Energetics of galactic nuclei

A model of compact galactic nuclei in statistical equilibrium was developed in [L. Sh. Grigorian and G. S. Sahakian, Astrofizika (in press)]. It was shown that they should consist predominantly of neutron stars (pulsars) and white dwarfs. The problem of the energy reserves of galactic nuclei is discussed in terms of this concept. The mechanism of conversion of a white dwarf into a neutron star due to the accretion of interstellar matter is considered. This means that a galactic nucleus has an energy reserve of some 5·10⁶⁰ N₈ erg (N is the number of stars in the nucleus). It is shown that galactic nuclei are powerful sources of hard γ radiation [power L » 2·10⁴⁴µ₃₀N₈(Ω/50)^17/7 erg/sec, where µ is the magnetic moment and Ω is the angular rotation rate of a neutron star ] due to curvature radiation from relativistic electron fluxes flowing along channels of open magnetic field lines of pulsars. The x-ray and ultraviolet emission are due to synchrotron emission from the same electron fluxes in the magnetic field of the galactic nucleus (L » 10⁴²-10⁴⁴ erg/sec). The optical (visible and infrared) and radio emission are due to bremsstrahlung from electrons in the interstellar medium [L » 6·10⁴⁶N ₈ ² (5/R_pc)³ erg/sec, where R is the radius of the galactic nucleus]. An equation is obtained for the magnetic moment of a pulsar: µ ≈ 3.4·10^-5L_γP^17/7, where P is the pulsar’s period and L_03B3; is the luminosity of the pulsar’s y radiation.     

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.     

On the nature of compact galactic nuclei

Spherically symmetric stellar clusters (compact galactic nuclei and globular clusters), far advanced toward the state of complete statistical equilibrium in the course of evolution, are investigated. The equation of state of such systems (a polytrope with an index k = 0.5) is derived and their main characteristics are calculated. It is shown that compact galactic nuclei must consist mainly of rapidly rotating neutron stars and white dwarfs. It is demonstrated that pulsars may be created by the evaporation of neutron stars from the nucleus of our Galaxy. The number of such pulsars is ~3.10⁶. Translated from Astrofizika, Vol. 41, No. 1, pp. 41–50, January-March, 1998.     

Candidate red clump giants in the PPMX catalogue

This paper presents the RCGP catalogue of more than 0.5 million candidate red clump stars with the limiting magnitude K _s = 9.5 ^m . These stars are selected from the PPMX catalogue as the most probable red clump members by analyzing the color-reduced proper motion diagrams built from the proper motions given in PPMX and J, K _s -photometry given in the 2MASS catalogue. Reddening of the selected stars is used to find extinction in the K _s -band and to consider it in the further analysis. The two-dimensional galactic rotation model generalized by Ogorodnikov is used to investigate the tangential velocity field of the selected red clump members, most of which are thin disk stars located within 1.5 kpc from the sun. The values of kinematic parameters and solar components are determined as a function of stellar heights above the galactic equatorial plane and their heliocentric distances.     

Meridional circulation and turbulence in surface layers of the stars

The meridional circulation is considered in the surface layers of the stars where the optical depth τ?1. It is shown that the radial component of circulation velocity reaches its maximum value at τ≈1 and decreases at τ→0. The tangential velocity reverses its sign at τ≈1 — i.e., the meridional flows are closed in stellar atmospheres. The tangential velocities can be of the order of 10⁶–10⁷ cm s^?1 in atmospheres of O-B-A stars. Such hydrodynamical motions can result in the generation of turbulence in the surface layers. Characteristic turbulent velocities are of the order of 10⁵–10⁶ cm s^?1 in early-type stars. The small-scale turbulent motions generate the acoustic waves and the flux of such waves may be the source of energy to heat coronae in O and B stars.     

Flares of red dwarf stars and the galactic X-ray background

The mean density of the UV Cet-type flare stars in the solar neighbourhood is estimated. If differences of activity levels on different flare stars are taken into account, their summary flare activity is equivalent to 0.03 YZ CMi's flare activity per cubic parsec or to 4×10²⁶ erg s^–1 pc^–3 in U-passband. From the X-ray flare observation on YZ CMi of 19.10.74 we estimate the luminosity of stellar flares in soft and intermediate X-ray. The ratio of X-ray to optical radiation for stellar flares is close to the respective ratio for strong solar chromospheric flares. It is shown the set of red-dwarf flare stars has all essential features of an ensemble of discrete X-ray sources to represent the galactic diffuse X-ray background.     

