Observation of microlensing toward the galactic spiral arms. EROS II 3 year survey ^*

¹ Laboratoire de l'Accélérateur Linéaire, , Université de Paris-Sud, BP 34, 91898 Orsay Cedex, France
² , , , Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
³ , , 77 avenue de l'Observatoire, 75014 Paris, France
⁴ Astronomical Observatory, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
⁵ Collège de France, , , 11 place Marcellin Berthelot, 75231 Paris Cedex, France
⁶ Institut d'Astrophysique de Paris, , 98bis Boulevard Arago, 75014 Paris, France
⁷ Universidad de la Serena, Facultad de Ciencias, Departamento de Fisica, Casilla 554, La Serena, Chile
⁸ Observatoire de Paris, DESPA, 92195 Meudon Cedex, France
⁹ European Southern Observatory (ESO), Casilla 19001, Santiago 19, Chile
¹⁰ Department of Astronomy, Ohio State University, Columbus, Ohio 43210, USA
¹¹ Department of Physics, Ohio State University, Columbus, Ohio 43210, USA
¹² Observatoire de Marseille, , 2 place Le Verrier, 13248 Marseille Cedex 04, France

Corresponding author: R. Ansari, ansari@lal.in2p3.fr See also our web server at http://eros.in2p3.fr

Received: 4 January 2000
Accepted: 19 March 2001

Abstract

We present an analysis of the light curves of 9.1 million stars observed during three seasons by EROS (Expérience de Recherche d'Objets Sombres), in the Galactic plane away from the bulge. Seven stars exhibit luminosity variations compatible with gravitational microlensing effects due to unseen objects. The corresponding optical depth, averaged over four directions, is . While this value is compatible with expectations from simple Galactic models under reasonable assumptions on the target star distances, we find an excess of events with short timescales toward the direction closest to the Galactic centre. We discuss a possible interpretation involving the contribution of an elongated bar.