Stability analysis of C−N−O Nuclear reaction inside the stars

In view of the fact that the nuclear reactions inside the stars are very strong, we introduce the effects of convection and diffusion in the fundamental equation of the problem in modeling to the C–N–O nuclear reaction. The stability of the nuclear reaction inside the star is investigated, and the results of some interest are obtained.     

Oscillations in nuclear reaction systems inside stars

Oscillations related to the particle number density in the nuclear reaction systems inside stars are investigated, which include the oscillations in the helium nuclear reaction and in the P-PI branch of proton-proton reaction under the (g - T) effect and the gas pressure effect. The investigation shows that there is a supercritical oscillating bifurcation with time in the helium nuclear reaction system. If the oscillations are excited by the pressure effect, the nuclear reaction must occur in the convecting zone with a radially reducing convective velocity; if the oscillations are excited by the (g - T) effect, the nuclear reaction may occur in the convecting zone or no-convection zone. The supercritical oscillating bifurcation with time also exists in the P-PI reaction system, which is excited only by the (g - T) effect, while the oscillation excited by the pressure effect is decaying.     

Activity cycle periods in late-type stars

Period of magnetic activity versus the stellar angular velocity for stars of given spectral type having extended convective shells is estimated within the framework of mean field dynamo theory. The dependence appears to be not monotonous, and can be checked by observations.     

Infrared hydrogen emission lines in the spectra of Be stars

For the interpretation of observed hydrogen emission lines in the spectrum of Cas calculations of the relative intensities of spectral lines of the Balmer, Paschen, Brackett and Pfund series are carried out, by using the theory of moving stellar envelopes.It is shown that in the spectra of Be stars, which have opaque envelopes and therefore a slow Balmer decrement, infrared hydrogen lines must be relatively strong.It is also found that the observed relative line intensities of Cas are in qualitative agreement with those calculated for the case of a simple model-envelope which consists of low and high opacity regions.     

Electric and magnetic fields inside the superconducting core of neutron stars

Feynman's approach has been used to derive the equation of dynamics for type II superconductors from the Schr?dinger equation. A closed set of equations for the study of vortex dynamics has been obtained. These equations have been used for calculating electric and magnetic fields inside the core of neutron stars. In particular, the contribution of vortices to the generation of electric and magnetic fields inside the core of the star is explicitly displayed.     

Stability of superdense stars in the bimetric theory of gravitation

Calculations of superdense star models in the Rosen bimetric theory of gravitation are presented. There is a family of quark stars whose members are probably stable even if their masses do not exceed the maximum mass of neutron stars.     

Stability of resonant planetary orbits in binary stars

This paper contains a numerical study of the stability of resonant orbits in a planetary system consisting of two planets, moving under the gravitational attraction of a binary star. Its results are expected to provide us with useful information about real planetary systems and, at the same time, about periodic motions in the general four-body problem (G4) because the above system is a special case of G4 where two bodies have much larger masses than the masses of the other two (planets). The numerical results show that the main mechanism which generates instability is the destruction of the Jacobi integrals of the massless planets when their masses become nonzero and that resonances in the motion of planets do not imply, in general, instability. Considerable intervals of stable resonant orbits have been found. The above quantitative results are in agreement with the existing qualitative predictions     

Further VLA observations of hydrogen deficient stars

VLA observations at 6 cm have been obtained for three hydrogen-deficient objects υ Sgr, V 348 Sgr, and A bell 58. A bell 58 was also observed at 2 cm. Only upper limits to the flux density could be set for these sources. A new radio source at 6 cm was found in the field of υ Sgr. The upper limit for 6 cm flux density of V348 Sgr sets an upper limit to its reddening asE(B–V) ≤ 0.65. The hydrogen deficient planetary nebula A 58 shows much lower radio flux than expected from the infrared-radio flux density relationship of planetary nebulae. National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National Science Foundation, USA.     

