Binary Stars (binary + star)

Distribution by Scientific Domains

Selected Abstracts

Tidal interaction in High-Mass X-ray Binaries

K.A. Stoyanov
Abstract Our aim is to investigate tidal interaction in High-Mass X-ray Binary stars in order to determine in which objects the rotation of the mass donors is synchronized or pseudosynchronized with the orbital motion of the compact companion. We calculate the pseudosynchronization period (Pps) and compare it with the rotational period of the mass donors (Prot). We find that (1) the Be/X-ray binaries are not synchronized, the mass donors rotate faster than the orbital period and the ratio Pps/Prot is 2,300; (2) the giant and supergiant systems are close to synchronization and for them the ratio Pps/Prot is 0.3,2 (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Tracing intermediate-mass black holes in the Galactic Centre

U. Löckmann
ABSTRACT We have developed a new method for post-Newtonian, high-precision integration of stellar systems containing a super-massive black hole (SMBH), splitting the forces on a particle between a dominant central force and perturbations. We used this method to perform fully collisional N -body simulations of inspiralling intermediate-mass black holes (IMBHs) in the centre of the Milky Way. We considered stellar cusps of different power-law indices and analysed the effects of IMBHs of different masses, all starting from circular orbits at an initial distance of 0.1 pc. Our simulations show how IMBHs deplete the central cusp of stars, leaving behind a flatter cusp with slope consistent with what has recently been observed. If an additional IMBH spirals into such a flat cusp, it can take 50 Myr or longer to merge with the central SMBH, thus allowing for direct observation in the near future. The final merger of the two black holes involves gravitational wave radiation which may be observable with planned gravitational wave detectors. Furthermore, our simulations reveal detailed properties of the hypervelocity stars (HVSs) created, and how generations of HVSs can be used to trace IMBHs in the Galactic Centre. We find that significant rotation of HVSs (which would be evidence for an IMBH) can only be expected among very fast stars (v > 1000 km s,1). Also, the probability of creating a hypervelocity binary star is found to be very small. [source]

The tightening of wide binaries in dSph galaxies through dynamical friction as a test of the dark matter hypothesis

X. Hernandez
ABSTRACT We estimate the time-scales for orbital decay of wide binaries embedded within dark matter haloes, due to dynamical friction against the dark matter particles. We derive analytical scalings for this decay and calibrate and test them through the extensive use of N -body simulations, which accurately confirm the predicted temporal evolution. For density and velocity dispersion parameters as inferred for the dark matter haloes of local dSph galaxies, we show that the decay time-scales become shorter than the ages of the dSph stellar populations for binary stars composed of 1 M, stars, for initial separations larger than 0.1 pc. Such wide binaries are conspicuous and have been well measured in the solar neighbourhood. The prediction of the dark matter hypothesis is that they should now be absent from stellar populations embedded within low velocity dispersion, high-density dark mater haloes, as currently inferred for the local dSph galaxies, having since evolved into tighter binaries. Relevant empirical determinations of this will become technically feasible in the near future, and could provide evidence to discriminate between dark matter particle haloes or modified gravitational theories, to account for the high dispersion velocities measured for stars in local dSph galaxies. [source]

Observational studies of early-type binary stars: VV Orionis

Dirk Terrell
ABSTRACT New and previously published observations of the bright eclipsing binary VV Orionis are analyzed. We present new radial velocities and interstellar reddening measurements from high-resolution spectra of this detached, short-period (P= 1.48 d) binary. We discuss the validity of prior claims for the existence of a third body and show that our new velocities and light-curve solution cast doubt on them. The components of VV Ori are shown to be a B1 V primary with a mass M1= 10.9 ± 0.1 M, and a radius R1= 4.98 ± 0.02 R, and a B4.5 V secondary with a mass M2= 4.09 ± 0.05 M, and a radius R2= 2.41 ± 0.01 R,. [source]

