The ancient stellar population of M32: RR Lyr Variable stars confirmed

TL;DR

This study confirms the presence of an ancient stellar population in M32 through the identification of RR Lyrae stars, and also detects younger stars and Cepheids, providing insights into the galaxy's stellar history.

Contribution

First comprehensive identification of RR Lyrae stars in M32 confirming its ancient population using archival HST data.

Findings

416 RR Lyrae stars identified, with 83 associated to M32

Detection of 4 Anomalous Cepheids indicating younger stars

Distance modulus of 24.33 mag confirmed by Population II Cepheids

Abstract

Using archival multi--epoch ACS/WFC images in the F606W and F814W filters of a resolved stellar field in Local Group dwarf elliptical galaxy M32 we have made an accurate Colour-Magnitude Diagram and a careful search for RR Lyr variable stars. We identified 416 bona fide RR Lyr stars over our field of view, and their spatial distribution shows a rising number density towards the centre of M32. These new observations clearly confirm the tentative result of Fiorentino et al. (2010), on a much smaller field of view, associating an ancient population of RR Lyr variables to M32. We associate at least 83 RR Lyr stars in our field to M32. In addition the detection of 4 Anomalous Cepheids with masses in the range 1.2-1.9 Mo indicates the presence of relatively young, 1-4 Gyr old, stars in this field. They are most likely associated to the presence of the blue plume in the Colour-Magnitude Diagram. However these young stars are unlikely to be associated with M32 because the radial distribution of the blue plume does not follow the M32 density profile, and thus they are more likely to belong to the underlying M31 stellar population. Finally the detection of 3 Population II Cepheids in this field gives an independent measurement of the distance modulus in good agreement with that obtained from the RRLyr, mu0=24.33 +- 0.21 mag.