UV Cet-type Stars Research Articles

On negative infrared flares of flare stars

The basic formulation of the so-called fast electron hypothesis is presented. According to this hypothesis, the optical flares of UV Cet-type stars are caused by the inverse compton effect, i.e., nonthermal interactions of infrared photons with fast electrons (E equal to about 1.5 MeV), as well as nonthermal bremsstrahlung. The spontaneous appearance of fast electrons on the outer regions of the star is the essence of the flare. The fast electron hypothesis elicits two important predictions: (1) the possibility of X-ray emission during the stellar flares and (2) the inevitability of negative infrared flares. Both predictions are confirmed by direct observations. The importance of negative infrared flares is especially striking; it may be taken as convincing evidence that the inverse Compton effect may really work in the stellar atmospheres. 25 references.

Flares on opposite sides of the star: Observational aspects

The problem of the flare taking place on opposite sides of a star is considered. Such a screened flare, diffused through the star's atmosphere (chromosphere), may also be registered. The theoretical light curve for diffused flare event is derived, which differs strongly from the usual flare light curves. The light curve of diffused flare is characterized first of all by its very slow rise of brightness. This result opens quite a new direction to understand the nature of the so-called ‘slow’ flares, observed often among the UV Cet-type stars as well as flare stars in aggregates. All slow flares can be interpreted as quite ordinary flares of quite ordinary flare stars — taking place, however, on the opposite sides of the star. The results of interpretation of some slow flare events of YY Gem and three flare stars in Orion are presented. An attempt is made for the determination of actual amplitudes of screened flares taking place on the opposite sides of a star.

On Activities of UV Cet-Type Flare Stars and of T Tau-Type Stars

Comparisons of the activities of UV Cet-type flare stars and of T Tau-type stars with solar activity permits the conclusion that non-sta-tionary processes in the UV Cet-type stars and in the Sun are of an indent ical physical nature but that they differ qualitatively from active events in the T Tau-type stars. The identity of the activity in flare stars and the Sun makes it possible to study successfully stellar activity with the help of known models of various solar events and, on the other hand, to have a more general approach to the physics and evolution of solar activity on the basis of established features of numerous flare stars of different ages and masses. The hypothesis on hydromagnetic activity of the T Tau-type stars is sketched; within this framework, one supposes that the main feature of such stars is an occurrence at every point of the stellar surface, of conditions necessary for the existence of dark spots, i.e. a lowering of the photospheric brightness due to strong local magnetic fields.

Empirical Models of Stellar Flares: Constraints on Flare Theories

ABSTRACTA review is presented on empirical flare models of UV Cet type stars based upon optical,UV, X-ray, and radio observations. The observational constraints on the flare energetics, nature of radiation sources, and flare structures are discussed, with special attention to the geometrical dimension and the magnetic field of the flaring region. The hot-plasma model in the solar analogy is critically examined by comparing it with observations. Possible future observations are suggested to tighten the constraints on the stellar flare theories.

Observations of Hβ and HE II λ 4686 Lines in the Spectra of Flares of UV Cet-Type Stars

We have carried out 45.4 hours of continuous spectroscopic and photoelectric B-band observations of AD Leo, DT Vir and YZ CMi. The spectroscopic resolution was about 1 A in wavelength and 5-10 min in time. 6 flares were recorded. Light curves and line profiles are to be presented in the Izv.Crim. Astrophys. Obs., Vol.69. In this paper only the main results are discussed.

Flare stars and the fast electron hypothesis

An extensive analysis is made of the theory of flare stars based on the ‘fast electron hypothesis’, in the light of the latest observational evidence. It is shown that an adequate agreement of theory with the observations obtains regarding the internal regular features in the flare amplitude data inUBV rays, as well as the changes of the colour characteristics of stars during the flares; in the latter case the analysis is made not only in respect of the UV Cet-type stars, but flare stars as well, forming a part of the Orion association. Problems bearing on the ‘negative flare’ and the screening effect are dealt with. New properties of the light curves of flares are revealed, based on the above theory.

