«Flare Stars».

«Fuors».

The physics and the evolution of the flare stars

From the L.V.Mirzoyans book «V.A. Ambartsumian»

Proceeding from the particularities of the flare stars radiation, first of all from the appearance of a continuous emission in the radiation of these stars during the flares, Ambartsumian concluded in 1953 that the flare stars physical nature is related to T Taauri type stars. The discovery of the flare stars in the stellar systems — in the associations and the young star clusters by the Mexican astronomer Guillermo Haro confirmed this conclusion and indicated the evolutionary importance of the flare stars.

In 1968 Ambartsumian proved that the flare stars repsent one of the earliest stages of the evolution of the dwarf stars. He developed an original statistical method for the estimation of the full number of the flare stars in a physical system based on the flare stars observational data.

Using this method the scientist established that in the relatively young Pleiades star cluster (70 millions years old) there should be at least a few hundred flare stars. Then using the known total mass of the cluster and the mass of the bright non-flare stars of the cluster he determined the mass of the other stars of the cluster. This mass coincided with the estimation of the mass of the all possible flare stars obtained by the scientist. That proved that all the low luminosity stars of the cluster should be flare.

Taking into account that these stars form a physical system — a cluster, their joint formation does not leave doubts. Consequently, it follows that the ability to show flares is a typical feature of the stars on this stage of their evolution and this stage in its turn is a regular stage in the life of the dwarf stars. So it was established that a stage when a flare star is able to show flares from time to time is an evolutionary stage, one of the earliest in the evolution of the stars. This stage is a regular one for all the dwarf stars.

This conclusion important for the study of the stars evolutioninitiated regular and systematic photographic observations of the flare stars in the stellar associations and clusters mainly in the observatories of Asiago(Italy), Budapest, Byurakan and Tonantzintla (Mexico). These observation results fully confirmed Ambartsumian’ s conclusion and gave a valuable data on the stars evolution early stage.

Following these observations Ambartsumian studied the question of the genetic relation between the two early stages of the evolution — T Tauri type stars and the flare stars. He proved that the stage of a flare star follows the stage of T Tauri type and starts even before the termination of the last one. During this period of a star life the stages of T Tauri type and of a flare star are mutually overlapped and T Tauri stars along with the continuous and irregular changes of brightness show flare type changes.

Among Amabrtsumians flare systems studies the work on the function derivation of the average flare frequencies in a given system (based on observations of stellar flares) is remarkably original. Here the problem of the mentioned functions definition is reduced to a solution of an inverse problem with the help of the chronology of the flare stars discovery (the first flare) and the chronology of their flare nature confirmation (observations of the second flares). The new method was tested by Ambartsumian on the sample of the flare stars in the Pleiades cluster. The obtained function of the flare average frequencies distribution satisfactory repsents the observations of this most studied flare stars system and confirms their abundance in the cluster.

Ambartsumian pdicted the existence of the «fast» and «slow» flares with different properties discovered later and gave the explanation to the surprising phenomenon of the Fuor.

From L.V. Mirzoyan’ s book «Victor Ambartsumian» (« Виктор Амбарцумян», Ереван, 1985).

The following two papers on this topic that enter in «A Life in Astrophysics» (Allerton Press, 1998), were originally published in:

«Flare stars,» Inter.Astron.Union,Colloq.no.15, Bamberg,31 August-3 Sept. 1971, in New Directions and New Frontiers in Variable Star Research, Bamberg, pp. 98-108, 1971.

«Фуоры,»Астрофизика, vol. 7, no. 4, pp. 557-572, 1971.