Ö. Çakırlı

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.

The pulsation modes of the pre-white dwarf PG 1159-035

Context. PG 1159-035, a pre-white dwarf with Teff � 140 000 K, is the prototype of both two classes: the PG 1159 spectroscopic class and the DOV pulsating class. Previous studies of PG 1159-035 photometric data obtained with the Whole Earth Telescope (WET) showed a rich frequency spectrum allowing the identification of 122 pulsation modes. Analyzing the periods of pulsation, it is possible to measure the stellar mass, the rotational period and the inclination of the rotation axis, to estimate an upper limit for the magnetic field, and even to obtain information about the inner stratification of the star. Aims. We have three principal aims: to increase the number of detected and identified pulsation modes in PG 1159-035, study trapping of the star’s pulsation modes, and to improve or constrain the determination of stellar parameters. Methods. We used all available WET photometric data from 1983, 1985, 1989, 1993 and 2002 to identify the pulsation periods. Results. We identified 76 additional pulsation modes, increasing to 198 the number of known pulsation modes in PG 1159-035, the largest number of modes detected in any star besides the Sun. From the period spacing we estimated a mass M/M� = 0.59 ± 0.02 for PG 1159-035, with the uncertainty dominated by the models, not the observation. Deviations in the regular period spacing suggest that some of the pulsation modes are trapped, even though the star is a pre-white dwarf and the gravitational settling is ongoing. The position of the transition zone that causes the mode trapping was calculated at rc/R� = 0.83 ± 0.05. From the multiplet splitting, we calculated the rotational period Prot = 1.3920 ± 0.0008 days and an upper limit for the magnetic field, B < 2000 G. The total power of the pulsation modes at the stellar surface changed less than 30% for � = 1 modes and less than 50% for � = 2 modes. We find no evidence of linear combinations between the 198 pulsation mode frequencies. PG 1159-035 models have not significative convection zones, supporting the hypothesis that nonlinearity arises in the convection zones in cooler pulsating white dwarf stars.

Carbon deficiencies in the primaries of some classical Algols

The equivalent widths (EWs) of the C IIλ4267 A line were measured for the mass-gaining primary stars of 18 Algol-type binary systems. The EWs of the gainers were compared with the EWs of single standard stars that have the same effective temperature and luminosity class. This comparison clearly indicates that the EWs of the gainers are systematically smaller than those of the standard stars. The primary components of the classical Algols, located in the main-sequence band of the Hertzsprung–Russell diagram, appear to be carbon-poor stars. We estimate [NC/Ntot] relative to the Sun as −1.91 for GT Cep, −1.88 for AU Mon and −1.41 for TU Mon, indicating poorer carbon abundance. An average differential carbon abundance has been estimated to be −0.82 dex relative to the Sun and −0.54 dex relative to the main-sequence standard stars. This result is taken to be an indication of material transferring from the evolved less-massive secondary components to the gainers, such that the CNO cycle processed material changed the original abundance of the gainers. There appear to be relationships between the EWs of the CII λ4267 A line and the rates of orbital period increase and mass transfer in some Algols. As the mass transfer rate increases, the EW of the C II line decreases. This indicates that accreted material has not yet been completely mixed in the surface layers of the gainers. This result supports the idea of mixing as an efficient process to remove the abundance anomaly built up by accretion. The chemical evolution of the classical Algol-type systems could lead to constraints on the initial masses of the less massive, evolved, mass-losing stars.

T-Cyg1-12664: a low-mass chromospherically active eclipsing binary in the Kepler field

The eclipsing binary T-Cyg1-12664 has been observed both spectroscopically and photo- metrically, and the radial velocities of both the components and the ground-based VRI light curves have been obtained. The Kepler R data and radial velocities for the system have been analysed simultaneously. Masses and radii have been obtained as 0.680 ± 0.021 Mand 0.613 ± 0.007 Rfor the primary star and 0.341 ± 0.012 Mand 0.897 ± 0.012 Rfor the secondary star. The distance to the system has been estimated as 127 ± 14 pc. The ob- served wave-like distortion at out-of-eclipse is modelled with two separate spots on the more massive star, which is also confirmed by the Ca II K and H emission lines in its spectra. The locations of the components in the mass-radius and mass-effective temperature planes have been compared with the well-determined low-mass components of eclipsing binaries as well as with the theoretical models. While the radius of the primary star is consistent with the main-sequence stars, the radius of the less massive component appears to be 2.8 times larger than that of the main-sequence models. A comparison of the radii of low-mass stars with the models reveals that the observationally determined radii begin to deviate from the models with a mass of 0.27 M� , and suddenly reach to maximum deviation at a mass of 0.34 M� . Then, the deviations begin to decrease up to the solar mass. The maximum deviation, seen at a mass of about 0.34 M� , is very close to the mass of fully convective stars, as suggested by theoretical studies. A third star in the direction of the eclipsing pair has been detected from our VRI images. The observed infrared excess of the binary most probably arises from this star, which can be radiated mostly in the infrared bands.

