A Global Physical Model for Cepheids

Abstract

We perform a global fit to ∼5, 000 radial velocity and ∼177, 000 magnitude measurements in 29 photometric bands covering 0.3 µm to 8.0 µm 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 −1 . 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.

Cepheid distances from the SpectroPhoto-Interferometry of Pulsating Stars (SPIPS)

“…Using the total χ 2 would have given more importance to data in highest numbers. Contrary to the approach taken by Pejcha & Kochanek (2012), we did not fit the zero points of photometric systems, so we do not suffer degeneracy. After we found the best fit, we estimated the uncertainties in the derived parameters by using the covariance matrix around the best-fit solution.…”

The Nickel Mass Distribution of Normal Type II Supernovae

“…The model is based on the expanding photosphere method (e.g. Kirshner & Kwan 1974;Eastman et al 1996), a generalization of the Baade-Wesselink technique (Pejcha & Kochanek 2012). Global fitting of all observations removes manual procedures such as correction for reddening and construction of L bol , and replaces them with functions that are consistently and mechanically applied to all objects.…”

Forbidden Iron Lines and Dust Destruction in Supernova Remnant Shocks: The Case of N49 in the Large Magellanic Cloud

“…All data cubes are on the same wavelength grid in the data reduction process, so that no shift is required along that axis. The final mosaic has dimensions 98″ × 91″ on the sky, which for an assumed distance of the LMC of 49 kpc (Pejcha & Kochanek 2012) corresponds to a field of 23.5 × 21.8 pc.…”