Local stellar stability. II

The existence proof of continuous spectra of eigenvaluess developed in the framework of the function space ofq-regularizations (Perdang, 1976) is extended in this paper by relaxing the severe restrictions previously imposed o the mathematical structure of the stellar stability equations. It is stressed that these local modes depend on the variable system in terms of which the linearized stellar structure equations are set up. We therefore search for a systematic procedure to select the most satisfactory system to analyze Local Stability. Our procedure is illustrated in great detail in the case of nonradial adiabatic stability. Moreover when applied to nonadiabatic perturbations it reveals the existence of two new types of local instability which seem to prevail in the majority of stars in a thermonuclear burning phase: (a) a nonrdial local secular instability; (b) a radial local nuclear instability. Numerical test calculations exhibit that the latter helps us to understand certain evolutionary features of stars, in particular it provides an interpretation of Hayashiet al.'s (1962) rule.     

Equilibrium Structures of Differentially Rotating Primary Components of Binary Stars

In this paper a method is proposed for computing the equilibrium structures and various other observable physical parameters of the primary components of stars in binary systems assuming that the primary is more massive than the secondary and is rotating differentially about its axis. Kippenhahn and Thomas averaging approach (1970) is used in a manner earlier used by Mohan, Saxena and Agarwal (1990) to incorporate the rotational and tidal effects in the equations of stellar structure. Explicit expressions for the distortional terms appearing in the stellar structure equations have been obtained by assuming a general law of differential rotation of the typeω² = b ₀+b ₁ s ²+b ₂ s ⁴, where ω is the angular velocity of rotation of a fluid element in the star at a distance s from the axis of rotation, and b ₀, b ₁, b ₂ are suitably chosen numerical constants. The expressions incorporate the effects of differential rotation and tidal distortions up to second order terms. The use of the proposed method has been illustrated by applying it to obtain the structures and observable parameters of certain differentially rotating primary components of the binary stars assuming the primary components to have polytropic structures. This revised version was published online in July 2006 with corrections to the Cover Date.     

Investigation of the new Local Group galaxy VV 124

We present the results of our stellar photometry and spectroscopy for the new Local Group galaxy VV124 (UGC4879) obtainedwith the 6-m BTAtelescope. The presence of a fewbright supergiants in the galaxy indicates that the current star formation process is weak. The apparent distribution of stars with different ages in VV 124 does not differ from the analogous distributions of stars in irregular galaxies, but the ratio of the numbers of young and old stars indicates that VV 124 belongs to the rare Irr/Sph type of galaxies. The old stars (red giants) form the most extended structure, a thick disk with an exponential decrease in the star number density to the edge. Definitely, the young population unresolvable in images makes a great contribution to the background emission from the central galactic regions. The presence of young stars is also confirmed by the [OIII] emission line visible in the spectra that belongs to extensive diffuse galactic regions. The mean radial velocity of several components (two bright supergiants, the unresolvable stellar population, and the diffuse gas) is υ _h = −70 ± 15 km s⁻¹ and the velocity with which VV 124 falls into the Local Group is υ _LG = −12 ± 15 km s⁻¹. We confirm the distance to the galaxy (D = 1.1 ± 0.1 Mpc) and the metallicity of red giants ([Fe/H] = −1.37) found by Kopylov et al. (2008). VV 124 is located on the periphery of the Local Group approximately at the same distance from M31 and our Galaxy and is isolated from other galaxies. The galaxy LeoA nearest to it is 0.5 Mpc away.     

The accurate determination of stellar angular diameters using broad-band stellar interferometry

The angular diameter of a star can be estimated from interferometric observations by fitting the data with the visibility function for a uniformly illuminated disc and then using published correction factors to convert the uniform-disc angular diameter to the limb-darkened angular diameter. The correction factors are strictly valid only for monochromatic light. We investigate the effect of using a broad bandwidth, and present a simple method for calculating broad-band correction factors from the monochromatic factors.
The technique of fitting the data with a uniform-disc visibility function is only useful for stars with compact atmospheres and 'typical' limb-darkening profiles. It should not be applied to stars with extended atmospheres or that show extreme limb darkening. These stars have visibility functions that are qualitatively different from a uniform-disc visibility function, so they can be distinguished observationally from compact-atmosphere stars.     

