Osman Demircan

Stellar mass-luminosity and mass-radius relations

The new binary star data have been used together with new theoretical estimates in the study of mass-luminosity and mass-radius relations (MLR and MRR) for the Main-Sequence (MS) stars. The slopes of the relations change at certain critical values, e.g., at ∼0.5M⊙ for MLR and at ∼1.7M⊙ for MRR. The first point is the indication of transition from fully-convective internal structure to the developing radiative core, and the second point, the transition from proton-proton cycle to the carbon-nitrogen cycle as the dominant source of nuclear energy in these stars. It seems that these critical points are important in constraining on the stellar models. Improved set of more parameters for stars particularly around the critical points where the slopes of the relations change, may help bringing the theory and observations closer.

A period study of V502 Ophiuchus

All available published times of minima of a W-type contact binary V502 Oph have been compiled and critically examined. Ten times of minima were revisted from original observations, and five new times of minima derived from our own observations. A period study covering 53 yr of observations including 84 timings of minima light (63 of them from photoelectric observations) reveals that the orbital period of the system oscillates with a period of about 35 ± 2 yr and amplitude of about 0.020 ± 0.005 day while it undergoes a constant period decrease of about dP/dt ∼ −1.45 10 −7 d/yr

A light and period study of SV Camelopardalis

New BVR light curves and times of minimum light for the short period RS CVn system SV Cam were analysed to derive the physical parameters of the system and the parameters of the third body orbit. The light curves obtained at the TUBITAK National Observatory during two nights in 2000 show considerable asymmetry and night-to-night variations. The analysis of the light curves is made using Djurasevics inverse problem method. The Roche model with spotted areas on the hotter primary component yields a good fit to observations. The extensive series of published photoelectric minima times indicate that the eclipsing pair orbits around the common mass center of the triple system with a period of 41.32 yr.

A period study of AM Leonis

The period variation of a W-type contact binary AM Leo is studied. It was shown mostly by our own data that (i) the orbital period of the system oscillates with a period of about 33 yr and an amplitude of about 0.008 day, and (ii) the secondary minima occur always (in the last 35 yr) slightly (∼1 min) earlier than expected. The latter point is explained in terms of a large persistent dark region on the leading hemisphere close to the neck region of the cooler primary component. Existence of such dark region is in agreement with observed OConnell effect as fainter secondary maxima of the system. The period oscillation is critically studied in terms of the (i) mass transfer, (ii) third body, (iii) magnetic activity, and (iv) internal motions

Period Changes as a Tool to Study Unseen Components Around Eclipsing Binaries

Observational detection of a strically periodic orbital period variation of a binary star system can be considered as a strong evidence on the existence of an unseen component stars around the binary system. Unseen components of eclipsing binary systems cause the light time effect as displacement of the times of eclipse minimum light in a sinousoidal fasion with the period(s) equal to the period(s) of revolution of the unseen component(s). The method may reveal strong evidence even on the unseen large planets or completely dark black hole components of close binary systems. But in the absence of any additional evidence supporting the presence of third component(s), the conclusion that these systems are triple (or multiple) is still considered to be premature at present. In the present study I will review the recent work on the periodic orbital period variation of eclipsing binaries considered to be the light time effect due to unseen component stars around binary systems.

A Detached Binary System V505 Persei

The UBV photometry of a detached F-type eclipsing binary V505 Persei is presented. The light curve solution by a simple spherical model combined with the radial velocity data from two high resolution spectra by Marschall et al. (1990) reveals that the system is formed with two identical component stars of M ≈ 1.2 Mθ in the main sequence close to ZAMS in evolution. They should have about solar metallicity (z ≈ 0.017). The isochrones with solar metallicity by Van denBerg (1985) yield an age of about (2.2± 0.5) × 109 yr for the system. The distance of the system should be about 60 pc.

UBV photometry of BX andromedae

The UBV observations of a marginal contact system BX And obtained during 1987, 1989 and 1990 seasons at the Ankara University Observatory are presented. Based on all available light curves of the system we studied light curve variations, particularly at the extrema points. Time scale of the variations was found as short as a few cycles of the orbital revolution. Six new estimates of the minima times are given. Some indications are found that the system might be detached or semi-detached.

Asymmetry in the Light Curves of Massive Binaries

A visual survey of the HIPPARCOS light curves of massive binaries with a component of OB or WR-type, showed that most of the well defined light curves exhibit strong asymmetries as irregular depressions which were most probably caused by the light absorbtion of dense wind material around the systems. The phase dependent optical depth of the wind material around two massive eclipsing binaries: CQ Cep and V376 And were estimated by using the residuals between theoretical and observational light curves. The resulting bipolar wind structure around CQ Cep agree well with the spectroscopic results. The density and size of the bipolar wind materials were explored by assuming the elctron scattering as dominant absorbing mechanism.

A detached binary system BK Pegasi

TheUBV photometry of the detached F-type eclipsing binary BK Pegasi is presented. The light curve solution by a simple spherical model assumption suggests that a slightly hotter, larger and more luminous primary was eclipsed during the primary minimum. Combined with the radial velocity curves in Popper (1983), it was used to determine anew the systems parameters. The age and the metal abundance of the system were estimated as (3.3 ± 0.2) × 109 yr andz = 0.028 ± 0.003 from a comparison with the new grids of stellar models and the isochrones by Schalleret al. (1993). The distance of the system was estimated as 290 pc.

A period study of GK Cephei

A period study of the times of minima from 1900 to 1988, of a W UMa system GK Cep was performed. The system was found to have no significant secular period change, but the photoelectric observations reveal a short term (∼18 yaer) periodic oscillation of the photometric period with an amplitude of about 0.035 day around the mean value of P = 0.9361653 day. Such oscillation of the photometric period has been discussed in terms of (i) the magnetically induced deformations in the atmosphere, (ii) mass motions in the core of a component star, (iii) cyclic behavior in the mass transfer or mass loss rate, and (iv) the light time effect due to a thrid body in the system