Rebuilding the Cepheid Distance Scale. I. A Global Analysis of Cepheid Mean Magnitudes

Abstract

We develop a statistical method for using multicolor photometry to determine distances using Cepheid variables including the effects of temperature, extinction, and metallicity and apply it to UBVRIJHK photometry of 694 Cepheids in 17 galaxies. We derive homogeneous distance, extinction and uncertainty estimates for four models, starting from the standard extragalactic method and then adding the physical effects of temperature distributions, extinction distributions, requiring positive definite extinctions, and metallicity.While we find general agreement with published distances when we make similar systematic assumptions, there is a clear problem in the standard distances because they require Cepheids with negative extinctions, particularly in low metallicity galaxies, unless the mean LMC extinction exceeds E(B − V ) > ∼ 0.25. The problem can be explained by the physically expected metallicity dependence of the Cepheid distance scale, where metal-poor Cepheids are hotter and fainter than metal-rich Cepheids. For V and I we found that the mean magnitude change is −0.14 ± 0.14 mag/dex and the mean color change is 0.13 ± 0.04 mag/dex, with the change in color dominating the change in distance. The effect on Type Ia supernova estimates of the Hubble constant is dramatic because most were found in the metal poor galaxies with the bluest Cepheids. The Type Ia Multi-color Light Curve Shape (MLCS) method estimate for H 0 formally rises from 69 ± 8 km s −1 Mpc −1 to 80 ± 6 km s −1 Mpc −1 with the metallicity correction.

Final Results from theHubble Space TelescopeKey Project to Measure the Hubble Constant

“…A comparison of the V IH photometry for the inner and outer Ðelds is consistent with a metallicity sensitivity of the PL relations, but artiÐcial star tests in the inner Ðeld indicate that crowding is signiÐcant, and precludes an accurate determination of the magnitude of the e †ect. Other recent studies (e.g., Sasselov et al 1997 ;Kochanek 1997) conclude that a metallicity e †ect is extant, and all of the empirical studies agree on the sign, if not the magnitude of the e †ect. Considering all of the evidence currently available and the (still considerable) uncertainties, we therefore adopt c VI \ [0.2^0.2 mag dex~1, approximately the midrange of current empirical values, and correct our Cepheid distances accordingly.…”

THE TIP OF THE RED GIANT BRANCH DISTANCES TO TYPE Ia SUPERNOVA HOST GALAXIES. II. M66 AND M96 IN THE LEO I GROUP

“…Furthermore, Tully et al Tanvir et al (1995) found seven Cepheids from HST/WFPC2 F555W and F814W images and derived a distance modulus of (m − M) 0 = 30.32 ± 0.16. Later Cepheid estimates showed a significant spread, ranging from (m−M) 0 = 29.94 ± 0.13 (Willick & Batra 2001) to 30.42 ± 0.15 (Kochanek 1997). Surprisingly, Mould & Sakai (2009b) presented a much smaller TRGB distance estimate (m − M) 0 = 29.65 ± 0.28 derived from the HST images.…”

USING ULTRA LONG PERIOD CEPHEIDS TO EXTEND THE COSMIC DISTANCE LADDER TO 100 Mpc AND BEYOND

“…We overlay this relation (normalized to our data set) in Figure 6 for reference. If the I Zw 18 Cepheids are confirmed, it suggests a stronger correlation between PL offset and metallicity than is evident in lower period Cepheids (e.g., Kochanek 1997;Kennicutt et al 1998). However, we note that we do not take into account any reddening or DM errors in this analysis.…”