On the separation between RR Lyrae and Type II Cepheids and their importance for distance determination: the case of $\omega$ Cen

TL;DR

This study investigates the separation criteria between RR Lyrae and Type II Cepheids in  Cen, demonstrating their similar period-luminosity relations in the NIR and proposing combined use for distance measurements.

Contribution

It challenges the traditional period threshold for classifying RRLs and T2Cs, and shows their unified PL relation in NIR, supporting their joint use in distance calibration.

Findings

Classical period threshold at 1 day is not universal.

RRLs and T2Cs obey the same NIR PL relations.

Derived a distance modulus of 13.65 mag for  Cen.

Abstract

The separation between RR Lyrae (RRLs) and Type II Cepheid (T2Cs) variables based on their period is debated. Both types of variable stars are distance indicators and we aim to promote the use of T2Cs as distance indicators in synergy with RRLs. We adopted new and existing optical and Near-Infrared (NIR) photometry of \wcen~to investigate several diagnostics (colour-magnitude diagram, Bailey diagram, Fourier decomposition of the light curve, amplitude ratios) for their empirical separation. We found that the classical period threshold at 1 day is not universal and does not dictate the evolutionary stage: V92 has a period of 1.3 days but is likely to be still in its core Helium-burning phase, typical of RRLs. We also derived NIR Period-Luminosity relations and found a distance modulus of 13.65$\pm$0.07 (err.)$\pm$0.01 ($\sigma$) mag, in agreement with the recent literature. We also found that RRLs and T2Cs obey the same PL relations in the NIR. This equivalence gives the opportunity to adopt RRLs+T2Cs as an alternative to classical Cepheids to calibrate the extragalactic distance scale.