Very low luminosity stars with very large amplitude flares

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In 1976, M. Lovas noted an apparent 13.5 mag stellar image on a blue Schmidt plate. Two hours later, another Schmidt plate showed the object at 18.0 mag. This variable source (designated CZ Cnc) is usually fainter than 19-21 mag. I have examined Lovas's position for CZ Cnc and find a faint and extremely red star. The measured colors are V = 21.15, B-V= 1.86, V-R = 1.39, V-I = 3.86, V-J = 5.98, V-H = 6.59, and V-K = 6.92. I interpret this star as a very low luminosity main-sequence star which had a very large amplitude flare in 1976. The colors indicate an absolute magnitude of 16.1 and a distance of 100 pc. The flare amplitude is larger than 9.5 mag in the blue, as compared with the previous record flare amplitude of 7.7 mag. In addition, a possible counterpart of KY Cep is suggested, which if true would imply a flare with an amplitude of 16 mag. The existence of a class of very large amplitude flare stars poses grave difficulties for widefield gamma-ray burst optical-flash searches.

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Astrophysical Journal

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