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Astron. Astrophys. 321, 549-556 (1997)

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1. Introduction

Many of our general conceptions concerning the analogy between flares on the Sun and the UV Cet-type stars (active red dwarf stars) derive from Gershberg & Pikel'ner (1972). When an impulsive flare on the Sun is considered, it is necessary to keep in mind the hard ([FORMULA]) X-ray burst. Several spikes can be observed during [FORMULA]. As a rule, a single short spike is an event, which occurs in a low-lying ([FORMULA]) loop. The total duration of a hard X-ray burst is controlled by the development time of the flare process as a whole in the system of coronal loops.

The hard X-ray radiation is due to the bremsstrahlung emission of accelerated electrons. These particles and the heat fluxes which arise in the region of primary energy release affect the chromosphere. The response of the solar chromosphere to the impulsive heating, the so called secondary process in a flare, was computed by Kostyuk & Pikel'ner (1974), where the importance of gas-dynamic motions was demonstrated for the first time. Later, a number of scientific groups have carried out analogous computations for the Sun (Kopp et al. 1989, and, in particular, Fisher et al. 1985). The main results of this modelling were confirmed by many solar observations.

Direct observations of the hard X-ray flare emission on red dwarf stars are in their infancy. However, our knowledge of, and the observational data on, stellar flares, makes it possible to assume that impulsive heating of the upper chromosphere takes place also during short events on red dwarf stars. This heating can be caused by electron beams as well as other factors, and it can lead to a development of secondary processes analogous to the solar ones. The first numerical simulations of a flare on a red dwarf star was carried out by Livshits et al. (1981) and Katsova et al. (1981). There, it was shown that the optical continuum emission sometimes accompanies the gas-dynamic response of impulsive heating of the chromosphere.

We consider impulsive flares identified with optical continuum bursts. The shortest stellar flares can be the result of a single heating by conduction or by the impact of accelerated electrons onto the chromosphere. This process is referred to as an 'elementary event' which lasts from a few tenths of a second to ten seconds. The real impulsive flare can continue longer, about 100 s and sometimes more, and can be considered as a set of such elementary bursts. In this case the process can envelop several low-lying loops. This is an important point in the interpretation of observational data on stellar flares.

It should be noted that Cheng & Pallavicini (1991) have carried out a series of computations of the response of a stellar chromosphere to heating in the top of a magnetically confined loop. This case of prolonged (a few hundreds of seconds) heating, located in coronal layers, describes well the important physical processes in the corona. These authors calculated the expected X-ray emission and compared it with the X-ray flare light curves obtained with EXOSAT. Trying to interpret the X-ray flare data, Cheng & Pallavicini were restricted to only a qualitative consideration of physical processes in the chromospheric layers close to the footpoints of the loop. Unfortunately, they didn't develop their modelling as applied to chromospheric layers or to an interpretation of optical radiation of stellar flares.

Therefore, we propose a gas-dynamic model for an impulsive flare which is able to interpret data not only in the soft X-ray, but also in the UV range, optical line and continuum emission.

We will discuss here the results of some new numerical modelling of an elementary flare event, when it occurs in the outer atmosphere of the red dwarf star AD Leo. In addition we consider the total effect from the influence of a number of such bursts, and some features of the behaviour of flaring radiation over a wide range of wavelengths.

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© European Southern Observatory (ESO) 1997

Online publication: June 30, 1998