A multi-wavelength analysis of BL Her stars: Models versus Observations

TL;DR

This paper develops theoretical period-luminosity and period-radius relations for BL Her stars across multiple wavelengths, comparing models with observations, and explores their implications for distance scale calibration.

Contribution

It provides new multi-wavelength theoretical relations for BL Her stars using detailed models and examines their agreement with empirical data, also suggesting combined PL relations for distance measurements.

Findings

Theoretical relations match well with most empirical data.

No significant metallicity effects on PR relations.

Supports using combined PL relations of RR Lyrae and Type II Cepheids for distance calibration.

Abstract

We present new theoretical period--luminosity (PL) and period--radius (PR) relations at multiple wavelengths (Johnson--Cousins--Glass and {\sl Gaia} passbands) for a fine grid of BL~Herculis models computed using {\sc mesa-rsp}. The non-linear models were computed for periods typical of BL~Her stars, i.e. $1\leq P ({\rm days}) \leq4$, covering a wide range of input parameters: metallicity ($-$2.0 dex $\leq$ [Fe/H] $\leq$ 0.0 dex), stellar mass (0.5--0.8 M$_{\odot}$), luminosity (50--300 L$_{\odot}$) and effective temperature (full extent of the instability strip; in steps of 50K). We investigate the impact of four sets of convection parameters on multi-wavelength properties. Most empirical relations match well with theoretical relations from the BL~Her models computed using the four sets of convection parameters. No significant metallicity effects are seen in the PR relations. Another important result from our grid of BL~Her models is that it supports combining PL relations of RR Lyrae and Type~II Cepheids together as an alternative to classical Cepheids for the extragalactic distance scale calibration.