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

We study the heating and dynamics of Cepheid chromospheres utilizing t he He I lambda10830 line, which due to its high excitation potential i s not formed in the photospheres of such cool stars. We construct time -dependent models of the chromospheres of the Cepheids by coupling hyd rodynamics with non-LTE radiative line transfer calculations for H, He , Ca II, and Mg II. We conclude that Cepheids have chromospheres heate d 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 observe d upper limits.