of mass-loss for stars in the upper left part of the HR diagram are the UV P Cygni profiles observed in the resonance lines of common ions such as N V, Si IV, and C IV. We present here some results from a study of these lines in the high resolution IUE spectra of 197 Ï stars. Profile fits were carried out in the manner described by Prinja & Howarth (1986) for all unsaturated P Cygni resonance doublets. The parameterisations adopted enable the product of mass-loss rate (Ṁ) and ion fraction (qi) to be determined at a given velocity, such that Ṁ qi°C Ni R* v∞, where Ni is the column density of the observed ion i, v∞ is the terminal velocity, and R⋆ is the stellar radius. The accompanying figures illustrate the behaviour of Ṁ qi (evaluated at 0.5 v∞) for N V and C IV.


1989 ◽  
Vol 106 ◽  
pp. 35-50 ◽  
Author(s):  
Harvey B. Richer

AbstractStudy of the late-type stellar content in external galaxies provides numerous clues for the theory of stellar evolution, for star-formation scenarios in galaxies, and for proper models of the luminosity evolution of galaxies which are then used in cosmological studies. In addition, these late-type stars can be used as distance indicators themselves and yield a local value of the Hubble constant consistent with recent Cepheid determinations.


1921 ◽  
Vol 10 (155) ◽  
pp. 363-368
Author(s):  
F. G. Brown

The question of sign constitutes a real difficulty to the intelligent boy at the outset of his study of Coordinate Geometry. At the beginning of his Trigonometry he is told OP must be considered always positive, but later on he will find some authorities giving a point in the third quadrant as ( - r, θ), while others prefer (r, θ + π). The perpendicular distance of (h, k) from ax + by + c = 0 is given by ± (ah + bk +c)/(a2 + b2)½, and sign seems to matter, but usually the pupil is told that he only wants to know how far off (h, k) is, and he is advised to stick to the absolute value. But a little later on he wants the equations of the bisectors of the angles between two given lines, and then he is blamed for not remembering that signs matter a good deal.


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