Gravity-darkening in eclipsing binaries

A quantitative analysis of the ellipticity effect in close binary systems of the W Ursae Maioris type with spectroscopically known mass-ratios discloses that the photometric ellipticities of these variables are, in general, more than twice as large as the dynamical ellipticities (due to the geometrical distortion alone) computed for contact models of centrally condensed stars. If, moreover, this excess of the photometric over dynamical ellipticity is attributed to the phenomenon of gravity-darkening over distorted surfaces of the constituent components, this darkening must be present to a considerablylarger degree than that predicted by the theory of radiative transfer — a result difficult to reconcile with the existence of extensive sub-surface convection zones in late-type Main-Sequence stars. If, on the other hand, gravity-darkening is present in W UMa-type systems in the amount predicted by the theory (let alone in the presence of sub-surface convection), the only effective way of reconciling the theory with the observations would be to assume that the mean fractional radii of their constituent components are appreciablylarger than those appropriate for contact models in which the two components just fill the largestclosed Roche equipotentials capable of containing their mass. In other words, it would appear that the W UMa-type stars considered in this paper may constitute a single dumb-bell figure rather than two distinct components; and that the observed variations of light, colour or radial velocity are invoked by the axial rotation (and consequent variable cross-section) or this dumb-bell figure rather than to the contribution of individual stars.     

Gravity-darkening in eclipsing binaries

An analysis of the photometric ellipticity effect in close binary systems consisting of early-type Main Sequence stars discloses that its excess over dynamical ellipticity is, on the whole, within limits explainable by linear laws of limb-and gravity-darkening over distorted stellar surfaces, with coefficients consistent with theoretical expectations based on the absence of convection in sub-surface layers. This contrasts with the situation encountered in our earlier investigation of W U Ma-type systems, and reveals that whereas the photometric properties of early-type Main Sequence systems appear to be normal in the light of the existing theories of their light changes, it is the W U Ma-type systems which confront us with unexplained phenomena in the form of anomalously large ellipticities; but their solution is as yet nowhere near in sight.     

Photometric analysis of two eclipsing binaries in the magellanic clouds

B, V andI light curves of the eclipsing binaries HV12634 in the LMV, and HV2208 in the SMC (cf. Westet al., 1992), have been analysed with the aid of optimal curve-fitting techniques. Parameter derivation involves a successive approximations procedure. The results of this are combined with a standard mass:luminosity relation to explore tentative absolute parameters. The combined evidence shows some disparity with standard MS-like models. It also demonstrates some relative insensitivity of photometric analysis, in isolation, to elucidate clear facts on such possibly atypical, remote, hot stars. Auxilliary spectroscopic data are required to resolve remaining ambiguities.     

First frequency analysis for three new members of the group of eclipsing binaries with a pulsating component

We present the first light curves and pulsation analysis results for V729 Aql and two newly discovered eclipsing binaries, namely USNO-A2.0 0975-17281677 and USNO-A2.0 1200-03937339. Frequency search was applied on the residuals of their light curves and the results showed that their primary components pulsate in multiperiodic modes and lie well inside the frequency and temperature range of δ Scuti stars. Moreover, for USNO-A2.0 1200-03937339 two frequencies inside the γ Dor frequency range were also detected, but their origin is discussed. The photometric models of USNO-A2.0 1200-03937339 and V729 Aql are also presented, while their absolute parameters as well as the evolutionary status of their components were roughly estimated.     

Study of the apsidal motion in the eclipsing binary MZ Lac

A series of highly accurate photoelectric observations of the eclipsing binary MZ Lac was obtained with a 48-cm AZT-14 reflector at the Tien-Shan High-Altitude Station of the Sternberg Astronomical Institute from 1985 to 2004 to study its apsidal motion. We constructed a consistent system of physical and geometrical parameters of the components and the binary’s orbit: we determined their masses (M₁ = 1.50M_⊙, M₂ = 1.29M_⊙), radii (R₁ = 1.86R_⊙, R₂ = 1.35R_⊙), luminosities (L₁ = 0.79L_⊙, L₂ = 0.45L_⊙), surface gravities (log_g1 = 4.06, log_g2 = 4.27), age (t = 1.9 × 10⁹ yr), and the distance to the binary (d = 510 pc). The binary exhibits apsidal motion with the period U_obs = 480 ± 40 yr, while its theoretically expected value is U_th = 450 ± 40 yr. Spectroscopic studies of MZ Lac and calculations of the absolute parameters of the components are required to test our conclusions.     

