Detailed analysis of the variable star population in the globular cluster NGC 7006

TL;DR

This study provides a comprehensive analysis of variable stars in NGC 7006, correcting classifications, identifying new variables, estimating cluster parameters, and exploring stellar evolution with Gaia data and Fourier analysis.

Contribution

It offers the first detailed variable star analysis in NGC 7006, including new classifications, refined cluster parameters, and insights into stellar evolution and variable star origins.

Findings

Identified ten new cluster member variables.

Estimated cluster metallicity [Fe/H]$_{ZW}=-1.53\, ext{dex}$ and distance 41.2 kpc.

Clarified variable star distribution and origins in the cluster.

Abstract

A five year CCD photometric \emph{VI} time-series of NGC 7006 is employed to perform a detailed analysis of the known population of variable stars. In the process we have corrected inconsistent classifications, sky coordinates and found ten new cluster member variables. An independent reddening estimate with a value $E(B-V)=0.08\pm0.05$ is made. Using Fourier decompositions of RR Lyrae light curves and well established calibrations, the cluster mean metallicity and distance [Fe/H]$_{\rm ZW}= -1.53\pm0.15$ and $41.2\pm1.4$ kpc are estimated based on an extended sample of cluster member RRab stars. Using the $Gaia$-DR3 data we performed an extensive membership analysis which leads to a clean Colour-Magnitude diagram, and hence to the identification of variables that are likely field stars, and to considerations on the variables distribution in the Horizontal Branch (HB). A double mode RR Lyrae and three CW star are discussed. The origin of CW stars from precursors in the blue tail of the HB with very thin ($\sim 0.06 \pm 0.01 M_{\odot}$) envelopes is argued. Our models indicate that the Main Sequence predecessor of RR Lyrae stars had a mass of 0.82-0.85 $M_{\odot}$ and lost about 25-35\% of its mass during the red giant branch events before settling in the HB some 12-13.5 Gyrs later.