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Astron. Astrophys. 359, 103-112 (2000)

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5. Conclusions

We have shown that, with the data available up to now, the spheroid star distribution follows a power law with an index smaller than previously thought. It is moderately flattened, as already found by several investigations. The best fit power law index is found 2.44 for a flattening of 0.76. We cannot exclude a spheroid population having a power law as low as 2 and an axis ratio of 0.5 at the 2 sigma level. Assuming a high exponent of 3.5, as suggested by some globular cluster and RR Lyrae data, model predictions deviate significantly from observations in the external parts of the galaxy whatever reasonable flattening is adopted.

The IMF slope of the spheroid is found to be [FORMULA], value which gives a local density of 1.64 10-4 stars pc-3 and a mass density of 4.15 10-5 [FORMULA] pc-3 for the stellar halo, yet this value ignores possible old white dwarfs. With this slope the expected mass density of brown dwarfs in the halo makes a negligible part of the dark matter halo, as already estimated from microlensing surveys.

Recent searches for ancient halo white dwarfs have given a hope to identify the microlensing events with such objects (Ibata et al., 1999; Ibata et al., 2000; Hodgkin et al., 2000). Ibata et al. (2000) conclude that old white dwarfs may constitute a significant fraction of about 10% of the dark matter halo. In the mean time, microlensing experiments have narrowed the range of the estimated halo baryonic fraction to 20 to 50% (Alcock et al., 2000). These two results are well in agreement according to the uncertainties.

So, as star count data progresses in depth and extent, the picture of the spheroid star population that comes out points to characteristics quite compatible with what we know about the distribution of baryonic dark matter if it is made of stellar remnants, suggesting a common dynamical origin. The visible spheroid and its heavy counterpart of dark remnants can make a significant but not dominant part of the so-called dark matter halo.

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© European Southern Observatory (ESO) 2000

Online publication: June 30, 2000