A Multiband Photometric Study of RR Lyrae Stars in M53 (NGC 5024)

TL;DR

This work presents a detailed multiband UBVRI time-series analysis of RR Lyrae stars in M53, combining 22 years of archival data with Lomb-Scargle period searches and Stripe 82 template fitting to extract accurate pulsation parameters. The authors establish M53 as an Oosterhoff II cluster, quantify mean periods $<P_{ab}>=0.649$ d and $<P_c>=0.346$ d, and find a high RRc fraction $N_c/N_{ m tot}=0.547$, including a substantial Blazhko incidence. They derive I-band PLZ and multiple PWZ relations, with the $(V,B ext{-}I)$ Wesenheit relation providing the most reliable distance, yielding a weighted modulus $ar{ }=16.24$ mag and a distance of $d \, ext{≈}\,17.7$ kpc, in good agreement with Gaia DR3 results. The study also identifies a set of blended or Blazhko-affected outliers, discusses the implications for distance estimates, and strengthens the distance framework for metal-poor globular clusters using RR Lyrae stars.

Abstract

We present a multiband (UBVRI) time-series photometric study of RR Lyrae (RRL) stars in the globular cluster Messier 53 (NGC 5024) to refine their pulsation properties and determine a precise cluster distance. The archival photometric data includes images taken over 22 years and 3 months using different optical telescopes, providing an excellent time baseline to investigate light curves of variable stars. Using Lomb-Scargle periodogram, we derived accurate periods for 29 fundamental-mode (RRab) and 35 first-overtone (RRc) RRLs. Template-fitting to phase-folded light curves provided robust mean magnitudes and amplitudes. The refined periods confirm M53 as an Oosterhoff II cluster, with a mean period of 0.649 days for RRab and 0.346 days for RRc, and a high RRc fraction (54.7\%). Most RRLs align with the horizontal branch in the color-magnitude diagram, while a few outliers result from blending effects. Period-amplitude diagrams show RRab stars following the Oosterhoff II locus. We derived I-band period-luminosity and multi-band period-Wesenheit relations, comparing them with theoretical predictions. A weighted mean distance modulus of 16.242 $\pm$ 0.05 mag yields a cluster distance of 17.717 $\pm$ 0.408 kpc, in agreement with recent estimates based on parallaxes from Gaia data.

Figures (27)

• Figure 1: Light curves of RR Lyrae stars V4 (top) and V40 (bottom) in the $U$, $B$, $V$, $R$, and $I$ bands. The derived periods show typical RRc-type variations with noisy background; see Figures \ref{['img:temp1to10']} and \ref{['img:tempv32to42']} for template fits.
• Figure 2: Light curve of V4 (RRc) in the $V$ band, displayed before (left) and after (right) phasing the data such that maximum light occurs at phase 0.
• Figure 3: Intensity averaged $V_{mean}$ distribution for both light curve data (left panel) and template fitted data (right panel). It can be clearly seen that the scatter of $V_{mean}$ is much lower in template fit in agreement with the discussion of \ref{['sec:validtemp']}.
• Figure 4: Difference in intensity-averaged mean magnitudes ($\Delta mag_{\mathrm{avg}}$) between the observed light curve data and the template fits, plotted against the intensity-averaged mean magnitude in each band. The vertical colored lines mark the $3\sigma$ limits of the mean magnitude distribution. Panels from left to right correspond to the $U$, $B$, $V$, $R$, and $I$ bands.
• Figure 5: Distribution of mean RMS scatter between observed light curves and template fits for RRab and RRc variables in NGC 5024.