Yazar "Tas, G" seçeneğine göre listele
Listeleniyor 1 - 13 / 13
Sayfa Başına Sonuç
Sıralama seçenekleri
Öğe The activity pattern on MM Herculis: spots and faculae(Edp Sciences S A, 1999) Tas, G; Evren, S; Ibanoglu, CThe RS CVn type eclipsing binary MM Herculis was observed using B, V, and R filters in 1997 and the light and colour curves were obtained. The long-term (1976-1997) variations in the brightness and colour of the system were revealed by examining 12 light and colour curves Thp reason fnr thp variations in the B-V colour curves may be the photospheric bright facular structures that surround starspots. The system is bluest when at its faintest. Migration periods of 5.8 and 5.9 years have been found fur the two spots or spot groups, which are located on the cooler component. The spots that are located within the longitude difference of 180 degrees seem to approach each other with a period of six years. In this case the amplitudes of the light curves increase, the system's mean brightness decreases and its mean colour becomes bluer.Öğe Apsidal motion studies for DI Her(Springer, 2000) Yildiz, M; Ibanoglu, C; Devlen, A; Tas, G; Ulas, B; Saygan, S; Evren, S; Cakirli, O; Ibanoglu, CEclipsing binary system DI Her has an apsidal motion and its observed apsidal advance seems to be less than even the relativistic advance. Therefore many debates are centered about DI Her, which may also be extended to more or less similar systems showing such a discrepancy between the theory and the observations. Computed value of the apsidal motion rate from the eclipse timings of DI Her, up to now, is very small and about one fourth of the relativistic advance. In this work we show that such a small observed rate is due to use of visual and photographic data having low precision, and due to disregarding the large orbital eccentricity of the system. As a result, we found that the observed apsidal advance is still less than the relativistic advance, but is larger than the previous results by a factor of about two.Öğe The brightness variations and orbital period changes of RT Lacertae(Edp Sciences S A, 2001) Ibanoglu, C; Evren, S; Tas, G; Devlen, A; Cakirli, OThe light curves of the chromospherically active eclipsing binary RT Lacertae obtained from 1993 to 1999 are analyzed here. The variation of the brightness at mid-eclipses and at maxima is carefully re-examined. The largest variation was obtained at mid-primary, where the more massive, hotter component occults the less massive cooler secondary star. Therefore, we suggest that the variation of the system's brightness mainly arises from the more massive star. The mean brightness of the system indicates a cyclic change. It showed at least two jumps during the last 22 years. The first occurred in 1984 and the second in 1994. Therefore, the length of the magnetic cycle appears to be about ten years. All the timings of the mid-eclipses obtained so far were collected and analyzed under the assumption of the third body hypothesis. A period of 94 yr was found for the third body orbit. The variation of the systemic velocity of the eclipsing pair seems to confirm this suggestion. The time delay and advance due to the orbit of the eclipsing pair around the third component were computed and subtracted from the original residuals obtained with the linear light elements. The remaining residuals also show a quasi-periodic change. The period of this change was calculated to be about 18 yr. This second O-C change may be related to the magnetic activity of the more massive component.Öğe BVR photometry and H alpha spectroscopy of RS CVn type binary MM Herculis(Edp Sciences S A, 2001) Tas, G; Evren, S; Marino, G; Frasca, A; Ibanoglu, C; Catalano, SThe RS CVn type eclipsing binary MM Herculis was observed photo-electrically using B, V and R filters in 1998 and 1999 and the light and colour curves were obtained. Spectroscopic observations were carried out in 1999. The new light and colour curves are anti-correlated with the observations of 1997, i.e. the system is bluer when it is faintest. The variations of the brightnesses at each special phase (0.0, 0.25, 0.5, and 0.75) show an almost cyclic change with a period of about 6 years. This value is in good agreement with the migration periods of the spots suggested by us previously. The outside-of-eclipse wave in the light curve shows a minimum at phase approximate to0.50, and the mean colour of the system is the bluest at the same phase. When the spots located on the cooler component are seen around phase 0.50, the amplitude of the light variation outside eclipse is larger than the others. Such variation may be caused by the effect of the hotter component. Spectroscopic observations of the system were carried out in the spectral range 5860-6700 Angstrom. The subtraction of a "synthetic" spectrum, built up with spectra of inactive standard stars, allows us to detect an H alpha emission excess only from the cool component. New radial velocity measurements of the system were obtained and analyzed for the orbital parameters.