W Ursae Majoris-type (W UMa-type) variable stars are contact eclipsing binary stars whose evolution is unknown. Modelling to determine the physical parameters of as many W UMa-type variable stars as possible might provide some insight as to how these contact binaries form and evolve. The All Sky Automated Survey (ASAS) has discovered over ve thousand of these systems. Using data from the ASAS and from the Wide Angle Search for Planets (SuperWASP) project, models of selected ASAS contact binaries are being created to determine their physical parameters. Some W UMa-type variable stars are known to undergo changes in orbital period. For selected ASAS contact binaries, a period analysis has been performed using SuperWASP data to determine if the systems are undergoing changes in orbital period. Results of the modelling and period analyses of selected systems are presented. / Thesis (M. Sc. (Astronomy))

We present new models for two low-mass X-ray binaries (LMXB), 4U 1822-371 and V1408 Aql (= 4U 1957+115). The eclipsing LMXB 4U 1822-371 is the prototypical accretion disk corona (ADC) system. We have obtained new time-resolved UV spectroscopy of 4U 1822-371 with the Advanced Camera for Surveys/Solar Blind Channel on the Hubble Space Telescope and new V- and J- band photometry with the 1.3-m SMARTS telescope at Cerro Tololo Inter-American Observatory. We use the new data to construct the UV/optical spectral energy distribution of 4U 1822-371 and its orbital light curve in the UV, V , and J bands. We derive an improved ephemeris for the optical eclipses and confirm that the orbital period is changing rapidly, indicating extremely high rates of mass flow in the system; and we show that the accretion disk in the system has a strong wind with projected radial velocities up to 4400 km s⁻¹. We show that the disk has a vertically extended, optically thick component at optical wavelengths. This component extends almost to the edge of the disk and has a height equal to ~0.5 of the disk radius. As it has a low brightness temperature, we identify it as the optically thick base of the disk wind, not as the optical counterpart of the ADC. Like previous models of 4U 1822-371, ours needs a tall obscuring wall near the edge of the accretion disk, but we interpret the wall as a layer of cooler material at the base of the disk wind, not as a tall, luminous disk rim. V1408 Aql is a black hole candidate. We have obtained new optical photometry of this system in 2008 and 2009 with the Argos photometer on the 2.1-m Otto Struve telescope and optical spectra with the low resolution spectrometer on the Hobby Eberly telescope. From the data we derive an improved optical orbital ephemeris and a new geometric model for the system. The model uses only a simple thin disk without the need for a warped disk or a large disk rim. The orbital variation is produced by the changing aspect of the irradiated secondary star with orbital phase. The new model leaves the orbital inclination unconstrained and allows for inclinations as low as 20 degrees. The spectra is largely featureless continuum with He II and occasionally H[alpha] emission lines, and an absorption line from Na D. The lines are highly variable in strength and wavelength, but the variations do not correlate with orbital phase. / text

This thesis reports photometric and spectroscopic studies of three close binary systems; BP Velorum, V392 Carinae and V752 Centauri. BP Velorum, a W UMa-type binary, was observed photometrically in February 2007. The light curves in four filters were fitted simultaneously with a model generated in the eclipsing binary modeling software package PHOEBE. The best model was one with a cool star spot on the secondary larger component. The light curves showed additional cycle-to-cycle variations near the times of maximum light which may indicate the presence of star spots that vary in strength and/or location on a time scale comparable with the orbital period, (P = 0.265 d). The system was confirmed to belong to the W-type subgroup of W UMa binaries for which the deeper primary minimum is due to an occultation. V392 Carinae, a detached binary with an orbital period of 3.147 d, was observed photometrically by Michael Snowden in 1997. These observations were reduced and combined with the published light curve from Debernardi and North (2001). High resolution spectroscopic images were taken using the University of Canterbury's HERCULES spectrograph. The radial velocities measured from these observations were combined with velocities from Debernardi and North (2001). The radial velocity and light curves were fit simultaneously, confirming that V392 Car is a detached system of two main sequence A stars with a mass-ratio of 0.95. The derived systematic velocity is consistent with V392 Car being a member of the open cluster NGC 2516. The W UMa-type binary V752 Centauri was observed photometrically and spectroscopically during 2007. The high resolution spectra displayed weak sharp lined features superimposed over the strong broad lined spectrum expected from the 0.370 d contact binary. Fourier methods were used to separate the broad and sharp spectral features and radial velocities for each were measured by cross-correlation. A fit to the photometry and radial velocities for the contact binary implied a system of two late F stars with a mass-ratio of 3.38 in an over-contact configuration. The derived systematic velocity (−13.8km/s), has changed significantly from the 1972 value (29.2km/s). The third (sharp lined) component's radial velocities were measured and found to have a period of 5.147 d, semi-amplitude of 43.4km/s and systematic velocity of −7.3km/s. The likely configuration of the entire system is that of a contact binary in a long period orbit about a lower mass detached binary. V752 Cen is thus a triple lined spectroscopic quadruple.

