This paper presents an extensive overview of known and proposed RR Lyrae stars in binaries. The aim is to revise and extend the list with new Galactic field systems. We utilized maxima timings for 11 RRab type stars with suspicious behaviour from the GEOS database, and determined maxima timings from data of sky surveys and our own observations. This significantly extended the number of suitable maxima timings. We modelled the proposed Light Time Effect (LiTE) in O-C diagrams to determine orbital parameters for these systems. In contrast to recent studies, our analysis focused on decades-long periods instead of periods in the order of years. Secondary components were found to be predominantly low-mass objects. However, for two of the stars, RZ Cet and AT Ser, the mass of the suspected companion of more than one solar mass suggests that it is a massive white dwarf, a neutron star or even a black hole. We found that the semi-major axes of the proposed orbits are between 1 and 20au. Because the studied stars belong to the closest RR Lyraes, maximal angular distances between components during orbit should at least be between 1 and 13mas and this improves the chance to detect both stars using current telescopes and interferometers. However, our interpretation of the O-C diagrams as a consequence of the LiTE should be considered as preliminary without reliable spectroscopic measurements. On the other hand, our models give a prediction of the period and radial velocity evolution which should be sufficient for plausible proof of binarity.
The First Byurakan Survey (FBS), also known as the Markarian survey, covers about 17000 sq. deg. It has been used by Markarian and his collaborators to search for UV excess galaxies (see Cat. VII/172), by Abrahamyan and his collaborators to search for late type stars and UV excess or emission line point sources (stars and starlike objects). The identification, classification, and investigation of late type stars and blue stellar objects constitutes the second part of the First Byurakan Survey and is a natural continuation of it. For this second program, at the present time, a catalogue of 1103 blue objects has been built (see Cat. II/223). In a series of 14 papers, referenced in the "References" section below, have been published lists of 1000 late M type and carbon stars, selected on the FBS plates. We have revised and updated this 14 lists with the new data from recently published optical and infrared catalogs to give access to all available data. As a result, the catalog of the First Byurakan Survey (FBS) Late Type Stars, was compiled. In this catalogue are presented high accurate coordinates, proper motions, spectral, photometric and infrared dates about of 995 FBS late M type and carbon stars. Are given cross-identifications of FBS late type stars with their counterparts from the most popular catalogs (2MASS, USNO-B1, NOMAD, UCAC2, GCVS4.2, NSVS, ASAS, IRAS PS and IRAS FS). On the base of 2MASS (J-K) color indices were determined a luminosity classes for 988 of them. For the 55 stars were determined a spectral classes, and for the another 129 were determined a spectral subclasses on the base of DFBS (http://byurakan.phys.uniroma1.it/) These data were not presented in original lists.
A total of 18 lists of the First Byurakan Survey of Late-Type Stars (FBS LTS) were published between 1990 and 2016. The stars were found on FBS low-dispersion spectroscopic plates. A systematic search and selection were carried out on a surface of ~16000deg^2^ on almost the whole area of the FBS. As a result, the "Revised and Updated Catalogue of the First Byurakan Survey of Late-Type Stars" was generated (FBS LTS v1; see Cat. III/266 -- 1045 objects). We present the second version of the catalogue of FBS LTS with new data (FBS LTS v2), comprising 1471 objects. It is a homogeneous and complete data base for high-Galactic-latitude, late-type stars, including M and C types. Since 2007, all FBS low-resolution spectral plates have been digitized. All digitized FBS (DFBS) spectral plates have been analysed with FITSVIEW and SAO IMAGE DS9, and numerous relatively faint LTSs have been discovered. We have performed cross-correlations with DFBS, USNO-B1.0, 2MASS, AllWISE, IRAS PSC/FSC, AKARI, ROSAT BSC/FSC, GCVS, SDSS and added updated SIMBAD data. For numerous new detected objects, we present accurate DSS2 positions, approximate spectral subtypes refined from the DFBS low-dispersion spectra, luminosity classes estimated from 2MASS colours, and available proper motions for 1471 FBS LTSs. The Revised and Updated Catalogue v2 lists a large number of completely new objects, which promises to extend very significantly the census of M giants, faint N-type asymptotic giant branch carbon stars, CH-type carbon giants at high Galactic latitudes, and M dwarfs in the Solar vicinity up to 16.0-17.0mag in the visual. Some important data from Gaia DR2 and supplementary spectra from the Byurakan Astrophysical Observatory 2.6m and LAMOST telescopes are presented.
By using the full span of multicolor data on a representative sample of first overtone RR Lyrae stars in the Large Magellanic Cloud (LMC), we revisit the problem of the incidence rate of the amplitude/phase-modulated (Blazhko) stars. Methods. Multicolor data, obtained by the MAssive Compact Halo Objects (MACHO) project, are utilized through a periodogram averaging method. Multicolor data, obtained by the MAssive Compact Halo Objects (MACHO) project, are utilized through a periodogram averaging method.
We present I and B photometry of five distinct transits of the exoplanet OGLE-TR-10b. By modeling the light curves, we find the planetary radius to be R_P_=(1.06+/-0.08)R_Jup_ and the stellar radius to be R_S_=1.10+/-0.07R_{sun}_. The uncertainties are dominated by statistical errors in the photometry. Our estimate of the planetary radius is smaller than previous estimates that were based on lower precision photometry, and hence the planet is not as anomalously large as was previously thought. We provide updated determinations of all the system parameters, including the transit ephemerides.
