Separation between RR Lyrae and type II Cepheids and their importance for a distance determination: the case of omega Cen^⋆

¹ INAF-Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone, Italy
e-mail: vittorio.braga@inaf.it
² Space Science Data Center, Via del Politecnico snc, 00133 Roma, Italy
³ Dipartimento di Fisica, Università di Roma Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
⁴ Herzberg Astronomy and Astrophysics, National Research Council, 5071 West Saanich Road, Victoria, British Columbia V9E 2E7, Canada
⁵ INAF-Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Napoli, Italy
⁶ Astrophysics Research Institute, Liverpool John Moores University, IC2, Liverpool Science Park, 146 Brownlow Hill, Liverpool L3 5RF, UK
⁷ Department of Astronomy, University of Michigan, Ann Arbor, MI, USA
⁸ Department of Astronomy, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁹ Instituto de Astrofísica de Canarias, Calle Via Lactea s/n, 38205 La Laguna, Tenerife, Spain
¹⁰ Department of Astronomy, University of Washington, Seattle, WA 98195, USA

Received: 10 August 2020
Accepted: 12 October 2020

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 ω Cen to investigate several diagnostics (color-magnitude diagram, Bailey diagram, Fourier decomposition of the light curve, and amplitude ratios) for their empirical separation. We found that the classical period threshold at one 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, which is typical of RRLs. We also derived NIR period-luminosity relations and found a distance modulus of 13.65 ± 0.07 (err.) ± 0.01 (σ) mag, in agreement with the recent literature. We also found that RRLs and T2Cs obey the same period-luminosity relations in the NIR. This equivalence provides the opportunity of adopting RRLs+T2Cs as an alternative to classical Cepheids to calibrate the extragalactic distance scale.