Synthetic properties of bright metal-poor variables. II. BL Her stars

TL;DR

This study models BL Her stars, a type of Population II Cepheids, to derive their Period-Magnitude and Period-Wesenheit relations, confirming their consistency with observed data and RR Lyrae-based distances.

Contribution

It provides updated pulsation and evolutionary models for BL Her stars, deriving their PM and PW relations across multiple bands and validating these against observed and RR Lyrae distances.

Findings

Predicted PM and PW relations match observed slopes in globular clusters.

Distances from models agree with RR Lyrae-based distances within errors.

No clear change in relations around P~10 days for W Vir stars.

Abstract

We investigate the properties of the so-called BL Her stars, i.e., Population II Cepheids with periods shorter than 8 days, using updated pulsation models and evolutionary tracks computed adopting a metal abundance in the range of Z=0.0001 to Z=0.004. We derive the predicted Period-Magnitude (PM) and Period-Wesenheit (PW) relations at the various photometric bands and we show that the slopes of these relations are in good agreement with the slopes determined by observed variables in Galactic globular clusters, independently of the adopted M_V(RR)-[Fe/H] relation to get the cluster RR Lyrae-based distance. Moreover, we show that also the distances provided by the predicted PM and PW relations for BL Her stars agree within the errors with the RR Lyrae based values. The use of the predicted relations with W Vir stars, which are Population II Cepheids with periods longer than 8 days, provides no clear evidence for or against a change in the PM and PW slopes around P~10 days.