A study of the stellar photosphere-hydrogen ionisation front interaction in pulsating variables using period-color relations

TL;DR

This paper investigates the interaction between the stellar photosphere and hydrogen ionization front in various pulsating variable stars using period-color relations, extending previous studies to include type II Cepheids and supporting the theory with theoretical models.

Contribution

It extends the HIF-photosphere interaction study to type II Cepheids and uses MESA models to support the theoretical framework across multiple variable star types.

Findings

W Vir and BL Her stars show PC relations similar to classical Cepheids.

Theoretical models align with the HIF-photosphere interaction theory.

Different PC behaviors at light phases are explained by star location on the HR diagram.

Abstract

Period-color (PC) relations may be used to study the interaction of the stellar photosphere and the hydrogen ionization front (HIF). RR Lyraes (RRLs) and long period classical Cepheids (P > 10d) have been found to exhibit different PC behavior at minimum and maximum light which can be explained by the HIF-photosphere interaction based on their location on the HR diagram. In this work, we extend the study to include type II Cepheids (T2Cs) with an aim to test the HIF-photosphere interaction theory across a broad spectrum of variable star types. We find W Vir stars and BL Her stars to have similar PC relations as those from long period and short period classical Cepheids, respectively. We also use MESA to compute RRL, BL Her and classical Cepheid models to study the theoretical HIF-photosphere distance and find the results to be fairly consistent with the HIF-photosphere interaction theory.