UV Cet-type Stars Research Articles

Unexpected stratification in the equivalent-duration distributions of flare stars

ABSTRACT In this study, the results obtained from the statistical models of flares observed with ground-based telescopes and TESS observations for the UV Ceti-type stars AD Leo and V1005 Ori are presented. The distribution of flare equivalent duration versus total flare duration for the flares of both stars, observed with different telescopes, has been modelled using the one-phase exponential association (OPEA) model. Although no bifurcation was observed in the models derived with the flare data of V1005 Ori, the OPEA model exhibited bifurcation in almost all observation data of AD Leo. While many similar studies have shown that such models derived for a star do not vary over time and exhibit a single distribution in the same observation season, the stratification in AD Leo’s distributions and consequently in its model is an unexpected result.

The flare cumulative frequencies of UV Ceti stars from different spectral types

ABSTRACT In this study, as a result of the photometric observations continued on nine UV Ceti type stars, examining the parameters obtained from the models of distributions of the flare equivalent durations in the logarithmic scale versus flare total durations, it is found that the highest flare equivalent durations occurring on the stars (Plateau values) are varying with the B − V colour indexes of those stars in agreement with the variations of both the magnetic field strength (B) of the magnetic loop and especially the electron density of its plasma (ne). The distributions of flare cumulative frequencies were derived by using the flare equivalent durations, instead of the flare energy. The main result of this study is a secondary variation, which can be just modelled with the damping sine waves, behind the main exponential variation of the flare cumulative frequency. The models indicate that this secondary variation in the flare cumulative frequency distributions is in the same shape with the periodic or quasi-periodic oscillations detected in the light curves of the stellar and solar flares. This caused to ask new questions about the flare mechanism working on stellar atmospheres.

Discovery of the Sub-second Linearly Polarized Spikes of Synchrotron Origin in the UV Ceti Giant Optical Flare

AbstractDuring our optical monitoring of UV Ceti, iconic late-type flaring star, with high temporal resolution using the Russian 6-m telescope in 2008, we detected a giant flare with the amplitude of about 3 magnitudes in U band. Near flare maximum, more than a dozen of spike bursts have been discovered with triangular shapes and durations from 0.6 to 1.2 s and maximal luminosities in the range (1.5–8) × 1027 erg s−1. For the half of these events, the linear polarization exceeds 35% with significance better than 5σ. We argue that these events are synchrotron emission of electron streams with the energies of several hundred MeV moving in the magnetic field of about 1.4 kG. Emission from such ultra-relativistic (with energies far exceeding 10 MeV) particles is being routinely observed in solar flares, but has never been detected from UV Ceti-type stars. This is the first ever detection of linearly polarized optical light from the UV Ceti-type stars which indicates that at least some fraction of the flaring events on these stars is powered by a non-thermal synchrotron emission mechanism.

Statistical Analyses of White-Light Flares: Two Main Results about Flare Behaviour

Abstract We present two main results, based on models and the statistical analyses of 1672 $ U$-band flares. We also discuss the behaviour of white-light flares. In addition, the parameters of the flares detected from two years of observations on CR Dra are presented. By comparing with flare parameters obtained from other UV Ceti-type stars, we examine the behaviour of the optical flare processes along with the spectral types. Moreover, we aimed, using large white-light flare data, to analyse the flare time-scales with respect to some results obtained from X-ray observations. Using SPSS V17.0 and GraphPad Prism V5.02 software, the flares detected from CR Dra were modelled with the OPEA function, and analysed with the $ t$-Test method to compare similar flare events in other stars. In addition, using some regression calculations in order to derive the best histograms, the time-scales of white-light flares were analysed. Firstly, CR Dra flares have revealed that white-light flares behave in a similar way as their counterparts observed in X-rays. As can be seen in X-ray observations, the electron density seems to be a dominant parameter in white-light flare process, too. Secondly, the distributions of the flare time-scales demonstrate that the number of observed flares reaches a maximum value in some particular ratios, which are 0.5, or its multiples, and especially positive integers. The thermal processes might be dominant for these white-light flares, while non-thermal processes might be dominant in the others. To obtain better results for the behaviour of the white-light flare process along with the spectral types, much more stars in a wide spectral range, from spectral type dK5e to dM6e, must be observed in white-light flare patrols.

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 222 flares detected in the B band observations of five stars and the results derived by statistical analysis and modeling of these data. Six basic properties have been found with a statistical analysis method applied to all models and analyses for the flares detected in the B band observation of UV Ceti type stars. We have also compared the U and B bands with the analysis results. This comparison allowed us to evaluate the methods used in the analyses. The analyses provided the following results. (1) The flares were separated into two types, fast and slow flares. (2) The mean values of the equivalent durations of the slow and the fast flares differ by a factor of 16.2 ± 3.7. (3) Regardless of the total flare duration, the maximum flare energy can reach a different Plateau level for each star. (4) The Plateau values of EV Lac and EQ Peg are higher than the others. (5) The minimum values of the total flare duration increase toward the later spectral types. This value is called the Half-Life value in models. (6) Both the maximum flare rise times and the total flare duration obtained from the observed flares decrease toward the later spectral types.

