Observations of double stars at Pulkovo with 65 cm Zeiss refractor

Some results of the photographic observations of double stars with 65 cm refractor of Pulkovo observatory are presented. We use the apparent motion parameters (AMP) method which allows to determine the orbits and to carry out the dynamical investigation of wide binaries on the basis of a short arc of their orbital motion. We have determined more than 40 orbits for wide pairs and also the sum of masses and in some cases—the mass-ratio of components. The references to our works and the basic results of observations are contained in Kisselev et al. [2004. Catalogue of relative positions of visual double stars made on the observations with 26^″ refractor of Pulkovo observatory. Strassbourg, I/297]. We apply two ways of revealing the hidden mass of our stars, namely: revealing of possible perturbations from comparison of observational and calculated positions using differences O-C (for instance, perturbations in the orbital motion of ADS 15571) and also by means of comparison of the sum of the masses obtained by us and the sum of the masses obtained by means of the mass-luminosity relation. An excess of masses of about 1-3 solar masses is detected for binaries: ADS 497, ADS 8450 and ADS 10329 by means of last method.The estimations of the masses for some binaries are discussed. Also we justify the necessity of precise parallaxes and relative radial velocities of stars, which could be measured by space telescopes such as the GAIA as the additional parameters for determination of orbits of binaries.     

Transneptunian Binaries

The discovery of binaries among the population of transneptunian objects isa landmark advance in the study of this remote region of the solar system.Determination of binary orbits will enable direct determination of systemmasses, fundamental for determination of density, internal structure, and bulkcomposition. The mere existence of binaries with the observed separations andapparent masses constrains models of planetary formation.     

双星轨道拟合的研究进展

双星轨道拟合是天文学的一项基础性研究工作。其主要目的是给出双星系统的二体轨道参数,这些参数不仅是高精度、高网格密度星表参考架的必要组成部分,而且也为理解各种有关观测现象提供了必要的动力学基础;更重要的是,双星轨道拟合可以直接估计恒星物理和星系天文学等领域极有应用价值的恒星质量参数。因此,长期以来双星轨道拟合工作一直受到研究者的广泛关注。近年来,随着高精度的恒星运动学观测资料的大量积累,双星轨道拟合更成为天体测量和天体力学的一个共同的热点课题,有关研究也有了长足的进展。综述了双星轨道拟合的历史及现状,其中着重介绍了目前所用的主要观测资料和各种具体的拟合模型、拟合方法;简要描述了几种主要的双星星表;展望了今后双星轨道拟合工作的发展趋势。     

Detached binaries in the Large Magellanic Cloud

Analysis of the radial-velocity and light curves of detached eclipsing binaries allows to derive stellar masses and radii and, in consequence, enables to find their distances. This method has been already applied to several LMC binaries, but in order to have the distance to the LMC determined with good accuracy, the study needs to be extended to a larger number of eclipsing systems. As a first step, we present results of the analysis of the photometry of over 80 detached binaries in the LMC selected from the OGLE-II catalog of 53,000 candidates for variables. If possible, we combine the OGLE-II data with the photometry from other projects (MACHO and EROS). As a result, we present the list of the brightest eclipsing binaries in the LMC suitable for distance determination.     

Eclipsing Binaries in Local Group Galaxies

A review is presented of the progress that has been made in the last 3 years towards quantifying the properties of high-mass detached and semi-detached eclipsing binaries in Local Group galaxies. Comparisons between these observational results on masses, radii, temperatures and luminosities for stars in detached binaries and evolution models for single stars at the appropriate metallicity are found to be very good. New evolution models for interacting binaries passing through case A mass exchange are being calculated, and indicate a requirement for some mass loss to find agreement with the observational data. The observational data on such semi-detached systems show similar properties to those in the Milky Way galaxy. The directly-determined distances to all these eclipsing binaries are proving to be most valuable for strengthening the distance scale amongst the Local Group galaxies.     

Extragalactic eclipsing binaries: astrophysical laboratories

Extragalactic eclipsing binaries open a new perspective on the study of stellar structure and evolution. Stars in different galaxies have formed and evolved in environments with chemical histories that may differ from those of the solar neighborhood. For example, the LMC and SMC contain low-metallicity, young massive stars that are no longer found in our Galaxy. Eclipsing binaries, yielding accurate determinations of masses, radii and temperatures, allow for critical tests of, e.g., convective overshooting, mass loss, and internal structure. In addition, they provide an empirical calibration of the mass-luminosity relationship. In this paper, I present some results from ongoing programs on the determination of physical properties and distances to eclipsing binaries in the LMC, the SMC, and M31. In particular, I focus on aspects relevant to stellar astrophysics, and on the contributions of these eclipsing binaries to our understanding of the structure and chemical evolution of the host galaxies.     