Öğe Ground-based photometry of the contact binary V1128 Tauri(Edp Sciences S A, 2003) Tas, G; Evren, S; Cakirli, O; Ibanoglu, CV1128 Tau is a short period W UMa type eclipsing binary which has a visual companion with a separation of 14" and a difference in brightness of about 1 mag. We observed the system in B and V filters during two observing seasons using three different telescopes and detectors. We obtained a total of 6063 observational points in each colour. The light curve reveals that V1128 Tau has a totality in the primary eclipse, which lasts about 16 min. The shape of the light curve indicates that V1128 Tauri is a W-type W UMa binary. We subtracted the visual component's light contribution to the total light of the system. We used the latest version of the Wilson-Devinney code for the analysis of the light curves and determined for the first time the geometric and physical parameters of the system. We found that the system consists of late G and early K type components. The more massive, larger component is cooler by about 300 K than its companion. The system has a circular orbit with an inclination of 85degrees. The light curves show a typical O'Connell effect, maximum I being brighter than maximum II. This difference may arise from a cool or hot region on either component and/or an accretion process between the components. Since the components appear to be later than G2, we assumed a cool spot or group of spots on the cooler component. Therefore, the light curves were also analyzed using the spot hypothesis and the results were compared with those obtained with the no-spot model.Öğe Ground-based photometry of the contact binary V1128 Tauri(Edp Sciences S A, 2003) Tas, G; Evren, S; Cakirli, O; Ibanoglu, CV1128 Tau is a short period W UMa type eclipsing binary which has a visual companion with a separation of 14" and a difference in brightness of about 1 mag. We observed the system in B and V filters during two observing seasons using three different telescopes and detectors. We obtained a total of 6063 observational points in each colour. The light curve reveals that V1128 Tau has a totality in the primary eclipse, which lasts about 16 min. The shape of the light curve indicates that V1128 Tauri is a W-type W UMa binary. We subtracted the visual component's light contribution to the total light of the system. We used the latest version of the Wilson-Devinney code for the analysis of the light curves and determined for the first time the geometric and physical parameters of the system. We found that the system consists of late G and early K type components. The more massive, larger component is cooler by about 300 K than its companion. The system has a circular orbit with an inclination of 85degrees. The light curves show a typical O'Connell effect, maximum I being brighter than maximum II. This difference may arise from a cool or hot region on either component and/or an accretion process between the components. Since the components appear to be later than G2, we assumed a cool spot or group of spots on the cooler component. Therefore, the light curves were also analyzed using the spot hypothesis and the results were compared with those obtained with the no-spot model.Öğe H alpha spectroscopy and BV photometry of RT Lacertae(E D P Sciences, 2002) Frasca, A; Cakirli, O; Catalano, S; Ibanoglu, C; Marilli, E; Evren, S; Tas, GContemporaneous spectroscopic and photometric BV observations of the RS CVn type eclipsing binary RT Lacertae were performed in summer 2000. The photometric observations were obtained at the Ege University Observatory, while the spectroscopic ones were carried out at Catania Astrophysical Observatory in the spectral range 5860-6700 Angstrom. We obtained a high quality radial velocity curve of the system that allowed us to give more accurate values of the orbital parameters. A steady decrease of the barycentric velocity from 1920 to 2000 has been pointed out and has been discussed in the context of a third body hypothesis. Through the subtraction of a "synthetic" spectrum, built up with spectra of inactive standard stars, we detected Halpha excess emission which fills in the photospheric absorption profiles of both components. With the exception of a few spectra, taken close to the eclipses, in which some extra absorption or a faint double-peaked broad emission appears, there is no further evidence of circumstellar matter in this system, as suggested in previous works. The hotter and more massive star appears also as the more active at a chromospheric level, since it has a Halpha flux about ten times greater than the companion, on average. Rotational modulation of the Halpha emission has been detected in both stars. The hemisphere of the more massive star facing the observer at phase 0.