Ongoing Astrometric Microlensing Events of Two Nearby Stars
  Context. Astrometric microlensing is an excellent tool to determine the mass
of a stellar object. By measuring the astrometric shift of a background source
star in combination with precise predictions of its unlensed position and of
the lens position, gravitational lensing allows to determine the mass of the
lensing star with a precision of 1 percent, independent of any prior knowledge.
  Aims. Making use of the recently published Gaia Data Release 2 (Gaia DR2) we
predict astrometric microlensing events by foreground stars of high proper
motion passing by a background star in the coming years.
  Methods. We compile a list of ~148.000 high-proper-motion stars within Gaia
DR2 with $\mu_{tot}$ > 150 mas/yr. We then search for background stars close to
their paths and calculate the dates and separations of the closest approaches.
Using color and absolute magnitude, we determine approximate masses of the
lenses. Finally, we calculate the expected astrometric shifts and
magnifications of the predicted events.
  Results . We detect two ongoing microlensing events by the high proper motion
stars Luyten 143-23 and Ross 322 and predict closest separations of (108.5
$\pm$ 1.4) mas in July 2018 and (125.3 $\pm$ 3.4) mas in August 2018,
respectively. The respective expected astrometric shifts are (1.74 $\pm$ 0.12)
mas and (0.76 $\pm$ 0.06) mas. Furthermore, Luyten 143-23 will pass by another
star in March 2021 with a closest separation of (280.1 $\pm$ 1.1) mas, which
results in an expected shift of (0.69 $\pm$ 0.05) mas.
