A search for period changes of eight short-period Type II Cepheids

TL;DR

This paper analyzes over a century of observational data to detect and confirm period changes in eight short-period Type II Cepheids, aligning findings with stellar evolution models.

Contribution

It provides the first comprehensive analysis of period changes in short-period Type II Cepheids using extensive historical and modern data.

Findings

Detected both increasing and decreasing periods in the stars.

Period changes are consistent with theoretical stellar evolution models.

Period variations are confirmed to be real through stability tests.

Abstract

In this study, we investigate the period changes of eight short-period Type II Cepheids of the BL Her subtype, i.e., with periods in the 1-4 day range. The $O-C$ diagrams for these stars are constructed using all suitable observational data from ground and space surveys. This spans a time interval of over one century and includes digitized photographic plates as well as photometry from the literature. The $O-C$ diagrams show parabolic evolutionary trends, which indicate the presence of both increasing and decreasing periods for these eight short period stars. These period changes are in good agreement with the recent theoretical evolutionary framework and stellar evolution models for BL Her stars. The pulsation stability test proposed by Lombard and Koen also suggests that the changes in the periods are real.