1 Pontificia Universidad Católica de Chile, Facultad de Física, Departamento de Astronomía y Astrofísica, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
2 Las Campanas Observatory, Carnegie Institution of Washington, Colina El Pino, Casilla 601, La Serena, Chile
3 The Milky Way Millennium Nucleus, Av. Vicuña Mackenna 4860, 782-0436 Macul, Santiago, Chile
4 Dipartimento di Astronomia, Università di Bologna, via Ranzani 1, 40127 Bologna, Italy
5 Department of Physics & Astronomy, University of Wisconsin-Oshkosh, Oshkosh, WI 54901, USA
6 Dept. of Physics and Astronomy, Michigan State University, East Lansing, MI 48824, USA
Received: 25 November 2011
Accepted: 20 June 2012
Context. We present a new search for variable stars in the Galactic globular cluster M 28 (NGC 6626).
Aims. The search is based on a series of BVI images obtained with the SMARTS Consortium’s 1.3 m telescope at Cerro Tololo Inter-American Observatory, Chile.
Methods. The search was carried out using the ISIS v2.2 image subtraction package.
Results. We find a total of 25 variable stars in the field of the cluster, nine being new discoveries. Of the newly found variables, one is an ab-type RR Lyrae star, six are c-type RR Lyrae, and two are long-period/semi-regular variables. V22, previously classified as a type II Cepheid, appears as a bona-fide RRc in our data. In turn, V20, previously classified as an ab-type RR Lyrae, could not be properly phased with any reasonable period.
Conclusions. The properties of the ab-type RR Lyrae stars in M 28 appear most consistent with an Oosterhoff-intermediate classification, which is unusual for bona-fide Galactic globulars clusters. However, the cluster’s c-type variables do not clearly support such an Oosterhoff type, and a hybrid Oosterhoff I/II system is accordingly another possibility, thus raising the intriguing possibility of multiple populations being present in M 28. Coordinates, periods, and light curves in differential fluxes are provided for all the detected variables.
Key words: globular clusters: individual: M 28 (NGC 6626) / stars: evolution / stars: variables: RR Lyrae
Based on observations obtained with the SMARTS Consortium 1.3 m telescope at the Cerro Tololo Inter-American Observatory, Chile.
Figures 2–5 are available in electronic form at http://www.aanda.org
© ESO, 2012