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 kinematics of Trapezium-type systems

Summary In this paper the results of the research of the stars proper motions Trapezium components are reported. They are: the galactic coordinates of the solar aprx and the Sun velocity (L _⊙=43±18°,B _⊙=+28±13°,V _⊙=13±4 km s⁻¹), the dispersion of peculiar velocities in the direction of the galactic coordinates for the above mentioned stars (σ _l =±11 km s⁻¹, σ _b =±7 km s⁻¹).The attained accuracy of the proper motions (±0.005″ yr⁻¹) is shown to be insufficient to the study of internal space motions in these systems. At present the work to increase the relative proper motions accuracy for multiple system components and to improve reductions from the relative to absolute proper motions, is being carried out in the Main Astronomical Observatory (Academy of Sciences of the Ukrainian SSR). The new catalogue of the AGK3 stars is composed now in the vicinity of the galactic equator in order to improve reductions from the relative to absolute proper motions. The r.m.s. errors of the proper motions, obtained in the AGK3 system, are ±0.005″ yr⁻¹.     

Gamma-ray bursts from nearby neutron stars

We interpret the puzzling-ray bursts as emitted by cooling sparks from the surface of spasmodically accreting, old neutron stars. Their spiky, anisotropic radiation is oriented w.r.t. the galactic disk via interstellar accretion, whose orbital angular momentum tends to counteralign with the galactic spin; in this way, larger source numbers in directions of the galactic disk are compensated by smaller beaming probabilities, resulting in a near-isotropic arrival distribution, as observed by BATSE. The source distances range between 10 pc and 500 pc. Their radiated energies are of order 10³⁵ erg, corresponding to accreted clumps (blades) of typical mass 10¹⁵ g per burst. Magnetic surface field strengths range between 10¹⁰ and 10¹² G, somewhat weaker than those of newborn neutron stars.     

Formation of stellar associations

The Rayleigh-Taylor instability forms massive complexes. When 10²¹ atoms cm^–2 are gathered, X-rays which heat the gas and UV-rays which ionize carbon are absorbed. A layer should appear with temperatures as low as 6 K and density to 4×10³ cm^–3. Finally the layer is fragmented into stars whose masses may even be less than one solar mass. The temperature of the layer should increase with time because part of free carbon is gradually absorbed by dust. Therefore more massive stars should appear after less massive stars. The stars which are formed kept near the layer by its gravitation. When their total mass becomes comparable with the mass of the layer, they should fall to the galactic plane in agreement with observed proper motions of several studied stellar systems.     

Effect of partial mixing of matter on the hydrodynamic angular momentum transport processes in massive main-sequence stars

We consider the evolution of a rotating star with a mass of 16M _⊙ and an angular momentum of 3.25 × 10⁵² g cm² s^?1, along with the hydrodynamic transport of angular momentum and chemical elements in its interiors. When the partial mixing of matter of the turbulent radiative envelope and the convective core is taken into account, the efficiency of the angular momentum transport by meridional circulation in the stellar interiors and the duration of the hydrogen burning phase increase. Depending on the Schmidt number in the turbulent radiative stellar envelope, the ratio of the equatorial rotational velocity to the circular one increases with time in the process of stellar evolution and can become typical of early-type Be stars during an additional evolution time of the star on the main sequence. Partial mixing of matter is a necessary condition under which the hydrodynamic transport processes can increase the angular momentum of the outer stellar layer to an extent that the equatorial rotational velocity begins to increase during the second half of the evolutionary phase of the star on the main sequence, as shown by observations of the brightest stars in open star clusters with ages of 10–25 Myr. When the turbulent Schmidt number is 0.4, the equatorial rotational velocity of the star increases during the second half of the hydrogen burning phase in the convective core from 330 to 450 km s^?1.     

