Effects of rotation and tidal distortions on the periods of adiabatic oscillations of composite models of stars

Mohan and Saxena's approach of using the averaging technique of Kippenhahn and Thomas in conjunction with Kopal's method of evaluating various parameters on the Roche equipotentials has been used to compute the effects of rotation and tidal distortions on the periods of small adiabatic radial and nonradial modes of oscillations of a series of composite models of stars. In these stars the density decreases slowly in the core from the centre to the interface and then falls of rapidly in the envelope from the interface to the outer surface.     

A comment on jovian greenhouse models

Radiative greenhouse models of Jupiter's atmosphere seriously overestimate the temperature of the lower cloud level because they neglect the convective transport of heat.     

Effects of rotation and tidal distortion on the periods of small adiabatic oscillations of the polytropic models of the stars

The averaging technique of Kippenhahn and Thomas (1970) has been used in conjunction with Kopal's method of evaluating various parameters on the Roche equipotentials to determine the effects of rotation and tidal distortions on the periods of small adiabatic radial and nonradial modes of oscillations of polytropic models of the stars.     

Chemical spots and oscillatory diffusion modes in magnetic stars

The stars of the middle main sequence often have spot‐like chemical structures at their surfaces. We consider diffusion caused by electric currents and argue that such current‐driven diffusion can form chemical inhomogeneities in a plasma. The considered mechanism can contribute to a formation of element spots in Hg‐Mn and Ap‐stars. Due to the Hall effect, diffusion in the presence of electric currents can be accompanied by the propagation of a particular type of magnetohydrodynamic modes in which only the impurity number density oscillates. Such modes exist if the magnetic pressure is much greater than the gas pressure and can be the reason for variations of the abundance peculiarities in stars. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Radii of some Ap,Am and A stars

Based on the observed energy curves of nine Ap stars, three Am stars, four normal A stars and one F0 V magnetic star, their radii have been estimated.Thence, the bolometric magnitudesM _bo1 have been obtained and a plot between logT _e andM _bo1 of these stars shows that a majority of Ap and Am stars are a little above the zero-age Main-Sequence, suggesting that they are slightly more evolved as compared to the normal A stars.The bolometric corrections derived from the aboveM _bo1 are much closer to those computed by Mihalas than to the ones given by Davis and Webb, the latter being about O ^m 1 more negative than the former.     

Absolute energy distributions of some A and F stars

The absolute energy distribution of fourteen A and F type stars lying in the Sct-region of the H-R diagram have been given. Their effective temperatures and gravities have been estimated from comparisons with model atmospheres. The masses derived for these stars are found to agree well with those inferred from the evolutionary tracks.     

Spectrophotometry of some Be stars

The continuum energy distribution of six Be stars, namely 25 Cyg, 31 Peg, HR 8758, 14 Lac, 12 Vul, and Psc, in the wavelength region 3200–7800 Å, are presented. Comparing the observed energy distributions with those of theoretical models given by Kurucz (1979), their effective temperatures are determined.A near-infrared excess emission at wavelengths above 6000 Å is seen in most of the stars.     

Spectral models of AM Her stars

Most of the optical light from AM Her systems is cyclotron radiation. Simple models for the spectrum and polarization are successful for some systems, but in others the spectrum rolls over more gently at high frequencies and is polarized over a wider range of frequencies than the models predict. This paper considers the emission from both the sides and top of the accretion column and the effects of oscillations in the shock height. These features lead to a slower roll over at high frequencies than is found in the simple models, but it is still not as flat as in some of the AM Her systems.Paper presented at the IAU Colloquium No. 93 on Cataclysmic Variables. Recent Multi-Frequency Observations and Theoretical Developments, held at Dr. Remeis-Sternwarte Bamberg, F.R.G., 16–19 June, 1986.     

Eigenfrequencies of small adiabatic barotropic modes of oscillations of rotationally and tidally-distorted stars

In this paper a method is proposed for computing the eigenfrequencies of small adiabatic barotropic modes of oscillations of rotationally and tidally-distorted stars. The method utilizes Kippenhahn and Thomas approach and concepts of Roche equipotentials to incorporate up to second-order the effects of rotation and tidal distortion terms on the eigenfrequencies. The proposed method has also been used to compute the eigenfrequencies of certain barotropic modes of oscillation of some rotationally and tidally distorted models of 10M , and 2.5M Main-Sequence stars.     

On the models of core-helium-burning stars

Summary Some aspects concerning the core-mass increase and the appearance and development of an intermediate semiconvective zone in helium-burning stars are discussed. An iterative method of computation of core increase and semiconvection is also presented. Details of this procedure are given with regard to the horizontal-branch stars of globular clusters.

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Riassunto Sono presi in esame alcuni aspetti relativi all'aumento in massa del nucleo ed alla comparsa e sviluppo di una zona semiconvettiva intermedia in modelli di stelle con combustione centrale dell'elio. E'inoltre schematizzato il metodo iterativo di calcolo con particolare riferimento alle stelle di ramo orizzontale degli ammassi globulari.

     

A comment on ‘Far ultraviolet photometry of orion stars’ by G.R. Carruthers

It is shown that the ultraviolet flux-deficiency, recently discovered by Carruthers, in early-type supergiants compared to dwarfs of the same spectral type can be explained in terms of the effects of differences in surface-gravity and effective temperature between stars of these two luminosity classes. It thus appears that this deficiency does not represent a serious discrepancy between theory and observation.     

