A B S T R A C TTo investigate the kinematics of the neutral material around the Gum nebula, emission from hydroxyl at 1667 MHz was observed at many positions over the region. Fitting models of expanding shells to these data together with previously published molecular line data shows that the diffuse molecular clouds and cometary globules form a single expanding shell centred on G26122.5. The mean angular radius is 10.58 and its maximum radius is 148. The models show that the distance range to the expansion centre is from 200 pc to 500 pc.The path of the runaway O-star z Puppis passed within ,0.58 of the expansion centre of the neutral shell ,1.5 Myr ago. The supernova of the erstwhile binary companion of z Puppis is the probable origin of the Gum nebula and the swept up expanding neutral shell. The 500-pc distance 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 observed 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 21 respectively.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 primarily ionized by z Puppis, which is still within the neutral shell. No evidence was found for the IRAS-Vela shell as a separate expanding shell.

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The 1667‐MHz ground‐state hydroxyl (OH) line has been observed towards positions on Puppis A as well as in the immediate vicinity of this supernova remnant (SNR). The spectra at two positions on Puppis A show absorption lines at velocities less than +7.6 km s−1, and emission lines above this velocity. Analysis of all four 18 cm OH lines indicates that Puppis A must lie between the OH clouds seen in absorption and emission, giving it a vlsr of +7.6 km s−1. This implies that the kinematic distance to Puppis A is The OH clouds in front of Puppis A are slightly anomalously excited, as shown by analysis of the four 18 cm OH lines at the two observed positions on Puppis A. There is no evidence of any interaction of the SNR with the clouds in front of and behind it.

A B S T R A C THydrogen recombination lines in the H156a and H139a transitions have been detected at four widely separated positions in the Gum nebula. This confirms that the radio continuum emission seen in parts of the nebula is predominantly bremsstrahlung rather than synchrotron emission.The derived electron temperatures and emission measures are in the range 4200 to 8500 K and 220 to 470 pc cm 26 respectively. This is consistent with the presence of a low-density, photoionized plasma. The linewidth observed at the position away from the edge of the nebula is significantly larger than those near the edge of the nebula. Together with the negative line velocity observed at this position, this suggests there is systematic expansion of the near side of the nebula.