Analysis of theU B V light curves of TT hydrae by Kopal’s frequency domain method

The light curves of the totally eclipsing system TT Hya inUBV colours observed by Kulkarni and Abhyankar during 1973–77 have been analysed by Kopal’s frequency domain method with slight modification. We find y_s (primary) = 0.104 ± 0.005, y_g (secondary) = 0.215 ± 0.008 and i = 89‡ ± 1‡. The value of y_g obtained in this study is smaller than that determined earlier by Kulkarni and Abhyankar by the method of Russell and Merrill; this confirms the undersized nature of the secondary component.The ultraviolet colour excess of the secondary is also confirmed.     

Orbital period variation of the eclipsing binary system TT Herculis

New photoelectric UBV observations were obtained for the eclipsing binary TT Her at the Ankara University Observatory (AUO) and three new times of minima were calculated from these observations. The (O – C) diagram constructed for all available times of minima of TT Her exhibits a cyclic character superimposed on a quadratic variation. The quadratic character yields an orbital period decrease with a rate of dP /dt = –8.83 × 10^–8 day yr^–1 which can be attributed to the mass exchange/loss mechanism in the system. By assuming the presence of a gravitationally bound third body in the system, the analysis of the cyclic nature in the (O – C) diagram revealed a third body with a mass of 0.21M_⊙ orbiting around the eclipsing pair. The possibility of magnetic activity cycle effect as a cause for the observed cyclic variation in the (O – C) diagram was also discussed. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Microlensing of close binary stars

The gravity due to a multiple-mass system has a remarkable gravitational effect: the extreme magnification of background light sources along extended so-called caustic lines. This property has been the channel for some remarkable astrophysical discoveries over the past decade, including the detection and characterization of extrasolar planets, the routine analysis of limb darkening, and, in one case, limits set on the apparent shape of a star several kiloparsec distant. In this paper, we investigate the properties of the microlensing of close binary star systems. We show that in some cases it is possible to detect flux from the Roche lobes of close binary stars. Such observations could constrain models of close binary stellar systems.     

Flare variability in the close binary FL Vir

The results of U -filter flare monitoring of the binary flare star FL Vir = Wolf 424 is presented. 57 flares with energies between  2 × 10²⁸  and  2 × 10³¹ erg  were recorded in 20 h of observation. The properties of flare occurrence and flare time-scales are analysed, and the flare activity level in 1980 April is determined to be   L _f( U ) = 8.0 × 10²⁶ erg s⁻¹  . This is larger than previously published results and may indicate a variation in the flare activity level on a time-scale of years. An analysis of existing data indicates that the flare activity level correlates with the relative orbital positions of the stars.     

A binary model for the UV-upturn of elliptical galaxies

The discovery of a flux excess in the far-ultraviolet (FUV) spectrum of elliptical galaxies was a major surprise in 1969. While it is now clear that this UV excess is caused by an old population of hot helium-burning stars without large hydrogen-rich envelopes, rather than young stars, their origin has remained a mystery. Here we show that these stars most likely lost their envelopes because of binary interactions, similar to the hot subdwarf population in our own Galaxy. We have developed an evolutionary population synthesis model for the FUV excess of elliptical galaxies based on the binary model developed by Han et al. for the formation of hot subdwarfs in our Galaxy. Despite its simplicity, it successfully reproduces most of the properties of elliptical galaxies with a UV excess: the range of observed UV excesses, both in  (1550 − V )  and  (2000 − V )  , and their evolution with redshift. We also present colour–colour diagrams for use as diagnostic tools in the study of elliptical galaxies. The model has major implications for understanding the evolution of the UV excess and of elliptical galaxies in general. In particular, it implies that the UV excess is not a sign of age, as had been postulated previously, and predicts that it should not be strongly dependent on the metallicity of the population, but exists universally from dwarf ellipticals to giant ellipticals.     

Spot signatures of a solar‐type dynamo in close binary stars

Magnetic activity signatures in the atmosphere of active stars can be used to place constrains on the underlying processes of flux transport and dynamo operation in its convective envelope. The ‘solar paradigm’ for magnetic activity suggests that the magnetic field is amplified and stored at the base of the convection zone. Once a critical field strength is exceeded, perturbations initiate the onset of instabilities and the growth of magnetic flux loops, which rise through the convection zone, emerge at the stellar surface, and eventually lead to the formation of starspots and active regions. In close binaries, the proximity of the companion star breaks the rotational symmetry. Although the magnitude of tidal distortions is rather small, non‐linear MHD simulations have nevertheless shown in the case of main‐sequence binary components that they can cause non‐uniform surface distributions of flux tube eruptions. The present work extends the investigation to post‐mainsequence components to explore the specific influence of the stellar structure on the surface pattern of erupting flux tubes. In contrast to the case of main‐sequence components, where the consistency between simulation results and observations supports the presumption of a solar‐like dynamo mechanism, the numerical results here do not recover the starspot properties frequently observed on evolved binary components. This aspect points out an insufficiency of the applied flux tube model and leads to the conclusion that additional flux transport and possibly amplification mechanisms have to be taken into account. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Theoretical study of partial frequency redistribution function in irradiated, moving atmospheres of close binary components

We have studied the effects of partial frequency redistribution function with angle-averaged   R _II-A  in irradiated and moving atmospheres of close binary components. We have considered the atmospheric extension of the primary component to be twice the radius of the primary component in a close binary system. We have considered two cases: (i) when the atmosphere is at rest and (ii) when the atmosphere is moving. In both the cases, we have computed the line profiles along the line of sight for a given optical depth. The irradiation from the secondary component is assumed to be one, five and 10 times the self-radiation. The line fluxes in the line of sight are calculated by using the total source functions due to self-radiation of the primary component and due to the irradiation from the secondary component. We have noted double-peaked emission lines in the case of a static medium and a reduction of emission peaks in the case of velocity field.     

