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

J. B. Marquette (1) , P. Tisserand (2) , P. François , J. P. Beaulieu (1) , V. Doublier , É. Lesquoy (1, 2) , A. Milsztajn (2) , J. Pritchard , A. Schwarzenberg-Czerny , C. Afonso (2) , J. N. Albert (3) , J. Andersen , R. Ansari (3) , É. Aubourg , P. Bareyre (2) , X. Charlot (2) , C. Coutures (1) , R. Ferlet (1) , P. Fouqué (4) , J. F. Glicenstein (2) , B. Goldman , A. Gould , D. Graff , M. Gros , J. Haissinski (3) , C. Hamadache (2) , J. de Kat (2) , L. Le Guillou (2) , C. Loup (1) , C. Magneville (2) , É. Maurice (5) , A. Maury (6) , M. Moniez (3) , N. Palanque-Delabrouille (2) , O. Perdereau (3) , Y. R. Rahal (3) , J. Rich (2) , M. Spiro (2) , A. Vidal-Madjar (1) , S. Zylberajch (2)

1 IAP - Institut d'Astrophysique de Paris
2 DAPNIA - Département d'Astrophysique, de physique des Particules, de physique Nucléaire et de l'Instrumentation Associée
3 LAL - Laboratoire de l'Accélérateur Linéaire
4 LATT - Laboratoire Astrophysique de Toulouse-Tarbes
5 LAM - Laboratoire d'Astrophysique de Marseille
6 ESO - European Southern Observatory

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. 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. The photometric data is only available in electronic form at the CDS via anonymous ftp to cdsarc.u-strasbg.fr (130.79.128.5) or via http://cdsweb.u-strasbg.fr/cgi-bin/qcat?J/A+A/486/891

Mots clés: Cepheids; Galaxy: disk; Magellanic Clouds; stars: peculiar

Domaines: Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Cosmologie et astrophysique extra-galactique [astro-ph.CO]; Astrophysique [astro-ph]

J. B. Marquette, P. Tisserand, P. François, J. P. Beaulieu, V. Doublier, et al.. Discovery of a peculiar Cepheid-like star towards the northern edge of the Small Magellanic Cloud. Astronomy and Astrophysics - A&A, 2008, 486, pp.891-898. ⟨10.1051/0004-6361:20066915⟩. ⟨hal-00325464⟩