Two new tests of the metallicity sensitivity of the cepheid period-luminosity relation (the leavitt law).
FREEDMAN W.L. and MADORE B.F.
Abstract (from CDS):
We undertake a new test of the metallicity sensitivity of the Leavitt Law for classical Cepheids. We derive an empirical calibration of the apparent luminosities of Cepheids as measured from the optical through the mid-infrared (0.45-8.0 µm) as a function of spectroscopic [Fe/H] abundances of individual Cepheids in the Large Magellanic Cloud (LMC) from Romaniello et al. The cumulative trend over the entire wavelength range shows a nearly monotonic behavior. The sense of the trend is consistent with differential line blanketing in the optical, leading to stars of high metallicity being fainter in the optical. This is followed by a reversal in the trend at longer wavelengths, with the crossover occurring near the K band at about 2.2 µm, consistent with a subsequent redistribution of energy resulting in a mild brightening of Cepheids (with increased metallicity) at mid-infrared wavelengths. This conclusion agrees with that of Romaniello et al. based on a differential comparison of the mean V- and K-band Leavitt Laws for the Galaxy, Small Magellanic Cloud, and LMC, but is opposite in sign to most other empirical tests of the sensitivity of Cepheid distances to mean [O/H] H II region abundances. We also search for a correlation of Cepheid host-galaxy metallicity with deviations of the galaxy's Cepheid distance from that predicted from a pure Hubble flow. Based on Cepheid distances to 26 nearby galaxies in the local flow, only a very weak signal is detected giving δµo= -0.17(± 0.31)([O/H] - 8.80) - 0.21(± 0.10). This is in agreement with previous determinations, but statistically inconclusive.