THE CHROMOSPHERIC STRUCTURE OF CLASSICAL CEPHEIDS - CONSTRAINTS FROM THE HE I LAMBDA-10830 LINE

We study the heating and dynamics of Cepheid chromospheres utilizing the He I lambda10830 line, which due to its high excitation potential is not formed in the photospheres of such cool stars. We construct time-dependent models of the chromospheres of the Cepheids by coupling hydrodynamics with non-LTE radiative line transfer calculations for H, He, Ca II, and Mg II. We conclude that Cepheids have chromospheres heated by acoustic (or magnetic) wave dissipation in addition to transient heating by the pulsation dynamics. Judging by the height of formation of the He I lambda10830 line in Cepheid atmospheres, the existence of hot coronae around Cepheids seems unlikely. We use the Cepheid models to compute X-ray and EUV fluxes, and these are consistent with observed upper limits.