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Astron. Astrophys. 351, 87-96 (1999)

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Observation of microlensing towards the galactic spiral arms

EROS II. 2 year survey *

The EROS collaboration F. Derue 1, C. Afonso 2, C. Alard 3, J.-N. Albert 1, A. Amadon 2, J. Andersen 4, R. Ansari 1, É. Aubourg 2, P. Bareyre 5, F. Bauer 2, J.-P. Beaulieu 6, A. Bouquet 5, S. Char  **  7, X. Charlot 2, F. Couchot 1, C. Coutures 2, R. Ferlet 6, J.-F. Glicenstein 2, B. Goldman 2, A. Gould  ***  8, D. Graff 2,9, M. Gros 2, J. Ha"ssinski 1, J.-C. Hamilton 5, D. Hardin 2, J. de Kat 2, A. Kim 5, T. Lasserre 2, É. Lesquoy 2, C. Loup 6, C. Magneville 2, B. Mansoux 1, J.-B. Marquette 6, É. Maurice 10, A. Milsztajn 2, M. Moniez 1, N. Palanque-Delabrouille 2, O. Perdereau 1, L. Prévot 10, N. Regnault 1, J. Rich 2, M. Spiro 2, A. Vidal-Madjar 6, L. Vigroux 2 and S. Zylberajch 2

1 Laboratoire de l'Accélérateur Linéaire, IN2P3-CNRS , Université de Paris-Sud, B.P. 34, 91898 Orsay Cedex, France
2 CEA , DSM , DAPNIA , Centre d'Études de Saclay, 91191 Gif-sur-Yvette Cedex, France
3 DASGAL , INSU-CNRS , 77 avenue de l'Observatoire, 75014 Paris, France
4 Astronomical Observatory, Copenhagen University, Juliane Maries Vej 30, 2100 Copenhagen, Denmark
5 Collège de France, LPCC , IN2P3-CNRS , 11 place Marcellin Berthelot, 75231 Paris Cedex, France
6 Institut d'Astrophysique de Paris, INSU-CNRS , 98 bis Boulevard Arago, 75014 Paris, France
7 Universidad de la Serena, Facultad de Ciencias, Departamento de Fisica, Casilla 554, La Serena, Chile
8 Department of Astronomy, Ohio State University, Columbus, Ohio 43210, USA
9 Department of Physics, Ohio State University, Columbus, Ohio 43210, USA
10 Observatoire de Marseille, INSU-CNRS , 2 place Le Verrier, 13248 Marseille Cedex 04, France

Received 13 March 1999 / Accepted 2 August 1999


We present the analysis of the light curves of 8.5 million stars observed during two seasons by EROS (Expérience de Recherche d'Objets Sombres), in the Galactic plane away from the bulge. Three stars have been found that exhibit luminosity variations compatible with gravitational microlensing effects due to unseen objects. The corresponding optical depth, averaged over four directions, is [FORMULA]. All three candidates have long Einstein radius crossing times ([FORMULA] 70 to 100 days). For one of them, the lack of evidence for a parallax or a source size effect enabled us to constrain the lens-source configuration. Another candidate displays a modulation of the magnification, which is compatible with the lensing of a binary source.

The interpretation of the optical depths inferred from these observations is hindered by the imperfect knowledge of the distance to the target stars. Our measurements are compatible with expectations from simple galactic models under reasonable assumptions on the target distances.

Key words: Galaxy: halo – Galaxy: kinematics and dynamics – Galaxy: stellar content – Galaxy: structure – cosmology: dark matter – cosmology: gravitational lensing

* This work is based on observations made at the European Southern Observatory, La Silla, Chile. In memory of Sergio Char
** deceased
*** Alfred P. Sloan Foundation Fellow

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© European Southern Observatory (ESO) 1999

Online publication: November 2, 1999