We have shown that the microlensing event, OGLE-1999-CAR-1, has a light curve shape that is significantly modified by the earth motion around the Sun. This event is still ongoing at the time of writing (Aug. 12, 1999); later evolutions of the light curve will test our predictions and reduce the uncertainties in the parameters. For comparison, the three microlensing events seen by the EROS collaboration (Derue et al. 1999) are toward different spiral arms which are somewhat closer to the Galactic center direction. The three events have between 70 to 100, and none of the events show parallax effects.
We have assumed a source distance of in the previous section. We show now that this is a reasonable assumption. From and , and taking into account the blending, we find that the lensed star has . Assuming an extinction of mag (see Fig. 4 in Wramdemark 1980), and , we obtain the intrinsic magnitude and color . The star is consistent with being a main sequence star (with mass ), as can be seen from the color-magnitude diagram (Udalski 1999, private communication) in the Carina region.
We can combine the expression for and to obtain the lens mass as a function of its distance
Using Eq. (6) in Alcock et al. (1995), we have calculated the likelihood of obtaining the observed transverse velocity and direction as a function of the lens distance. The result is shown in the right panel of Fig. 2 for the Gould et al. (1997) mass function. The lens distance is approximately ; we caution that this likelihood function is sensitive to the assumed kinematics. For example, if we take , then the best-fit lens distance changes to . It is possible that a lens is a (massive) dark lens such as a white dwarf, neutron star and black hole; in this case the blended light is contributed by another unrelated star. Another possibility is that the blending is contributed by the lens itself. If the lens is a main sequence star and , which implies , then the lens will contribute about the right amount of light to explain the light curves (including the color change in the V and I bands). Further high-resolution imaging of the lensed object should allow us to differentiate between these two possibilities.
© European Southern Observatory (ESO) 1999
Online publication: October 4, 1999