VLA observations of extreme hydrogen deficient stars

VLA observations at 2 and 6 cm have been obtained for six hydrogen-deficient stars R CrB, HD 160641, BD — 9°4395, V348 Sgr, MV Sgr and Sgrv Upper limits to the massloss rates have been estimated for some of these using the upper limits to the radio flux density. National Radio Astronomy Observatory’s Very Large Array is operated by Associated Universities Inc. under contract with National Science Foundation, USA     

A new, complete sample of faint B stars in the Galactic halo

A new sample of 31 faint B and A0 stars is reported, 30 of which comprise a complete sample within the limits ( U − V )<0 and 10.0< B ≲18.0. The sample is based on low- and intermediate-resolution spectrophotometry of colour-excess objects selected in the US survey. Atmospheric parameters for the stars are derived through the use of synthetic colours, Balmer-line strengths, and model-atmosphere fitting. The atmospheric parameters and preliminary metallicity estimates indicate that most of the stars are distributed along the blue horizontal branch, with low metallicities ([Fe/H]∼−1.0) and with both the first and second Newell gaps present. However, nine of the B/A0 stars can be identified as candidate main-sequence stars, based on evidence of high metallicities ([Fe/H]∼0) and/or derived effective temperatures and surface gravities which place them close to the main-sequence relation. The completeness characteristics of the sample are discussed, and its surface density is compared to that of other recently isolated B-star samples. The sample exhibits a shallow integral number-count slope. This new sample will help provide increased statistical coverage of the B-star population in the Galactic halo through its relatively faint magnitude-completeness limits and its relatively red colour-completeness limit.     

Stability of magnetic flux tubes in close binary stars

Photometric and Doppler imaging observations of active binaries indicate the existence of starspots at preferred longitudes (position angles with respect to the companion star). We investigate the stability of magnetic flux tubes in the convection zone of close, fast‐rotating binary stars and explore whether the observed preferred longitudes could be caused by tidal forces and the deformation of the active star. We assume a synchronized binary system with spin axes perpendicular to the orbital plane and a rotation period of a few days. The tidal force and the deviation from spherical structure are considered in lowest‐order perturbation theory. The magnetic field is in the form of toroidal magnetic flux rings, which are stored in mechanical equilibrium within the stably stratified overshoot region beneath the convection zone until the field has grown sufficiently strong for the undulatory instability to initiate the formation of rising loops. Frequencies and geometry of stable as well as growth rates of unstable eigenmodes are determined by linear stability analysis. Particular consideration is given to the question whether the effects of tidal forces and perturbations of the stellar structure can force a rising flux loop to enter the convection zone at specific longitudes.     

Radiative accelerations, accumulation of iron and thermohaline convection inside stars

The radiative acceleration on iron inside stars may lead to an accumulation of this element in stellar internal layers. As discussed by several authors, this iron accumulation has many important consequences. It may lead to an extra convective zone, and in some cases it may help triggering stellar pulsations. However, the computations which have been done up to now ignore an important effect: the double-diffusive, or “thermohaline” convection induced by the inverse μ gradient. Detailed computations of all these processes have been introduced in the TGEC stellar evolution code. We show how thermohaline convection modifies the profiles of iron inside stars, with important consequences     

Stability of the system of stars,gas and magnetic fields

Linear stability of a system of stars, gas and magnetic fields under the existence of a relative motion between the stars and the gas is investigated by the use of the magnetohydrodynamic and the polytropic equations for the gas and the collisionless Boltzmann equation for the stars together with the Poisson equation. The star system is supposed to have the anisotropic Schwarzschild distribution. The critical wavenumber is calculated and it is found that the system becomes universally unstable under some conditions.     

Stability of polar oscillations of non-rotating Newtonian superfluid stars

In this paper, we shall study the problem of stability of polar oscillations of a non-rotating Newtonian superfluid star. We find that the stability of the latter system is guaranteed by the positive definiteness of a matrix quantity. We also find conditions which characterize the occurrence of zero-frequency polar modes, which marks the onset of polar instability. We find that the negative definiteness of the mentioned matrix quantity implies instability of the system. We apply our results to show that the polar oscillations of a non-rotating Newtonian superfluid star in the zero-temperature approximation are marginally stable.     

