Kinematics of the Gum nebula region

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.