Star cluster ecology , V. Dissection of an open star cluster: spectroscopy

Simon F. Portegies Zwart
ABSTRACT We have modelled in detail the evolution of rich open star clusters such as NGC 2516, NGC 2287, Pleiades, Praesepe, Hyades, NGC 2660 and 3680, using simulations that include stellar dynamics as well as the effects of stellar evolution. The dynamics is modelled via direct N -body integration, while the evolution of single stars and binaries is followed through the use of fitting formulae and recipes. The feedback of stellar and binary evolution on the dynamical evolution of the stellar system is taken into account self-consistently. Our model clusters dissolve in the tidal field of the Galaxy in a time-span of the order of a billion years. The rate of mass loss is rather constant, ,1 M, per million years. The binary fraction at first is nearly constant in time, then increases slowly near the end of a cluster's lifetime. For clusters which are more than about 108 yr old the fractions of stars in the form of binaries, giants and merger products in the inner few core radii are considerably higher than in the outer regions, beyond the cluster's half-mass radius. When stars with masses ,2 M, escape from the cluster, they tend to do so with velocities higher than average. The stellar merger rate in our models is roughly one per 30 million years. Most mergers are the result of unstable mass transfer in close binaries (,70 per cent), but a significant minority are caused by direct encounters between single and binary stars. While most mergers occur within the cluster core, even beyond the half-mass radius stellar mergers occasionally take place. We notice a significant birth rate of X-ray binaries, most containing a white dwarf as the mass acceptor. We also find one high-mass X-ray binary with a neutron-star accretor. If formed and retained, black holes participate in many (higher-order) encounters in the cluster centre, resulting in a large variety of exotic binaries. The persistent triple and higher-order systems formed in our models by dynamical encounters between binaries and single stars are not representative for the multiple systems observed in the Galactic disc. We conclude that the majority of multiples in the disc probably formed when the stars were born, rather than through later dynamical interactions. [source]

Short-period near-contact binary systems at the beginning of the overcontact phase

Shengbang Qian
Abstract A detailed analysis of orbital period changes of seven near-contact binary stars (NCBs) (BL And, V473 Cas, XZ CMi, BV Eri, RU Eri, UU Lyn and GR Tau) with period less than 1 d has been performed and their respective O,C diagrams are formed and discussed. It is found that all systems analysed show secular period decreasing. For V473 Cas, the analysis of the period change was performed based on data collected by Moschner, Frank & Bastian. For XZ CMi, its period shows some complex changes, a possible cyclic oscillation is discovered to superpose on the secular decrease that can be explained either by the presence of a third body or by magnetic activity cycles of the components. Since the third-body assumption is consistent with the photometric solution of Rafert, XZ CMi may be a truly triple system. For BV Eri, the period decrease is only supported by weak evidence. All the seven systems are short-period NCBs with AF-type primary components where both components are filling or nearly filling the critical Roche lobe. As the period decreases, the separation between both components will be reducing and thus these systems will evolve into A-type overcontact binaries. The period decrease may be caused by mass transfer or/and by angular momentum loss via magnetic braking. Combined with the published data on the other systems of the same type, a possible statistical connection between orbital period P and its rate of decrease dP/dt is obtained: dP/dt=,5.3 × 10,7×P+ 1.3 × 10,7 d yr,1. This correlation indicates that the smaller the orbital period P is, the smaller its rate of change dP/dt will be. The correlation found in this paper indicates that there may be a smooth transition from A- and F-type NCBs with period decreases to the A- and F-type overcontact binaries that have period increases, and in that sense one may postulate that the NCBs may be the progenitors of the A-type W UMa systems and will be oscillating around a marginal-contact state as predicted by thermal relaxation oscillation (TRO) theory. [source]

The influence of binary stars on the kinematics of low-mass galaxies

S. De Rijcke
In this paper, the influence of binary stars on the measured kinematics of dwarf galaxies is investigated. Using realistic distributions of the orbital parameters (semi-major axis, eccentricity, etc.), analytical expressions are derived for the changes induced by the presence of binary stars in the measured velocity moments of low-mass galaxies (such as the projected velocity dispersion and the fourth-order Gauss,Hermite coefficient h4). It is shown that there is a noticeable change in the observed velocity dispersion if the intrinsic velocity dispersion of a galaxy is of the same order as the binary velocity dispersion. The kurtosis of the line-of-sight velocity distribution (LOSVD) is affected even at higher values of the intrinsic velocity dispersion. Moreover, the LOSVD of the binary stars (i.e. the probability of finding a star in a binary system with a particular projected velocity) is given in closed form, approximating the constituent stars of all binaries to revolve on circular orbits around each other. With this binary LOSVD, we calculate the observed LOSVD, the latter quantifying the movement of stars along the line of sight caused both by the orbits of the stars through the galaxy and by the motion of the stars in binary systems. As suggested by the changes induced in the moments, the observed LOSVD becomes more peaked around zero velocity and develops broader high-velocity wings. These results are important in interpreting kinematics derived from integrated-light spectra of low-mass galaxies and many of the intermediate results are useful for comparison with Monte Carlo simulations. [source]

