An Empirical Investigation of the Effect of Metallicity on Linear vs. Non-Linear Cepheid Period-Luminosity Relations D. Crain, G. Feiden, S. McCabe, R. Stevens, S. Scott, M. Schoonmaker, D. Wallace, S. Kanbur (Physics Department, SUNY Oswego) and C. Ngeow (Astronomy Department, UIUC) 209 th AAS Meeting, January 2007, Seattle, WA Abstract: In recent years, evidence has emerged that the Cepheid period-luminosity (PL) relation in the LMC may be non-linear with two distinct slopes for short (P 10 days) respectively. This non-linearity of the PL relation is more profound at certain phases. We launch an ongoing study to investigate possible non-linearity in the Cepheid PL relation due to metallicity effects. The first step is the accurate reconstruction of light curves. We present preliminary results for M31 and IC 1613 Cepheids. Introduction: Cepheids are intrinsically variable stars which vary in brightness with regular periods of the order of days. This variation in brightness is called a light curve. Their period of oscillation is related to their absolute brightness. This relation between period and absolute brightness (the PL relation) is the key to measure distances to other galaxies and hence the size of the Universe via the Hubble relation. Recent evidence has emerged that the LMC Cepheid PL relation (at mean light) is non- linear with two relations for short and long period Cepheids, respectively. This non-linearity is more profound at the phase near 0.8 (Ngeow & Kanbur 2006; See Fig 1 as well). Thus it is of great interest to see if other galaxies exhibit such a break at a phase near 0.8. We use Cepheids in M31 (the Andromeda galaxy) and IC 1613 to study such a break in the PL relation. M31 has a higher metallicity (comparable to our Galaxy), while IC 1613 is a metal-deficit dwarf galaxy in the local group. This permits the study of the metallicity dependence of this break in the PL relation. Figure 1: Plot of the LMC Cepheid PL relation at phase of ~0.8, where the maximum light has phase of zero. The data has been corrected for extinction before this plot is produced. This plot is adopted from Ngeow & Kanbur (2006). Analysis & Preliminary Results: In order to characterize the light curve, we fit a Fourier sum to the observed data points, where A 0 is the mean magnitudes, A k and φ k are the amplitudes and phases of the harmonics, respectively. For each star with data in V and I bands, we found the largest N such that the above expression made a good fit to the observed data points. Too large a value of N will introduce unphysical wiggles in the light curves. Too small a value for N will not be a realistic representation of the light curve. Preliminary results of the fits are presented in Fig 2 for M31 and Fig 3 for IC This will permit studying the PL relations in M31 and IC 1613 as a function of phase and whether a break like that depicted in Fig 1 for the LMC is present for other galaxies. These fits will be used to construct PC/PL relations as a function of phase and metallicity. phase magnitude Figure 2: Results for Cepheids in M31. The data points are taken from the DIRECT project Figure 3: Preliminary fits to Cepheids in IC The open and solid circles are data point from Udalski et al (2001) and Antonello et al (2006), respectively. References: Antonello et al 2006 A&A 445:901 Ngeow & Kanbur 2006 MNRAS 376:723 Udalski et al 2001 Acta Astron 51:221 Acknowledgements: SK would like to acknowledge the HST Grant HST-AR-10673