Guray Tas

Long-term starspot evolution, activity cycle and orbital period variation of RT Lacertae

A 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 Kuruczs 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 ts were obtained assuming spots on both components and the unspotted V -band luminosity ratio: LG5=LG9 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 of40, 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 of8: 5y r 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 dierential rotation, with a lower limit of = 3:2 10 3 . The G9 IV secondary does not show evidence for an activity cycle, its spot coverage appearing rather constant at15 20% of its surface. The relative amplitude of its surface dierential rotation, as indicated by the variation of the spot migration rate, is = 2:7 10 3 . The variation of the orbital period shows a correlation with the activity level of the primary component. Specically, 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 gure of equilibrium of the active components, due to the operation of non-linear hydromagnetic dynamos in their extended convective envelopes.

Spots, plages, and flares on λ Andromedae and II Pegasi ⋆

Aims. We present the results of a contemporaneous photometric and spectroscopic monitoring of two RS CVn binaries, namely λ And and II Peg. The aim of this work is to investigate the behavior of surface inhomogeneities in the atmospheres of the active components of these systems that have nearly the same temperatures but different gravities. Methods. The light curves and the modulation of the surface temperature, as recovered from line-depth ratios (LDRs), were used to map the photospheric spots, while the Hα emission was used as an indicator of chromospheric inhomogeneities. The spot temperatures and sizes were derived from a spot model applied to the contemporaneous light and temperature curves. Results. We find larger and cooler spots on II Peg (Tsp � 3600 K) than on λ And (Tsp � 3900 K); this could be the result of both the difference in gravity and the higher activity level of the former. Moreover, we find a clear anti-correlation between the Hα emission and the photospheric diagnostics (temperature and light curves). We have detected a modulation in the intensity of the He i D3 line with the star rotation, suggesting surface features also in the upper chromosphere of these stars. A rough reconstruction of the 3D structure of their atmospheres was also performed by applying a spot/plage model to the light and temperature curves and to the Hα flux modulation. In addition, a strong flare affecting the Hα ,t he Hei D3, and the cores of Na i D1,2 lines has been observed on II Peg. Conclusions. The spot/plage configuration has been reconstructed in the visible component of λ And and II Peg, which have nearly the same temperature but very different gravities and rotation periods. A close spatial association of photospheric and chromospheric active regions, at the time of our observations, was found in both stars. Larger and cooler spots were found on II Peg, the system with the active component of higher gravity and a higher activity level. The area ratio of plages to spots seems to decrease when the spots get bigger. Moreover, with both this and literature data, a correlation between the temperature difference ∆T = Tph −Tsp and the surface gravity is also suggested.

The brightness variations and orbital period changes of RT Lacertae

The 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 systems 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.

Long-term photometric behaviour of the RS CVn binary RT Lacertae

A 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 Djuray sevics 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 45 ◦ -170 ◦ and at high latitudes (above 45 ◦ ). Our analysis indicates two spots with diameters of ∼10 ◦ -50 ◦ 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.

LO Peg: Surface Differential Rotation, Flares, and Spot-topographic evolution

Using the wealth of ~24 yr multiband data, we present an in-depth study of the star-spot cycles, surface differential rotations (SDR), optical flares, evolution of star-spot distributions, and coronal activities on the surface of young, single, main-sequence, ultrafast rotator (UFR) LO Peg. From the long-term V -band photometry, we derive rotational period of LO Peg to be 0.4231 +/- 0.0001 d. Using the seasonal variations on the rotational period, the SDR pattern is investigated, and shows a solar-like pattern of SDR. A cyclic pattern with period of ~2.7 yr appears to be present in rotational period variation. During the observations, 20 optical flares are detected with a flare frequency of 1 flare per two days and with flare energy of 10^{31-34} erg. The surface coverage of cool spots is found to be in the range of 9-26 per cent. It appears that the high- and low-latitude spots are interchanging their positions. Quasi-simultaneous observations in X-ray, UV, and optical photometric bands show a signature of an excess of X-ray and UV activities in spotted regions.

Apsidal Motion Studies for DI Her

Eclipsing 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.