Neutron stars and the dense matter equation of state

Neutron stars provide a unique laboratory with which to study cold, dense matter. The observational quantities of primary astrophysics interest are the maximum mass and the typical radius of a neutron star. These quantities are related to the relative stiffness of neutron-rich matter at supernuclear densities and the density dependence of the nuclear symmetry energy near the nuclear saturation density. The measurements of these nuclear properties via nuclear systematics and structure, heavy-ion collisions and parity-violating electron scattering from neutron-rich nuclei, are discussed. Several new observations, including mass measurements of binary pulsars and a confirmed distance determination for a nearby cooling neutron star, will be summarized. Additionally addressed will be observations of thermal emissions from cooling neutron stars in globular clusters and thermonuclear explosions from accreting stars. It will be demonstrated how this astrophysical data is shedding light on the pressure-density relation of extremely dense matter.     

Small dense molecular clouds which envelope groups of T-Tauri stars

Dense molecular clouds within the Taurus and NGC 2264 regions have undergone gravitational collapse and fragmentation to form groups of low mass (1M ) T-Tauri stars which are still embedded within the clouds and which are kinematically associated with them. Molecular column densities on the order of 10¹⁴ cm^–2 are inferred from the emission lines of OH and NH₃. Emission line widths are 2 km s^–1 and the antenna beamwidths include linear extents of order 0.1 pc. The OH emission appears to be in a condition of local thermodynamic equilibrium, and it cannot arise from circumstellar sheils similar to those surrounding the masing infrared stars. The OH and NH₃ emission occurs in clouds of 1 pc in extent with optical depths of 0.1 to 1.0 and excitation temperatures of the order of 10 K. The molecular clouds have radii of 0.5 pc, molecular hydrogen densities of 4000 cm^–3, masses of 100 solar masses, and kinetic temperatures of 20 K. The observed data are not inconsistent with the molecular clouds being in a state of hydrostatic equilibrium.Paper presented at the Conference on Protostars and Planets, held at the Planetary Science Institute, University of Arizona, Tucson, Arizona, between January 3 and 7, 1978.     

Models of dusty envelopes around Vega-like stars

Einfache Entwicklungsmodelle der Staubhüllen um Wega, die Sonne, β Pictoris und Ross 128 werden konstruiert. Die Infrarotspektren dieser Hüllen werden unter der Annahme berechnet, daß der Minimalradius der Teilchen durch den Poynting-Robertson-Effekt bestimmt wird, während ihr Wachstum bis zur maximalen Größe durch Koagulation erfolgt. Ein Vergleich mit den Beobachtungen der Staubhüllen durch IRAS zeigt, daß das Modell die Grundzüge der Entwicklung protoplanetarer Scheiben zufriedenstellend beschreibt.     

Models of strange stars having a crust

Models of strange quark stars with a crust consisting of atomic nuclei and degenerate electrons, maintained by an electrostatic barrier at the surface of the strange quark matter, are investigated for a realistic range of parameters of the MIT bag model. The density at which neutrons escape from nuclei, ρ = ρ_drip, is taken as the maximum possible boundary density of the crust. Series of strange stars are calculated as a function of central density. Configurations with masses of 1.44 and 1.77 M{ie330-1} and a gravitational redshift Z_s = 0.23, corresponding to the best-known observational data, are investigated. The presence of a crust results in the existence of a minimum mass for strange stars, and also helps to explain the glitch phenomenon of pulsars within the framework of the existence of strange quark matter. Translated from Astrofizika, Vol. 42, No. 3, pp. 439–448, July–September, 1999.     

Fast rotation of neutron stars and equation of state of dense matter

Fast rotation of compact stars (at sub-millisecond period) and, in particular, their stability, are sensitive to the equation of state (EOS) of dense matter. Recent observations of XTE J1739-285 suggest that it contains a neutron star rotating at 1122 Hz. At such rotational frequency the effects of rotation on star’s structure are significant. We study the interplay of fast rotation, EOS, and gravitational mass of a sub-millisecond pulsar. We discuss the EOS dependence of spin-up to a sub-millisecond period, via mass accretion from a disk in a low-mass X-ray binary.     

Models of neutron stars in generalized bimetric theory of gravitation

Yerevan State University; Institute of Applied Physics Problems, Armenian Academy of Sciences. Translated fromAstrofizika, Vol. 35, No. 1, pp. 121–129, July–August, 1991.     

