Outward from Cepheids

Nancy Remage Evans, Scott Engle, Edward Guinan, Hilding Neilson, Massimo Marengo, Lynn Matthews and H. Moritz Guenther

Pulsation in Cepheids is driven by a wave generated in the envelope of the supergiants. We have long had diagnostics to observe the wave as it passes through the photosphere and chromosphere. At minimum radius there are many signatures of disturbance by the wave passage including ultraviolet lines in emission and an increase in microturbulence. X-ray observations, however, provided a surprise. They have a low level of flux for most phases which persists unchanged through minimum radius. However, there is a burst of X-ray flux just after maximum radius, with a rapid rise, short duration and rapid fall similar to a stellar flare. This has been observed four times, including twice for δ Cep itself. The occurrence at a specific phase ties this to the pulsation process. This may be an upper atmosphere phenomenon (at about 0.13 stellar radii) which is related to circumstellar envelopes found via interferometry, at larger radii particularly in the infrared. These new diagnostics may ultimately answer questions about possible pulsation driven mass-loss.

Proceedings of the Polish Astronomical Society, vol. 6, 253-257 (2018)

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