Photometric data from the ASAS - South (declination less than 29 degree ) survey have been used for identification of bright stars located near the sources from the ROSAT All Sky Survey Bright Source ...Catalog (RBSC). In total 6028 stars brighter than 12.5 mag in I- or V-bands have been selected and analyzed for periodicity. Altogether 2302 variable stars have been found with periods ranging from 0.137 d to 193 d. Most of these stars have X-ray emission of coronal origin with a few cataclysmic binaries and early type stars with colliding winds. Whenever it was possible we collected data available in the literature to verify periods and to classify variable objects. The catalog includes 1936 stars (1233 new) considered to be variable due to presence of spots (rotationally variable), 127 detached eclipsing binary stars (33 new), 124 contact binaries (11 new), 96 eclipsing stars with deformed components (19 new), 13 ellipsoidal variables (4 new), 5 miscellaneous variables and one pulsating RR Lyr type star (blended with an eclipsing binary). More than 70% of new variable stars have amplitudes smaller than 0.1 mag, but for ASAS 063656-0521.0 we have found the largest known amplitude of brightness variations due to the presence of spots (up to Delta V=0.8 mag). The table with the compiled data and figures with light curves can be downloaded from Acta Astronomica Archive.
Barnard's star is a red dwarf, and has the largest proper motion (apparent motion across the sky) of all known stars. At a distance of 1.8 parsecs
, it is the closest single star to the Sun; only the ...three stars in the α Centauri system are closer. Barnard's star is also among the least magnetically active red dwarfs known
and has an estimated age older than the Solar System. Its properties make it a prime target for planetary searches; various techniques with different sensitivity limits have been used previously, including radial-velocity imaging
, astrometry
and direct imaging
, but all ultimately led to negative or null results. Here we combine numerous measurements from high-precision radial-velocity instruments, revealing the presence of a low-amplitude periodic signal with a period of 233 days. Independent photometric and spectroscopic monitoring, as well as an analysis of instrumental systematic effects, suggest that this signal is best explained as arising from a planetary companion. The candidate planet around Barnard's star is a cold super-Earth, with a minimum mass of 3.2 times that of Earth, orbiting near its snow line (the minimum distance from the star at which volatile compounds could condense). The combination of all radial-velocity datasets spanning 20 years of measurements additionally reveals a long-term modulation that could arise from a stellar magnetic-activity cycle or from a more distant planetary object. Because of its proximity to the Sun, the candidate planet has a maximum angular separation of 220 milliarcseconds from Barnard's star, making it an excellent target for direct imaging and astrometric observations in the future.
Based on analysis of photometric observations of nearby M type stars obtained with ASAS, 31 periodic variables were detected. The determined periods are assumed to be related to rotation periods of ...the investigated stars. Among them 10 new variables with periods longer than 10 days were found, which brings the total number of slowly rotating M stars with known rotation periods to 12 objects. X-ray activity and rotation evolution of M stars follows the trends observed in G-K type stars. Rapidly rotating stars are very active and activity decreases with increasing rotation period but the period-activity relation is mass-dependent which suggests that the rotation period alone is not a proper measure of activity. The investigated stars were grouped according to their mass and the empirical turnover time was determined for each group. It increases with decreasing mass more steeply than for K type stars for which a flat dependence had been found. The resulting Rossby number-activity relation shows an exponential decrease of activity with increasing Rossby number. The analysis of space motions of 27 single stars showed that all rapidly rotating and a few slowly rotating stars belong to young disk (YD) whereas all old disk (OD) stars are slowly rotating. The median rotation period of YD stars is about 2 days and that of OD stars is equal to 47 days, i.e., nearly 25 times longer. The average X-ray flux of OD stars is about 1.7 dex lower than YD stars in a good agreement with the derived Rossby number-activity formula supplemented with rotation-age relation and in a fair agreement with recent observations but in a disagreement with the Skumanich formula supplemented with the activity-rotation relation.
Context. It was recently demonstrated that contact binaries occur in globular clusters (GCs) only immediately below turn-off point and in the region of blue straggler stars (BSs). In addition, ...observations indicate that at least a significant fraction of BSs in these clusters was formed by the binary mass-transfer mechanism. Aims. The aim of our present investigation is to obtain and analyze a set of evolutionary models of cool, close detached binaries with a low metal abundance, which are characteristic of GC. Methods. We computed the evolution of 975 models of initially detached, cool close binaries with different initial parameters. The models include mass exchange between components as well as mass and angular momentum loss due to the magnetized winds for very low-metallicity binaries with Z = 0.001. The models are interpreted in the context of existing data on contact binary and blue straggler members of GCs. The models are based on our recently developed code for evolutionary modeling of cool close and contact binaries. Results. The model parameters agree well with the observed positions of the GC contact binaries in the Hertzsprung-Russell diagram, and we achieve detailed parameters of several individual contact and near-contact binaries. Contact binaries in the lower part of the cluster main sequence are absent because there are no binaries with initial orbital periods shorter than 1.5 d. Contact binaries end their evolution as mergers that appear in the BS region. Binary-formed BSs populate the whole observed BS region in a GC, but a gap is visible between low-mass mergers that are concentrated along the zero-age main sequence and binary BSs occupying the red part of the BS region. Very few binary mergers are expected to rotate rapidly and/or possess chemical peculiarities resulting from the exposure of the layers processed by CNO nuclear reactions. All other binary mergers are indistinguishable from the collisionally formed mergers. The results show that binary-formed BSs may constitute at least a substantial fraction of all BSs in a GC.