Speckle observations with PISCO in Merate: IV. Astrometric measurements of visual binaries in 2005

We present relative astrometric measurements of visual binaries made during the second semester of 2005, with the speckle camera PISCO at the 102 cm Zeiss telescope of Brera Astronomical Observatory, in Merate. Our sample contains orbital couples as well as binaries whose motion is still uncertain. The purpose of this long term program is to improve the accuracy of the orbits and determine the masses of the components. We performed 130 new observations of 120 objects, with most of the angular separations in the range 0″.1–4″, and with an average accuracy of 0″.01. Most of the position angles could be determined without the usual 180. ambiguity with the application of triple‐correlation techniques, and their mean error is 0°8. We have found a possible new triple system: ADS 11077. The measurements of the closest binaries were made with a new data reduction procedure, based on model fitting of the background of the auto‐correlations. As this procedure proved to be very efficient, we have re‐processed the old observations of close binaries made with PISCO in Merate since 2004. We thus improved 20 measurements already published and obtained 7 new measurements for observations that were previously reported as “unresolved”. We finally present revised orbits for ADS 684, MCA 55Aac (in the Beta 1 Cyg–Albireo multiple system) and ADS 14783 for which the previously published orbits led to large residuals with our measurements and for which the new observations made since their computation allowed a significant improvement of those old orbits. The sum of the masses that we derived for those systems are consistent with the spectral type of the stars and the dynamic parallaxes are in good agreement with the parallaxes measured by Hipparcos. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Parametrization of single and binary stars

Stellar masses and ages are not directly observable parameters, and the methods used to determine them are based on the calibrating relations. In particular, the mass–luminosity relation, based on the masses of less than 200 well-studied binaries, is virtually the only way to estimate the mass of single stars. Thus, the development of methods for estimating stellar masses with accuracy comparable to direct methods is a problem of vital importance.
Here, we describe a method for estimating stellar masses and ages, which is based on the geometric similarity of evolutionary tracks for the stars at the same evolutionary stage in the Hertzsprung–Russell (HR) diagram. To examine the proposed approach, it has been applied to various test data sets. Application of the method, using synthetic stellar spectra Basel Stellar Library (of theoretical spectra; BaSeL), demonstrates that it allows determination of masses and ages of stars with a predictable distribution of uncertainties.
This statistical approach allows us to demonstrate the viability of the method using it on the set of double-lined eclipsing binaries with intermediate-mass and low-mass components which allows us to compare calculated characteristics with observational ones. As a result, the uncertainties of the stellar masses estimated with the proposed method are comparable with the accuracy of ones obtained from direct observations. This allows us to recommend the method for mass estimates of masses of single stars by the localization in the HR diagram.
As for the ages, the estimates for intermediate-mass stars are more reliable, while those obtained for low-mass stars are very uncertain, due both to slower movement of these stars in the HR diagram with age at stages close to the main sequence and to certain disagreements between theoretical models for this mass range.     

A New Algorithm in Computations of Binary Stars’ Observational Quantities and Its Application

The determination of the orbital parameters of binaries is an important constituent of fundamental astronomy. It is the only reliable way to determine the stellar mass, which is crucial for the studies of stellar dynamics and evolution. And the orbital parameters can be used to compute the motions of component stars, which could improve the population of reference stars in the bright star catalogue reference frame. However, the previous studies on the determination of binaries’ orbital parameters adopted some approximations in the computation of observational quantities. According to the statistical results of hundreds of binaries with bright component stars, it could be found that the approximations in the variation of the tangent plane and the prospective effect of the reference plane have made the deviations of computed observational quantities become larger than the present observational precision (1 mas). In this paper, a new algorithm based on the two-body model and the rigorous relation of solid geometry for computing observational quantities is presented. This new algorithm has been verified by fitting the simulation and actually-observed data. And the fitting results show that the fitted parameters solved by the new algorithm usually have a higher confidence than that solved by the approximate algorithm. Therefore, the new algorithm has certain improvements in the determination of the orbital parameters of binaries, especially, for the wide binaries nearby the solar system.     

Companions to sdB binaries−degenerate or non-degenerate?

We report the results of an on-going programme to collect lightcurves for EHB binaries with orbital periods P ? 1 d. Degenerate and non-degenerate companions are clearly distinguished by the presence or absence of a reflection effect in these binaries. The amplitude of the reflection effect can be combined with other data to provide an estimate of the mass of the companion star. We find that the fraction of sdB binaries in our sample which have non-degenerate companions is 0.08±0.06 (1?σ error). These non-degenerate companions have very low masses (? 0.1M⊙). This property is not predicted by existing population synthesis models.     