Gravitational collapse of magnetic neutron stars

Pulsars are presently believed to be rotating neutron stars with frozen-in magnetic fields. Because of the high density of neutron stars, general relativistic effects are important since they effect both the structure and stability of such stars. Besides this, the magnetic field outside the star is also affected. Instead of falling of asr ^(2+l) as in flat space, it is shown that each magnetic multipole varies as a hypergeometric function of radius. A closed form of these hypergeometric functions is given in terms of Legendre functions of the second kind. If the mass of a neutron star exceeds about 2.4m , the star becomes unstable and coliapses. For a quasistatically collapsing body, it is shown that the magnetic field seen by a distant observer vanishes as the radius approaches the gravitational radius.This work was supported in part by the Air Force Office of Scientific Research, Office of Aerospace Research under AFOSR Grant 70-1866.     

The <Emphasis Type="Italic">r</Emphasis>- and <Emphasis Type="Italic">s</Emphasis>-process contributions to heavy-element abundances in the halo star HD 29907

The abundances of 22 heavy elements from Sr to Pb have been determined for the halo star HD 29907 (T _eff = 5500 K, log g = 4.64) with [Fe/H] = −1.55 using high-quality VLT/UVES spectra (ESO, Chile). The star has a moderate enhancement of r-process elements (Eu-Tm) with [r/Fe] = 0.63. In the range from Ba to Yb, the derived abundance pattern agrees well with those for strongly r-process enhanced stars (r-II stars with [Eu/Fe] > 1 and [Ba/Eu] < 0), such as CS 22892-052 and CS 31082-001, as well as with the scaled solar r-process curve and the r-process model HEW. Thus, Ba-Yb in HD 29907 originate in the r-process. Just as other moderately r-process enhanced stars studied in the literature, HD 29907 exhibits higher Sr, Y, and Zr abundances than those for r-II stars. These results confirm the assumption by other authors about the existence of an additional Sr-Zr synthesis mechanism in the early Galaxy before the onset of nucleosynthesis in asymptotic giant branch (AGB) stars. The same mechanism can be responsible for the enhancement of Mo-Ag in the star being investigated compared to r-II stars. There are no grounds to suggest the presence of s-nuclei of lead in the material of the star being investigated, because its measured abundance ratio log ɛ(Pb/Eu) = 1.20 lies within the range for the comparison stars: from log ɛ(Pb/Eu) = 0.17 (CS 31082-001) to < 1.55 (HE 1219-0312). Thus, even if there was a contribution of AGB stars to the heavy-element enrichment of the interstellar medium at the epoch with [Fe/H] = −1.55, it was small, at the level of the abundance error.     

Characterization of intermittent chaos and oscillating frequencies in some pulsating stars

If one zone models are available for some pulsating stars and the motion remains close to adiabaticity, we show the existence of odd frequencies in the power spectrum of these pulsating variable stars, and the frequency of free oscillation thus becomes dependent on the amplitude. We find 3 appearing in the power spectrum of the white dwarf PG 1351+489. Furthermore, the intrinsic stochasticity in the basic equations describing the structure of stars is discussed in this paper. We found that the pulsation of intermittent chaos can occur spontaneously in the basic equations of a star within certain intervals of values of its control parameters, and the intermittency can be used as an interpretation of pulsation in variable stars.     

The Crust Properties and Cooling of Strange Stars

We investigate the influence of the following parameters on the crust properties of strange stars: the strange quark mass (m _s), the strong coupling constant (α_c) and the vacuum energy density (B). It is found that the mass density at the crust base of strange stars cannot reach the neutron drip density. For a conventional parameter set of m _s=200 MeV, B ^1/4 = 145 MeV and α_c = 0.3, the maximum density at the crust base of a typical strange star is only 5.5 × 10¹⁰ gcm^-3, and correspondingly the maximum crust mass is 1.4 ×10^-6 M_⊙. Subsequently, we present the thermal structure and the cooling behavior of strange stars with crusts of different thickness, and under different diquark pairing gaps. Our work might provide important clues for distinguishing strange stars from neutron stars.     

Inhomogeneous chemical evolution of dwarf spheroidal galaxies

Stellar abundance pattern of n-capture elements such as barium is used as a powerful tool to infer how the star formation proceeded in dwarf spheroidal (dSph) galaxies. It is found that the abundance correlation of barium with iron in stars belonging to dSph galaxies orbiting the Milky Way, i.e., Draco, Sextans, and Ursa Minor have a feature similar to that in Galactic metal-poor stars. The common feature of these two correlations can be realized by our in homogeneous chemical evolution model based on the supernova-driven star formation scenario if dSph stars formed from gas with a velocity dispersion of ∼ 26 km s^-1. This velocity dispersion together with the stellar luminosities strongly suggest that dark matter dominated dSph galaxies. The tidal force of the Milky Way links this velocity dispersion with the currently observed value ≲ 10 km s^-1 by stripping the dark matter in dSph galaxies. As a result, the total mass of each dSph galaxy is found to have been originally ∼ 25 times larger than at present. In this model, supernovae immediately after the end of the star formation can expel the remaining gas over the gravitational potential of the dSph galaxy. This revised version was published online in August 2006 with corrections to the Cover Date.     

