On the use of field RR Lyrae as galactic probes – VI. Mixed mode RR Lyrae variables in Fornax and in nearby dwarf galaxies

Abstract

We investigate the properties of the mixed-mode (RRd) RR Lyrae (RRL) variables in the Fornax dwarf spheroidal (dSph) galaxy by using B- and V-band time series collected over twenty-four years. We compare the properties of the RRds in Fornax with those in the Magellanic Clouds and in nearby dSphs, with special focus on Sculptor. We found that the ratio of RRds over the total number of RRLs decreases with metallicity. Typically, dSphs have very few RRds with 0.49≲ P0 ≲0.53 days, but Fornax fills this period gap in the Petersen diagram (ratio between first overtone over fundamental period versus fundamental period). We also found that the distribution in the Petersen diagram of Fornax RRds is similar to SMC RRds, thus suggesting that their old stars have a similar metallicity distribution. We introduce the Period-Amplitude RatioS (PARS) diagram, a new pulsation diagnostics independent of distance and reddening. We found that LMC RRds in this plane are distributed along a short- and a long-period sequence that we identified as the metal-rich and the metal-poor component. These two groups are also clearly separated in the Petersen and Bailey (luminosity amplitude versus logarithmic period) diagrams. These circumstantial evidence indicates that the two groups have different evolutionary properties. All the pulsation diagnostics adopted in this investigation suggest that old stellar populations in Fornax and Sculptor dSphs underwent different chemical enrichment histories. Fornax RRds are similar to SMC RRds, while Sculptor RRds are more similar to the metal-rich component of the LMC RRds.

Precise Empirical Determination of Metallicity Dependence of Near-infrared Period–Luminosity Relations for RR Lyrae Variables

“…1. Therefore, the firstovertone periods of RRc and RRd stars were fundamentalized using the equation: log(P f ) = log(P o ) + 0.127 (e.g., Coppola et al 2015;Braga et al 2022) for the combined sample of all 1310 RR Lyrae stars.…”

“…8 displays the histograms of period distributions of all 964 cluster RR Lyrae variables in our sample. We derived PL relations for different subtypes of RR Lyrae stars and the combined sample of RRab+RRc+RRd stars by fundamentalizing the periods of RRc/RRd stars using: log(P f ) = log(P f o ) + 0.127 (Braga et al 2022). We assume that the RRd stars exhibit the first-overtone mode as the dominant mode of pulsation.…”

RR Lyrae Mid-infrared Period–Luminosity–Metallicity and Period–Wesenheit–Metallicity Relations Based on Gaia DR3 Parallaxes

“…The RRc stars had their period fundamentalized by adding 0.127 to P log (Iben & Huchra 1971;Braga et al 2022). The explicit, nonlinear fit, which takes into account the uncertainties in all of the observed quantities, was performed using a weighted orthogonal distance regression (Boggs et al 1989;Zwolak et al 2007) and the nonlinear fitting function nls in R (R Core Team 2018).…”

Period–Luminosity–Metallicity relations for Classical Pulsators at Near-infrared Wavelengths

“…8 displays the histograms of period distributions of all 964 cluster RR Lyrae variables in our sample. We derived PL relations for different subtypes of RR Lyrae stars and the combined sample of RRab+RRc+RRd stars by fundamentalizing the periods of RRc/RRd stars using: log(P f ) = log(P fo ) + 0.127 (Braga et al 2022). We assume that the RRd stars exhibit the first-overtone mode as the dominant mode of pulsation.…”