Multiwavelength Characteristics of Period-Luminosity Relations

TL;DR

This paper explains the wavelength-dependent behavior of the Cepheid Period-Luminosity relation, linking it to surface brightness sensitivities and calibrating it with observations across optical to mid-infrared wavelengths.

Contribution

It provides a physically motivated model for the wavelength dependence of the PL relation and calibrates the Period-Radius relation using observational data.

Findings

Slope of the Period-Radius relation is 0.724 +/- 0.006.

Wavelength dependence of scatter matches theoretical predictions.

The model applies to both Cepheids and RR Lyrae stars.

Abstract

We present a physically motivated explanation for the observed, monotonic increase in slope, and the simultaneous (and also monotonic) decrease in the width/scatter of the Leavitt Law (the Cepheid Period-Luminosity (PL) relation) as one systematically moves from the blue and visual into the near and mid-infared. We calibrate the wavelength-dependent, surface-brightness sensitivities to temperature using the observed slopes of PL relations from the optical through the mid-infrared, and test the calibration by comparing the theoretical predictions with direct observations of the wavelength dependence of the scatter in the Large Magellanic Cloud Cepheid PL relation. In doing so we find the slope of the Period-Radius (PR) relation is c = 0.724 +/- 0.006. Investigating the effect of differential reddening suggests that this value may be overestimated by as much as 10%; however the same slope of the PR relation fits the (very much unreddened) Cepheids in IC1613, albeit with lower precision. The discussion given is general, and also applies to RR Lyrae stars, which also show similarly increasing PL slopes and decreasing scatter with increasing wavelength.