The Effect of Metallicity on Cepheid Magnitudes and the Distance to M33

We present the results from a multi‐epoch survey of two regions of M33 using the 3.5 m WIYN telescope. The inner field is located close to the center of the galaxy, with the outer region situated about 5.1 kpc away in the southern spiral arm, allowing us to sample a large metallicity range. We have data for 167 fundamental mode Cepheids in the two regions. The reddening‐free Wesenheit magnitude $Wvi$ period‐luminosity relations were used to establish the distance modulus of each region, with $μinner = 24.37±0.02 mag$ and $μouter = 24.54±0.03 mag.$ The apparent discrepancy between these two results can be explained by the significant metallicity gradient of the galaxy. We determine a value for the metallicity parameter of the Period‐Luminosity relation $γ = δ(m−M)0δ log Z = −0.29±0.11 mag dex−1,$ consistent with previous measurements. This leads to a metallicity corrected distance modulus to M33 of $μγ = 24.53±0.11 mag.$