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The nature of flare activity on dMc stars (red dwarfs with strong chromospheric IIα emission lines) has been the subject of many studies. Some years ago Lacy et al. (1976) demonstrated a relationship (see also Doyle et al., 1986) between mean flare power and quiescent luminosity, in the photometric U-band. This study was extended, independently, by Skumanich (1985, 1986) and Doyle and Butler (1985) to show that the time averaged U-band power-loss due to flaring is linearly related to a star's quiescent X-ray luminosity. Skumanich also showed an inverse relationship between a star's flaring-rate and its quiescent X-ray luminosity.
We present an overview of recent observations of stellar X-ray flares obtained with the EXOSAT Observatory. We discuss a few examples of flares from M dwarf flare stars, from RS CVn and Algol-type binaries, from single late-type stars (including a G0 dwarf and an A-type visual binary), and from pre-main-sequence objects. We also draw some general conclusions from the pieliminary analysis of the EXOSAT data sample.
Simultaneous optical spectroscopy and X-ray monitoring of stellar and solar flares shows that a well-defined linear correlation exists between the integrated Hγ and soft X-ray flux that extends over four orders of magnitude. The existence of this relationship implies a direct proportionality between the emission from the cooler and denser regions (T ≈ 104K) responsible for Balmer lines and the emission from the hot plasma (T ≈ 107K) responsible for soft X-rays. The consequences are considered for (a) several models which have been proposed for solar flares, and (b) the suggestion that the Balmer emission results from irradiation by soft X-rays.
EXOSAT observed the flare star EV Lacertae for 17 hr over 2 days in October 1985. Two flaring episodes were recorded. During a significant fraction of these observations, IUE, photometric and spectroscopic coverage was available. A 2-hour long period of X-ray flaring was observed during which there was no U-band activity and almost no chromospheric activity. On the other hand, two ΔU ~ 1° m° 5 optical flares produced normal chromospheric enhancements, but only a weak X-ray response. We suggest that these and a few other observations of stellar flares may imply the occasional existence of magnetically isolated regions in M-dwarf atmospheres.
The Danish experiment WATCH (Wide Angle Telescope for Cosmic Hard X-rays) is to be flown on board the Soviet satellite GRANAT in middle of 1989. The performence characteristics of the WATCH instrument is described. It is estimated that WATCH can detect about 100 solar hard X-ray bursts per day. WATCH can also detect about 40 energetic stellar soft X-ray flares, similar to the fast transient X-ray emissions detected by the Ariel V satellite.
Radio emission from dMe flare stars has both a flaring and a quiescent component. When we compare stellar radio emission with the Sun, however, we find that the apparent brightness temperature of the quiescent component often exceeds the temperature of non-thermal solar radio flares, and so it is likely that stellar quiescent emission also comes from non-thermal electrons. The duration of stellar quiescent emission is much longer than solar non-thermal emission. Obvious questions to ask are, what is the source of the non-thermal electrons, where do they reside, and how can non-thermal emission last so long? Here we briefly review the observations of quiescent emission, argue that the emitting regions are small, show that such small regions can still account for the observed fluxes, and discuss the source of electrons.
We describe high-sensitivity VLA observations of rapidly varying radio emission (‘flares’) from two stars of very different types, one of which (λ And) is a Long-Period RS CVn system, and the other (HR 5942) is a magnetic Bp star. In both cases, however, the physical mechanism producing the radio emission is most likely to be gyrosynchrotron radiation from mildly relativistic, power-law electrons.
Ultraviolet continua observed in IUE spectra of dMc stars in a flaring state are compared with those in solar Hares. There is evidence that, as may be the case with solar flares, stellar-flare continua in particularly the λ < 1683 Å region are due to free-bound transitions in neutral silicon excited by ultraviolet lino emission.
Photometric and spectroscopic observations of a very large flare on AD Leo are presented. A self consistent model of a flare corona, transition region and chromosphere is developed; in particular the chromospheric temperature distributions resulting from X-ray and EUV irradiation by coronae of various temperatures are determined. The predicted line fluxes in Hγ are compared to the observed line fluxes to find the coronal temperature as a function of time during the flare. This run of temperature with time is then compared with the predictions of an independent theoretical flare model based on a dynamic scaling law (see paper by Fisher and Hawley, these proceedings).
We report on the first successful coordinated observations of stellar flares carried out on March 28, 1984 simultaneously over a wide range of wavelengths, from UV to microwaves, using the IUE satellite, three ESO telescopes at La Silla (Chile) and the VLA at Socorro (NM, USA).
