Classical Cepheids: Chromospheric Heating and Formation of the Helium Spectrum

摘要

During the period covered by this report the results from our last paper on the ROSAT observations of zeta Gem (Sasselov & Sabbey 1994) became the basis for multi-wavelength comparison of energy distributions of Cepheid variables and nonvariable supergiants. The findings were that the non-radiative ultraviolet flux in the IUE data on nonvariable supergiants is nonexistent or much reduced in Cepheids. At the same time, the energy distributions at longer wavelengths are identical. The difference between X-ray fluxes of Cepheids and their non-variable counterparts, discovered by our ROSAT project is in the same sense and stronger. We discussed this new developments in a poster at the IAU Colloquium No.155 last February. Our plans are to understand the physical connection between the UV and X-ray flux estimates and differences, by extending the Cepheid atmosphere modelling from last year. During the period covered by this report, the PI was invited to make two presentations based on the findings from this project - a colloquium talk at the University of Toronto, Canada; and a poster paper at IAU Colloquium No. 155., on Astrophysical Applications of Stellar Pulsation in Cape Town, South Africa.