The Hydrogen Ionization Front-Stellar Photosphere Interaction and the Period-Color Relations of Variable Stars

Abstract

Recent evidence has emerged that the Cepheid PL relation in the LMC is nonlinear in the sense that the existing data are more consistent with two lines of differing slope with a break at a period of 10 days. We review the statistical evidence for this, the implications for the extra-galactic distance scale and CMB independent estimations of Hubble's constant and briefly outline one possible physical mechanism which could cause this nonlinearity.

5 pages, 3 figures, to appear in Cefalu 2007 Workshop XXI Century Challenges for Stellar Evolution, Eds. S. Cassisi and M. Salaris, Memorie della Societa Astronomica Italiana, Vol. 79