AUSTRALIA TELESCOPE COMPACT ARRAY OBSERVATIONS OF RADIO RECOMBINATIONLINES TOWARD 30-DORADUS

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 Austral ia Telescope Compact Array toward the 30 Doradus Nebula, the giant H I I region in the Large Magellanic Cloud. In this paper, emphasis is pla ced on the more sensitive H90 alpha observations, which also include t he He90 alpha and H113 beta lines. A spatial resolution of 15 '' was o btained 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 he liocentric 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 profil e is double, suggesting that sections of expanding shells are being ob served. The mean (LTE) electron temperature is dose to 7700 K over mos t of the nebula. The intensity ratio of H113 beta/H90 alpha is close t o the expected LTE value of 0.28, implying that the calculated LTE ele ctron temperatures are close to the true electron temperatures; in one area to the north the ratio is enhanced, 0.39 +/- 0.03. The helium ab undance (Y+ = He+/H+) shows no significant variation across the source , with a mean value of 0.13 +/- 0.02.