Chemical composition of H II regions in the Large Magellanic Cloud and its cosmological implications

Abstract

Photoelectric spectrophotometry of emission lines in the A range is presented for NGC 1714, NUC 2079, IC 2111, and 30 Dor in the LMC. The observations were obtained at CTIO with the 36-inch (91-cm) and 60-inch (1.5-rn) telescopes and the Harvard scanner. From these observations the chemical abundances of He, N, 0, Ne, S, and Ar relative to hydrogen are derived. It is found that N(He)/N(H) = 0.084 i 0.005 for the four H ii regions in the Large Magellanic Cloud (LMC), this value is about 20 percent smaller than the values for the Orion and the Carina Nebulae. This is the first time that a helium-abundance difference among normal H II regions considerably higher than the observational errors is presented. From the chemical composition of H ii regions in our Galaxy and other galaxies it is found that the primeval value of N(He)/N(H) is 0.069 i 0.008 (Y = 0.216 :l:: 0.02) and that the chemical evolution of galaxies is characterized by A Y 3.3 AZ. From the big-bang computations by Wagoner for no neutrino degeneracy we obtain for the present density of the Universe, - 31.41 < log Po <- - 30.6. The primeval deuterium abundance is 4-80 times than that observed in the interstellar matter of the solar neighborhood. By adopting H0 = 55 km 1 Mpc -1 we find 0.003 < q0 < 0.02, and consequently an age of 16.8 < t0 < 17.5 billion years for the Universe. Subject headings: abundances, nebular cosmology - Magellanic Clouds - nebulae