We present a catalog of 307 optical counterparts of the bright ROSAT X-ray sources, identified with the ASAS North survey data and showing periodic brightness variations. They all have declination ...north of -25 degree . Other data available from the literature for the listed stars are also included. All the tabulated stars are new variables, except for 13 previously known, for which the revised values of periods are given.
The aim of the study was to indicate the procedure of using laboratory physical model tests of scour around bridge piers for the purposes of determining the potential scour of a riverbed on field ...bridge crossings. The determination of the uniform modeling scale coefficient according to the criterion of reliable sediment diameter limits the application of the results of tests on physical models to selected types of sediment. The projected depths of scouring of the riverbed at the pier in nature were determined for an object reproduced in the scale of 1:15 determined from the relationship of flow resistance, expressed by hydraulic losses described by the Chézy velocity coefficient, the value of which, in the model and in nature, should be the same. Expressing the value of the Chézy velocity coefficient with the Manning roughness coefficient and introducing the Strickler parameter, it was shown that the coarse sand used in the laboratory bed models the flow resistance corresponding to the resistance generated by gravel in nature. The verification of the calculated size of scouring was based on popular formulas from Russian literature by Begam and Volčenkov 16, Laursen and Toch’s 20 from the English, and use in Poland according to the Regulation ... (Journal of Laws of 2000, No. 63, item 735) 32.
Observations of cool stars with very high activity levels show a tendency of magnetic regions to be concentrated near the poles. In close binaries a preference of low latitude regions to appear at ...fixed longitudes has also been suggested. We modeled a 3D process of emergence of a thin magnetic tube through a convective envelope of a solar type component of a close binary. In no case the emerging tube approached a pole by more than 20 degree , compared to its initial latitude. Assuming that dynamo operates mainly at low and moderate latitudes, the polar magnetic regions cannot be formed unless an additional poleward transport of magnetic fields exists in rapidly rotating stars. Preferred longitudes in active close binaries result in our model from an azimuthal variation of effective gravity, hence buoyancy, throughout the convection zone of each component. The variation is strong enough in a contact binary with orbital period of 0.3 days to force parts of the tube emerging in the regions of enhanced buoyancy to approach stellar surface substantially sooner than the rest of the tube, thus forming two preferred longitudes 180 degree apart. The precise positions of the preferred longitudes depend on binary parameters. The gravity variation is, however, too weak in a close binary with a period of 2 days to influence significantly the emergence rate of different tube parts. A magnetic region can form at any longitude, depending mostly on an initial perturbation, provided the original tube is placed on an equipotential surface. Such a location of the tube seems to be a correct initial condition. The tube originally placed on a spherical surface has parts lying closer to the unstable zone, where the tube instability preferentially develops. This results in magnetic flux emergence at preferred longitudes.
Abstract
The search for Earth-like planets around late-type stars using ultrastable spectrographs requires a very precise characterization of the stellar activity and the magnetic cycle of the star, ...since these phenomena induce radial velocity (RV) signals that can be misinterpreted as planetary signals. Among the nearby stars, we have selected Barnard’s Star (Gl 699) to carry out a characterization of these phenomena using a set of spectroscopic data that covers about 14.5 yr and comes from seven different spectrographs: HARPS, HARPS-N, CARMENES, HIRES, UVES, APF, and PFS; and a set of photometric data that covers about 15.1 yr and comes from four different photometric sources: ASAS, FCAPT–RCT, AAVSO, and SNO. We have measured different chromospheric activity indicators (H α, Ca ii HK, and Na i D), as well as the full width at half-maximum (FWHM), of the cross-correlation function computed for a sub-set of the spectroscopic data. The analysis of generalized Lomb–Scargle periodograms of the time series of different activity indicators reveals that the rotation period of the star is 145 ± 15 d, consistent with the expected rotation period according to the low activity level of the star and previous claims. The upper limit of the predicted activity-induced RV signal corresponding to this rotation period is about 1 m s−1. We also find evidence of a long-term cycle of 10 ± 2 yr that is consistent with previous estimates of magnetic cycles from photometric time series in other M stars of similar activity levels. The available photometric data of the star also support the detection of both the long-term and the rotation signals.
We test the evolutionary model of cool close binaries developed by one of us (KS) on the observed properties of near contact binaries (NCBs). These are binaries with one component filling the inner ...critical Roche lobe and the other almost filling it. Those with a more massive component filling the Roche lobe are SD1 binaries whereas in SD2 binaries the Roche lobe filling component is less massive. Our evolutionary model assumes that, following the Roche lobe overflow by the more massive component (donor), mass transfer occurs until mass ratio reversal. A binary in an initial phase of mass transfer, before mass equalization, is identified with SD1 binary. We show that the transferred mass forms an equatorial bulge around the less massive component (accretor). Its presence slows down the mass transfer rate to the value determined by the thermal time scale of the accretor, once the bulge sticks out above the Roche lobe. It means, that in a binary with a (typical) mass ratio of 0.5 the SD1 phase lasts at least 10 times longer than resulting from the standard evolutionary computations neglecting this effect. This is why we observe so many SD1 binaries. Our explanation is in contradiction to predictions identifying the SD1 phase with a broken contact phase of the Thermal Relaxation Oscillations model. The continued mass transfer, past mass equalization, results in mass ratio reversed. SD2 binaries are identified with this phase. Our model predicts that the time scales of SD1 and SD2 phases are comparable to one another. Analysis of the observations of 22 SD1 binaries, 27 SD2 binaries and 110 contact binaries (CBs) shows that relative number of both types of NCBs favors similar time scales of both phases of mass transfer. Total masses, orbital angular momenta and orbital periods of SD1 and SD2 binaries are indistinguishable from each other whereas they differ substantially from the corresponding parameters of CBs. We conclude that the results of the analysis fully support the model presented in this paper.
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