Photometry of the bright stars inside and outside associations in the spiral arm S5 of the andromeda galaxy

Photometry of more than 1000 bright stars of 20 associations and some fields in its vicinity is made, usingUBV plates taken with a 2-m Ritchey-Chrétien telescope of the National Observatory at the Bulgarian Academy of Sciences with limited magnitudes of about 20.6 inU andV, 21.6 inB. The colour-magnitudes of about 20.6 inU andV, 21.6 inB. The colour-magnitude and colour-colour diagrams of 12 associations and 8 fields are constructed and the brightest blue and red stars colour excesses, ages, and integrated luminosities of the associations are determined. The composite diagrams for all association and nonassociation measured stars are constructed too. Inside associations bright stars as well as non-stellar objects occur more often than outside. The brightest stars in S5 are withM _v7 and do not differ considerably from the brightest ones in the spiral arm S4 of M31. Approximately the same are the absolute magnitudes of the brightest stars in our Galaxy and M33. A small difference between the brightest stars of the latter galaxy with those of M31 is possible because the stars in M33 are younger. This is in agreement with the concept that Sc galaxies (for example M33) are younger than Sb ones (for example M31). The same could be the reason for the smaller ratio of blue to red supergiants in M33 than in M31. The age estimations of the associations in S5 together with those in S4 in M31 show that they are older than the associations in M33 which is in agreement with the above mentioned concept.     

Ionization state of cosmic hydrogen by early stars and quasars

Cosmic hydrogen is reionized and maintained in its highly ionized state by the ultraviolet emission attributed to an early generation of stars and quasars. The Lyα opacity observed in absorption spectra of high-redshift quasars permits more stringent constraints on the ionization state of cosmic hydrogen. Based on density perturbation and structure formation theory, we develop an analytic model to trace the evolution of the ionization state in the post-overlap epoch of reionization, in which the bias factor is taken into ac-count. For quasars, we represent an improved luminosity function by utilizing a hybrid approach for the halo formation rate that is in reasonable agreement with the published measurements at 2 z 6. Comparison with the classic Press-Schechter mass function of dark matter halos, we demonstrate that the biased mass distribution indeed enhances star formation efficiency in the overdense environment by more than 25 per cent following the overlap of ionized bubbles. In addition, an alternative way is introduced to derive robust estimates of the mean free path for ionizing photons. In our model, star-forming galax-ies are likely to dominate the ionizing background radiation beyond z = 3, and quasars contribute equally above a redshift of z ～ 2.5. From 5 ≤ z ≤ 6, the lack of evolution in photoionization rate can thus be explained by the relatively flat evolution in star formation efficiency, although the mean free path of ionizing photons increases rapidly. Moreover, in the redshift interval z ～ 2 - 6, the expected mean free path and Gunn-Peterson optical depth obviously evolve by a factor of ～ 500 and ～ 50 respectively. We find that the rel-ative values of critical overdensities for hydrogen ionization and collapse could be 430% at z ≈ 2 and 2% at z ≈ 6, suggesting a rapid overlap process in the overdense regions around instant quasars following reionization. We further illustrate that the absolute esti-mates of the fraction of neutral hydrogen computed from theoretical models may not be important because of comparable uncertainties in the computation.     

Magnetic field measurements of CP stars from hydrogen line cores

We present the results of measurements of magnetic fields of chemically peculiar (CP) stars, performed from the shifts between the circularly polarized components of metal and hydrogen lines (the Babcock method). The observations are carried out with an analyzer of circular polarization at the 6‐m telescope of the SAO RAS. We found that for the absolute majority of the objects studied (in 22 CP stars out of 23), the magnetic fields, determined from the Zeeman shifts in the hydrogen line cores, are significantly lower than those obtained from metal lines in the same spectra. This disparity varies between the stars. We show that instrumental effects can not produce the above features, and discuss the possible causes of the observed effect. The discovered condition reveals a more complicated structure of magnetic fields of CP stars than a simple dipole, in particular, a reduction of the field strength in the upper atmosphere with the vertical gradient, significantly higher than the dipole (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)