Περίπου το 70% των αστέρων που παρατηρούνται στο σύμπαν είναι μέλη πολλαπλών συστημάτων και η μελέτη τους είναι υψίστης σημασίας για τους αστρονόμους τις τελευταίες δεκαετίες. Τα διπλά εκλειπτικά συστήματα προσφέρουν μοναδικές δυνατότητες μελέτης των αστρικών παραμέτρων και κατ’ επέκταση των θεωριών εξέλιξης εξαιτίας της γεωμετρίας τους και αποτελούν τη βάση της μεθόδου προσδιορισμού αστρονομικών αποστάσεων ως «πρότυπα κεριά» (standard candles). Στην παρούσα εργασία πραγματοποιείται η μοντελοποίηση των καμπυλών φωτός 36 λαμπρών αποχωρισμένων διπλών εκλειπτικών συστημάτων που παρατηρήθηκαν κατά την τρίτη φάση του προγράμματος OGLE στο Μεγάλο Νέφος του Μαγγελάνου και εξάγονται οι φωτομετρικές τους παράμετροι, ώστε τα πιο ενδιαφέροντα από αυτά να επιλεχθούν για περαιτέρω φασματοσκοπικές παρατηρήσεις ως υποψήφιοι δείκτες απόστασης. Στο πρώτο κεφάλαιο πραγματοποιείται μία εισαγωγή στους μεταβλητούς αστέρες, μία αναφορά στην ταξινόμηση των διπλών εκλειπτικών συστημάτων σύμφωνα με το μοντέλο Roche και με τη μορφή της καμπύλης φωτός τους καθώς και στοιχεία που αφορούν τη γεωμετρία τους και τις εξισώσεις της τροχιάς τους. Στο δεύτερο κεφάλαιο περιγράφεται το πρόγραμμα OGLE (Optical Gravitational Lensing Experiment) σκοπός του οποίου είναι η ανίχνευση συμβάντων βαρυτικής μικροεστίασης με κύριους στόχους τα Νέφη του Μαγγελάνου, το Γαλαξιακό Δίσκο και την Κεντρική Περιοχή του Γαλαξία. Ως αποτέλεσμα της χαρτογράφησης των πεδίων αυτών είναι η δημιουργία μίας μεγάλης βάσης δεδομένων μεταβλητών αστέρων για περεταίρω μελέτη. Στο τρίτο κεφάλαιο περιγράφεται αναλυτικά το πρόγραμμα PHOEBE (PHysics Of Eclipsing BinariEs) με το οποίο πραγματοποιήθηκε η μελέτη των καμπυλών φωτός των 36 συστημάτων. Το πρόγραμμα αυτό αποτελεί μία βελτιωμένη έκδοση του προγράμματος WD (Wilson & Devinney 1979) που στηρίζεται στη μέθοδο των διαφορικών διορθώσεων για της εξαγωγή των φωτομετρικών παραμέτρων των συστημάτων. Τέλος στο τέταρτο κεφάλαιο παρουσιάζεται η μέθοδος επιλογής των 36 διπλών εκλειπτικών συστημάτων που μελετήθηκαν, ο προσδιορισμός των βασικών παραμέτρων που εισήχθησαν στο πρόγραμμα, οι φωτομετρικές παράμετροι που διεξήχθησαν καθώς και τα επιστημονικά συμπεράσματα της μελέτης. / Over 70% of the stars in the universe are components of multiple systems. The study of these systems is crucial for the astronomers over the last decades because their parameters can be derived due to their geometry and assumptions can be made about stellar evolution. Moreover these systems are very promising for measuring distances in the universe as “standard candles”. In the present work 36 DEBs were chosen from the third phase of OGLE program from the Large Magellanic Cloud and Light Curve modeling was held via PHOEBE program which is an improved version of WD code (Wilson & Devinney 1979). Photometric parameters of these systems were derived so that the most interesting of these would be potential targets for further spectroscopic observations as distance indicators for the Large Magellanic Cloud.