Der offene sternhaufen NGC 6067

The open star cluster NGC 6067 was investigated by the strip method on charts of photographs with different exposure times taken with the 1-m Schmidt telescope of the European Southern Observatory. Because of the large distance (1820 pc) of the cluster its luminosity function is known up to date only betweenM _v =–4.5 andM _v =+1.5. In this paper it is continued toM _v =4.4 and further extrapolated by means of 2 variants. The cluster contains 419 respectively 476 stars with total masses of 1453 resp. 1483. On account of its radius of 5.9 pc the cluster is a rather extended object, which can be described by the generalized density law of Schuster withn=4.42 and the central star density 8.9 stars pc^–3. The mean velocity of the stars amounts to 1.03 km s^–1, the massbrightness relation is 0.031 in solar units.

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Mitteilungen Serie A.     

Magnetic field evolution in OBA stars

The applications of the spectral analysis methods discovered by Kirchhoff for the investigation of stellar magnetic fields are considered. The statistical properties of the mean magnetic fields for OBA stars have been investigated by analyzing data from two catalogs of magnetic fields. It is shown that the mean effective magnetic field ℬ of a star can be used as a statistically significant characteristic of its magnetic field. The magnetic field distribution functions F(ℬ) have been constructed for B-type and chemically peculiar (CP) stars, which exhibit a power-law dependence on ℬ. A sharp decrease in F(ℬ) in the range of weak magnetic fields has been found. The statistical properties of the magnetic fluxes for main-sequence stars, white dwarfs, and neutron stars are analyzed.     

Effects of external magnetic field on the outflows of an accretion disc

The aim of this work is to study the effects of an external magnetic field generated by a magnetized compact star on the outflows of its accretion disc. For this purpose, we solve a set of magneto-hydrodynamic (MHD) equations for an accretion disc in spherical coordinates to consider the disc structure along the θ-direction. We also consider the magnetic field of a compact star beyond its surface as a dipolar field, producing a toroidal magnetic field inside the disc. We convert the equations to a set of ordinary differential equations (ODEs) as a function of the θ only by applying self-similar assumptions in the radial direction. Then, this set of equations is solved under symmetrical boundary conditions in the equatorial plane to obtain the velocity field. The results are considered in the gas-pressure-dominated (GPD) region and radiation-pressure-dominated (RPD) region as well. The dipolar field of the compact stars can significantly enhance the speed of outflows. It also can change the structure of the disc. The results of this work would be useful in the study of X-ray binaries, the origin of ultra-relativistic outflows, and jet formation around the compact stars.     

Model of wind accretion in extrinsic AGB stars under conservation of total angular momentum

The equations of wind accretion and variation of orbital parameters are rederived under conservation of total (rather than tangential) angular momentum, and applied to extrinsic AGB stars. These equations, together with nucleosynthesis in intrinsic AGB stars, are used to calculate heavy element overabundance for the scenario of successive pulses and mixing. The results show that wind accretion pertains when the orbital period is longer than 1300 d or 1600 d according as the initial mass of the Ba star is 2.5 M_⊙ or 1.3 M_⊙, while cataclysmic variables will result if the period is shorter than 600 d in either case. The results are advantageous for interpreting a) the observations on the overabundance and orbital parameters, b) the observed lower limit of 600 d in the period of extrinsic S stars, and c) the observed fact that the mean eccentricity of Ba stars is greater than the mean eccentricities of extrinsic S stars and CH stars.     

