Brief History of Investigation of Flare Red Dwarfs

In 1924, Hertzsprung noticed that one of the faint stars in Carina was brighter by 2m on one of the photographs taken on the night of January 29. The rate of brightness increase suggested that this star did not belong to novae or oscillating RR Lyrae-type stars. Thus, Hertzsprung decided that the effect could be produced by the fall of an asteroid on star. Apparently, it was the first recorded flare on stars of this type.

Studying the spectra of faint stars in Orion in December 1938, Wachmann (1939) found an unusual variable star with an abnormal spectrum. The spectrum was obtained using an objective prism, the spectra being discretely broadened in one direction at 0.02 mm every 6 min during an hour. During the first third of the exposure the stellar spectrum resembled that of a nucleus of a planetary nebula of the WR type: on a background of continuous radiation of the type of continuum of B or A stars one could see strong emission lines Hy, Eg, He, and Hz. Then, the brightness of the object decreased by at least one and a half stellar magnitudes, and in the band corresponding to the other part of the exposure mainly emission lines became visible. At the same time, the spectrum of the adjacent star displayed uniform darkening throughout its width. In the images obtained a month later, the star had a regular K spectrum without emission. Apparently, Wachmann was the first to record the spectrum of a flare on a UV Cet-type star.

In 1940, while examining the parallaxes of faint stars van Maanen (1940) noticed that the brightness of the M6e star Lalande 21258 (= WX UMa) was about 16m on more than 20 plates, but in two images obtained on 11 May 1939 with an interval of about half an hour it was 14"}2 and 14"}5 (Fig. 1). Several years later, van Maanen (1945) detected a similar phenomenon in measuring the parallax of Ross 882 (= YZ CMi). Reporting this fact, he noted that both variables had low luminosity, belonged to a late spectral class, and should be objects of the same type.

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