Starspots are not well understood for contact binary star systems. The following properties of spots were systematically investigated: temperature, radius, colatitude, and longitude. Spots were modeled on an AE Phe like contact binary system. The spots were changed in a systematic manner. The light curve phases of primary minimum and primary maximum were affected by these parameter changes in a systematic manner, as well as the secondary minimum and maximum. It will be shown that it is possible to use the shift in these phases to study starspots over time. This information can also be used to identify the presence of spots in binary star systems.Starspots on contact binary systems are not commonly found at a longitude near 180°. The results of this study show that starspots near 180° should be the easiest to detect using photometric techniques. This is the most significant result from this study. Either there is an unknown physical reason why contact binary stars do not have starspots near a longitude of 180°, or the starspots are there and the photometric data has been misinterpreted. / Department of Physics and Astronomy

This thesis reports photometric and spectroscopic studies of three close binary systems; BP Velorum, V392 Carinae and V752 Centauri. BP Velorum, a W UMa-type binary, was observed photometrically in February 2007. The light curves in four filters were fitted simultaneously with a model generated in the eclipsing binary modeling software package PHOEBE. The best model was one with a cool star spot on the secondary larger component. The light curves showed additional cycle-to-cycle variations near the times of maximum light which may indicate the presence of star spots that vary in strength and/or location on a time scale comparable with the orbital period, (P = 0.265 d). The system was confirmed to belong to the W-type subgroup of W UMa binaries for which the deeper primary minimum is due to an occultation. V392 Carinae, a detached binary with an orbital period of 3.147 d, was observed photometrically by Michael Snowden in 1997. These observations were reduced and combined with the published light curve from Debernardi and North (2001). High resolution spectroscopic images were taken using the University of Canterbury's HERCULES spectrograph. The radial velocities measured from these observations were combined with velocities from Debernardi and North (2001). The radial velocity and light curves were fit simultaneously, confirming that V392 Car is a detached system of two main sequence A stars with a mass-ratio of 0.95. The derived systematic velocity is consistent with V392 Car being a member of the open cluster NGC 2516. The W UMa-type binary V752 Centauri was observed photometrically and spectroscopically during 2007. The high resolution spectra displayed weak sharp lined features superimposed over the strong broad lined spectrum expected from the 0.370 d contact binary. Fourier methods were used to separate the broad and sharp spectral features and radial velocities for each were measured by cross-correlation. A fit to the photometry and radial velocities for the contact binary implied a system of two late F stars with a mass-ratio of 3.38 in an over-contact configuration. The derived systematic velocity (−13.8km/s), has changed significantly from the 1972 value (29.2km/s). The third (sharp lined) component's radial velocities were measured and found to have a period of 5.147 d, semi-amplitude of 43.4km/s and systematic velocity of −7.3km/s. The likely configuration of the entire system is that of a contact binary in a long period orbit about a lower mass detached binary. V752 Cen is thus a triple lined spectroscopic quadruple.

Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model stellar atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model stellar atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model stellar atmospheres. We find that sphericity is important even for dwarf model atmospheres, leading to significant differences in the predicted coefficients.

Limb darkening is a fundamental ingredient for interpreting observations of planetary transits, eclipsing binaries, optical/infrared interferometry and microlensing events. However, this modeling traditionally represents limb darkening by a simple law having one or two coefficients that have been derived from plane-parallel model stellar atmospheres, which has been done by many researchers. More recently, researchers have gone beyond plane-parallel models and considered other geometries. We previously studied the limb-darkening coefficients from spherically symmetric and plane-parallel model stellar atmospheres for cool giant and supergiant stars, and in this investigation we apply the same techniques to FGK dwarf stars. We present limb-darkening coefficients, gravity-darkening coefficients and interferometric angular diameter corrections from Atlas and SAtlas model stellar atmospheres. We find that sphericity is important even for dwarf model atmospheres, leading to significant differences in the predicted coefficients.

No description available.

New CCD photometric observations of ten short-period LMB (Low Mass Binaries) were carried out. Data were obtained using 65 cm telescope in Ondřejov in the filters I, R and V from 2014 to 2016. Light curves were analysed using the program PHOEBE. The obtained masses and radii of the stars were compared with the theoretical mass-radii relation. There arises that the short-period binaries have the radii of about 4 % larger than the theoretical prediction from the stellar evolution models. Powered by TCPDF (www.tcpdf.org)