High time-resolution observations of UV CET stars.

view Abstract Citations (47) References (12) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS High time-resolution observations of UV Ceti stars. Moffett, T. J. ; Bopp, B. W. Abstract Simultaneous, high time-resolution spectroscopic and photometric observations of eight flare events on four UV Ceti-type stars are presented. These observations firmly establish the existence of two phases during a flare: the spike phase, which is dominated by continuum emission, and the slow phase, which shows strong emission-line radiation with decreasing continuum radiation. The time sequence of spectral features during a flare is that the continuum reaches maximum strength first, followed 1 to 19 minutes later by maximum strength in the Balmer emission lines, and finally the peak of the Ca ii K-line strength some 4 to 52 minutes after the continuum peak. The flare phenomenon is spectroscopically complex, with no apparent simple relation between the various flare parameters. The emission lines produce a time-varying contribution to the broad-band color indices of flare light. Subject headings: stars: emission4ine - stars: flare Publication: The Astrophysical Journal Supplement Series Pub Date: May 1976 DOI: 10.1086/190374 Bibcode: 1976ApJS...31...61M full text sources ADS | data products SIMBAD (4)

Simultaneous radio and optical observations of UV Ceti-type flare stars

The UV Ceti-type stars, YZ Canis Minoris, AD Leonis, and Wolf 424 AB, were monitored for 57.8 hours from January 15 to February 22, 1974, using the 1000-foot (305-m) radio telescope at Arecibo, Puerto Rico, and the 91-cm or 76-cm reflector at McDonald Observatory. Radio observations were made at frequencies of 430, 318, and 196 MHz, and optical monitoring was done in the Johnson U or B band, the Stromgren u band, or white light. During the period of simultaneous observations, 62 optical flares were detected. A total of 13 RF flares were independently identified, of which 10 reached maximum flux within 10 min of the peak time of an optical flare. Using times of optical-flare events as reference data, an additional 15 cases of radio enhancements associated with optical flares are identified. The most probable delay interval between the optical-flare maximum and 318-MHz radio peak was found to be 0 to 5 min, with the optical flare occurring first. Aside from this statistical delay of peak occurrence and a general tendency for radio and optical flares to be associated, no systematic correlations of optical and radio amplitudes or morphology were found. The possible consequences of the radio emission's being due to a coherent process are discussed.

High-Speed Photometry of Cluster Flare Stars

Recent photoelectric observations made at Catania Observatory show a high occurrence of short-lived faint flares on the Hyades member II Tau (H II 2411). Its activity level turns out to be higher than for UV Cet-type stars of equal absolute luminosity.II Tau could represent a transition prototype between UV Cet-type flare star in the solar neighbourhood and cluster ones.The importance of coordinated photoelectric patrol of properly selected flare stars in clusters of different age is emphasized.

Flare Colours and Luminosities

Flare colours determined from simultaneous UBV observations made at Catania Observatory and from sequential UBV observations made at McDonald Observatory are presented. They fit fairly well with the theoretical colours computed according to the Gurzadian's (1970) non-thermal model. Only part of the observed flare colours are consistent with the solar type models by Gershberg (1967) and Kunkel (1970).From a B-band patrol of UV Cet-type stars carried out from 1967 to 1972, some quantitative estimates of flare frequencies and luminosities and of their average contribution to the stellar radiation are given.The corresponding parameters for the Sun, which were estimated from ‘white light’ flare activity, are also given for comparison. The Sun and V 1216 Sgr can be regarded as low-activity flare stars of the type found by Kunkel (1973).

Short-Duration Radio Flares of UV Ceti Stars

view Abstract Citations (32) References (9) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Short-Duration Radio Flares of UV Ceti Stars Spangler, Steven R. ; Shawhan, Stanley D. ; Rankin, John M. Abstract The UV Ceti-type stars YZ Canis Minoris, AD Leonis, Woll 424, and V371 Orionis were monitored with the Arecibo radio telescope at meter wavelengths. A total of 26 flares were recorded on YZ CMi, AD Leo, and Wolf 424. The observed flares had shorter durations and smaller amplitudes than those previously reported in the literature. Rates of radio-flare detection are similar to those found optically. Subject headings: flare stars - radio sources, variable Publication: The Astrophysical Journal Pub Date: June 1974 DOI: 10.1086/181523 Bibcode: 1974ApJ...190L.129S full text sources ADS | data products SIMBAD (6)

Some characteristics of the flare activity of UV Cet type stars. I

Some characteristics of the flare activity of UV Cet type stars. I

Some characteristics of the flare activity of UV Ceti type stars. II

Some characteristics of the flare activity of UV Ceti type stars. II

Report of the Working Group on Flare Stars

The Working Group was organized in order to promote and develop the cooperative studies of flare stars. Up to the present time our observation programmes were restricted to the UV Ceti-type stars only. Organizational activities followed the aims:(1) To extend the existing network of photoelectric observers in order to provide 24-hour coverage of a star during selected sessions lasting usually 14 days.(2) To collect data using standardized techniques in order to study frequency of stellar flares and secondary variations.(3) To provide optical coverage for simultaneous radio, spectroscopic and polarimetric observations.