Stars In Binary Systems Research Articles

Evolution of Wolf-Rayet stars in binary systems: An analysis of the mass and orbital-eccentricity distributions

We have undertaken a statistical study of the component mass ratios and the orbital eccentricities of WR + O close binary, detached main-sequence (DMS), contact early-type (CE), and semidetached (SD) systems. A comparison of the characteristics of WR + O systems and of DMS, CE, and SD systems has enabled us to draw certain conclusions about the evolutionary paths of WR + O binaries and to demonstrate that up to 90% of all known WR + O binaries formed as a result of mass transfer in massive close O + O binary systems. Since there is a clear correlation between the component masses in SD systems with subgiants, the absence of an anticorrelation between the masses of the WR stars and O stars in WR + O binaries cannot be considered evidence against the formation of WR + O binaries via mass transfer. The spectroscopic transitional orbital period P tr sp corresponding to the transition from nearly circular orbits (e sp<0.1) to elliptical orbits (e sp≥0.1) is ∼14d for WR + O systems and ∼2d–3d for OB + OB systems. The period range in which all WR + O orbits are circular \((1\mathop d\limits_. 6 \leqslant P \leqslant 14^d )\) is close to the range for SD systems with subgiants, \(0\mathop d\limits_. 7 \leqslant P \leqslant 15^d \). The large difference between the P tr sp values for WR + O and OB + OB systems suggests that a mechanism of orbit circularization additional to that for OB + OB systems at the DMS stage (tidal dissipation of the orbital energy due to radiative damping of the dynamical tides) acts in WR + O binaries. It is natural to suggest mass transfer in the parent O + O binaries as this supplementary orbit-circularization mechanism. Since the transitional period between circular and elliptical orbits for close binaries with convective envelopes and ages of 5×109 years is \(P_{tr} = 12\mathop d\limits_. 4\), the orbits of most known SD systems with subgiants had enough time to circularize during the DMS stage, prior to the mass transfer. Thus, for most SD systems, mass transfer plays a secondary role in circularization of their orbits.

X-ray observations of massive colliding wind binaries

Massive stars in binary systems can generate X-ray emission in the region between the two stars where stellar winds collide. Colliding wind X-ray emission acts as an in-situ probe of important wind parameters like mass-loss rates, chemical abundances, wind velocities, and possibly magnetic field strengths. Variations in observed colliding-wind X-ray emission can be produced by the changing line-of-sight to the colliding wind region as the stars move in orbit and/or the changing emission measure of the shocked gas in the wind collision zone produced if the separations between the two stars change. X-ray variability depends on the wind and orbital parameters and so can in principal allow the direct measurement of mass functions even for low inclination systems and provide a connection between the stellar and wind parameters. X-ray emission from colliding wind systems can thus be used to refine our understanding of the evolutionary state of massive stars and to help test evolutionary models. We discuss recent advances in defining the X-ray spectral and temporal signatures of colliding wind X-ray emission using ROSAT, ASCA, RXTE, Chandra and XMM-Newton data.

An X-ray survey of Wolf-Rayet stars in the Magellanic Clouds

We have used archival Chandra-acis observations to search for X-ray emission from WR stars in the Magellanic Clouds (MCs), where the low interstellar absorption and known distances make it easy to compare X-ray luminosities with spectral types and binary status of WR stars. We have detected X-ray emission from ~20 WR stars in the MCs with X-ray luminosities of 6.6×1032-1.4×1035 ergs s–1. The analysis of the X-ray spectra of the brightest sources indicates that the X-ray emitting gas has temperatures ≥ 1.0×107 K, as expected in colliding winds. We will compare these results with the binary status of the WR stars to determine the amounts of the X-ray emission produced in the colliding winds for WR stars in binary systems and to assess the X-ray emission from single WR stars.

The evolution of close helium star to neutron star binaries

We present the evolution of helium stars in binary systems with a 1.4 M⊙ neutron-star companion which will produce double neutron-star binaries, i.e., systems with helium stars more massive than 2.5 M⊙. We found that mass transfer from helium star less massive than ~ 3.3 M⊙ will end up in a common-envelope phase. If the neutron star has enough time to complete the spiral-in before the core of the helium star collapses, the system will become a very tight double neutron-star binary (P ≈ 0d.01). More massive helium stars do not go through a dynamically-unstable mass transfer. The outcome of binaries with helium star in this range of mass is double-neutron star systems with period of 0d.1-1d, suggesting them to be the progenitor of the observed Galactic double neutron-star pulsars B 1913+16 and B 15344+12. Wide DNS pulsars like J 1518+4904 are produced from helium star-neutron star binaries which avoid Roche-lobe overflow. We are also able to distinguish the progenitors of Type Ib supernovae (as the high-mass helium stars or systems in wide orbits) from those of Type Ic supernovae (as the lower-mass helium stars or systems in close orbits).