Productivity and impact of astronomical facilities: A recent sample

V. Trimble
Abstract The papers published in 11 key astronomical journals in 2008, and a year of citations to those from the first half of the year, have been associated with the telescopes, satellites, and so forth where the data were gathered using a form of fractional counting. Some numbers are also given by journal, by subfield, and by wavelength band. The largest numbers of papers, and generally also quite highly cited ones, in their respective wavelength bands come from the Very Large Array, the Hubble Space Telescope, the Sloan Digital Sky Survey, the Spitzer Space Telescope, and the Chandra X-ray Telescope. Optical astronomy is still the largest sector; and papers about cosmology and exoplanets are cited more often than papers about binary stars and planetary nebulae. The authors conclude that it is of equal importance to recognize (a) that a very large number of papers also come from less famous facilities, (b) that a very large fraction of papers (and their authors) are concerned with the less highly-cited topics, (c) that many facilities are quite slow in achieving their eventual level of influence, and (d) that one really needs at least three years of citation data, not just one or two, to provide a fair picture of what is going on (© 2010 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Velocity curve analysis of the spectroscopic binary stars RZ Cas, CC Cas, HS Her, HD 93917, V921 Her and Y Cygni by the artificial neural networks

K. Karami
Abstract We use an Artificial Neural Network (ANN) to derive the orbital parameters ofspectroscopic binary stars. Using measured radial velocity data of six double-lined spectroscopic binary systems RZ Cas, CC Cas, HS Her, HD 93917, V921 Her and Y Cygni, we find corresponding orbital and spectroscopic elements. Our numerical results are in good agreement with those obtained by others using more traditional methods (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The Knorre astronomers' dynasty

G. Pinigin
Abstract We attempt to throw light upon the poorly known astronomical dynasty of Knorre and describe its contribution to astronomy. The founder of the dynasty, Ernst Christoph Friedrich Knorre (1759,1810), was born in Germany in 1759, and since 1802 he was a Professor of Mathematics at the Tartu University, and observer at its temporary observatory. He determined the first coordinates of Tartu by stellar observations. Karl Friedrich Knorre (1801,1883) was the first director of the Naval Observatory in Nikolaev since the age of 20, provided the Black Sea navy with accurate time and charts, trained mariners in astronomical navigation, and certified navigation equipment. He compiled star maps and catalogues, and determined positions of comets and planets. He also participated in Bessel's project of the Academic Star Charts, and was responsible for Hora 4, published by the Berlin Academy of Sciences. This sheet permitted to discover two minor planets, Astraea and Flora. Viktor Knorre (1840,1919) was born in Nikolaev. In 1862 he left for Berlin to study astronomy. After defending his thesis for a doctor's degree, he went to Pulkovo as an astronomical calculator in 1867. Since 1873 Viktor worked as an observer of the Berlin Observatory Fraunhofer refractor. His main research focussed on minor planets, comets and binary stars. He discovered the minor planets Koronis, Oenone, Hypatia and Penthesilea. Viktor Knorre also worked on improving astronomical instrumentation, e.g. the Knorre & Heele equatorial telescope mounting (© 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

Pairing mechanisms for binary stars

M.B.N. Kouwenhoven
Abstract Knowledge of the binary population in stellar groupings provides important information about the outcome of the star forming process in different environments. Binarity is also a key ingredient in stellar population studies and is a prerequisite to calibrate the binary evolution channels. In these proceedings we present an overview of several commonly used methods to pair individual stars into binary systems, which we refer to as the pairing function.Many pairing functions are frequently used by observers and computational astronomers, either for the mathematical convenience, or because they roughly describe the expected outcome of the star forming process. We discuss the consequences of each pairing function for the interpretation of observations and numerical simulations. The binary fraction and mass ratio distribution generally depend strongly on the selection of the range in primary spectral type in a sample. These quantities, when derived from a binary survey with a mass-limited sample of target stars, are thus not representative for the population as a whole. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]

The empirical upper limit for mass loss of cool main sequence stars

A. Lednicka
Abstract The knowledge of mass loss rates due to thermal winds in cool dwarfs is of crucial importance for modeling the evolution of physical parameters of main sequence single and binary stars. Very few, sometimes contradictory, measurements of such mass loss rates exist up to now. We present a new, independent method of measuring an amount of mass lost by a star during its past life. It is based on the comparison of the present mass distribution of solar type stars in an open cluster with the calculated distribution under an assumption that stars with masses lower than Mlim have lost an amount of mass equal to ,M. The actual value of ,M or its upper limit is found from the best fit. Analysis of four clusters: Pleiades, NGC 6996, Hyades and Praesepe gave upper limits for ,M in three of them and the inconclusive result for Pleiades. The most restrictive limit was obtained for Praesepe indicating that the average mass loss rate of cool dwarfs in this cluster was lower than 6 × 10,11 M,/yr. With more accurate mass determinations of the solar type members of selected open clusters, including those of spectral type K, the method will provide more stringent limits for mass loss of cool dwarfs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) [source]