Astron. Astrophys. 317, L17-L20 (1997)

3. The Shell Phase of Cas

Be stars were discovered when Secchi (1867) noted visually that H was in emission in the spectrum of Cas (B0IVe). Astronomers have observed Cas regularly since that time, initially with the unaided eye and then later with both photometric and spectroscopic instruments.

Edwards (1944) assembled observations of Cas obtained over a time interval of about 100 years. He concluded that any magnitude changes between 1830 and 1930 were generally small, probably less than . His claim that these variations were periodic, however, does not seem to be very convincing. Photoelectric measures of the brightness of Cas between 1931 and 1939, which include the period when it brightened to magnitude , are given by Huffer (1939). Following this variation in the 1930s, there was little change in magnitude until the late 1950s; a slow increase of about 0.3 has occurred between that time and 1970 (Cowley et al. 1976). In summary, the magnitude of Cas was approximately constant, varying only by a small amount from 1830 to about 1931. There followed a short period when it brightened by about , faded back to its preoutburst level, brightened again by a few tenths of a magnitude, then decreased somewhat and has not varied significantly since that time.

Greaves and Martin (1938) determined temperatures from the optical continuum of 16,100 K in 1926.7, 10,500 K in 1936.9 and 9200 K in 1937.7. Edwards (1956) discussed spectral changes between 1929 and 1942, and noted in particular that the colour temperature, obtained from the slope of the Paschen continuum, had decreased to 8500 K in 1937. Cowley and Marlborough (1968) showed changes in the spectrum over the period 1911 to 1966. Specifically their spectra show the two distinct shell phases of Cas, between October 1935 and August 1936, and between June 1939 and October 1940, as indicated by the presence of He I 3889. All the variations between 1933 and 1942 are described in greater detail in Underhill and Doazan (1982). This phase of variability certainly appears to be rare. Only one example of it has occurred in the 165 years of recorded observations of Cas.

© European Southern Observatory (ESO) 1997

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