(p)75 appears brighter (in Halpha) than that seen at phase 0.(p)25, while for the less massive G9 IV star the maximum Halpha emission is seen around phase 0.(p)0-0.(p)1. From the analysis of the contemporaneous light curve (Lanza et al. 2002), the more massive G5 IV star results to be more active than the companion at a photospheric level, in agreement with the chromospheric behaviour observed in Halpha. In addition, the starspots of the G5 IV star are mainly located in the Halpha brighter hemisphere, suggesting a close spatial association of spots and plages in this star. The G9 IV star displays instead the maximum Halpha emission at the phase of maximum visibility of the smaller spotted area found from the light-curve analysis. The minimum Halpha emission occurs when the more heavily spotted region is visible.Öğe High-speed photometry of the pre-cataclysmic binary HW Virginis and its orbital period change(Edp Sciences S A, 2004) Ibanoglu, C; Cakirli, O; Tas, G; Evren, SThe broad band B and V light curves of the pre-cataclysmic eclipsing binary HW Vir were obtained. All the available eclipse timings, including the new ones, spanning 19 years were analyzed under the third-body hypothesis. The residuals between the observed and calculated times of mid-eclipse show a long-term sinusoidal variation. The analysis yields the parameters of the third-body orbit, as well as limiting mass for the tertiary object. The result of this analysis gives a light-time semi-amplitude of 112 s. an orbital period of 18.8 yr and an eccentricity of 0.12. The mass of the third star is below the theoretical threshold of 0.07 M(.) for a hydrogen burning star. Its minimum mass is about 0.022 M(.) and for a wide range of inclinations of third-body orbit, i.e. i greater than or equal to 19degrees, the mass is smaller than 0.07 M(.). Therefore, we suggest that the third star may be a brown dwarf candidate. Combining the semi-amplitudes of the radial velocities and the photometric light curve solution has allowed us to model the short-period detached binary HW Vir. The luminosity and radius of the dM companion are slightly larger than that given by low-mass models.Öğe Long-term photometric behaviour of the RS CVn binary RT lacertae(Edp Sciences S A, 2003) Cakirli, O; Ibanoglu, C; Djurasevic, G; Erkapic, S; Evren, S; Tas, GA sequence of the seasonal light curves of RT Lac, covering the period 1978-2000, is analysed in the framework of the starspot hypothesis to define the spot distribution, based on the interpretation of the B-band observations. The analysis of the corresponding light curves is made using Djurasevic's inverse-problem method. To explain the light-curve variations we modelled the binary system using a Roche model that involved regions containing spots on both components. Satisfactory fits were obtained assuming spots on both components. The more-massive G5 primary appears to be the most active star in the system and its spotted areas are mainly responsible for the light-curve distortions. Spots are concentrated around longitudes 45degrees-170degrees and at high latitudes (above 45degrees). Our analysis indicates two spots with diameters of similar to10degrees-50degrees on both hemispheres of the primary. However, the less-massive cool component seems to have only one spot which covers a relatively small area. Total spotted area of the more-massive primary component indicates clear evidence for a short-term activity cycle with a period of 8.4 yr, and a possible long-term cycle with a period of 33.5 yr. The G9IV secondary does not show any evidence for an activity cycle, its spot coverage appearing rather constant at about 10% of its surface. The variation of the orbital period seems to be correlated with the total activity level of the system. In particular, the decrement of the orbital period appears to be associated with minimum spottedness and sizeable changes of the surface spot pattern distribution on the surface of each star. This result, if confirmed by the future observations, can provide further support for recently proposed models for connection between the magnetic activity and orbital period variations.