Evolution of Wolf-Rayet stars in binary systems: analysis of distributions of masses and orbital eccentricities

It is shown that up to 90% of the known WR+O binaries have been formed as a result of the mass transfer in O+O binary systems.

On the formation of Super-AGB stars in intermediate mass close binary systems

The evolution of a star of initial mass 9 , and metallicity in a Close Binary System (CBS) is followed in the presence of different mass companions in order to study their influence on the final evolutionary stages and, in particular, on the structure and composition of the remnant components. In order to do that, we study two extreme cases. In the first one the mass of the secondary is 8 , whereas in the second one the mass was assumed to be 1 . For the first of those cases we have also explored the possible outcomes of both conservative and non-conservative mass-loss episodes. During the first mass transfer episode, several differences arise between the models. The system with the more extreme mass ratio () is not able to survive the first Roche lobe overflow (RLOF) as a binary, but instead, spiral-in of the secondary onto the envelope of the primary star is most likely. The system formed by two stars of comparable mass undergoes two mass transfer episodes in which the primary is the donor. We have performed two sets of calculations corresponding to this case in order to account for conservative and non-conservative mass transfer during the first mass loss episode. One of our main results is that for the non-conservative case the secondary becomes a Super–AGB star in a binary system. Such a star undergoes a final dredge-up episode, similar to that of a single star of comparable mass. The primary components do not undergo a Super–AGB phase, but instead a carbon-oxygen white dwarf is formed in both cases (conservative and non-conservative), before reversal mass transfer occurs. However, given the extreme mass ratios at this stage between the components of the binary system, especially for the conservative case, the possibility of merger episodes remains likely. We also discuss the presumable final outcomes of the system and possible observational counterparts.

On the anomaly of Balmer line profiles of A-type stars

In previous work, Gardiner et al. ([CITE]) found evidence for a discrepancy between the obtained from Balmer lines with that from photometry and fundamental values for A-type stars. An investigation into this anomaly is presented using Balmer line profiles of stars in binary system with fundamental values of both and . A revision of the fundamental parameters for binary systems given by Smalley & Dworetsky ([CITE]) is also presented. The obtained by fitting Hα and Hβ line profiles is compared to the fundamental values and those obtained from uvby photometry. We find that the discrepancy found by Gardiner et al. ([CITE]) for stars in the range 7000 K 9000 K is no longer evident.

A general three-dimensional fluid dynamics code for stars in binary systems

We describe the theory and implementation of a three-dimensional fluid dynamics code that we have developed for calculating the surface geometry and circulation currents in the secondaries of interacting binary systems. The main method is based on an Eulerian-Lagrangian scheme to solve the advective and force terms in Euler's equation. Surface normalized spherical polar coordinates are used to allow the accurate modelling of the surface of the star, as is necessary when free surfaces and irradiation effects are to be considered. The potential and its gradient are expressed as sums of Legendre polynomials, which allows a very efficient solution of Poisson's equation. The basic solution scheme, based on operator splitting, is outlined, and standard numerical tests are presented.

Conformally flat smoothed particle hydrodynamics application to neutron star mergers

We present a new 3D smoothed particle hydrodynamics code that solves the general relativistic $\mathrm{field}+\mathrm{hydrodynamic}$ equations in the conformally flat approximation. Several test cases are considered to test different aspects of the code. We then apply the code to the coalescence of a neutron star binary system. The neutron stars are modeled by a polytropic equation of state with adiabatic indices $\ensuremath{\Gamma}=2.0, \ensuremath{\Gamma}=2.6,$ and $\ensuremath{\Gamma}=3.0.$ We calculate the gravitational wave signals, luminosities, and frequency spectra by employing the quadrupole approximation for emission and back reaction in the slow motion limit. In addition, we consider the amount of ejected mass.

