Poster Abstracts

Name/Affiliation:  George Wallerstein (University of washington)

Title:  The Behavior of the Paschen and Balmer Lines in Cepheids

In 1939 Struve, Wurm and Henyey recognized that the forbidden transition from the 2s to the 1s level in hydrogen may result in the over-population of the 2s level and would enhance the strength of the Balmer lines in stellar atmospheres. The effect should be greatest in low density atmospheres where collisional redistribution is least effective. In the same year Lyman Spitzer found that the Balmer lines in the M supergiant Alpha Orionis required an excitation temperature of 12,000 K though the effective temperature of the star was known to be 3400 K. Cepheid variables are high luminosity stars with low density atmospheres so departures from LTE in the population of the 2s level should be evident by comparing the profiles of the Balmer and Paschen absorption lines. Hence the modelling of a cepheid's atmosphere should be constrained by the line profiles as the pulsation waves move through. I have assembled velocity curves and line profiles for Delta Cep and 10 cepheids with periods between 10 and 20 days over their pulsation cycles. In my talk I will show the data and discus the problem of modelling dynamic atmospheres rather than just attaching a standard model with the appropriate Teff and logg and letting it rise and fall with the pulsation cycle.