AAVSO: American Association of Variable Star Observers

The Correlation between Hα and HeI 6678 Emission Activity in the Be Star γ Cassiopeiae from 1995 to 2021

Volume 49 number 1 (2021)

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Ernst Pollmann
Observatory “Vereinigung der Sternfreunde Köln”, Germany; ernst-pollmann@t-online.de


The Be star γ Cas is among others well known for erratic rapid variations in its HeI emission at 6678 Å. Recently, this emission has become an important diagnostic feature to investigate the regions of disc activity close to the star. It was recognized that several factors—mass loss from the photosphere of the primary star, density variations, and collision excitations of the HeI atoms—force the emissions in HeI (at 6678 Å) to be restricted to about to 2.3 stellar radii from the center of the system. In contrast, the Hα emission may originate from anywhere in the disc. The different loci of these emissions have consequences for any potential correlation between them. Investigating such a phenomenon in this star does not require large telescopes and associated instrumentation, but does need an extended program of monitoring over many years. This is precisely the kind of program that is amenable to an amateur spectroscopic study. Thus, using data sets from long-term observation campaigns by an international consortium of amateurs over 15 years, the study presented here used equivalent width analysis to show that the emission activities of Hα and HeI 6678 are, indeed, strongly correlated.