Runaways and shells around the CMa OB1 association
  The origin of the arc-shaped Sh2-296 nebula is still unclear. Mainly due to
its morphology, the nebula has been suggested to be a 0.5 Myr-old supernova
remnant (SNR) that could be inducing star formation in the CMa OB1 association.
We aim to show, for the first time, that the nebula is part of a large,
shell-like structure, which we have designated the ``CMa shell'', enclosing a
bubble created by successive supernova (SN) explosions. We identified three
runaway stars, associated with bow-shock structures, in the direction of the
CMa shell and we investigate the possibility that they have originated in the
center of the shell. By analyzing images of the CMa OB1 association at several
wavelengths, we clearly see that the Sh2-296 nebula is in fact part of a large
structure, which can be approximated by a large (with a diameter of ~60 pc)
elliptical shell. Using the recent Gaia-DR2 astrometric data, we trace back the
path of the three runaway stars, in order to find their original position in
the past, with relation to the CMa shell. We also revise the heating and
ionization of the Sh2-296 nebula, by comparing the photon budget provided by
the O stars in the region with results from radio observations. We find that
the runaway stars have likely been ejected from a Trapezium-like progenitor
cluster on three successive SN explosions having taken place ~6, ~2 and ~1 Myr
ago. We also show that the few late-type O stars in the region cannot explain
the ionization of the Sh~2-296 nebula and other mechanisms need to be at work.
We argue that, though we now have evidence for several SNe events in the CMa
OB1 association, the SNe probably played a minor role in triggering star
formation in these clouds. In contrast, the CMa OB1 association, as it is now,
likely testifies to the last stages of a star-forming region.