Observations of binaries and evolutionary implications

Comparison of the characteristics of groups of stars in various evolutionary phases and the study of individual systems allow to make estimates of the parameters governing mass loss and mass transfer. Observations enable us in a few cases to determine geometric models for binaries during or after the mass transfer phase (disks, rings, common envelopes, symbiotics, interacting binaries, compact components).From spectra taken at different phases, radial velocity curves can be derived and masses and radii can be determined. In special cases spectra in different spectral ranges (visual, UV, X-ray) are required for the determination of the radial velocities of the two components (for X-ray binaries, for systems with hot and cool components). Information on parameters related to the mass transfer process enables us to consider non conservative evolution — i.e. the computation of evolutionary sequences with the assumption that mass and angular momentum not only are transferred from one of the components towards the other one, but that also mass and angular momentum can leave the system. Careful and detailed analysis of the observations allows in certain cases to determine the parameters governing this mass and angular momentum loss, and for contact phases, to determine the degree of contact.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.     

Companions of Post-Common Envelope sdB Binaries

Preliminary results are presented from two ongoing complementary surveys intended to investigate the nature and characteristics of the optically invisible secondaries in post-common envelope subdwarf B (sdB) binary stars. We obtain precise radial velocities to derive periods and minimum companion masses for bright field sdB stars. These data are combined with light curves to search for eclipses, reflection effects, or ellipsoidal variations. We emphasize the importance of using complete unbiased samples, without which it will not be possible to understand the details of the multiple processes that produce these stars. It remains true that all known secondary companions in short-period sdB binaries are nearly invisible, thus they must be either low mass main sequence (MS) stars or compact objects, e.g., white dwarfs. In our small, nearly-complete sample, white dwarf secondaries outnumber MS secondaries by about a factor of five. Known MS masses in short-period sdB binaries are all surprisingly low, indicating a possible bimodal mass distribution for all MS secondaries in sdB binaries.     

Masses and Radii of Low-Mass Stars: Theory Versus Observations

Eclipsing binaries with M-type components are still rare objects. Strong observational biases have made that today only a few eclipsing binaries with component masses below 0.6 M_⊙ and well-determined fundamental properties are known. However, even in these small numbers the detailed comparison of the observed masses and radii with theoretical predictions has revealed large disagreements. Current models seem to predict radii of stars in the 0.4--0.8 M_⊙ range to be some 5--15% smaller than observed. Given the high accuracy of the empirical measurements (a few percent in both mass and radius), these differences are highly significant. I review all the observational evidence on the properties of M-type stars and discuss a possible scenario based on stellar activity to explain the observed discrepancies.     

Short Gamma Ray Bursts as electromagnetic counterpart of coalescing binary systems

Coalescing binary systems, consisting of two collapsed objects, are among the most promising sources of high frequency gravitational waves signals detectable, in principle, by ground-based interferometers. Binary systems of Neutron Star or Black Hole/Neutron Star mergers should also give rise to short Gamma Ray Bursts, a subclass of Gamma Ray Bursts. Short-hard-Gamma Ray Bursts might thus provide a powerful way to infer the merger rate of two-collapsed object binaries. Under the hypothesis that most short Gamma Ray Bursts originate from binaries of Neutron Star or Black Hole/Neutron Star mergers, we outline here the possibility to associate short Gamma Ray Bursts as electromagnetic counterpart of coalescing binary systems.     

Eclipsing binaries as accurate distance indicators to nearby galaxies

We discuss the potential for using extragalactic eclipsing binaries with well-determined physical properties as standard candles to improve the extragalactic distance scale. The advent of high quantum efficiency/low noise CCDs has now made it possible to obtain high precision light and radial velocity curves for the more luminous OB-type eclipsing binaries in the Magellanic Clouds with even small to moderate size (1–2m) telescopes. This can lead to the determination of distance moduli (m-M)₀ to the LMC and SMC with precisions of about 0^m.15 for individual binaries.These distances are essentially free from the assumptions made using other distance indicators.     

The Hill stability of inclined small mass binary systems in three-body systems with special application to triple star systems, extrasolar planetary systems and Binary Kuiper Belt systems

The dynamical stability of a bound triple system composed of a small binary or minor planetary system moving on a orbit inclined to a central third body is discussed in terms of Hill stability for the full three-body problem. The situation arises in the determination of stability of triple star systems against disruption and component exchange and the determination of stability of extrasolar planetary systems and minor planetary systems against disruption, component exchange or capture. The Hill stability criterion is applied to triple star systems and extrasolar planetary systems, the Sun-Earth-Moon system and Kuiper Belt binary systems to determine the critical distances for stable orbits. It is found that increasing the inclination of the third body decreases the Hill regions of stability. Increasing the eccentricity of the binary also produces similar effects.These type of changes make exchange or disruption of the component masses more likely. Increasing the eccentricity of the binary orbit relative to the third body substantially decreases stability regions as the eccentricity reaches higher values. The Kuiper Belt binaries were found to be stable if they move on circular orbits. Taking into account the eccentricity, it is less clear that all the systems are stable.     