We present spectral diagnostics for the fluxes of emission lines, in the spectral range 3600-4400 Å, during the cooling phase of stellar flares on dMe stars. Using these diagnostics, electron temperatures have been computed for flares on AD Leonis, Proxima Centauri and UV Ceti. This preliminary model assumes a single flare loop containing a homogeneous, stationary optically-thin flare plasma.
We report a simultaneous IUE, VLA and ground-based photometric observation of a flare on the dMe star, AD Leo, on 2nd February 1903. The optical flare was extremely impulsive, lasting in total only about 3 mins. A relatively longlived 6 cm flare was observed with the VLA which was initially 100% polarized. An IUE spectrum, taken ≈ 8 min after the onset of the optical U band flare, shows motu than a factor of 2 increase in the Ha II λ 1640Å emission line. The other mid-transition region lines such as C IV λλ154B/52Å show almost no response.
While monitoring the flare star EV Lac with high time resolution using the Space Astrophysical Station ASTRON, a rather strong flare was recorded. During this event, flare emissions were detected in the C IV (λ1550 Å) UV line, in the narrow band continuum at λ2434 Å (28 Å bandwidth) and in the wide wavelength range from 1700 Å to 6500 Å, all emission enhancements taking place within 10 s. About 50 s after the flare start, a fast and very powerful burst of the C IV line took place.
Flare optical continuum with duration above one minute is emitted by a gas condensation with T ≈ 104 K, which may form in a gas-dynamic process. The light carve slope implies that this flare consists of several elementary events. The features of the initial C IV line burst during one elementary event are determined by numerical simulation. The comparison of theoretical intensity and duration of the C IV burst with the observations of the EV Lac flare on February 6, 1986 shows that the observed elementary event in the C IV line is consistent with the formation of a radiative shock wave with an explosive evaporation of the chromosphere. The possibility of the appearance of C IV doublet emission, accompanying the entire process, is also discussed.
Observations are reported of the Ca II resonance lines and II α in dK and dM stars, made with high S/N ratio and high spectral resolution. Ca II emission is found in all stars observed, and those having weak Ca II exhibit marked Hα absorption. It is found that the strengths of the two kinds of chromospheric, lines are not tightly correlated, an effect which can be shown to be independent of the effective temperature of the stars. The result implies that a one-parameter description (e.g. heating rate) of the chromospheres is not viable. While lateral inhomogeneities are likely to be an important second parameter, we also suggest that the Hα line may be formed in a region considerable higher that in which the Ca II lines are formed.
It has been shown for solar flares (Donati-Falchi et al. 1985) that the continuum emission at the Balmer discontinuity (the blue “pseudo-continuum”) is a very sensitive tool to determine the electron density. In order to use the same interpretative scheme for stellar flares (Falchi et al. 1988), spectroscopic observations of various flare stars have been performed in June 1987 at ESO observatory (Chile). In this paper we report the analysis of spectra of the star n. 729 (V 1216 Sgr) of the Gliese catalog.
High time resolution, moderate spectral resolution spectra were taken during the decay phase of a ΔU > 3 magnitude flare on the dM5.5e star Wolf 424. This flare shows intense, broad Balmer line emission with extended wings, narrow Call lines and numerous weak emission lines from neutral and singly ionized metals. The time history shows substantial, short-lived enhancements in the line emission. These variations are not always seen in association with continuum enhancements.
The spectra of a flare on YZ CMi, obtained with a temporal resolution of 60 seconds on March 4 1985 and over the range 3600-4400 A, are analysed using a gas-dynamic model. In this model, the optical radiation in the U-band, outside of flare maximum, is produced by a condensation formed during the gas-dynamic process. With the optical continuum described by a Planck function for a temperature of T ≈ 104K, the emitting source area S ≥ 5 10l7cm2. The hydrogen plasma kinetics of an “8 levels plus continuum” model atom are calculated, and it is shown that the low slope of the Balmer decrement, just after the flare maximum, is connected with a large population in the second level of the hydrogen atom.
Relative energies are given for the U, B, V, R and I bands for a-3.8 magnitude U-band flare observed on the dwarf dMe star Gl 234 AB on 28 Feb 1985. This flare had a 45 second rise time and 20 minute decay time. The total flare energy from all five bands during the flare was 7 1031 erg, 34% of this total was from the U-band and 20% from the two near infrared R and I bands. The energy density (per frequency interval) implied a rising continuum towards the red, however this only lasted for approximately 20-40 seconds, i.e. during the impulsive phase, after-which the excess flare emission could not be detected in tlie near infrared bands. Of the various models fitted to the flare data (i.e. optical synchrotron, bound-free emission and free-free emission), bound-free emission seems the most promising.
A 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.