Radio Recombination Lines from Compact Planetary Nebulae

Abstract

We summarize recent results of H76α and H110α recombination line observations, made with high angular resolution, toward five compact planetary nebula. The H76α line widths averaged over the source are greater than 40 km s^-1 for all, except one, nebulae. We attribute the large widths to expansion motions of the ionized gas. The H110α line widths are significantly wider than the H76α widths, suggesting that broadening by electron impacts is important at the higher quantum number. Electron densities derived from the collisionally broadened line widths are in good agreement with those determined from optical and radio continuum data. The peak H76α line to continuum ratios among the five nebulae are spread by more than a factor of four. We conclude that the observed large range is mainly due to the considerable extent of their electron temperatures. For three nebulae the H76α line maps are noticeable different from the continuum maps. We suggest that this result is primarily due to line enhancement by stimulated emission in regions with emission measure larger than average.