UV Ceti Research Articles

Variations of the H -emission line during a large flare on UV Ceti

Simultaneous high-speed photometry and photoelectric scanner observations of the Hβ-line were obtained for five flare events, one a major flare, on UV Ceti on 1975 January 6. The relative increase in the intensity of the Hβ-line during the large flare was much greater than the relative continuum rise as measured both by the scanner and by broad-band photometric observations. In Hβ the flare lasted nearly 30 times as long as in the continuum. Peak intensity in the Hβ-line occurred later than the continuum maximum. The possibility of using emission line observations to detect flare activity on early spectral-type stars ( dK– dG) is discussed. Some speculations on the mechanism of flare production are indulged.

Ionization zones around flare stars

The formation of an H II zone around a UV Ceti flare star and its H..cap alpha.. recombination emission after a powerful flare are discussed. Such zones might be accessible to observation.

Thermal X-rays from stellar flares - Reevaluation of scaling from solar flares

view Abstract Citations (39) References (20) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Thermal X-rays from stellar flares: reevaluation of scaling from solar flares. Mullan, D. J. Abstract A simple method of estimating the ratio of thermal X-ray luminosity to optical luminosity in stellar flares is presented which is valid at times close to and following peak flare luminosity. It is proposed that at such times, the X-ray and optical luminosities are, respectively, the energy radiated by the plasma and the energy conducted out of the plasma. If the conductive energy directed downward toward the photosphere and lower chromosphere is regarded as the energy source for optical flares, then the luminosity ratio is equal to the ratio of conductive to radiative cooling times in the hot flare plasma. A self-consistent model for red-dwarf flares is discussed in which the decay time of the optical light curve is controlled by the time scale for thermal conduction from the hot flare plasma. The condition that the two times must be approximately equal leads to estimates of electron densities in flares, provided the flare temperatures are known; it is argued that stellar flare temperatures are 1 to 4 times higher than the mean temperature of solar flares. Numerical results are given for the luminosity ratio in flares of seven UV Ceti stars. It is concluded that if the solar luminosity ratio is used in attempting to predict thermal X-ray fluxes from stellar flares, the predicted X-ray flux accompanying an optical flare of given amplitude will provide only an upper limit on the actual thermal X-ray flux. Publication: The Astrophysical Journal Pub Date: July 1976 DOI: 10.1086/154492 Bibcode: 1976ApJ...207..289M Keywords: Energy Dissipation; High Temperature Plasmas; Light Curve; Solar Flares; Solar X-Rays; Stellar Flares; Stellar Luminosity; Convective Heat Transfer; Dwarf Stars; Numerical Analysis; Radiant Cooling; Stellar Models; Stellar Radiation; Stellar Temperature; X Ray Astronomy; Space Radiation full text sources ADS |

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)

A search for slowly varying radio continuum emission from UV Ceti stars

The paper presents results of a search conducted at the Arecibo Observatory for variable 430-MHz emission from the active flare stars EQ Peg, YZ CMi, and AD Leo. No statistically significant evidence for slowly varying emission is found to a level of 0.05 Jy for AD Leo, 0.018-0.021 Jy for YZ CMi, and 0.009-0.018 Jy for EQ Peg. Upper limits of about 10 trillion K are determined for the brightness temperature of active radio regions on these stars at the time of the observations.

UV Ceti stars - Statistical analysis of observational data

UV Ceti stars - Statistical analysis of observational data

Evidence for X-ray emission from flare stars observed by ANS

Observations that detected the first X-ray emission from flare stars are described. An X-ray flare was detected from YZ CMi at 0.28 keV and approximately 1-7 keV, although no optical or radio coverage was available. During a very large optical flare from UV Ceti, X-ray emission at (only) 0.28 keV was detected. Upper limits for X-ray emission from several small optical flares of UV Ceti are presented. Implications for X-ray flare models, the diffuse X-ray background, and low-energy cosmic-ray flux are mentioned.

Comparison of UV Ceti Flares with Solar Flares

During several intervals totalling 26 h, 84 flares of UV Ceti were observed using a photometer with 1 s time resolution. Using solar flare terminology, all flares of UV Ceti observed here can be characterized by a typical light curve, consisting of a preflare, a flash and a slow phase. Observations of Bopp and Moffett (1973) show that the flash phase of UV Ceti is dominated by continuous and Hα emission. UV Ceti flare flash phases are compared with solar ‘white-light’ emission during the flash phase and it is suggested that during all UV Ceti flares strong-particle emission occurs, as is the case for solar white-light flares (Švestka, 1970). The characteristics of the solar flare particles are similar to those of the soft cosmic ray sources (Mogro-Campero and Simpson, 1972). If all flare stars emit particles with these characteristics they could produce the soft cosmic ray component in agreement with the work of Comstock (1969).

On Variations in the Flare Activity of V 1054 Oph

Four years ago I examined the flare incidence data then available for UV Ceti and V1054 OPH (Wolf 630) and concluded that no evidence could be found for periodic or secular variation in the levels of activity (Kunkel, 1971). Variations greater than 0.3 mag. (rms) should have been found.