Torus formation in neutron star mergers and well-localized short gamma-ray bursts

Merging neutron stars (NSs) are hot candidates for the still enigmatic sources of short gamma-ray bursts (GRBs). If the central engines of the huge energy release are accreting relic black holes (BHs) of such mergers, it is important to understand how the properties of the BH–torus systems, in particular disc masses and mass and rotation rate of the compact remnant, are linked to the characterizing parameters of the NS binaries. For this purpose, we present relativistic smoothed particle hydrodynamic simulations with conformally flat approximation of the Einstein field equations and a physical, non-zero temperature equation of state. Thick disc formation is highlighted as a dynamical process caused by angular momentum transfer through tidal torques during the merging process of asymmetric systems or in the rapidly spinning triaxial post-merger object. Our simulations support the possibility that the first well-localized short and hard GRBs 050509b, 050709, 050724, 050813 have originated from NS merger events and are powered by neutrino-antineutrino annihilation around a relic BH–torus system. Using model parameters based on this assumption, we show that the measured GRB energies and durations lead to estimates for the accreted masses and BH mass accretion rates which are compatible with theoretical expectations. In particular, the low-energy output and short duration of GRB 050509b set a very strict upper limit of less than 100 ms for the time interval after the merging until the merger remnant has collapsed to a BH, leaving an accretion torus with a small mass of only  ∼0.01 M_⊙  . This favours a (nearly) symmetric NS+NS binary with a typical mass as progenitor system.     

Circularization of Binary Orbits

In a previous paper Mayer and Hanna (1991) discussed the process of binary orbit circularization for detached eclipsing binaries. An agreement of observational data with Tassoul's circularization time formula for binaries with masses greater than 2.5 M was found. In the present paper we also examine orbital circularization in evolved spectroscopic binaries.     

Mass transfer in eccentric binaries: the new oil-on-water smoothed particle hydrodynamics technique

To measure the onset of mass transfer in eccentric binaries, we have developed a two-phase smoothed particle hydrodynamics (SPH) technique. Mass transfer is important in the evolution of close binaries, and a key issue is to determine the separation at which mass transfer begins. The circular case is well understood and can be treated through the use of the Roche formalism. To treat the eccentric case, we use a newly developed two-phase system. The body of the donor star is made up from high-mass water particles, whilst the atmosphere is modelled with low-mass oil particles. Both sets of particles take part fully in SPH interactions. To test the technique, we model circular mass-transfer binaries containing a  0.6 M_⊙  donor star and a  1 M_⊙  white dwarf; such binaries are thought to form cataclysmic variable (CV) systems. We find that we can reproduce a reasonable CV mass-transfer rate, and that our extended atmosphere gives a separation that is too large by approximately 16 per cent, although its pressure scale height is considerably exaggerated. We use the technique to measure the semimajor axis required for the onset of mass transfer in binaries with a mass ratio of   q = 0.6  and a range of eccentricities. Comparing to the value obtained by considering the instantaneous Roche lobe at pericentre, we find that the radius of the star required for mass transfer to begin decreases systematically with increasing eccentricity.     

On the formation and evolution of black hole binaries

We present the results of a systematic study of the formation and evolution of binaries containing black holes and normal-star companions with a wide range of masses. We first reexamine the standard formation scenario for close black hole binaries, where the progenitor system, a binary with at least one massive component, experienced a common-envelope phase and where the spiral-in of the companion in the envelope of the massive star caused the ejection of the envelope. We estimate the formation rates for different companion masses and different assumptions about the common-envelope structure and other model parameters. We find that black hole binaries with intermediate- and high-mass secondaries can form for a wide range of assumptions, while black hole binaries with low-mass secondaries can only form with apparently unrealistic assumptions (in agreement with previous studies).
We then present detailed binary evolution sequences for black hole binaries with secondaries of 2 to 17 M_⊙ and demonstrate that in these systems the black hole can accrete appreciably even if accretion is Eddington-limited (up to 7 M_⊙ for an initial black hole mass of 10 M_⊙) and that the black holes can be spun up significantly in the process. We discuss the implications of these calculations for well-studied black hole binaries (in particular GRS 1915+105) and ultraluminous X-ray sources of which GRS 1915+105 appears to represent a typical Galactic counterpart. We also present a detailed evolutionary model for Cygnus X-1, a massive black hole binary, which suggests that at present the system is most likely in a wind mass-transfer phase following an earlier Roche-lobe overflow phase. Finally, we discuss how some of the assumptions in the standard model could be relaxed to allow the formation of low-mass, short-period black hole binaries, which appear to be very abundant in nature.