Rotation Modulations and Distributions of the Flare Occurrence Rates on the Surface of Five UV Ceti Type Stars

Abstract In this study, we considered stellar spots, stellar flares, and also the relation between these two magnetic proccesses that take place on UV Cet stars. In addition, the hypothesis about slow flares described by Gurzadyan (1986 Ap&SS, 125, 127) was investigated. All of these discussions were based on the results of three years of observations of UV 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 the stellar surface of EV Lac, V1005 Ori, and EQ Peg, while AD Leo does not show any short-term variability and V1054 Oph does not exhibit any variability. We report on new ephemerides for EV Lac, V1005 Ori, and EQ Peg, obtained from time-series analyses. The phases, computed at intervals of 0.10 phase length, where the mean flare occurence rates to obtain maximum amplitude; also, the phases of rotational modulation were compared in order to investigate whether there is any longitudinal relation between stellar flares and spots. Although the results show that flare events are related with spotted areas on stellar surfaces during some of the observing seasons, we did not find any clear correlation among them. Finally, it was tested whether slow flares are fast flares occurring on the opposite side of the stars according to the direction of the observers, as mentioned in a hypothesis developed by Gurzadyan (1986). The flare occurence rates reveal that both slow and fast flares can occur in any rotational phases. The flare occurence rates of both fast and slow flares vary in the same way along the longitudes for all program stars. These results are not expected based on the case mentioned in the hypothesis.

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 statistical analyses of obtained parameters, we discuss flare activity levels and flare characteristics of five UV Ceti stars. We present the parameters of unpublished flares detected over two years of observations of V1005 Ori. We compare parameters of the U-band flares detected over several seasons of observations of AD Leo, EV Lac, EQ Peg, V1054 Oph, and V1005 Ori. Flare frequencies calculated for all program stars and maximum energy levels of the flares are compared, and we consider which is the most correct parameter as an indicator of flare activity levels. Using the One Phase Exponential Association function, the distributions of flare equivalent duration versus flare total duration are modeled for each program star. We use the Independent Samples t-Test in the statistical analyses of the parameters obtained from the models. The results reveal some properties of flare processes occurring on the surfaces of UV Ceti type stars. (1) Flare energies cannot be higher than a specific value regardless of the length of the flare total duration. This must be a saturation level for white-light flares occurring in flare processes observed in the U band. Thus, for the first time it is shown that white-light flares have a saturation in a specific energy range. (2) The span values, which are the difference between the equivalent durations of flares with the shortest and longest total durations, are almost equal for each star. (3) The half-life values, minimum flare durations for saturation, increase toward the later spectral types.

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 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.

USE OF PHOTOGRAPHIC PLATE ARCHIVES FOR STUDYING THE LONG-TERM BEHAVIOUR OF THE PLEIADES FLARE STARS

As a result of the cooperation between the Sofia Sky Archive Data Center (Bulgaria) and Bamberg Observatory (Germany), Royal Observatory of Edinburgh (UK), Konkoly Observatory (Hungary), Strasbourg Observatory, Institute of Astronomy in Cambridge (UK), etc., a programme for investigation of the long-term behaviour of the Pleiades active dwarf stars (in particular, flare stars) is in progress. Existing plates in the Pleiades are found in different European plate archives, made since the end of the nineteenth century, by the searching tool of the wide-field plate database (http://www.skyarchive.org), which provides detailed information for astronomical plate archives all over the world, as well as for plates themselves. We investigate the Pleiades flare stars (UV Cet-type stars) included in the flare Stars database and identified in the USNO A2.0 catalogue. 68 Pleiades plates are already digitized with different scanning machines such as PDS 1010 (Sofia), PDS 2020 (Munster), SUPER COSMOS (Edinburgh), APM (C...

Optical observations of flare stars in the galaxy

Observations of flare stars in the galaxy are considered. UV Ceti type stars in the solar vicinity and flare stars is star clusters and associations have almost the same properties. The differences between them are connected with the age. Flare stars are one of the richest populations in the galaxy. The evolutionary path for all flare stars is the same.

BD+47°819, an A-type Flare Star with Peculiar Spectroscopic Variation

Stellar flares are one of the most challenging problems in astrophysics. Up to now, most of the detected flare stars are of late spectral type (e.g. UV Cet type stars etc.). Non-classical flare stars with early spectral type are very rare.Recently, an A5-type flare star BD+47°819, near the open cluster α Persei, was discovered (Wang 1993). The flare U amplitude was 1m.5 in ten minutes. The V magnitude of the star was 9m.25 during the quiescent state.On 1993 February 28 and March 1, spectroscopic observations of BD+47°819 were madewithn the 2.16 m telescope of Beijing Astronomical Observatory. A grating with a dispersion of 5nm/mm was used. Figs. 1 (a) and (b) show the observed spectra in the two nights. The exposure time for each spectrum was 15 minutes. Fig. 1 (a) shows a strange feature, an “emission line” at around 676 nm. Is this a real feature?

