Australia Telescope Compact Array Observations of Radio Recombination Lines toward 30 Doradus

Abstract

Three hydrogen recombination lines---H90 alpha at 8.9 GHz, H92 alpha at 8.3 GHz, and H109 alpha at 5.0 GHz---have been observed with the Australia Telescope Compact Array toward the 30 Doradus Nebula, the giant H II region in the Large Magellanic Cloud. In this paper, emphasis is placed on the more sensitive H90 alpha observations, which also include the He90 alpha and H113 beta lines. A spatial resolution of 15" was obtained with a velocity resolution of 4.2 km s-1 and an rms noise of 1.6 mJy. Typical line-to-continuum ratios were 0.04 for the H90 alpha and H92 alpha , and 0.03 for the H109 alpha data. The distributions of radial velocity and electron temperature have been determined. The heliocentric velocity of the ionized gas varies between 245 and 290 km s-1 with a mean value of 262 km s-1. In some places the line profile is double, suggesting that sections of expanding shells are being observed. The mean (LTE) electron temperature is close to 7700 K over most of the nebula. The intensity ratio of H113 beta /H90 alpha is close to the expected LTE value of 0.28, implying that the calculated LTE electron temperatures are close to the true electron temperatures; in one area to the north the ratio is enhanced, 0.39 +/- 0.03. The helium abundance (Y+ = He+/H+) shows no significant variation across the source, with a mean value of 0.13 +/- 0.02.