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.

NSVS 06507557: a low-mass double-lined eclipsing binary

In this paper, we present the results of a detailed spectroscopic and photometric analysis of the V = 13.4 mag low-mass eclipsing binary NSVS 06507557 with an orbital period of 0.515 d. We have obtained a series of mid-resolution spectra covering nearly the entire orbit of the system. In addition, we have obtained simultaneous VRI broad-band photometry using a small aperture telescope. From these spectroscopic and photometric data, we have derived the systems orbital parameters and we have determined the fundamental stellar parameters of the two components. Our results indicate that NSVS 06507557 consists of a K9 pre-main-sequence star and an M3 pre-main-sequence star. These have masses of 0.66 ± 0.09 M ⊙ and 0.28 ± 0.05 M ⊙ and radii of 0.60 ± 0.03 and 0.44 ± 0.02 R ⊙ , respectively, and are located at a distance of 111 ± 9 pc. The radius of the less massive secondary component is larger than that of a zero-age main-sequence (ZAMS) star having the same mass. While the radius of the primary component is in agreement with ZAMS, the secondary component appears to be larger by about 35 per cent with respect to its ZAMS counterpart. Night-to-night intrinsic light variations up to 0.2 mag have been observed. In addition, the H α and H β lines and the forbidden line of [OI] are seen in emission. The Li 6708 A absorption line is seen in most of the spectra. These features are taken to be signs of the characteristics of classic T Tauri stars. The parameters we have derived are consistent with an age of about 20 Myr, according to stellar evolutionary models. The spectroscopic and photometric results are in agreement with those obtained using theoretical predictions.

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.

The young active star SAO 51891 (V383 Lacertae)

Aims. The aim of this work is to investigate the surface inhomogeneities of a young, late-type star, SAO 51891, at different atmospheric levels, from the photosphere to the upper chromosphere, analyzing contemporaneous optical high-resolution spectra and broad-band photometry. Methods. The full spectral range of FOCES@CAHA (R � 40 000) is used to perform the spectral classification and to determine the rotational and radial velocities. The lithium abundance is measured to obtain an age estimate. The BVRIJHKs photometric bands are used to construct the spectral energy distribution (SED). The variations in the observed BV fluxes and effective temperature are used to infer the presence of photospheric spots and observe their behavior over time. The chromospheric activity is studied applying the spectral subtraction technique to Hα ,C aii H& K, H� ,a nd Caii IRT lines. Results. We find SAO 51891 to be a young K0-1V star with a lithium abundance close to the Pleiades upper envelope, confirming its youth (∼100 Myr), which is also inferred from its kinematical membership of the Local Association. No infrared excess is detected from analysis of its SED, limiting the amount of remaining circumstellar dust. We detect a rotational modulation of the luminosity, effective temperature, Ca ii H& K, H� ,a nd Caii IRT total fluxes. A simple spot model with two main active regions, about 240 K cooler than the surrounding photosphere, fits the observed light and temperature curves very well. The small-amplitude radial velocity variations are also well reproduced by our spot model. The anti-correlation of light curves and chromospheric diagnostics indicates

MY Serpentis: a high-mass triple system in the Ser OB2 association

We present spectroscopic observations of the massive multiple system HD\,167971, located in the open cluster NGC\,6604. The brighter component of the triple system is the overcontact eclipsing binary MY\,Ser with an orbital period of 3.32\,days. The radial velocities and the previously published UBV data obtained by \citet{may10} and the UBVRI light curves by \citet{dav88} are analysed for the physical properties of the components. We determine the following absolute parameters: for the primary star M

A comprehensive study of the open cluster NGC 6866

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