Astron. Astrophys. 355, L39-L42 (2000)

1. Research context

The search for gravitational microlensing in our Galaxy has been going on for a decade, following the proposal to use this effect as a tool to probe the dark matter content of the Galactic halo (Paczynski 1986). The first microlensing candidates were reported in 1993, towards the LMC (Aubourg et al. 1993; Alcock et al. 1993) and the Galactic Centre (Udalski et al. 1993) by the EROS , MACHO and OGLE collaborations.

Because they observed no microlensing candidate with a duration shorter than 10 days, the EROS 1 and MACHO groups were able to exclude the possibility that a substantial fraction of the Galactic dark matter resides in planet-sized objects (Aubourg et al. 1995; Alcock et al. 1996; Renault et al. 1997; Renault et al. 1998; Alcock et al. 1998).

However a few events were detected with longer timescales. From 6-8 candidate events towards the LMC , the MACHO group estimated an optical depth of order half that required to account for the dynamical mass of the standard spherical dark halo; the typical Einstein radius crossing time of the events, , implied an average mass of about 0.5  for the lenses (Alcock et al. 1997a). Based on two candidates, EROS 1 set an upper limit on the halo mass fraction in objects of similar masses (Ansari et al. 1996), that is below that required to explain the rotation curve of our Galaxy ¹.

The second phase of the EROS programme was started in 1996, with a ten-fold increase in the number of monitored stars in the Magellanic Clouds. The analysis of the first two years of data towards the Small Magellanic Cloud (SMC ) allowed the detection of one microlensing event (Palanque-Delabrouille et al. 1998; see also Alcock et al., 1997b). This single event, out of 5.3 million stars, allowed EROS 2 to further constrain the halo composition, excluding in particular that more than 50% of the standard dark halo is made up of objects (Afonso et al. 1999). In contrast, an optical detection of a halo white dwarf population was reported (Ibata et al. 1999).

In this letter, we describe the analysis of the two-year light curves from 17.5 million LMC stars. We then combine these results with our previous limits, and derive the strongest limit obtained thus far on the amount of stellar mass objects in the Galactic halo.

© European Southern Observatory (ESO) 2000

Online publication: March 21, 2000
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