Öğe Long-term photometric behaviour of the RS CVn binary RT lacertae(Edp Sciences S A, 2003) Cakirli, O; Ibanoglu, C; Djurasevic, G; Erkapic, S; Evren, S; Tas, GA sequence of the seasonal light curves of RT Lac, covering the period 1978-2000, is analysed in the framework of the starspot hypothesis to define the spot distribution, based on the interpretation of the B-band observations. The analysis of the corresponding light curves is made using Djurasevic's inverse-problem method. To explain the light-curve variations we modelled the binary system using a Roche model that involved regions containing spots on both components. Satisfactory fits were obtained assuming spots on both components. The more-massive G5 primary appears to be the most active star in the system and its spotted areas are mainly responsible for the light-curve distortions. Spots are concentrated around longitudes 45degrees-170degrees and at high latitudes (above 45degrees). Our analysis indicates two spots with diameters of similar to10degrees-50degrees on both hemispheres of the primary. However, the less-massive cool component seems to have only one spot which covers a relatively small area. Total spotted area of the more-massive primary component indicates clear evidence for a short-term activity cycle with a period of 8.4 yr, and a possible long-term cycle with a period of 33.5 yr. The G9IV secondary does not show any evidence for an activity cycle, its spot coverage appearing rather constant at about 10% of its surface. The variation of the orbital period seems to be correlated with the total activity level of the system. In particular, the decrement of the orbital period appears to be associated with minimum spottedness and sizeable changes of the surface spot pattern distribution on the surface of each star. This result, if confirmed by the future observations, can provide further support for recently proposed models for connection between the magnetic activity and orbital period variations.Öğe Long-term starspot evolution, activity cycle and orbital period variation of RT Lacertae(Edp Sciences S A, 2002) Lanza, AF; Catalano, S; Rodono, M; Ibanoglu, C; Evren, S; Tas, G; Cakirli, O; Devlen, AA sequence of V-band light curves of the active close binary RT Lacertae (G5+G9 IV), extending from 1965 to 2000, is presented and analysed to derive the spot distribution and evolution on the component stars. In our modelling approach, the Roche geometry and Kurucz's atmospheric models were adopted. The resulting maps of the spot surface distribution were regularized by means of the Maximum Entropy and Tikhonov criteria to take full advantage of the increased geometrical resolution during eclipses. By comparing the maps obtained with these two criteria, it was possible to discriminate between surface features actually required by the data and artifacts introduced by the regularization process. Satisfactory fits were obtained assuming spots on both components and the unspotted V-band luminosity ratio: L-G5/L-G9 IV = 0.65+/-0.05. The more massive G5 primary appears to be the most active star in the system and its spotted areas are mainly responsible for the light curve distortions. The yearly spot distributions on both components indicate that their spot patterns consist of two components, one uniformly and the other non-uniformly distributed in longitude, the latter suggesting the presence of preferential longitudes. In particular, spots are concentrated around the substellar points and their antipodes on both stars. The eclipse scanning reveals spots with diameters of similar to40degrees, or possibly smaller, on the hemisphere of the primary star being occulted. The primary shows clear evidence for a short-term activity cycle with a period of similar to8.5 yr and a possible long-term cycle with a period of approximately 35 yr. The variation of the spot migration rate may be related with surface differential rotation, with a lower limit of DeltaOmega/Omega similar to 3.2x10(3). The G9 IV secondary does not show evidence for an activity cycle, its spot coverage appearing rather constant at similar to15-20% of its surface. The relative amplitude of its surface differential rotation, as indicated by the variation of the spot migration rate, is DeltaOmega/Omega similar to 2.7x10(3). The variation of the orbital period shows a correlation with the activity level of the primary component. Specifically, the decreases of the orbital period appear to be associated with minimum spottedness and sizeable changes of the surface spot distribution that may be related to increases of the rotation rate of the spot pattern. Conversely, an episode of increase of the orbital period was related to an increase of the spotted area on the primary star. Such results support the recently proposed models that connect the perturbations of the orbital dynamics with the variation of the figure of equilibrium of the active components, due to the operation of non-linear hydromagnetic dynamos in their extended convective envelopes.