The 5 degrees diameter ionized halo of the planetary nebula Abell 36

We have observed an ionized halo surrounding the planetary nebula Abell 36. It is barrel-shaped, with dimensions 4 degrees x 5 degrees, which is 17 pc x 21 pc at a distance of 240 pc. With an average H alpha surface brightnes s of 0.8 +/- 0.2 rayleighs, the halo's total H alpha flux, 8.3 x 10(-10) er gs cm(-2) s(-1), exceeds that of the previously known inner part of the neb ula by a factor of 30. The ionized mass of the halo is 48 root epsilon M ., where epsilon is the filling factor of the ionized gas. Velocity-resolved Ha spectra indicate that the halo is ambient interstellar matter and not ma tter ejected from the planetary nebula. The edges of the halo are evident i n both 100 mum emission and red optical continuum.