Hydrogen recombination lines from the Gum nebula

Abstract

Hydrogen recombination lines in the H156α and H139α 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−6 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.