The gradient of the brightest star age and density across the spiral arm S4 in the Andromeda galaxy

On a plate obtained with the 2-m RC telescope at the Bulgarian National Observatory about 1400 stars in the spiral arm S4 of the Andromeda galaxy were measured. The limit of completeness is 20 _. ^m 2 (B magnitudes). In the central part of S4 (Figure 3) a pronounced gradient of star luminosity and density is found (Figures 6 and 7a). Here the stars become fainter at about 2 ^m and their surface density decreases tenfold at the distance 1 kpc from the inner edge of the arm. We have interpreted the decline of star maximum brightness from this edge as age gradient and have evaluated from it the velocity of star formation propagation across the arm, which is about 60 km s^–1. If the Andromeda galaxy has trailing spiral arms and the pitch angle of S4 is about 25° in its central part, the pattern velocity p7–14 km s^–1 kpc^–1. This value is close to that obtained earlier with the help of the Cepheids in the same part of S4 (Efremov, 1980). The absence of a pronounced asymmetry in the star distribution across the arm in the OB82 region may be connected with the position of the strongest dust lanes in front of the stellar spiral arm here. We have stressed that in one part of the same spiral arm there may be a pronounced age gradient, and there may be no such gradient in the near-by one. In spite of the known difficulties in understanding the structure of the Andromeda galaxy it is possible to draw some conclusions which are important for the theory of spiral arms. The detailed investigations of the nearest galaxies are, therefore, most useful for understanding the spiral structure nature.     

Closed form Vaidya-Tikekar type charged fluid spheres with pressure

Recently, Bijalwan (Astrophys. Space Sci. doi:, 2011) discussed all important solutions of charged fluid spheres with pressure and Gupta et al. (Astrophys. Space Sci. doi:, 2010) found first closed form solutions of charged Vaidya-Tikekar (V-T) type super-dense star. We extend here the approach evolved by Bijalwan (Astrophys. Space Sci. doi:, 2011) to find all possible closed form solutions of V-T type super-dense stars. The existing solutions of Vaidya-Tikekar type charged fluid spheres considering particular form of electric field intensity are being used to model massive stars. Infact at present maximum masses of the star models are found to be 8.223931M _Θ and 8.460857M _Θ subject to ultra-relativistic and non-relativistic conditions respectively. But these stars with such are large masses are not well behaved due to decreasing velocity of sound in the interior of star. We present new results concerning the existence of static, electrically charged perfect fluid spheres that have a regular interior. It is observed that electric intensity used in this article can be used to model superdense stars with ultrahigh surface density of the order 2×10¹⁴ gm/cm³ which may have maximum mass 7.26368240M _Θ for ultra-relativistic condition and velocity of sound found to be decreasing towards pressure free interface. We solve the Einstein-Maxwell equations considering a general barotropic equation of state with pressure. For brevity we don’t present a detailed analysis of the derived solutions in this paper.     

Comparison of the Pulkovo compilation of radial velocities with the RAVE DR1 catalogue

The Data Release 1 of the Radial-Velocity Experiment (RAVE DR1 1, 24748 stars) (PCRV, 35 495 stars) is compared with the May 15, 2006 version of the Pulkovo Compilation of Radial Velocities (PCRV, 35 495 stars). RAVE DR1 includes mostly 9–13 ^m stars, while the PCRV contains brighter stars. Analysis of the “RAVE minus PCRV” radial-velocity differences for 14 common stars has revealed no systematic dependences on any factors, except the effect due to the RAVE radial-velocity zero-point offset known from the RAVE observations. This effect shows up for ten of these stars observed on a single night as a sine wave with an amplitude of 1.5 km s^?1 in the dependence of the radial-velocity difference on the ordinal number of the optical fiber used and, accordingly, on the star position angle in the field of view of the RAVE instrument. The detection of this dependence confirms a high radial-velocity accuracy in both catalogs: on average, better than 1 km s^?1 for stars brighter than 10^m (for the RAVE, after applying a correction for the zero-point offset). The RAVE zero-point offset can be corrected for with an accuracy better than 1 km s^?1 by observing several PCRV stars in each RAVE frame and by analyzing the “RAVE minus PCRV” radial-velocity differences.     

First stars. I. Evolution without mass loss

The first generation of stars was formed from primordial gas. Numerical simulations suggest that the first stars were predominantly very massive, with typical masses M≥100M _⊙. These stars were responsible for the reionization of the universe, the initial enrichment of the intergalactic medium with heavy elements, and other cosmological consequences. In this work, we study the structure of Zero Age Main-Sequence stars for a wide mass and metallicity range and the evolution of 100, 150, 200, 250 and 300M _⊙ galactic and pregalactic Pop III very massive stars without mass loss, with metallicity Z=10⁻⁶ and 10⁻⁹, respectively. Using a stellar evolution code, a system of 10 equations together with boundary conditions are solved simultaneously. For the change of chemical composition, which determines the evolution of a star, a diffusion treatment for convection and semiconvection is used. A set of 30 nuclear reactions are solved simultaneously with the stellar structure and evolution equations. Several results on the main sequence, and during the hydrogen and helium burning phases, are described. Low-metallicity massive stars are hotter and more compact and luminous than their metal-enriched counterparts. Due to their high temperatures, pregalactic stars activate sooner the triple alpha reaction self-producing their own heavy elements. Both galactic and pregalactic stars are radiation pressure dominated and evolve below the Eddington luminosity limit with short lifetimes. The physical characteristics of the first stars have significant influence in predictions of the ionizing photon yields from the first luminous objects; also they develop large convective cores with important helium core masses which are important for explosion calculations.     

