Abstract
The nearest stellar system consists of the stars Proxima, alpha Centauri A
and B and at least one planet Proxima b. The habitability of Proxima b and any
other planets are likely to be significantly influenced by the orbital
evolution of the system. To study the dynamical evolution of the system, we
simulate the motions of Proxima and alpha Centauri due to the perturbations
from the Galactic tide and stellar encounters in a Monte Carlo fashion. From
100 clones, we find that 74% orbits of Proxima Centauri are bound to alpha
Centauri while 17% and 9% orbits become unbound in the simulations over the
past and future 5 Gyr. The ratio of unstable clones increases with the
simulation time scale and encounter rate. The dynamical history of Proxima is
sensitive to the uncertainty of the astrometric data as well as the uncertainty
of the model of stellar encounters. This provides some evidence for a capture
scenario for the formation of the current triple system. We note that the
spectroscopic evidence in the literature indicates a super-solar metallicity
for alpha Centauri and a solar one for Proxima supporting different origins and
thus the capture hypothesis. Nonetheless, further improvements in the available
data and models will be necessary for a reliable assessment of the history of
the alpha Centauri system and its impact on the habitability of Proxima b.
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