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The statistical analyses of flares detected in B band photometry of UV Ceti type stars
In this study, we present the unpublished flare data collected from 222flares detected in the B band observations of five stars and the resultsderived by statistical analysis and modeling of these data. Six basicproperties have been found with a statistical analysis method applied toall models and analyses for the flares detected in the B bandobservation of UV Ceti type stars. We have also compared the U and Bbands with the analysis results. This comparison allowed us to evaluatethe methods used in the analyses. The analyses provided the followingresults. (1) The flares were separated into two types, fast and slowflares. (2) The mean values of the equivalent durations of the slow andthe fast flares differ by a factor of 16.2 ± 3.7. (3) Regardlessof the total flare duration, the maximum flare energy can reach adifferent Plateau level for each star. (4) The Plateau values of EV Lacand EQ Peg are higher than the others. (5) The minimum values of thetotal flare duration increase toward the later spectral types. Thisvalue is called the Half-Life value in models. (6) Both the maximumflare rise times and the total flare duration obtained from the observedflares decrease toward the later spectral types.
| Rotation Modulations and Distributions of the Flare Occurrence Rates on the Surface of Five UV Ceti Type Stars
In this study, we considered stellar spots, stellar flares, and also therelation between these two magnetic proccesses that take place on UV Cetstars. In addition, the hypothesis about slow flares described byGurzadyan (1986 Ap&SS, 125, 127) was investigated. All of thesediscussions were based on the results of three years of observations ofUV Cet-type stars: AD Leo, EV Lac, V1005 Ori, EQ Peg, and V1054 Oph.First of all, the results show that stellar spot activity occurs on thestellar surface of EV Lac, V1005 Ori, and EQ Peg, while AD Leo does notshow any short-term variability and V1054 Oph does not exhibit anyvariability. We report on new ephemerides for EV Lac, V1005 Ori, and EQPeg, obtained from time-series analyses. The phases, computed atintervals of 0.10 phase length, where the mean flare occurence rates toobtain maximum amplitude; also, the phases of rotational modulation werecompared in order to investigate whether there is any longitudinalrelation between stellar flares and spots. Although the results showthat flare events are related with spotted areas on stellar surfacesduring some of the observing seasons, we did not find any clearcorrelation among them. Finally, it was tested whether slow flares arefast flares occurring on the opposite side of the stars according to thedirection of the observers, as mentioned in a hypothesis developed byGurzadyan (1986). The flare occurence rates revealthat both slow and fast flares can occur in any rotational phases. Theflare occurence rates of both fast and slow flares vary in the same wayalong the longitudes for all program stars. These results are notexpected based on the case mentioned in the hypothesis.
| Bayesian inference of stellar parameters and interstellar extinction using parallaxes and multiband photometry
Astrometric surveys provide the opportunity to measure the absolutemagnitudes of large numbers of stars, but only if the individualline-of-sight extinctions are known. Unfortunately, extinction is highlydegenerate with stellar effective temperature when estimated frombroad-band optical/infrared photometry. To address this problem, Iintroduce a Bayesian method for estimating the intrinsic parameters of astar and its line-of-sight extinction. It uses both photometry andparallaxes in a self-consistent manner in order to provide anon-parametric posterior probability distribution over the parameters.The method makes explicit use of domain knowledge by employing theHertzsprung-Russell Diagram (HRD) to constrain solutions and to ensurethat they respect stellar physics. I first demonstrate this method byusing it to estimate effective temperature and extinction from BVJHKdata for a set of artificially reddened Hipparcos stars, for whichaccurate effective temperatures have been estimated from high-resolutionspectroscopy. Using just the four colours, we see the expected strongdegeneracy (positive correlation) between the temperature andextinction. Introducing the parallax, apparent magnitude and the HRDreduces this degeneracy and improves both the precision (reduces theerror bars) and the accuracy of the parameter estimates, the latter byabout 35 per cent. The resulting accuracy is about 200 K in temperatureand 0.2 mag in extinction. I then apply the method to estimate theseparameters and absolute magnitudes for some 47 000 F, G, K Hipparcosstars which have been cross-matched with Two-Micron All-Sky Survey(2MASS). The method can easily be extended to incorporate the estimationof other parameters, in particular metallicity and surface gravity,making it particularly suitable for the analysis of the 109stars from Gaia.
| Saturation Levels for White-light Flares of Flare Stars: Variation of Minimum Flare Duration for Saturation
Taking into account results obtained from models and from statisticalanalyses of obtained parameters, we discuss flare activity levels andflare characteristics of five UV Ceti stars. We present the parametersof unpublished flares detected over two years of observations of V1005Ori. We compare parameters of the U-band flares detected over severalseasons of observations of AD Leo, EV Lac, EQ Peg, V1054 Oph, and V1005Ori. Flare frequencies calculated for all program stars and maximumenergy levels of the flares are compared, and we consider which is themost correct parameter as an indicator of flare activity levels. Usingthe One Phase Exponential Association function, the distributions offlare equivalent duration versus flare total duration are modeled foreach program star. We use the Independent Samples t-Test in thestatistical analyses of the parameters obtained from the models. Theresults reveal some properties of flare processes occurring on thesurfaces of UV Ceti type stars. (1) Flare energies cannot be higher thana specific value regardless of the length of the flare total duration.This must be a saturation level for white-light flares occurring inflare processes observed in the U band. Thus, for the first time it isshown that white-light flares have a saturation in a specific energyrange. (2) The span values, which are the difference between theequivalent durations of flares with the shortest and longest totaldurations, are almost equal for each star. (3) The half-life values,minimum flare durations for saturation, increase toward the laterspectral types. (4) Both maximum total durations and maximum rise timescomputed from the observed flares decrease toward the later spectraltypes among the UV Ceti stars. According to the maximum energy levelsobtained from the models, both EV Lac and EQ Peg are more active thanthe other three program stars, while AD Leo is the most active flarestar according to the flare frequencies.
| A New Method for Classifying Flares of UV Ceti Type Stars: Differences Between Slow and Fast Flares
In this study, a new method is presented to classify flares derived fromthe photoelectric photometry of UV Ceti type stars. This method is basedon statistical analyses using an independent samples t-test. The dataused in analyses were obtained from four flare stars observed between2004 and 2007. The total number of flares obtained in the observationsof AD Leo, EV Lac, EQ Peg, and V1054 Oph is 321 in the standard JohnsonU band. As a result flares can be separated into two types, slow andfast, depending on the ratio of flare decay time to flare rise time. Theratio is below 3.5 for all slow flares, while it is above 3.5 for allfast flares. Also, according to the independent samples t-test, there isa difference of about 157 s between equivalent durations of slow andfast flares. In addition, there are significant differences betweenamplitudes and rise times of slow and fast flares.
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