Observations of the recombination-line region toward Sagittarius A

in an investigation of the properties of the recombination-line region toward Sgr A a search was conducted at A21 cm for the following lines: H166a, He166a, C166a, H208fl, and He+265e. Only the H166a line was detected, and this line had a peak antenna temperature of 0.43 K. The observational limits for the H208fl line and the He166a line are consistent with a model in which the lines are emitted in a region having an electron temperature of 20 K and an electron density of 5 cm -2 and in which the ratio of ionized helium to hydrogen n(He + )/n(H +) < 0.046. Assuming that the total helium-to-hydrogen ratio is "normal," [He/H] = 0.10, the observational n(He+)/ n(H+) ratio can be explained if either (1) the line-emitting region is ionized by a distribution of hot pre-white-dwarf stars (UV stars) having temperatures in the range 30,000 <% T <% 50,000 K or (2) the rate for He + recombination via the radiative charge-transfer reaction He + + H He + H + + hv exceeds the radiative recombination rate. These two interpretations are evaluated; the ionization rate and total cloud density that they require to explain the observational results are discussed. Subject heqdings: galactic nuclei - nebulae - radio lines