Pulsations and apsidal motion in the eclipsing variable system DI Herculis

In 2003–2008, highly accurate photoelectric and CCD observations of the close binary system DI Her were performed in the V band. The light curves of three primary and three secondary eclipses were constructed. These observations, along with the highly accurate photoelectric observations of other authors obtained in different years from 1963 to 1986, have confirmed the difference between the observed (1 _. ^o 3 ± 0 _. ^o 1/100 yr) and theoretical (4 _. ^o 3/100 yr) rates of apsidal motion. Our photometric data are indicative of a possible variability in the system with period P′ = 1.175 days and amplitude A′ = 0 _. ^m 011, which is probably related to the pulsations of one of the components. There may be a third body in the system that produces in-phase variations in the times of primary and secondary minima with a period of 10.5 yr and an amplitude of 1 _. ^m 5.     

Evaluation of the light changes for distorted eclipsing binaries

This paper discusses a method for improving on the numerical evaluation of the light changes exhibited by a distorted eclipsing binary system.In the theory formulated by Kopal (1959), certain boundary integrals due to the distortion of both components have been calculated in terms of the Appell hypergeometric series of the first kind. The values of the four parameters appearing in these series differ according as to whether one is dealing with a partial or an annular eclipse.To accelerate the numerical evaluation of the light changes one should avoid recomputing such infinite series for contiguous values of the parameters. This can be achieved by making use of certain recursion formulae which hold for the foregoing series.We have provided here a procedure that yields forty-eight recursion formulae for the Appell hypergeometric series and have specifically calculated four new independent recursion formulae relevant to the astrophysical problem.     

Automated analysis of eclipsing binary light curves – II. Statistical analysis of OGLE LMC eclipsing binaries

In the first paper of this series, we presented EBAS – Eclipsing Binary Automated Solver, a new fully automated algorithm to analyse the light curves of eclipsing binaries, based on the ebop code. Here, we apply the new algorithm to the whole sample of 2580 binaries found in the Optical Gravitational Lensing Experiment (OGLE) Large Magellanic Cloud (LMC) photometric survey and derive the orbital elements for 1931 systems. To obtain the statistical properties of the short-period binaries of the LMC, we construct a well-defined subsample of 938 eclipsing binaries with main-sequence B-type primaries. Correcting for observational selection effects, we derive the distributions of the fractional radii of the two components and their sum, the brightness ratios and the periods of the short-period binaries. Somewhat surprisingly, the results are consistent with a flat distribution in log P between 2 and 10 d. We also estimate the total number of binaries in the LMC with the same characteristics, and not only the eclipsing binaries, to be about 5000. This figure leads us to suggest that  (0.7 ± 0.4)  per cent of the main-sequence B-type stars in the LMC are found in binaries with periods shorter than 10 d. This frequency is substantially smaller than the fraction of binaries found by small Galactic radial-velocity surveys of B stars. On the other hand, the binary frequency found by Hubble Space Telescope ( HST ) photometric searches within the late main-sequence stars of 47 Tuc is only slightly higher and still consistent with the frequency we deduced for the B stars in the LMC.     

Photometric elements and apsidal motion of the eclipsing binary NSV 18773

The photometric elements of the eclipsing binary NSV 18773 (HD 99898) have been determined for the first time by analyzing its V-and I-band light curves from the ASAS-2 and ASAS-3 catalogs. Based on these elements and using other published spectroscopic and photometric data, we constructed a consistent system of geometrical and physical parameters for the system that consists of two stars (M ₁ = 20M _⊙, Sp₁=B0V, R ₁ = 5.0R _⊙ and M ₂ = 14M _⊙, Sp₂ = B1V, R ₂ = 6.5R _⊙) in elliptical orbits (P = 5 _. ^d 049, e = 0.365, a = 40.1R _⊙). The distance to the system is d = 3.3 kpc, the interstellar extinction is A _V = 2 _. ^m 0, and the age is t = 2.8 × 10⁶ yr. NSV 18773 is a visual binary with components V _A = 9 _. ^m 9 and V _B = 10 _. ^m 3 separated by 0 _. ^" 8. The third light (L ₃ = 0.61) that we found by analyzing the light curves shows that the eclipsing binary is the system’s fainter component B. We confirmed the rapid apsidal motion of the star detected by Otero and Wils (2006) and refined its observed period: U _obs = 150 ± 6 yr. Our photometric elements and physical parameters allowed the apsidal parameter $\bar k_2^{obs} = 0.0135(14)$ , which reflects the density distribution along the radii of the component stars, to be determined. Within the error limits, the derived parameter agrees with its theoretically expected value, $\bar k_2^{th} = 0.0119(8)$ , from current evolutionary models of stars of the corresponding masses and ages.     