Models of rapidly rotating neutron stars: remnants of accretion-induced collapse

Equilibrium models of differentially rotating nascent neutron stars are constructed, which represent the result of the accretion-induced collapse of rapidly rotating white dwarfs. The models are built in a two-step procedure: (1) a rapidly rotating pre-collapse white dwarf model is constructed; (2) a stationary axisymmetric neutron star having the same total mass and angular momentum distribution as the white dwarf is constructed. The resulting collapsed objects consist of a high-density central core of size roughly 20 km, surrounded by a massive accretion torus extending over 1000 km from the rotation axis. The ratio of the rotational kinetic energy to the gravitational potential energy of these neutron stars ranges from 0.13 to 0.26, suggesting that some of these objects may have a non-axisymmetric dynamical instability that could emit a significant amount of gravitational radiation.     

Models of dense cores in translucent regions of low mass star formation

We have constructed models for a region of low mass star formation where stellar winds ablate material from dark dense cores and return it to a translucent intercore medium from which subsequent generations of cores condense. Depletion of gas phase species onto grains plays a major role in the chemistry. For reasonable agreement between model core chemical fractional abundances and measured TMC-1 fractional abundances to obtain, the core collapse, once started, must be relatively uninhibited by turbulence or magnetic fields and the core lifetime must fall in a limited range determined by the assumed depletion rates. In a core with the TMC-1 fractional abundances, CH, OH, C₂H, H₂CO, HCN, HNC, and CN are the only simple species that have been detected in TMC-1 at radio and millimeter wavelengths to have fractional abundances that are roughly constant or increasing with time; this result bears considerably on previous work concerned with searches for spectroscopic evidence for and the diagnosis of collapse during protostellar formation, but depends on the fractions of the OH and CH emissions that are associated with the core centre rather than more extended gas or a core-stellar wind boundary layer. Model results for the abundance ratios of H₂O, CH₄, and NH₃ ices are in good agreement with those inferred for Halley's Comet.     

Convection in normal stars and metal-deficient stars

The depths at which convection starts have been calculated for main sequence stars and giant stars of various effective temperature and hydrogen-to-metal ratios. It is found that for late-type stars ofT _e4400 K convenction stars at 0.1. In metal-deficient stars convection starts at shallower optical depths compared to normal stars of the same spectral type. The larger the metal deficiency, the shallower is the depth at which convection starts. The importance of convection in the study of metal-deficient stars and late type stars are also discussed.     

Galactic orbits of Hipparcos stars: Classification of stars

The Galactic orbits of 27 440 stars of all classes with accurate coordinates and parallaxes of more than 3 mas from the Hipparcos catalogue, proper motions from the Tycho-2 catalogue, and radial velocities from the Pulkovo Compilation of Radial Velocities (PCRV) are analyzed. The sample obtained is much more representative than the Geneva-Copenhagen survey and other studies of Galactic orbits in the solar neighborhood. An estimation of the influence of systematic errors in the velocities on orbital parameters shows that the errors of the proper motions due to the duplicity of stars are tangible only in the statistics of orbital parameters for very small samples, while the errors of the radial velocities are noticeable in the statistics of orbital parameters for halo stars. Therefore, previous studies of halo orbits may be erroneous. The distribution of stars in selection-free regions of the multidimensional space of orbital parameters, dereddened colors, and absolute magnitudes is considered. Owing to the large number of stars and the high accuracy of PCRV radial velocities, nonuniformities of this distribution (apart from the well-known dynamical streams) have been found. Stars with their peri- and apogalacticons in the disk, perigalacticons in the bulge and apogalacticons in the disk, perigalacticons in the bulge and apogalacticons in the halo, and perigalacticons in the disk and apogalacticons in the halo have been identified. Thus, the bulge and the halo are inhomogeneous structures, each consisting of at least two populations. The radius of the bulge has been determined: 2 kpc.     

