Long term CCD photometry of the distant cluster NGC 2419: the CMD revisited

TL;DR

This study presents a 17-year photometric analysis of NGC 2419, utilizing Gaia data to refine its CMD, analyze variable stars, and estimate its metallicity and distance with improved accuracy.

Contribution

It provides a comprehensive long-term photometric dataset, a refined cluster membership analysis using Gaia data, and new estimates of NGC 2419's metallicity and distance.

Findings

RR Lyrae stars share the instability strip region, unusual for OoII clusters.

Theoretical models link Pop II cepheids to blue tail stars with thin envelopes.

Estimated metallicity [Fe/H] ≈ -1.89 and distance ≈ 84-86 kpc.

Abstract

Employing \emph{VI} images of NGC 2419 acquired over 17 years, light curves for most of the known variables in the field of the cluster are produced. A cluster membership analysis for about 3100 stars in the cluster field with proper motions from $Gaia$-DR3, revealed the presence of member stars as far as 140 pc from the cluster center and enabled the construction of a cleaner CMD free of field stars. It was found that RRab and RRc stars share the inter-order region in the instability strip, which is unusual for OoII clusters. Theoretical considerations confirm that Pop II cepheids are descendants of extreme ZAHB blue tail stars with very thin envelopes of about 10\% of the total mass. Member RR Lyrae stars were employed to calculate independent estimates of the mean cluster metallicity and distance; we found [Fe/H]$_{\rm UV}= -1.90 \pm 0.27$ and $D=86.3 \pm 5.0$ kpc from the RRab and [Fe/H]$_{\rm UV}= -1.88 \pm 0.30$ and $D=83.1 \pm 8.1$ kpc from the RRc light curves.