Spectroscopic observations of red dwarfs I. Flare stars in the Pleiades

Byurakan Astrophysical Observatory. Translated from Astrofizika, Vol. 33, No. 1, pp. 5–20, July–August, 1990.     

Variation of the Flare Frequency of Flare Stars. II. Flare Stars of the Pleiades Cluster

The possible variation of the flare frequencies of flare stars in the Pleiades cluster is considered. It is shown that of the 75 chosen stars that each exhibit five or more flares, 33 can be suspected of variation of flare frequency at the 0.1 significance level.     

Spectral classification of Pleiades flare stars

National Astrophysics, Optics, and Electronics Institute, Mexico, Byurakan Astrophysical Observatory. Translated from Astrofizika, Vol. 36, No. 4, pp. 501–515, October–December, 1993.     

Spectral observations of red dwarfs II. New observations of flare stars in the Pleiades cluster region

We studied the spectra of 24 flare stars of the Pleiades cluster region obtained on the 2.6m telescope of the Byurakan observatory in the years 1986–1989. From the spectral indices of titanium oxide (TiO) and other chemical elements of the stars investigated we determined their spectral classes and luminosities (absolute visual stellar magnitudes). The agreement of these moduli of the stars with the distance modulus of the Pleiades cluster confirms that 21 of them belong to the cluster. Only three of them (FSP 169, FSP 377, and FSP 435) are possibly not members of the cluster. This indicates that the flare activity of a star is a reliable criterion for its membership in a nearby stellar system (cluster or association).Translated fromAstrofizika, Vol. 37, No. 4, 1994.     

Flare stars in the taurus cloud region

In the Winters of 1975 and 1977, observations of flare stars in the Taurus Cloud region were made with the 40 cm double astrograph of the Purple Mountain Observatory. Two flare stars and one repeat outburst of FH Tau were identified. We give also a comparison between the flare stars of this region and the ϱ Oph region.     

Statistical investigation of flare stars. III. Flare stars in the general galactic star field

Byurakan Astrophysical Observatory. Translated from Astrofizika, Vol. 29, No. 3, pp. 531–540, November–December, 1988.     

Reconstruction of the Three-Dimensional Density Distribution of Flare Stars in the Pleiades

This paper deals with the determination of the three-dimensional distribution of flare stars in the Pleiades cluster. For this purpose a one-dimensional distribution is first constructed from the observed two-dimensional distribution of the stars. It is shown that reliable construction of one dimensioned distribution requires solution of the Abel equation. The last one used to determine the dependence of the true three-dimensional distribution of the density of flare stars on distance from the center of the cluster. This reveals a spherical layer of width 0.5 pc (2.8 R 3.5 pc) with a deficit in the number of flare stars. A profile of the three-dimensional density distribution of flare stars is constructed in the region of deficit. The characteristics of this region are described.     

Star formation in the Cepheus Flare region – II. A-type stars associated with IRAS sources

In order to reveal the star-forming history of the molecular cloud complex we studied the intermediate mass stellar population in the Cepheus Flare region. (Paper I dealt with the distance and the young stellar object candidates of the region.) Correlating the IRAS Point Source Catalogue and Faint Source Catalogue positions with those of 1214 B8–A8 and 1760 F0–F5 type stars brighter than     and classified during an objective prism survey, we identified 19 stars showing far-infrared excess emission in the Cepheus Flare region. In addition to the 16 stars whose counterparts are given in the IRAS catalogues, we found three more stars with infrared excess not recognized before. In order to identify the young medium-mass stars associated with the Cepheus Flare molecular clouds we observed the optical spectra of the IR-excess stars, and using published optical photometry and the IRAS data we examined their spectral energy distributions (SEDs) and IRAS two-colour diagram. The observations resulted in the discovery of a new Herbig Ae/Be star, BD +68°1118, coinciding with IRAS 21169+6842. More evolved HAe/Be stars may be SAO 19953, BD +67°1314 and BD +69°1231, whose H α lines showed weak emission components. Possible β Pictoris- or Vega-type stars may be HD 203854, HD 212826 and HD 216486, whereas the far-infrared fluxes at the positions of BD +72°1018, HD 210806 and HD 217903 can be attributed to the heating of the interstellar environment. We used distances and radial velocities of the stars derived from the spectroscopy and published optical photometry as indicators of their relations to the clouds. Information on the environment of the observed stars deduced from the diffuse interstellar band at λ 6613 is briefly 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.Particular emphasis is laid on the X-ray radiation from flare stars. It is shown that the observed spectrum of X-ray radiation of flare stars differs sharply from that of X-ray radiation both of the stellar corona and solar X-ray flares. At the same time, the observed X-ray spectrum of flares is in complete harmony with the previously calculated theoretical spectrum corresponding to nonthermal bremsstrahlung with the energy of monoenergetic fast electrons 1.5 MeV. The durations of X-ray flares should be essentially shorter than that of the optical flares. The very high momentary intensities of the X-ray brightness with the exceedingly small duration at the curve maximum is predicted. It is shown that the gamma-ray bursts recorded so far have no relation whatever to flare stars.     

Chromospheric activity in the late A- and early F-type stars of open clusters – II. Pleiades and Alpha Persei

We report observations of the He  i λ 5876 (D3) line in the late A- and early F-type stars in the Pleiades and Alpha Persei star clusters used to determine chromospheric activity levels. This represents the first sample of young stars in this temperature range with chromospheric activity measurements. We find the same average activity level in the young early F stars as in Hyades-age stars and field stars. In addition, the young star sample shows the same large star-to-star variation in activity as seen in the older stars. Thus, as a whole, chromospheric activity in this photospheric temperature range remains the same over nearly a factor of 100 in stellar age (50 Myr to 3 Gyr), in striking contrast to the behaviour of later-type stars. In the five late A stars we find three certain detections of D3 and one likely detection. This includes the bluest star yet observed with a chromospheric D3 line, Pleiades star HII 1362 at ( B − V )₀=0.22, making it one of the earliest stars with an observed chromosphere. The late A stars have D3 equivalent widths comparable to the weakest early F stars. However, when comparing D3 measurements in the young late A stars with older late A stars, we find evidence for a slight decrease in activity with age based on the large number of non-detections in the older stars. We find an apparently linear relationship between the activity upper limit and B − V over our entire range of B − V . Extrapolated blueward, this relationship predicts that the chromospheric D3 line would disappear for all stars at B − V ≈0.13.