Does the Oosterhoff Dichotomy Exist in The Andromeda Galaxy? I. The case of G11

TL;DR

This study provides the first evidence of Oosterhoff type II globular clusters in the Andromeda galaxy, using Hubble Space Telescope data to analyze RR Lyrae stars in G11, revealing its classification and similarities to Milky Way clusters.

Contribution

It demonstrates the existence of Oosterhoff type II clusters in M31 and applies detailed RR Lyrae analysis to classify G11's Oosterhoff type.

Findings

G11 contains RR Lyrae stars with periods indicative of Oosterhoff type II.

The properties of G11's RR Lyrae stars align with Milky Way Oosterhoff II clusters.

G11's classification supports the presence of similar globular cluster types in M31.

Abstract

We present the first evidence that Oosterhoff type II globular clusters exist in the Andromeda galaxy (M31). On the basis of time-series photometry of the moderately metal-poor ([Fe/H]\sim -1.6 dex) M31 globular cluster G11, obtained with the Wide Field Planetary Camera 2 on board the Hubble Space Telescope, we detected and derived periods for 14 RR Lyrae stars, of which five are found to lie inside the cluster tidal radius. They include three fundamental-mode (RRab) and two first-overtone (RRc) pulsators, with average periods <Pab> = 0.70 d, and <Pc> = 0.40 d, respectively. These mean periods and the position of the cluster variable stars in the period-amplitude and period-metallicity diagrams, all suggest that G11 is likely to be an Oosterhoff type II globular cluster. This appears to be in agreement with the general behavior of Milky Way globular clusters with similar metallicity and horizontal branch morphology.