AAS 205th Meeting, 9-13 January 2005
Session 34 Stellar Structure and Evolution
Oral, Monday, January 10, 2005, 10:00-11:30am, Royal Palm 4-6

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[34.02] Investigating the Break of the Cepheid Period-Luminosity Relation and its Implications

C. Ngeow, S. Kanbur (UMASS)

The Cepheid period-luminosity (PL) relation is a major component of the distance scale ladder. Recent work has strongly suggested that the LMC PL relation is broken at 10 days. The aim of this work is to further investigate the observed break as evidenced from LMC Cepheids and examine its implications for the distance scales, as well as for the stellar structure, pulsation and evolution. This work involved both data analysis of Cepheids in different metallicity environments (Galactic, LMC and SMC) to further characterize the PL break and the construction of stellar pulsation models to understand the physics causing the break in the PL relation.

The main reason that the LMC PL relation is broken because the LMC period-color (PC) relation is also broken, as the both of the PL and PC relations are connected via the PLC relation. We first examine the broken PL and PC relations with rigorous statistical test to the LMC Cepheids. The results from the F-test strongly suggest that the LMC PL and PC relations are indeed broken at 10 days. Similar test to the Galactic and SMC PC relations do not show such a break at 10 days. The working hypothesis for the physics behind the broken PC relation is that it is caused by the interaction of the hydrogen ionization front (HIF) with the photosphere at certain phases of pulsation (e.g., at maximum and/or minimum light) for some period ranges. For example, this interaction happens at maximum light for long period Galactic Cepheids, because the observed PC(max) relation is flat for these Cepheids. By using the Florida pulsation codes, which include 1-D recipes for time-dependent convection calculations, we confirmed that the flatness of PC(max) relation for the Galactic Cepheid is caused by the HIF-photosphere interaction. The LMC Cepheids also show similar flatness for the long period Cepheids at maximum light. However, the preliminary results from the LMC models suggest that the different behaviour around the phases of minimum light between the Galactic and LMC models might explain the linear and non-linear nature of the Galctic and LMC PC relations, respectively.

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