Symbiotic stars

I review our current knowledge of symbiotic stars. A great many papers have graced the literature in the fifty years of their study, and many data are available on the spectral variations at optical wavelengths these stars undergo. I do not give extensive references to those data, for previous reviews have done so quite adequately. Rather, I concentrate on the extensive widening of the wavebands within which symbiotic stars have been studied over the past few years, and attempt to synthesise the data into a coherent picture.Symbiotic stars are most readily explained as interacting binaries, though single star models may still be tenable for some systems. They are made much more complex than most other interacting binaries by the variety of accreting stars, and because gas flows may be highly structured. Moreover, their study is more difficult than that of dwarf novae because the orbital periods are long compared to the activity cycles of the accretion phenomena.Our data base has expanded enormously with our present spectral catholicism. But there remains much valuable work to be done with even simple equipment on small telescopes. I suggest in a final section areas for future work.Invited paper presented at the Lembang-Bamberg IAU Colloquium No. 80 on Double Stars: Physical Properties and Generic Relations, held at Bandung, Indonesia 3–7 June, 1983.     

Zero-population stars

The expected characteristics of stars with zero metal content are investigated for a large range of masses and in varying the original helium-content. It is shown that, in such an extreme case (Z=0), the Main Sequence locus is much less sensitive than expected to the He-content;Y=0 models are found to ignite 3α reactions in the Zero-Age Main Sequence at masses larger by ～ 5M _⊙ than in theY=0.2 case. A comparison with previous results discloses the M.S. slope to be a sensitive function of the metal content only. Evolutionary computations confirm that possible survivors of the zero population can develop a giant branch, akin to that of the well-known Population II stars.     

Formation rate of triple stars relative to double stars

A series of our and McAlisteret al.'s speckle observations of spectroscopic binaries show that certain fractions of them are triple systems. A typical configuration of those systems is that of two stars separated by about 0.1 AU with a third component in an orbit at a distance about 10 AU from the central stars. These results suggest that the third star was formed from the outer circumstellar envelope after forming the central double.     

Massive stars

We describe the present state of massive star research seen from the viewpoint of stellar evolution, with special emphasis on close binaries. Statistics of massive close binaries are reasonably complete for the Solar neighbourhood. We defend the thesis that within our knowledge, many scientific results where the effects of binaries are not included, have an academic value, but may be far from reality. In chapter I, we summarize general observations of massive stars where we focus on the HR diagram, stellar wind mass loss rates, the stellar surface chemistry, rotation, circumstellar environments, supernovae. Close binaries can not be studied separately from single stars and vice versa. First, the evolution of single stars is discussed (chapter I). We refer to new calculations with updated stellar wind mass loss rate formalisms and conclusions are proposed resulting from a comparison with representative observations. Massive binaries are considered in chapter II. Basic processes are briefly described, i.e. the Roche lobe overflow and mass transfer, the common envelope process, the spiral-in process in binaries with extreme mass ratio, the effects of mass accretion and the merging process, the implications of the (asymmetric) supernova explosion of one of the components on the orbital parameters of the binary. Evolutionary computations of interacting close binaries are discussed and general conclusions are drawn. The enormous amount of observational data of massive binaries is summarized. We separately consider the non-evolved and evolved systems. The latter class includes the semi-detached and contact binaries, the WR binaries, the X-ray binaries, the runaways, the single and binary pulsars. A general comparison between theoretical evolution and observations is combined with a discussion of specially interesting binaries: the evolved binaries HD 163181, HD 12323, HD 14633, HD 193516, HD 25638, HD 209481, Per and Sgr; the WR+OB binary V444 Cyg; the high mass X-ray binaries Vela X-1, Wray 977, Cyg X-1; the low mass X-ray binaries Her X-1 and those with a black hole candidate; the runaway Pup, the WR+compact companion candidates Cyg X-3, HD 50896 and HD 197406. We finally propose an overall evolutionary model of massive close binaries as a function of primary mass, mass ratio and orbital period. Chapter III deals with massive star population synthesis with a realistic population of binaries. We discuss the massive close binary frequency, mass ratio and period distribution, the observations that allow to constrain possible asymmetries during the supernova explosion of a massive star. We focuss on the comparison between observed star numbers (as a function of metallicity) and theoretically predicted numbers of stellar populations in regions of continuous star formation and in starburst regions. Special attention is given to the O-type star/WR star/red supergiant star population, the pulsar and binary pulsar population, the supernova rates. Received 17 July 1998     

Carbon stars

In this paper, the present state of knowledge of the carbon stars is discussed. Particular attention is given to issues of classification, evolution, variability, populations in our own and other galaxies, and circumstellar material.