The angular momentum of eclipsing binaries

The angular momentum-mass relationship for 1048 eclipsing binaries is presented.     

On evolution of contact eclipsing binaries

It is shown that during contact eclipsing binaries evolution under the influence of stellar wind, magnetic stellar wind and with matter transfer by gas flow, in binary stellar systems there may take place a process of star merger (low mass stars) within 10⁵–10⁷ yr and a fast increase of distance between stars of massive binaries. W UMa-type stars are a finite evolutionary stage of very close and low mass binary pairs. As for contact systems of early spectral types (CE-systems), they are more varied in evolution.     

Light curve solutions for eclipsing binaries in NGC 188

We present light curve solutions for the W UMa-type eclipsing binaries EP, EQ, ER, ES, and V369 Cep in the old open cluster NGC 188. Using light curve solution parameters combined with reasonable mass estimates, we determine the distance modulusV-M _vof the cluster. Our aim is to examine if current uncertainties in the cluster's distance and age can be resolved. Three binaries yield distance moduli close to 10 _. ^m 80 (±0 _. ^m 08), two others give values around 11 _. ^m 40(±0 _. ^m 09). Depending on the amount of reddening, we find a weighted mean distance modulus for all five binaries between 11 _. ^m 01 and 11 _. ^m 05 (±0 _. ^m 06), which lends modest support for the lower distance (1.65 kpc) and older age (10 Gyr) of the cluster.Participants in the Research Experience for Undergraduates (REU) program at Florida International University, sponsored by the National Science Foundation.     

Light-time orbit and apsidal motion of the eclipsing binary U Ophiuchi

A comprehensive period study of the times of minima observed from 1881 to 1985 has been performed. Previous interpretations of the O–C diagram based on light-time effect are confirmed. The light-time orbit of U Oph has been revised using a differential corrections procedure. We get an eccentric orbit withe=0.22±0.06,P=38.7±0.2 yr, and a mass function . In addition, our analysis has revealed short-period apsidal motion (U/P=4515±75) in a slightly eccentric close orbit (e=0.0031±0.0003), allowing a reliable determination of the density concentration coefficient,k ₂=0.0059±4. A comparison with stellar evolutionary models calculated by Jeffery (1984) yields the helium contentY=0.28±0.05 and an age of 3×10⁷ yr for the components of U Oph.     

Correlations among the parameters of the spherical model for eclipsing binaries

Correlation coefficients have been computed to investigate the parameters used to describe the spherical model of an eclipsing binary system. Regions in parameter hyperspace have been identified where strong correlations exist and, by implication, the solution determinacy is low. The results are presented in tabular form for a large number of system configurations.Originally presented at the IAU Colloquium No. 16, held at the University of Pennsylvania, Philadeiphia, Pa, U.S.A., September 8–11, 1971.     

Note on the analysis of light curves of eclipsing binaries with dissimilar components

The term l _per giving a correction needed to reduce the light curves of eclipsing binaries with dissimilar components to those of rectifiable models with similar components is discussed in view of a practical definition of the luminosities of the components. A new form of the term is proposed. Furthermore, a way is shown for applying this term also when the light curve has already been rectified in the usual manner.Mitteilung des Astronomischen Instituts der Universität Tübingen Nr. 108.     

The first light-curve analysis of eclipsing binaries observed by the INTEGRAL/OMC

Three Algol-type binaries in Cygnus constellation were selected for an analysis from a huge database of observations made by the INTEGRAL/OMC camera. These data were processed and analyzed, resulting in a first light-curve study of these neglected eclipsing binaries. The temperatures of the primary components range from 9500 K to 10,500 K and the inclinations are circa 73° (for PV Cyg and V1011 Cyg), while almost 90° for V822 Cyg. All of them seem to be main-sequence stars, well within their critical Roche lobes. Nevertheless, further detailed analyses are still needed.     

A new suspected apsidal period for HS Herculis

From a preliminary analysis of observed minima of the eclipsing binary HS Herculis, a new apsidal period is postulated. A general possible error in Oapsidal period is emphasized.     

Photometric light curves and physical paramters of the eclipsing binary systems IT Cas,CO Cep,AI Hya with possible apsidal motions

Multi-colourWBVR photoelectric observations of the eclipsing binaries IT Cas, CO Cep, and AI Hya were carried out. The photometric elements and the absolute parameters of the orbits and components were obtained. The valuation of the angular rate of the apsidal motion of AI Hya based on the comparison of our observations with the previous photoelectric light curve of this system was derived.