The ancient stellar population of M 32: RR Lyrae variable stars confirmed^⋆

¹ INAF-Osservatorio Astronomico di Bologna, via Ranzani 1, 40127 Bologna, Italy
e-mail: giuliana.fiorentino@oabo.inaf.it
² Dipartimento di Astronomia, Universitá degli studi di Bologna, via Ranzani 1, 40127 Bologna, Italy
³ Kapteyn Astronomical Institute, University of Groningen, Groningen, The Netherlands
⁴ NOAO, PO Box 26732, Tucson, AZ 85726, USA

Received: 7 June 2011
Accepted: 15 December 2011

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 M 32 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 M 32. These new observations clearly confirm the tentative result of Fiorentino et al. (2010b, ApJ, 708, 817), on a much smaller field of view, associating an ancient population of RR Lyr variables to M 32. We associate at least 83 RR Lyr stars in our field to M 32. In addition the detection of 4 anomalous Cepheids with masses in the range 1.2−1.9 M_⊙ 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 M 32 because the radial distribution of the blue plume does not follow the M 32 density profile, and thus they are more likely to belong to the underlying M 31 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 RR Lyr, μ₀ = 24.33 ± 0.21 mag.