RU Cen and SX Cen: Two strongly depleted RV Tauri stars in binary systems

We present a chemical abundance analysis on the basis of high signal-to-noise and high-resolution (= 48 000) optical spectra of two RV Tauri stars RU Cen and SX Cen. With an (Fe=H) = 1: 9a nd a (Zn=Fe) = +0:9 for RU Cen and a (Fe=H) = 1: 1a nd a (Zn=Fe) = +0:6 for SX Cen, both stars of spectroscopic class B display strong depletion of refractory elements in their photospheres. Our CORALIE radial velocity measurements prove the stars to be members of binary systems and a detailed construction of the spectral energy distribution indicate the presence of a large amount of hot circumstellar dust. Moreover, the orbital period of SX Cen of around 600 days is similar to the published period of mean magnitude variation in the light curve (RV Tauri phomometric class b phenomenon). All these observations indicate the presence of a stable circumbinary disk in the objects and strengthen the model that this is a necessary condition for the depletion process to take place.

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

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.

On the angular momentum transfer on to compact stars in binary systems

Results of three-dimensional numerical simulations of the gas transfer in close binary systems show that, in addition to the formation of a tidally induced spiral shock wave, it is also possible for accretion streams to be produced, having low specific angular momentum in a region close to the accreting star. These streams are mainly placed above the orbital disc but are also unevenly present in the equatorial plane. The relevance of such flows is related to formation of hot coronae or bulges in regions very close to the accretor centre. The eventual formation of such bulges and shock-heated flows is interesting in the context of advection-dominated solutions and for the explanation of spectral properties of the black hole candidates in binary systems.

The Frequency of Binary Stars in the Core of 47 Tucanae

Differential time series photometry has been derived for 46,422 main-sequence stars in the core of 47 Tucanae. The observations consisted of near-continuous 160 s exposures alternating between the F555W and F814W filters for 8.3 days in 1999 July with the Wide Field Planetary Camera 2 on the Hubble Space Telescope. Using Fourier and other search methods, 11 detached eclipsing binaries and 15 W Ursa Majoris stars have been discovered plus an additional 10 contact or near-contact noneclipsing systems. After correction for nonuniform area coverage of the survey, the observed frequencies of detached eclipsing binaries and W UMa stars within 90'' of the cluster center are 0.022% and 0.031%, respectively. The observed detached eclipsing binary frequency, the assumptions of a flat binary distribution with log period, and assuming that the eclipsing binaries with periods longer than about 4 days have essentially their primordial periods imply an overall binary frequency of 13% ± 6%. The observed W UMa frequency and the additional assumptions that W UMa stars have evolved to contact according to tidal circularization and angular momentum loss theory and that the contact binary lifetime is 109 yr imply an overall binary frequency of 14% ± 4%. An additional 71 variables with periods from 0.4 to 10 days have been found, which are likely to be BY Draconis stars in binary systems. The radial distribution of these stars is the same as that of the eclipsing binaries and W UMa stars and is more centrally concentrated than average stars but less so than the blue straggler stars. A distinct subset of six of these stars falls in an unexpected domain of the color-magnitude diagram, comprising what we propose to call red stragglers.

THE R-MODE INSTABILITY IN ROTATING NEUTRON STARS

In this review we summarize the current understanding of the gravitational-wave driven instability associated with the so-called r-modes in rotating neutron stars. We discuss the nature of the r-modes, the detailed mechanics of the instability and its potential astrophysical significance. In particular we discuss results regarding the spin-evolution of nascent neutron stars, the detectability of r-mode gravitational waves and mechanisms limiting the spin-rate of accreting neutron stars in binary systems.

Equilibrium and stability of supermassive stars in binary systems

We investigate the equilibrium and stability of supermassive stars of mass $M\ensuremath{\gtrsim}{10}^{5}{M}_{\ensuremath{\bigodot}}$ in binary systems. We find that corotating binaries are secularly unstable for close, circular orbits with $r\ensuremath{\lesssim}{4R(M/10}^{6}{M}_{\ensuremath{\bigodot}}{)}^{1/6}$ where r is the orbital separation and R the stellar radius. We also show that corotation cannot be achieved for distant orbits with $r\ensuremath{\gtrsim}{12R(M/10}^{6}{M}_{\ensuremath{\bigodot}}{)}^{\ensuremath{-}11/24},$ since the time scale for viscous angular momentum transfer associated with tidal torques is longer than the evolution time scale due to emission of thermal radiation. These facts suggest that the allowed mass range and orbital separation for corotating supermassive binary stars is severely restricted. In particular, for supermassive binary stars of large mass $M\ensuremath{\gtrsim}6\ifmmode\times\else\texttimes\fi{}{10}^{6}{M}_{\ensuremath{\bigodot}},$ corotation cannot be achieved, as viscosity is not adequate to mediate the transfer between orbital and spin angular momentum. One possible outcome for binary supermassive stars is the onset of quasi-radial, relativistic instability which drives each star to collapse prior to merger: We discuss alternative outcomes of collapse and possible spin states of the resulting black holes. We estimate the frequency and amplitude of gravitational waves emitted during several inspiral and collapse scenarios.