Flare Stars Database: Present Status

AbstractAn updated data collection for UV Cet type stars and their registered flare events in stellar aggregates and in the solar neighbourhood during more than 9580 hours observing time is described. The Flare Stars Database (FSDB) presently includes 3127 entries for 1622 stars.

Flare event statistics on UV Ceti-type stars

A statistical study of 228 flares on the three UV Ceti-type stars, i.e., YZ CMi, AD Leo, and EV Lac, is presented. Observations were gathered by Ichimura and Shimizu over a total monitoring time of 907 hours distributed over 18 years (1971 to 1988). Period analysis of flare activity was performed, and no periodicity was detected on the three stars for either the flare number rate or the energy rate in time-scales ranging from a year up to 14 years. Average colour of flares at peak was (U-B)=−0.98±0.17 and (B-V)=0.05±0.13. Cumulative number distributions of flare event time-integrated energies were solved by a least-squares method on a log-log plot for a power-law function to get both the constant of and the gradient β, which were found to be similar among the three stars. The gradient showed that rare large flare events radiate most of the energy released by all the flare events in the monitoring time. The flare number rate and energy rate are similar if the power-law distributions are extended up to a specific maximum energy. In reality, the Kolmogorov-Smirnov test showed that the observed cumulative number distributions of flare event energy were not necessarily a power-law function. The monte-Carlo simulation, however, indicates that the monitoring time and/or the patrol observation time interval may not be long enough to get the average flare number rate and energy rate, especially at the upper energy limits which are statistically unreliable.

Distribution of red dwarfs in general galactic field on the basis of stellar star statistics

The mathematical expectation for detection of stellar flare on UV Ceti type stars in the solar vicinity during photographic patrol observations with 40″ Shcmidt camera of the Byurakan Astrophysical Observatory is estimated. We use the luminosity function of the flaring red dwarfs the assume a uniform distribution in the general galactic field. Comparison with the results of photographic patrol supports this assumption. The numbers and total mass of the flare and non-flare red dwarf stars in the Galaxy for the uniform distribution are determined. They are not in contradiction with Oort's estimate of total mass of red dwarfs.

Statistics of flares observed for UV Ceti type stars YZ CMi, AD Leo, and EV Lac at the Okayama Observatory

Photoelectric monitoring of flare stars YZ CMi, AD Leo, and EV Lac has been done at the Okayama Observatory since early 1970s. This is a simultaneous UBV observations with a high time resolution. Some statistics of the flares of the UV Ceti type stars are presented.

Flare stars in star clusters, associations and solar vicinity

The observational data on flare stars observed in star clusters and associations as well as in the solar vicinity (the UV Ceti type stars) are discussed. The analysis of these data show that they constitute one common class of objects possessing flare activity and the differences between them are conditioned by the age differences. The stage of flare activity is an evolutionary stage, one of the earliest stages of evolution passed by all red dwarf stars. It comes before the end of their T Tau stage of evolution. The UV Ceti type flare stars in the solar vicinity seem to be the population of the general galactic field, which were formed in the systems, already desintegrated. Most probably the stellar flares are the result of the release of the the surplus energy having intra-stellar origin.

Review of the theoretical models of flares of the UV Ceti-type stars

The observations of stellar flares in optics, radio and X-ray show the general similarity of the flares on UV Ceti-type stars and on the Sun. At the same time there exist a lot of data showing that the analogy between these processes is not full. From this point of view the analysis of the theoretical models of stellar flares is given.

Some physical consequences from the flare statistics of the UV Cet type stars in the solar neighbourhood

The observational data permit us to establish statistical correlations between different parameters of stellar flare activity and the characteristics of quiet stars. We discusss physical consequences following from these relationships.

High Time Resolution Photometry of Red Dwarf Flare Stars II. The Shortest Flare Rise Time

AbstractWe have studied the initial phases of 65 optical flares on UV Cet type stars with a time resolution of 3x10–7 s. Ninety per cent of these flares have rapid brightenings shorter than 10 s and several are shorter than 1 s. We detected on UV Cet the shortest flare observed to date with a total duration of 1.7 s and a rise time of 0.3 s. The shortest flare rise times on the observed stars fit the predictions of the gas-dynamic flare model by Katsova et al. (1981) and Katsova and Livshits 1986).

High Time Resolution Photometry of Red Dwarf Flare Stars I. A Search For Fine Structures on the Optical Light Curves of Flares

AbstractA detailed analysis of 73 flares on UV Cet type stars observed at the 6 m telescope of the Special Astrophysical Observatory with 3x10–7 s time resolution shows no fine structures on flare light curves with time scales from 10–6 to 10–1 s.