Öğe New findings based on long-term photometric observations of the eclipsing binary V471 Tauri(Oxford Univ Press, 2005) Ibanoglu, C; Evren, S; Tas, G; Cakirli, OThe post-common envelope and pre-cataclysmic binary V471 Tau has been observed by the authors since 1973. At least a complete light curve in B and V bands and more than two eclipse timings were obtained in each year. All the available data published so far ( including the authors') have been collected and analysed for the brightness and orbital period changes. The system brightened about 0.22 mag in both B and V bands more or less regularly up to 1997 and started to decrease afterwards. A search for periodicity of this variation yields a period longer than 85 yr. In addition to this long-period variation, a small amplitude of about 0.08 mag and short time-interval fluctuations on the mean brightness have been detected. The variations of the mean brightness have been discussed and plausible causes suggested. The changes of the apparent period have been attributed to a third body. Analysis of all the 'observed-calculated' (O-C) data yields a period of 32.4 yr, with a semi-amplitude of 151 s and an eccentricity of 0.30 for the third-body orbit. For orbital inclinations greater than 34. the mass of the third body would possibly match to a brown dwarf. One of the most interesting features in the light curve of V471 Tau is the decrement of the eclipse depth with time. The depth of the eclipse in the B band has been decreased from 0.082 to 0.057 mag over 34 degrees yr. Subtracting the variation of the depth due to the brightening of the red dwarf star, the actual variation in depth, originated from from the white dwarf, was found to be about 0.012 mag. This change in the brightness of the compact object has been attributed to the mass accretion from its primary component via thermally driven wind and/or flare-like events.Öğe V 1162 Ori: A multiperiodic delta Scuti star with variable period and amplitude(Edp Sciences S A, 2001) Arentoft, T; Sterken, C; Handler, G; Freyhammer, LM; Bruch, A; Niarchos, P; Gazeas, K; Manimanis, V; Van Cauteren, P; Poretti, E; Dawson, DW; Liu, ZL; Zhou, AY; Du, BT; Shobbrock, RR; Garrido, R; Fried, R; Akan, MC; Ibanoglu, C; Evren, S; Tas, G; Johnson, D; Blake, C; Kurtz, DWWe present the results of multisite observations of the delta Scuti star V1162 Ori. The observations were done in the period October 1999 - May 2000, when 18 telescopes at 15 observatories were used to collect 253 light extrema during a total of 290 hours of time-series observations. The purpose of the observations was to investigate amplitude and period variability previously observed in this star, and to search for low-amplitude frequencies. We detect, apart from the main frequency and its two first harmonics, four additional frequencies in the light curves, all with low amplitudes (1-3 mmag). Combining the present data set with data obtained in 1998-99 at ESO confirms the new frequencies and reveals the probable presence of yet another pulsational frequency. All five low-amplitude frequencies are statistically significant in the data, but at least one of them (f(5)) suffers from uncertainty due to aliasing. Using colour photometry we find evidence for a radial main frequency (f(1)), while most or all low-amplitude frequencies are likely non-radial. We show that the main frequency of V1162 Ori has variable amplitude and period/phase, the latter is also displayed in the O-C diagram from light extrema. The amplitude variability in our data is cyclic with a period of 282 d and a range of nearly 20 mmag, but earlier amplitude values quoted in the literature cannot be explained by this cyclic variation. O-C analysis including data from the literature show that the period of V1162 Ori displays a linear period change as well as sudden or cyclic variations on a time scale similar to that of the amplitude variations.