Diffuse Gas and Low‐Mass X‐Ray Binaries in theChandraObservation of the S0 Galaxy NGC 1553

We have spatially and spectrally resolved the sources of X-ray emission from the X-ray-faint S0 galaxy NGC 1553 using an observation from the Chandra X-Ray Observatory. The majority (70%) of the emission in the 0.3-10.0 keV band is diffuse, and the remaining 30% is resolved into 49 discrete sources. Most of the discrete sources associated with the galaxy appear to be low-mass X-ray binaries (LMXBs). The luminosity function of the LMXB sources is well fitted by a broken power law with a break luminosity comparable to the Eddington luminosity for a 1.4 M☉ neutron star. It is likely that those sources with luminosities above the break are accreting black holes, and those below are mostly neutron stars in binary systems. Spectra were extracted for the total emission, diffuse emission, and sum of the resolved sources; the spectral fits for all require a model including both a soft and hard component. The diffuse emission is predominately soft, while the emission from the sources is mostly hard. Approximately 24% of the diffuse emission arises from unresolved LMXBs, with the remainder resulting from thermal emission from hot gas. There is a very bright source at the projected position of the nucleus of the galaxy. The spectrum and luminosity derived from this central source are consistent with it being an active galactic nucleus (AGN); the galaxy also is a weak radio source. Finally, the diffuse emission exhibits significant substructure with an intriguing spiral feature passing through the center of the galaxy. The X-ray spectrum and surface brightness of the spiral feature are consistent with adiabatic or shock compression of ambient gas but not with cooling. This feature may be due to compression of the hot interstellar gas by radio lobes or jets associated with the AGN.

Neutron Stars Imply Relativity's a Drag

A new finding, based on x-rays from distant neutron stars, could be the first clear evidence of a weird relativistic effect called frame dragging, in which a heavy chunk of spinning matter wrenches the space-time around it like an eggbeater. Using data from NASA's Rossi X-ray Timing Explorer, three astronomers in Amsterdam found circumstantial evidence for frame dragging in the flickering of three neutron stars in binary systems. They announced their results in the 1 September issue of The Astrophysical Journal.

The Mysterious X-Ray Emission from Late-B Type Stars: the Lindroos Sample

We present a study of the X-ray emission coming from B-type stars in binary systems. The sample has been extracted from the Lindroos catalogue (Lindroos, 1986) and consists of six visual pairs comprised of a B-type star and a later-type companion. Surprisingly, we have detected X-ray emission coming from the late-B primaries, suggesting the presence of a third unresolved late-type star or an unknown mechanism which generates X-rays in late-B stars.

Sources of Relativistic Jets in the Galaxy

▪ Abstract Black holes of stellar mass and neutron stars in binary systems are first detected as hard X-ray sources using high-energy space telescopes. Relativistic jets in some of these compact sources are found by means of multiwavelength observations with ground-based telescopes. The X-ray emission probes the inner accretion disk and immediate surroundings of the compact object, whereas the synchrotron emission from the jets is observed in the radio and infrared bands, and in the future could be detected at even shorter wavelengths. Black-hole X-ray binaries with relativistic jets mimic, on a much smaller scale, many of the phenomena seen in quasars and are thus called microquasars. Because of their proximity, their study opens the way for a better understanding of the relativistic jets seen elsewhere in the Universe. From the observation of two-sided moving jets it is inferred that the ejecta in microquasars move with relativistic speeds similar to those believed to be present in quasars. The simultaneous multiwavelength approach to microquasars reveals in short timescales the close connection between instabilities in the accretion disk seen in the X-rays, and the ejection of relativistic clouds of plasma observed as synchrotron emission at longer wavelengths. Besides contributing to a deeper understanding of accretion disks and jets, microquasars may serve in the future to determine the distances of jet sources using constraints from special relativity, and the spin of black holes using general relativity.

AGB stars in binaries and their progeny

An AGB star in a binary system is likely to pollute its companion with carbon- and s-process-rich matter. After the AGB star has faded into an unconspicuous white dwarf, the polluted companion enters the zoo of stars with chemical peculiarities. In this paper, the progeny of AGB stars in binary systems are identified among existing spectroscopic classes (Abell 35-like, binary post-AGB, WIRRing, dwarf Ba and C, subgiant CH, Ba, CH, S, yellow symbiotics) and their filiation is discussed from the properties of their eccentricity – period diagrams.