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

First results on RR Lyrae stars with the TESS space telescope: untangling the connections between mode content, colors and distances

“…We therefore do not yet have a wellcalibrated PL relation for Galactic ACEP stars, and too few have been studied so far with TESS (Clementini et al 2019;Plachy et al 2021). The importance of distance determination was also highlighted by Braga et al (2020), who showed that short-period ACEP and Type II Cepheids can overlap in period with long-period RR Lyrae stars. Period itself is therefore not a unique classifier, and a reliable separation of Galactic short-period ACEP stars from long-period RR Lyrae stars will require accurate light curve shape information, distances and colors.…”

RR Lyrae and Type II Cepheid Variables in Globular Clusters: Optical and Infrared Properties

“…We did not find any correlation of the residuals of PLRs with metallicity for T2Cs, implying a negligible dependence on metallicity. Braga et al [97] used T2C PLRs in ω Cen to obtain a distance to the cluster that is consistent with independent methods, including those based on RRL stars. Since T2Cs and RRL follow the same PLRs in NIR bands, their combined PLRs can be used as an alternative to classical Cepheids for providing the calibration of the first rung of the extragalactic distance scale.…”

“…Even though NGC 6441 is metal-rich, previous studies of its RRab have shown that they have periodamplitude diagrams similar to those in OoII GCs [96]. Braga et al [97] showed that the long-period RRab in ω Cen follows the locus of OoIII RRab, similar to NGC 6441 variables. Rich et al [98] first discovered that metal-rich NGC 6441 and NGC 6388 not only have expected red HB but also blue HB components that extend through the IS.…”

“…Matsunaga et al [137] used homogeneous NIR time-series for 46 T2Cs in 26 GCs to derive JHK s PLRs. While Pritzl et al [138] added T2Cs in NGC 6388 and NGC 6441 to an optical sample, Braga et al [97] provided an updated sample of T2Cs with NIR photometry by adding variables in ω Cen. We combined and updated the optical and NIR dataset of T2Cs from these studies to obtain a sample of 60 variables in GCs for a comparative study of their pulsation properties.…”

“…Similar trends to the optical bands are seen for W Vir and RV Tau at longer wavelengths. Braga et al [97] investigated amplitude ratios for T2Cs and found a small but significant dependence on pulsation period, unlike RRL stars, which seem to have constant amplitude ratios for RRc and RRab stars [48,50].…”