A Global Physical Model for Cepheids
Abstract
We perform a global fit to ~5000 radial velocity and ~177, 000 magnitude measurements in 29 photometric bands covering 0.3 μm to 8.0 μm distributed among 287 Galactic, Large Magellanic Cloud, and Small Magellanic Cloud Cepheids with P > 10 days. We assume that the Cepheid light curves and radial velocities are fully characterized by distance, reddening, and timedependent radius and temperature variations. We construct phase curves of radius and temperature for periods between 10 and 100 days, which yield lightcurve templates for all our photometric bands and can be easily generalized to any additional band. With only four to six 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^{1}. The model derives the mean Cepheid spectral energy distribution and its derivative with respect to temperature, which deviate from a blackbody in agreement with metalline and molecular opacity effects. We determine a mean reddening law toward the Cepheids in our sample, which is not consistent with standard assumptions in either the optical or nearIR. Based on stellar atmosphere models, we predict the biases in distance, reddening, and temperature determinations due to the metallicity and 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 metalpoor 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.
 Publication:

The Astrophysical Journal
 Pub Date:
 April 2012
 DOI:
 10.1088/0004637X/748/2/107
 arXiv:
 arXiv:1112.3038
 Bibcode:
 2012ApJ...748..107P
 Keywords:

 distance scale;
 stars: abundances;
 stars: variables: Cepheids;
 Astrophysics  Solar and Stellar Astrophysics;
 Astrophysics  Cosmology and Extragalactic Astrophysics
 EPrint:
 30 pages, 21 figures, 10 tables. Accepted to ApJ. Light curve templates and a code to find the best fit period and reference time as well as accurate mean magnitudes are available at http://www.astronomy.ohiostate.edu/~pejcha/cepheids . For a brief video explaining the key results of this paper, see http://www.youtube.com/user/OSUAstronomy#p/u/0/eJdJmQbQ_nM