Charged anisotropic models for quark stars

We perform a detailed physical analysis for a class of exact solutions for the Einstein–Maxwell equations. The linear equation of state consistent with quark stars has been incorporated in the model. The physical analysis of the exact solutions is performed by considering the charged anisotropic stars for the particular nonsingular exact model obtained by Maharaj, Sunzu and Ray. In performing such an analysis we regain masses obtained by previous researchers for isotropic and anisotropic matter. It is also indicated that other masses and radii may be generated which are in acceptable ranges consistent with observed values of stellar objects. A study of the mass-radius relation indicates the effect of the electromagnetic field and anisotropy on the mass of the relativistic star.     

Sensitivity in the models of massive stars

Two related problems are discussed in this article: The width of the Main-Sequence of massive stars and sensitivity peatures introduced into the evolutionary tracks of massive stars by mass loss and core-overshooting. It is suggested that core-overshooting may not necessarily be implied by the observations of the width of the Main-Sequence band. It is also noted that models evolved with both mass loss and/or core-overshooting reveal the presence of a large and unexplained expansion of the stellar models under certain conditions. This sensitivity feature would seem to be a fundamental feature inherent to the structure of massive stars.     

A study of some stars with circumstellar dust envelopes. I

We present the results of a study of circumstellar dust envelopes of 36 stars of early(O-B-A) types in the directions of the associations Cas OB1, Cas OB2, Per OB1, and Ori OB1. We determine the absorption at 1640 Å, the linear radius of the dust envelopes, the mean value of the coefficient k, and the masses of the envelopes. They differ significantly from one another.Translated fromAstrofizika, Vol. 38, No. 3, 1995.     

Towards self-consistent models of isolated neutron stars

We propose a self–consistent model to explain all observational properties reported so far on the isolated neutron star (INS) RX J0720-3125 with the aim of giving a step forward towards our understanding of INSs. For a given magnetic field structure, which is mostly confined to the crust and outer layers, we obtain theoretical models and spectra which account for the broadband spectral energy distribution (including the apparent optical excess), the X-ray pulsations, and for the spectral feature seen in the soft X-ray spectrum of RX J0720-3125 around 0.3 keV. By fitting our models to existing archival X-ray data from 6 different XMM–Newton observations and available optical data, we show that the observed properties are fully consistent with a normal neutron star, with a proper radius of about 12 km, a temperature at the magnetic pole of about 100 eV, and a magnetic field strength of 2–3×10¹³ G. Moreover, we are able to reproduce the observed long–term spectral evolution in terms of free precession which induces changes in the orientation angles of about 40 degrees with a periodicity of 7 years. In addition to the evidence of internal toroidal components, we also find strong evidence of non–dipolar magnetic fields, since all spectral properties are better reproduced with models with strong quadrupolar components.      

2-D models of rapidly rotating stars

Recent observational advances in the fields of asteroseismology and interferometry, have put in evidence the need of physically realistic models of rapidly rotating stars. In rapidly rotating stars, the centrifugal force affects dramatically the structure of the star and makes it necessary to use 2-D methods that take fully into account the deformation of the star. We compute the structure of rapidly rotating stars using 2-D spectral methods. The advantage of spectral methods compared with finite difference methods is that they can achieve the same accuracy while reducing significantly the number of grid points, thus saving computing time. The models include core convection and realistic microphysics (tabulated equation of state and opacity).     

NLTE wind models of hot subdwarf stars

We calculate NLTE models of stellar winds of hot compact stars (central stars of planetary nebulae and subdwarf stars). The studied range of subdwarf parameters is selected to cover a large part of these stars. The models predict the wind hydrodynamical structure and provide mass-loss rates for different abundances. Our models show that CNO elements are important drivers of subdwarf winds, especially for low-luminosity stars. We study the effect of X-rays and instabilities on these winds. Due to the line-driven wind instability, a significant part of the wind could be very hot.     

Main sequence models for massive Zero-metal stars

Zero-age main-sequence models for stars of 20, 10, 5 and 2M _⊙ with no heavy elements are constructed for three different possible primordial helium abundances:Y=0.00,Y=0.23, andY=0.30. The latter two values ofY bracket the range of primordial helium abundances cited by Wagoner. With the exceptions of the two 20M _⊙ models that contain helium, these models are found to be self-consistent in the sense that the formation of carbon through the triple-alpha process during pre-main sequence contraction is not sufficient to bring the CN cycle into competition with the proton-proton chain on the ZAMS. The zero-metal models of the present study have higher surface and central temperatures, higher central densities, smaller radii, and smaller convective cores than do the population I models with the same masses. If galaxies containing the zero-metal stars were formed as recently as one third the Hubble time, they would likely appear very blue today — perhaps bluer even that most known quasars — and their redshifted effective temperatures could range as high as 3×10⁴ K to 4×10⁴ K.     

Calibrating models of ultralow‐mass stars

Evolutionary and atmospheric models have become available for young ultralow‐mass objects. These models are being used to determine fundamental parameters from observational properties. TiO bands are used to determine effective temperatures in ultralow‐mass objects, and together with Na‐ and K‐lines to derive gravities at the substellar boundary. Unfortunately, model calibrations in (young) ultralow‐mass objects are rare. As a first step towards a calibration of synthetic spectral features, I show molecular bands of TiO, which is a main opacity source in late M‐dwarfs. The TiO ε ‐band at 8450 Å is systematically too weak. This implies that temperatures determined from that band are underestimated, and I discuss implications for determining fundamental parameters from high resolution spectra. (© 2005 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)