Capture of field stars by giant interstellar clouds: The formation of moving stellar groups

Abstract

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. As a result, a number of passing field stars is gravitationally captured by the cloud at the end of this phase; 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 comoving with the cloud have been solved analytically. We have assumed a homogeneous spheroidal cloud of fixed semimajor axis a = 300 pc and of an initial density of 7 atoms 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 are captured. The Sun itself could have been captured by the same cloud that originated the moving groups of the solar neighbourhood.