A Global Physical Model for Cepheids
  We perform a global fit to ~5,000 radial velocity and ~177,000 magnitude
measurements in 29 photometric bands covering 0.3 to 8.0 microns distributed
among 287 Galactic, LMC, and SMC Cepheids with P > 10 days. We assume that the
Cepheid light curves and radial velocities are fully characterized by distance,
reddening, and time-dependent radius and temperature variations. We construct
phase curves of radius and temperature for periods between 10 and 100 days,
which yield light curve templates for all our photometric bands and can be
easily generalized to any additional band. With only 4 to 6 parameters per
Cepheid, depending on the existence of velocity data and the amount of freedom
in the distance, the models have typical rms light and velocity curve residuals
of 0.05 mag and 3.5 km/s. The model derives the mean Cepheid spectral energy
distribution and its derivative with respect to temperature, which deviate from
a black body in agreement with metal-line and molecular opacity effects. We
determine a mean reddening law towards the Cepheids in our sample, which is not
consistent with standard assumptions in either the optical or near-IR. Based on
stellar atmosphere models we predict the biases in distance, reddening, and
temperature determinations due to the metallicity and we quantify the
metallicity signature expected for our fit residuals. The observed residuals as
a function of wavelength show clear differences between the individual
galaxies, which are compatible with these predictions. In particular, we find
that metal-poor Cepheids appear hotter. Finally, we provide a framework for
optimally selecting filters that yield the smallest overall errors in Cepheid
parameter determination, or filter combinations for suppressing or enhancing
the metallicity effects on distance determinations. We make our templates
publicly available.
