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Session 87 - Star Parameters.
Oral session, Thursday, June 13
Chemically peculiar (CP) stars of the upper main sequence show very large deviations from the solar abundance pattern. Some extreme cases occur in the hotter CP stars, where the dominant ions are not used for the abundance determinations. Lanthanide rare earths in silicon stars are a case in point. The dominant, doubly ionized species have been identified in a number of objects, but thus far transition probabilities have been unavailable. We have recently attempted to remedy this situation with the help of the Cowan (Los Alamos 1995) atomic structure code. Bord, Barisciano, and Cowley (MN 278, 997, 1996) discuss calculations for La II; we report results for the isoelectronic Ce III spectrum, and apply them in a study of abundances in the cerium-rich star HD 200311 studied by Adelman (ApJS, 28, 51, 1974). We analyzed 5 Ce II and 3 Ce III lines. For Ce II, we used gf-values from Kurucz (CDROM18 1993). The wavelength region is \lambda\lambda4460---4563. Models are based on work by Leushin, Ryabchikova, and Topil'skaya (Sov.AJ, 36, 279, 1992), as well as uvby-\beta photometry (B. Smalley, private comm. 1996). Abundances from the Ce II lines give an standard error of only 0.2 dex. The standard error of the Ce III lines is 0.5 dex. Our most consistent results are for T_eff = 13500K and log(g) = 4.0--both Ce II and III give -5.4 for the logarithm of the cerium to total abundance. This is an excess of 5 orders of magnitude above solar! We thank S. J. Adelman for the use of his tracings of HD 200311, and R. D. Cowan for help with his code.
Program listing for Thursday