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Astron. Astrophys. 350, L19-L22 (1999)

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1. Introduction

Gravitational microlensing was originally proposed as a method of detecting compact dark matter objects in the Galactic halo (Paczynski 1986). However, it also turned out to be a powerful method to study Galactic structure, mass functions of stars and extrasolar planetary systems (for a review, see Paczynski 1996). Earlier microlensing targets include the Galactic bulge, LMC, SMC and M31. Recently, the EROS and OGLE collaborations have started to monitor spiral arms. The EROS collaboration (Derue et al. 1999) has announced the discovery of three microlensing events toward two spiral arms. In this paper, we study the first spiral arm microlensing event discovered by the OGLE II experiment (Udalski, Kubiak & Szymanski 1997). This ongoing event, OGLE-1999-CAR-1, was discovered in real-time toward the Carina arm by the OGLE early-warning system (Udalski et al. 1994). We show that this is a unique event that exhibits strong parallax effects. Such events were predicted by Refsdal (1966) and Gould (1992). The first case was reported by the MACHO collaboration toward the Galactic bulge (Alcock et al. 1995). OGLE-1999-CAR-1 is the first parallax event discovered toward any spiral arm. The outline of the paper is as follows. In Sect. 2, we briefly describe the observational data that we use. In Sect. 3, we present two different fits for the light curve, with and without considering the effect of parallax and blending. In Sect. 4, we calculate the expected optical depth, event rate and duration distribution toward Carina. Finally in Sect. 5, we summarize and discuss the implications of our results.

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© European Southern Observatory (ESO) 1999

Online publication: October 4, 1999