The So-Called Antiflare Stars

There seems to exist a small subgroup of irregular variables with light curves similar to R CrB stars but with a shorter time scale. These stars have small-amplitude light variations around a ‘normal brightness’, but this is interrupted by Algol-like minima of about to at irregular intervals. The duration of the minimum phase is several percent of the total time. While at minimum the variables also show rapid light variation of greater amplitude. Moreover, RZ Psc shows a flare activity, which strongly resembles that of UV Cet stars. The minimum phase is followed by a rapid recovery to the ‘normal brightness’.

X rays predicted from flares of UV Ceti stars

KNOWN UV Ceti flare stars are all located within 20 pc of the Sun, and their distribution seems to be isotropic. They are cool, low-luminosity objects with masses less than 0.5 M⊙. Both optical1 and radio2 radiations have been observed during flare events. X rays have also been predicted3,4, but no positive observations have yet been reported. We present here new estimates of X-ray emission, based on the ratio between X-ray and radio flux from solar bursts. It is anticipated that the intensities of the X-ray flares will be more than 10 photons cm−2s−1; within the range of sensitivities of present satellite detectors (approximately 0.5–0.01 photon cm−2 s−1 for a flare duration of 3 min in the energy range 1–10 keV).

UV Ceti flare stars - Observational data

view Abstract Citations (169) References (14) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS UV Ceti flare stars: observational data. Moffett, T. J. Abstract Photometric observations of UV Ceti flare stars during the period 1971-1972 are presented. Most observations were obtained using integration times of 1 sec or less. Four-hundred and nine flares were detected during 469.2 hours of flare monitoring. The mean (B - V) of flare light for 77 flares was found to be +0.34 plus or minus 0.44, and the mean (U - B) for 153 flares was -0.88 plus or minus 0.31. A discussion of the usefulness and limitation of various flare parameters is presented. Publication: The Astrophysical Journal Supplement Series Pub Date: December 1974 DOI: 10.1086/190330 Bibcode: 1974ApJS...29....1M Keywords: Astronomical Catalogs; Stellar Mass Ejection; Stellar Spectrophotometry; Variable Stars; Astronomical Photometry; Data Reduction; Error Analysis; Flares; Tables (Data); Astronomy full text sources ADS | data products SIMBAD (1)

Large flare on the red dwarf star UV Ceti

ATTEMPTS to measure radio emission occurring simultaneously with the sporadic optical flares on the red dwarf dMe type stars have been hindered by the rarity of large flares. Apart from one case of a flare with an unusually long duration and a peak magnitude of 1.7 (ref. 1) the correlation so far established has been limited to flares of about 1 mag or less2,3. On October 11, 1972 we were able to obtain simultaneous radio and optical recordings of an unusually large flare on the star UV Ceti (L726–8AB), during which the apparent magnitude of the star increased by more than 4.55 mag.

Model for a flare on the star UV Ceti

ON October 11, 1972 a large flare occurred on the star UV Ceti. It was observed simultaneously at photoelectric frequencies and at the radio frequency of 408 MHz (ref. 1). These observations provide enough information to describe a reasonably good model of the outer layers of the star, and of the overall mechanics of the flare.

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)

Emission-Line Variability in Quiescent Flare Stars

Spectroscopic monitoring of five flare stars (UV Ceti, YZ CMi, BY Dra, YY Gem, and BD + 48 deg 1958A) reveals that the stars exhibit marked decreases in the Balmer and Ca II emission line strengths over intervals of one day or less. The variations can be explained by a nonuniform, patchy distribution of emitting regions on a rotating star. This is consistent with the starspot model from which in some other cases stellar rotation periods were inferred. Equatorial rotational velocities inferred are significantly greater than might be expected for ordinary stars at the same positions in the H-R diagram. (auth)

The nature of the flare radiation of UV Ceti stars

The nature of the flare radiation of UV Ceti stars

High time resolution studies of UV Cet.

High time resolution studies of UV Cet.

Wolf 424: A Neglected Flare Star

Four-color high-speed photometric observations of the UV Ceti type flare star Wolf 424AB are presented. This system may be one of the most active flare stars known. During a period of 2.62 hr 12 flares were detected and flare activity accounted for approximately 8% of the total energy received from the system in the U-bandpass. The observations of the largest flare are compared with the predictions of the fast-electron flare model proposed by Gurzadyan (Astrofiz.; 4: 154(1968)) which is remarkably successful though some discrepancies exist. The basic hypothesis of the fast-electron model is questioned. (auth)

TWO FLARE STARS BEHIND THE PLEIADES

ABSTRACT The flare stars H11 230 and H11 1069, both nonmembers of the Pleiades, were observed at 200 A rum-1. These stars are well behind the Pleiades and do not show Ca II or hydrogen emission. The spectral type of H11 230 is earlier than that of any other known UV Ceti-type star. Key words: UV Ceti stars - spectra - Pleiades