Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud *,**
Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France e-mail: firstname.lastname@example.org
2 CEA, DSM, DAPNIA, Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
3 Research School of Astronomy & Astrophysics, Mount Stromlo Observatory, Cotter Road, Weston ACT 2611, Australia
4 GEPI, Observatoire de Paris, 61 avenue de l'Observatoire, 75014 Paris, France
5 European Southern Observatory (ESO), Casilla 19001, Santiago 19, Chile
6 Centrum Astronomiczne im. M. Kopernika, Bartycka 18, 00-716 Warszawa, Poland
7 Obserwatorium Astronomiczne, Uniwersytet A. Mickiewicza, Sloneczna 36, 60-286 Poznan, Poland
8 Laboratoire de l'Accélérateur Linéaire, IN2P3 CNRS, Université de Paris-Sud, 91405 Orsay Cedex, France
9 The Niels Bohr Institute, Copenhagen University, Juliane Maries Vej 30, DK2100 Copenhagen, Denmark
10 Observatoire Midi-Pyrénées, Laboratoire d'Astrophysique (UMR 5572), 14 av. E. Belin, 31400 Toulouse, France
11 Department of Astronomy, Ohio State University, Columbus, OH 43210, USA
12 Observatoire de Marseille, 2 place Le Verrier, 13248 Marseille Cedex 04, France
Accepted: 13 April 2008
Context. For seven years, the EROS-2 project obtained a mass of photometric data on variable stars. We present a peculiar Cepheid-like star, in the direction of the Small Magellanic Cloud, which demonstrates unusual photometric behaviour over a short time interval.
Aims. We report on data of the photometry acquired by the MARLY telescope and spectroscopy from the EFOSC instrument for this star, called EROS2 J005135-714459(sm0060n13842), which resembles the unusual Cepheid HR 7308.
Methods. The light curve of our target is analysed using the Analysis of Variance method to determine a pulsational period of 5.5675 days. A fit of time-dependent Fourier coefficients is performed and a search for proper motion is conducted.
Results. The light curve exhibits a previously unobserved and spectacular change in both mean magnitude and amplitude, which has no clear theoretical explanation. Our analysis of the spectrum implies a radial velocity of 104 km s-1 and a metallicity of -0.4±0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4±6.0 mas yr-1 using EROS astrometry, which is compatible with data from the NOMAD catalogue.
Conclusions. The nature of EROS2 J005135-714459(sm0060n13842) remains unclear. For this star, we may have detected a non-zero proper motion, which would imply that it is a foreground object. Its radial velocity, pulsational characteristics, and photometric data, however, suggest that it is instead a Cepheid-like object located in the SMC. In such a case, it would present a challenge to conventional Cepheid models.
Key words: Cepheids / Galaxy: disk / Magellanic Clouds / stars: peculiar
Based on observations made by the EROS-2 collaboration with the MARLY, 1.54 m Danish and 3.60 m telescopes at the European Southern Observatory, La Silla, Chile.
© ESO, 2008