Theoretical Cepheid Period-Luminosity And Period-Color Relations In Spitzer IRAC Bands

TL;DR

This study investigates theoretical Cepheid period-luminosity and period-color relations in Spitzer IRAC bands, comparing models with observations to understand metallicity effects and improve distance measurements.

Contribution

It presents new synthetic P-L and P-C relations based on pulsation models with varying metallicity and helium, and compares them with recent empirical data.

Findings

Synthetic P-L relations agree with empirical data for specific model parameters.

Metallicity influences the slopes and intercepts of P-L relations.

Synthetic P-C relations show some discrepancies with observed Galactic relations.

Abstract

In this paper the synthetic period-luminosity (P-L) relations in Spitzer's IRAC bands, based on a series of theoretical pulsation models with varying metal and helium abundance, were investigated. Selected sets of these synthetic P-L relations were compared to the empirical IRAC band P-L relations recently determined from Galactic and Magellanic Clouds Cepheids. For the Galactic case, synthetic P-L relations from model sets with (Y = 0.26, Z = 0.01), (Y = 0.26, Z = 0.02) and (Y = 0.28, Z = 0.02) agree with the empirical Galactic P-L relations derived from the Hubble Space Telescope parallaxes. For Magellanic Cloud Cepheids, the synthetic P-L relations from model sets with (Y = 0.25, Z = 0.008) agree with both of the empirical Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC) P-L relations. Analysis of the synthetic P-L relations from all model sets suggested that the IRAC band P-L relations may not be independent of metallicity, as the P-L slopes and intercepts could be affected by the metallicity and/or helium abundance. We also derive the synthetic period color(P-C) relations in the IRAC bands. Non-vanishing synthetic P-C relations were found for certain combinations of IRAC band filters and metallicity. However, the synthetic P-C relations disagreed with the [3.6]-[8.0] P-C relation recently found for the Galactic Cepheids. The synthetic [3.6]-[4.5] P-C slope from (Y = 0.25, Z = 0.008) model set, on the other hand, is in excellent agreement to the empirical LMC P-C counterpart, if a period range of 1.0 < log(P) < 1.8 is adopted.