# Photometric and radial-velocity time-series of RR Lyrae stars in M3: analysis of single-mode variables

## TL;DR

This study provides the first combined photometric and spectroscopic analysis of RR Lyrae stars in M3, revealing Blazhko modulation, consistent velocities, and precise distance measurements, advancing understanding of variable star properties in globular clusters.

## Contribution

It offers the largest RV sample for RR Lyrae stars in a cluster, introduces a new $A_{puls}-A_V$ relation, and analyzes phase differences and color dependencies with high precision.

## Key findings

- Blazhko modulation detected in 47% of RRab stars
- Cluster distance determined as 10,480±210 pc
- No period or metallicity dependence of ${\

## Abstract

We present the first simultaneous photometric and spectroscopic investigation of a large set of RR Lyrae variables in a globular cluster. The radial-velocity data presented comprise the largest sample of RVs of RR Lyrae stars ever obtained. The target is M3; $BVI_{\mathrm{C}}$ time-series of 111 and $b$ flux data of further 64 RRab stars, and RV data of 79 RR Lyrae stars are published. Blazhko modulation of the light curves of 47 percent of the RRab stars are detected. The mean value of the center-of-mass velocities of RR Lyrae stars is $-146.8$ km s$^{-1}$ with 4.52 km s$^{-1}$ standard deviation, which is in good agreement with the results obtained for the red giants of the cluster. The ${\Phi_{21}}^{\mathrm RV}$ phase difference of the RV curves of RRab stars is found to be uniformly constant both for the M3 and for Galactic field RRab stars; no period or metallicity dependence of the ${\Phi_{21}}^{\mathrm RV}$ is detected. The Baade-Wesselink distances of 26 non-Blazhko variables with the best phase-coverage radial-velocity curves are determined; the corresponding distance of the cluster, $10480\pm210$ pc, agrees with the previous literature information. A quadratic formula for the $A_{\mathrm{puls}}-A_V$ relation of RRab stars is given, which is valid for both OoI and OoII variables. We also show that the $(V-I)_0$ of RRab stars measured at light minimum is period dependent, there is at least 0.1 mag difference between the colours at minimum light of the shortest- and longest-period variables.