Low- and intermediate-mass close binary evolution and the initial–final mass relation

Using Eggleton's stellar evolution code, we carry out 150 runs of Population I binary evolution calculations with the initial primary mass between 1 and 8 M_⊙, the initial mass ratio     between 1.1 and 4, and the onset of Roche lobe overflow (RLOF) at an early, middle or late Hertzsprung-gap stage. We assume that RLOF is conservative in the calculations, and find that the remnant mass of the primary may change by more than 40 per cent over the range of initial mass ratio or orbital period, for a given primary mass. This is contrary to the often-held belief that the remnant mass depends only on the progenitor mass if mass transfer begins in the Hertzsprung gap. We fit a formula, with an error less than 3.6 per cent, for the remnant (white dwarf) mass as a function of the initial mass M _1i of the primary, the initial mass ratio q _i and the radius of the primary at the onset of RLOF. We also find that a carbon–oxygen white dwarf with mass as low as 0.33 M_⊙ may be formed if the initial mass of the primary is around 2.5 M_⊙.     

SS Ari: a shallow-contact close binary system

Two CCD epochs of light minimum and a complete R light curve of SS Ari are presented. The light curve obtained in 2007 was analyzed with the 2003 version of the W-D code. It is shown that SS Ari is a shallow contact binary system with a mass ratio q=3.25 and a degree of contact factor f=9.4%(±0.8%). A period investigation based on all available data shows that there may exist two distinct solutions about the assumed third body. One, assuming eccentric orbit of the third body and constant orbital period of the eclipsing pair, results in a massive third body with M ₃=1.73M _⊙ and P ₃=87.0 yr. On the contrary, assuming continuous period changes of the eclipsing pair the orbital period of tertiary is 37.75 yr and its mass is about 0.278M _⊙. Both of the cases suggest the presence of an unseen third component in the system.     

Photometric and spectroscopic investigation of the oscillating Algol type binary: EW Boo

We obtained the physical and geometrical parameters of the EW Boo system, which exhibits short period and small amplitude pulsations as well as brightness variations due to orbital motion of components. Towards this end we carried out photometric observations at Ankara University Kreiken Observatory (AUKO) as well as spectroscopic observations at TUBITAK National Observatory (TNO). The light and radial velocity curves obtained from these observations have been simultaneously analyzed with PHOEBE and the absolute parameters of the system along with the geometric parameters of the components have been determined. Using model light curves of EW Boo, light curve regions in which the pulsations are active have been determined and as a result of analyses performed in the frequency region, characteristic parameters of pulsations have been obtained. We find that the results are compatible with current parameters of similar systems in the literature. The evolutionary status of the components is propounded and discussed.     

The close binary system SU Ind

Photometric data on the southern mid‐F type close eclipsing binary system SU Ind from the Carter Observatory (NZ) were examined, together with a light curve obtained from the Hipparcos Epoch Photometry (HEP) database. The system appears detached, although appreciably evolved from the Zero Age Main Sequence. The HEP period must be accurate for its own epoch, but it is significantly different from the reference (Hoffmeister 1956) value. This is difficult to explain, given the uncomplicated nature of the light curve. The apparent discrepancy can be reconciled with a period that has remained almost constant over the 40 year interval between the epochs by assuming some confusion between the almost equal depth minima. The near‐equality of components raises issues of determinacy in light curve analysis, which are reviewed against the presentation of relevant numerical curve‐fitting details. The system is of physical interest, in that it is close to the start of the ‘active’ range of spectral types. Its stars have relatively rapid rotation speeds and probably shallow convective subphotospheric layers. (© 2004 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Photometric study of an eclipsing binary in the field of M37

CCD photometric observations with B and V passbands were performed on the contact binary V3 in the field of open cluster M37. The solutions were obtained for data from both B and V passbands along with R passband given by Hartman et al.using the Wilson-Devinney code. The positive O'Connell effect was observed in all the three passbands and its associated cool spot parameters were derived. The results indicate that the spot parameters have not shown any significant variability during the last four years. The spot radius was found to be 40and located close to the equator of the secondary component. The absolute parameters of the system were derived using the empirical relations given by Gazeas et al.     

A photometric study of the contact binary AZ Virginis

New light curves and photometric solutions of the contact binary AZ Vir are presented in this paper. The light curves appear to exhibit a typical O'Connell effect, with Maximum I being 0.021 mag (V) and 0.023 mag (B) brighter than Maximum II, respectively. From the observations, six times of minimum light were determined and from the present times of minimum light and those collected from the references, the light elements of the system were improved. The light curves were analyzed by means of the Wilson‐Devinney program. The results suggest that AZ Vir is a W‐subtype contact binary with a mass ratio of q = 0.623(2). The asymmetry of the light curves is explained by star spot models. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Updated catalogue of the light curve solutions of contact binary stars

The catalogue of the light curve solutions of contact binary stars was updated. Based on the catalogue data we call the attention to a remarkable gap in the temperature distribution of contact binaries which remained unexplained. The absolute dimensions of the components were calculated in a reliable way. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)     

Resolving the Algol Paradox and Kopal’s Classification of Close Binaries with Evolutionary Implications

The Algol paradox posed a serious challenge to the theory of evolution of close binaries. We indicate that it has been resolved by the collective efforts of a whole generation of astronomers rather than by anyone’s individual ingenious accomplishment. We discuss the role of illuminating ideas put forward by Zdeněk Kopal in solving the Algol paradox.