We study the connection between the chromospheric and photospheric behaviour of the active late-type star FK Comae. We use spot temperature modelling, light curve inversion based on narrow- and wide-band photometric measurements, Halpha observations from 1997-2010, and Doppler maps from 2004-2010 to compare the behaviour of chromospheric and photospheric features. Investigating low-resolution H{alpha} spectra we find that the changes in the chromosphere seem to happen mainly on a time scale longer than a few hours, but shorter variations were also observed. According to the Halpha measurements prominences are often found in the chromosphere that reach to more than a stellar radius and are stable for weeks, and which seem to be often, but not every time connected with dark photospheric spots. The rotational modulation of the H{alpha} emission seems to typically be anticorrelated with the light curve, but we did not find convincing evidence of a clear connection in the long-term trends of the Halpha emission and the brightness of the star. In addition, FK Com seems to be in an unusually quiet state in 2009-2010 with very little chromospheric activity and low spot contrast, that might indicate the long-term decrease of activity.
Based on one-month long MMT time-series observations of the open cluster M37, we monitored light variations of nearly 2500 red dwarfs and successfully identified 420 flare events from 312 cluster M dwarf stars. For each flare light curve, we derived observational and physical parameters, such as flare shape, peak amplitude, duration, energy, and peak luminosity. We show that cool stars produce serendipitous flares energetic enough to be observed in the r-band, and their temporal and peak characteristics are almost the same as those in traditional U-band observations. We also found many large-amplitude flares with inferred {Delta}u>6mag in the cluster sample which had been rarely reported in previous ground-based observations. Following the ergodic hypothesis, we investigate in detail statistical properties of flare parameters over a range of energy (E_r_~=10^31^-10^34^erg). As expected, there are no statistical differences in the distributions of flare timescales, energies, and frequencies among stars of the same age and mass group. We note that our sample tend to have longer rise and decay timescales compared to those seen in field flare stars of the same spectral type and be more energetic. Flare frequency distributions follow power-law distributions with slopes {beta}~0.62-1.21 for all flare stars and {beta}~0.52-0.97 for stars with membership information (P_mem_>=0.2). These are in general agreement with previous works on flare statistics of young open clusters and nearby field stars. Our results give further support to the classical age-activity relations.
Magnetic fields are a key component in the main sequence evolution of low mass stars. Flares, energetic eruptions on the surfaces of stars, are an unmistakable manifestation of magnetically driven emission. The occurrence rates and energy distributions of flares trace stellar characteristics such as mass and age. But before flares can be used to constrain stellar properties, the flaring-age-mass relation requires proper calibration. This work sets out to quantify flaring activity of independently age-dated main sequence stars for a broad range of spectral types using optical light curves obtained by the Kepler satellite. Drawing from the complete K2 archive, we searched 3435 80 day long light curves of 2111 open cluster members for flares using the open-source software packages K2SC to remove instrumental and astrophysical variability from K2 light curves, and AltaiPony to search and characterize the flare candidates. We confirmed a total of 3844 flares on high probability open cluster members with ages from zero age main sequence (Pleiades) to 3.6Gyr (M67). We extended the mass range probed in the first study of this series to span from Sun-like stars to mid-M dwarfs. We added the Hyades (690Myr) to the sample as a comparison cluster to Praesepe (750Myr), the 2.6Gyr old Ruprecht 147, and several hundred light curves from the late K2 Campaigns in the remaining clusters. We found that the flare energy distribution was similar in the entire parameter space, following a power law relation with an exponent between 1.84 and 2.39. We confirmed that flaring rates declined with age, and declined faster for higher mass stars. Our results are in good agreement with most previous statistical flare studies. We found evidence that a rapid decline in flaring activity occurred in M1-M2 dwarfs around Hyades/Praesepe age, when these stars spun down to rotation periods of about 10 d, while higher mass stars had already transitioned to lower flaring rates, and lower mass stars still resided in the saturated activity regime. We conclude that some discrepancies between our results and flare studies that used rotation periods for their age estimates could be explained by sample selection bias toward more active stars, but others may hint at limitations of using rotation as an age indicator without additional constraints from stellar activity.
In this study, a new method is presented to classify flares derived from the photoelectric photometry of UV Ceti type stars. This method is based on statistical analyses using an independent samples t-test. The data used in analyses were obtained from four flare stars observed between 2004 and 2007. The total number of flares obtained in the observations of AD Leo, EV Lac, EQ Peg, and V1054 Oph is 321 in the standard Johnson U band. As a result flares can be separated into two types, slow and fast, depending on the ratio of flare decay time to flare rise time. The ratio is below 3.5 for all slow flares, while it is above 3.5 for all fast flares. Also, according to the independent samples t-test, there is a difference of about 157 s between equivalent durations of slow and fast flares. In addition, there are significant differences between amplitudes and rise times of slow and fast flares.
From Kepler data, we show that the incidence of flares on stars drops by only a factor of 4 from K-M dwarfs to A-F stars. Allowing for visibility effects, this implies that the true relative number of flare stars does not change very much from cool dwarfs to hot A stars. The idea that flares on A stars can be attributed to a cool companion has to be rejected because it leads to flare amplitudes two orders of magnitude smaller than actually observed. We confirm that spots on flare stars are generally larger than those on non-flare stars and that flare stars rotate significantly faster than non-flare stars. Analysis of 209 flare stars observed in Kepler short-cadence mode allows accurate measurements of flare shapes and duration. We find that about one-third of the flares have a bump or slope discontinuity on the decaying branch and that flares of long duration are to be found in stars with low surface gravities. Flare energies are strongly correlated with stellar luminosity and radius. The correlation with radius leads to a rough estimate of several tens of gauss for the typical magnetic field associated with a flare. The correlation with stellar luminosity can be understood if the typical flare loop length-scales approximately as the stellar radius. We examined the flare frequency as a function of orbital phase in three eclipsing binaries in which a large number of flares are visible. There appears to be no correlation of flaring with orbital phase, which weakens the hypothesis that flares in close binaries could be a result of reconnection of field lines connecting the two stars.