Nucleosynthesis in supernova outbursts and the chemical composition of the envelopes of neutron stars

The formation of chemical elements in the envelopes of neutron stars is considered at the densities ?=10⁷ to 10¹³ g cm^?3. It is shown, that the compression of cold and hot matter leads to different chemical compositions. The compression of cold matter is accompanied by a decrease of atomic weightA, up to ?≈3×10¹² g cm^?3. One may distinguish the following stages during the compression of hot matter: quasi-equilibrium, when there exists both nuclear equilibrium and kinetic equilibrium in β-processes; and limited equilibrium, when the total number of nuclei is constant. It is shown that a nonequilibrium chemical composition may be formed in the envelopes of neutron stars where there is an excess of neutrons in the presence of superheavy nuclei. The nuclear energy, stored in the neutron star envelope may be sufficient to support neutron star luminosity at a level of ～ 10³⁶ erg s^?1 over a period of ～ 10⁵ yr. Possible applications to the problem of X-ray sources and pulsars are discussed. The formation of the heavy nuclei in Supernovae explosions is considered briefly. Rough estimates are made for the differences in chemical composition of ejected matter during the explosions of stars of different masses and Supernovae of different types.     

Early-Type Stars

Away from the young disk, several classes of early type stars are found. They include (i) the old, metal-poor blue horizontal branch stars of the halo and the metal-poor tail of the thick disk; (ii) metal-rich young A stars in a rapidly rotating subsystem but with a much higher velocity dispersion than the A stars of the young disk, and (iii) a newly discovered class of metal-poor young main sequence A stars in a subsystem of intermediate galactic rotation (V_rot ≈ 120 km s⁻¹). The existence and kinematics of these various classes of early type stars provide insight into the formation of the metal-poor stellar halo of the Galaxy and into the continuing accretion events suffered by our Galaxy. This revised version was published online in July 2006 with corrections to the Cover Date.     

A chemically decoupled nucleus and inner polar ring of the SBb galaxy NGC 4548

Our inzvestigation of the central region in NGC 4548, a bright Sb galaxy with a large-scale bar, using the Multipupil Field Spectrograph of the 6-m telescope revealed a chemically decoupled compact stellar nucleus with [Fe/H]=+0.6 and [Mg/Fe]=+0.1...+0.2 and with a mean stellar-population age of 5 Gyr. This nucleus, a probable circumnuclear disk coplanar with the global galactic disk, is embedded in the bulge whose stars are generally also young, T≈4 Gyr, although they are a factor of 2.5 more metal-poor. The bulge of NGC 4548 is triaxial and has a de Vaucouleurs surface-brightness profile; the unusual characteristics of its stellar population suggest the bulge formation or completion in the course of secular evolution in the triaxial potential of the global bar. The ionized gas within 3″ of the NGC 4548 nucleus rotates in a plane inclined to the principal symmetry plane of the galaxy, possibly, even in its polar plane, which may also result from the action of the large-scale bar.     

Dynamical instability in accreting white dwarfs

We study the evolution of solid, CO white dwarfs after explosive carbon ignition at central densities around 10¹⁰ g cm^–3 triggered by steady accretion in a close binary system, in order to elucidate whether these stars can collapse to form a neutron star. We show that as long as the velocity of the burning front remains below a critical value of 0.006c _s (60 km s^–1), gravitational collapse is the final fate. These calculations support the accretion-induced collapse (AIC) scenario for the origin of a fraction of low-mass X-ray binaries.Paper presented at the 11th European Regional Astronomical Meetings of the IAU on New Windows to the Universe, held 3–8 July, 1989, Tenerife, Canary Islands, Spain.     

Schmidt plate survey in the galactic centre direction. 1. Investigation of open clusters in the Sagittarius-Carina spiral arm

A study of four open clusters on Tautenburg Schmidt plates in the direction to the Galactic Centre (l = 17.0°, b = +0.8°) is presented. In a field of 8.95 square degrees proper motions and B, V magnitudes were determined for about 36 500 stars up to the limiting magnitudes B = 17^m.9, V = 16^m.8. For stars brighter than B = 16^m an accuracy of about 3 mas/year has been estimated for proper motions. The rms errors of stellar magnitudes and (B—V) colours is about 0.1 mag. There are two open clusters already known in this sky area: Trumpler 32 (C 1814-133) and NGC 6611 (C 1816-138). By the inspection of the plates two additional anonymous clusters were identified near the double star ADS 11285 or BD −14°5014 (C 1819-146) and near the star BD −14°5016 (C 1820-146). The cluster membership determination was carried out using information on spatial and proper motion distributions of stars in the field. The colour-magnitude diagrams up to the limiting stellar magnitude were constructed. For each cluster interstellar extinction A_V, diameters of the core and corona, ages and spatial velocity components (V, W) relative to the LSR in the Y, Z-galactic directions were derived. The distances to the clusters were obtained to 1720 pc, 2260 pc, 2130 pc and 2130 pc for Trumpler 32, NGC 6611, C 1819-146 and C 1820-146, respectively. All clusters are situated in the Sagittarius-Carina spiral arm. Their spatial location confirms the assumption that galactic spiral arms are close to logarithmic spirals with a characteristic angle of about 20 degrees.     