Spectral variability of LBV star V 532 (Romano’s star)

We present the results of studying the spectral and photometric variability of the luminous blue variable star V532 in M33. The photometric variations are traced from 1960 to 2010, spectral variations—from 1992 to 2009. The star has revealed an absolute maximum of visual brightness (1992–1994, high/cold state) and an absolute minimum (2007–2008, low/hot state) with a brightness difference of ΔB ≈ 2.3 ^m . The temperature estimates in the absolute maximum and absolute minimum were found to be T ∼ 22000 K and T ∼ 42000 K, respectively. The variability of the spectrum of V532 is fully consistent with the temperature variations in its photosphere, while both permitted and forbidden lines are formed in an extended stellar atmosphere. Broad components of the brightest lines were found, the broadening of these components is due to electron scattering in the wind parts closest to the photosphere. We measured the wind velocity as a difference between the emission and absorption peaks in the PCyg type profiles. The wind velocity clearly depends on the size of the stellar photosphere or on the visual brightness, when brightness declines, the wind velocity increases. In the absolute minimum a kinematic profile of the V532 atmosphere was detected. The wind velocity increases and its temperature declines with distance from the star. In the low/hot state, the spectral type of the star corresponds to WN8.5h, in the high/cold state—to WN11. We studied the evolution of V532 along with the evolution of AGCar and the massive WR binary HD5980 in SMC. During their visual minima, all the three stars perfectly fit with the WNL star sequence by Crowther and Smith (1997). However, when visual brightness increases, all the three stars form a separate sequence. It is possible that this reflects a new property of LBV stars, namely, in the high/cold states they do not pertain to the bona fide WNL stars.     

Correlation and variability of the HeI 4471 and MgII 4481 absorption lines in Be stars: A possible diagnostic tool for nonradial pulsations

In a total of 181 photographic spectrograms, obtained at the Manuel Foster Observatory of the Catholic University in Santiago (Chile), the equivalent widths of the absorption lines HeI 4471 and MgII 4481 have been determined for 10 selected southern Be stars and 4 normal B-type stars. The mean equivalent widths of each star are compared to published mean values from largers samples of B stars. On the average, Be stars coincide with normal B-type stars in their equivalent width. However, a striking difference between rapidly and slowly rotating stars in their line ratios (HeI/MgII) was found, which is probably an artifact due to a systematic misclassification of spectral types of rapidly rotating B stars (v sini>200 km s^–1). Six of the ten Be stars show evidence for variability in HeI and/or MgII with time-scales shorter than a few days and amplitudes up to a factor 2 in equivalent width. Line variability occurs in the entire range of projected rotation velocity (70 km s^–1<v sini<350 km s^–1), but seems to be restricted to spectral types B2-B4. In addition, variations at longer time-scales (years) were observed in two cases. Three of the variable stars (HR 4074, HR 4537, and Ara) present correlated variations in both lines, one ( Cen) anti-correlated variability. We interpret our results in terms of nonradial pulsations and stress the importance to study the HeI/MgII lines and their variability in a larger sample of Be, Bn, and normal B stars, including archival material in order to follow-up line variability for several decades in the past.     

Note on a remarkable object close to the star LSS 3027

The star LSS 3027, embedded in a parabolic-shaped dark nebula, and several stars in its vicinity, were observed photometrically and spectroscopically to investigate whether they are candidates of very young Herbig Ae/Be stars. Only the star LSS 3027B is without doubt a very good candidate. Its H-emission is very strong, indicating an extended atmosphere around the star, and its near- and far-infrared spectral energy distributions exhibit a strong excess radiation, which most probably is caused by a thick dust shell in the close environment of the star. The ratio of total to selective extinction is found to be equal toR-5.1. It can be shown that it is embedded in the dark cloud mentioned above. From its properties we believe that the remarkable object LSS 3027B is a typical Hergig Be pre-Main-Sequence object.Based on data obtained at the ESO, La Silla, Chile and with the Infrared Astronomical Satellite (IRAS).