Capture of field stars by giant interstellar clouds: the formation of
  moving stellar groups
  In the solar neighbourhood, there are moving groups of stars with similar
ages and others of stars with heterogeneous ages as the field stars. To explain
these facts, we have constructed a simple model of three phases. Phase A: a
giant interstellar cloud is uniformly accelerated (or decelerated) with respect
to the field stars during a relatively short period of time (10 Myr) and the
cloud's mass is uniformly increased; phase B: the acceleration (or
deceleration) and mass accretion of the cloud cease. The star formation spreads
throughout the cloud, giving origin to stellar groups of similar ages; and
phase C: the cloud loses all its gaseous component at a constant rate and in
parallel is uniformly decelerated (or accelerated) until reaching the initial
velocity of phase A (case 1) or the velocity of the gas cloud remains constant
(case 2). Both cases give equivalent results. The system equations for the star
motions governed by a time-dependent gravitational potential of the giant cloud
and referred to a coordinate system co-moving with the cloud have been solved
analytically. We have assumed a homogeneous spheroidal cloud of fixed
semi-major axis a=300 pc and of an initial density of 7 at cm^{-3}, with a
density increment of 100 per cent and a cloud's velocity variation of 30 km
s^{-1}, from the beginning to the end of Phase A. The result is that about 4
per cent of the field stars that are passing within the volume of the cloud at
the beginning of phase A is captured. The Sun itself could have been captured
by the same cloud that originated the moving groups of the solar neighbourhood.