A young cooling neutron star in the remnant of Supernova 1987A

We describe the cooling theory for isolated neutron stars that are several tens of years old. Their cooling differs greatly from the cooling of older stars that has been well studied in the literature. It is sensitive to the physics of the inner stellar crust and even to the thermal conductivity of the stellar core, which is never important at later cooling stages. The absence of observational evidence for the formation of a neutron star during the explosion of Supernova 1987A is consistent with the fact that the star was actually born there. It may still be hidden in the dense center of the supernova remnant. If, however, the star is not hidden, then it should have a low thermal luminosity (below ～10³⁴ erg s^?1) and a short internal thermal relaxation time (shorter than 13 yr). This requires that the star undergo intense neutrino cooling (e.g., via the direct Urca process) and have a thin crust with strong superfluidity of free neutrons and/or an anomalously high thermal conductivity.     

A hydrodynamic model for asymmetric weak explosion of collapsing supernovae with rapid initial rotation

We further investigate the two-dimensional hydrodynamic explosion model for rapidly rotating and collapsing supernovae (Aksenov et al. 1997), in which the initial energy release inside a fragmenting low-mass neutron star of critical mass ≈0.1M _⊙ moving in a circular orbit at a velocity of ≈18000 km s^?1 is reduced considerably. This velocity closely corresponds to a pulsar escape velocity of ≈1000 km s^?1 (at a total mass of ≈1.9M _⊙ for the binary of neutron stars). Compared to our previous study (Zabrodina and Imshennik 1999), this energy release was reduced by more than a half. Otherwise, the model in question does not differ from the explosion model with a self-consistent chemical composition of nuclides investigated in the above paper. In particular, the initial energy release was carefully reconciled with a chemical composition. Our numerical solution shows that the reduction in energy release due to the time scales of β processes and neutrino energy losses being finite does not alter the qualitative results of our previous studies (Aksenov et al. 1997; Imshennik and Zabrodina 1999). An intense undamped diverging shock wave (with a total post-shock energy ? 10⁵¹ erg at a front radius of ≈10 000 km) is formed; a large asymmetry of explosion with a narrow cone (with a solid angle of ≈π/4) around the leading direction, which coincides with the velocity direction of the low-mass neutron star at the instant of its explosive fragmentation in the two-dimensional model, emerges. A jet of synthesized radioactive nickel, whose mass is estimated by using simple threshold criteria to be M _Ni≈(0.02?0.03)M _⊙ is concentrated inside this cone. This appears to be the integrated parameter that is most sensitive to the specified reduction in initial energy release; it is also reduced by almost a half compared to our previous estimate (Imshennik and Zabrodina 1999). The time of propagation of the shock wave inferred in our model to the presupernova surface was estimated for SN 1987A to be 0.5–1.0 h, in agreement with observations.     

Radial pulsations of red giant branch stars

We performed hydrodynamic computations of nonlinear stellar pulsations of population I stars at the evolutionary stages of the ascending red giant branch and the following luminosity drop due to the core helium flash. Red giants populating this region of the Hertzsprung–Russel diagram were found to be the fundamental mode pulsators. The pulsation period is the largest at the tip of the red giant branch and for stars with initial masses from 1.1 M _⊙ to 1.9 M _⊙ ranges from ∏ ≈ 254 day to ∏ ≈ 33 day , respectively. The rate of period change during the core helium flash is comparable with rates of secular period change in Mira type variables during the thermal pulse in the helium shell source. The period change rate is largest (∏?/∏ ≈ ?10^?2 yr^?1) in stars with initial mass M ^ZAMS = 1.1 M ^⊙ and decreases to ∏?/∏ ～ ?10^?3 yr^?1 for stars of the evolutionary sequence M ^ZAMS = 1.9 M ^⊙. Theoretical light curves of red giants pulsating with periods ∏ > 200 day show the presence of the secondary maximum similar to that observed in many Miras.