The TYCHO database as a control microlensing experiment

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The amount of dark matter in the solar neighbourhood is a poorly constrained quantity which is nevertheless used to normalise the properties of the dark halo. While the current surveys towards the LMC, SMC and the Galactic bulge probe the dark matter content at large distances from the Sun, the HIPPARCOS satellite provides a unique way to constrain the dark matter within about 1 kpc. The TYCHO program observed about a million stars in two passbands down to a magnitude of about 12, all over the sky. The large number of observations taken during the 3 year mission make such a catalogue a useful probe to validate the microlensing technique to detect baryonic compact objects, since the results can be compared to the local dynamical constraints. We predict here that about 0.1 events could be present in the TYCHO database, but after extensive Monte Carlo simulations that take into account both temporal sampling and photometric errors, we find that the efficiency is only 9% for typical events of 15 days, reducing the expected number of events to 0.01.