To investigate the kinematics of the neutral material around the Gum nebula
, emission from hydroxyl at 1667 MHz was observed at many positions over th
e region. Fitting models of expanding shells to these data together with pr
eviously published molecular line data shows that the diffuse molecular clo
uds and cometary globules form a single expanding shell centred on G261-2.5
. The mean angular radius is 10.5 degrees and its maximum radius is 14 degr
ees. The models show that the distance range to the expansion centre is fro
m 200pc to 500pc.
The path of the runaway O-star zeta Puppis passed within <0.5<degrees> of t
he expansion centre of the neutral shell similar to 1.5 Myr ago. The supern
ova of the erstwhile binary companion of zeta Puppis is the probable origin
of the Gum nebula and the swept up expanding neutral shell. The 500-pc dis
tance to the supernova. is adopted as the distance to the expansion centre
of the neutral shell. At this distance the energy required to produce the o
bserved expansion could have been met with a single supernova. The radii of
the front and back faces of the shell are 130 and 70 pc respectively. The
front face is expanding faster than the back face, at 14 and 8.5 km s(-1) r
espectively.
The extent of the neutral shell matches the radio continuum and Ha emission
of the Gum nebula well. The photoionized gas in the nebula is probably pri
marily ionized by Puppis, which is still within the neutral shell. No evide
nce was found for the IRAS-Vela shell as a separate expanding shell.