EDP Sciences
Free access
Volume 486, Number 3, August II 2008
Page(s) 891 - 898
Section Stellar structure and evolution
DOI http://dx.doi.org/10.1051/0004-6361:20066915
Published online 29 May 2008

A&A 486, 891-898 (2008)
DOI: 10.1051/0004-6361:20066915

Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud

J. B. Marquette1, P. Tisserand2, 3, P. François4, J. P. Beaulieu1, V. Doublier5, É. Lesquoy2, 1, A. Milsztajn2, J. Pritchard5, A. Schwarzenberg-Czerny6, 7, C. Afonso2, J. N. Albert8, J. Andersen9, R. Ansari8, É. Aubourg2, P. Bareyre2, X. Charlot2, C. Coutures2, 1, R. Ferlet1, P. Fouqué10, J. F. Glicenstein2, B. Goldman2, A. Gould11, D. Graff11, M. Gros2, J. Haissinski8, C. Hamadache2, J. de Kat2, L. Le Guillou2, C. Loup1, C. Magneville2, É. Maurice12, A. Maury5, M. Moniez8, N. Palanque-Delabrouille2, O. Perdereau8, Y.R. Rahal8, J. Rich2, M. Spiro2, A. Vidal-Madjar1, and S. Zylberajch2

1  Institut d'Astrophysique de Paris, UMR 7095 CNRS, Université Pierre & Marie Curie, 98 bis Boulevard Arago, 75014 Paris, France
    e-mail: marquett@iap.fr
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

Received 11 December 2006 / 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$\pm$0.2 dex. In the direction of right ascension, we measure a proper motion of 17.4$\pm$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

© ESO 2008