Faint Emission Lines and Temperature Fluctuations in M8

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Abstract
We present echelle spectroscopy in the 3500-10300 Å range of the Hourglass Nebula, which is embedded in the Galactic H II region M8. The data were obtained using the 2.1 m telescope at Observatorio Astronómico Nacional in San Pedro Mártir, Baja California. We have measured the intensities of 274 emission lines, in particular 88 permitted lines of C+, N0, N+, O0, O+, Ne0, S0, S+, Si0, Si+, and Si++, some of them produced by recombination only and others mainly by fluorescence. We have determined electron temperatures and densities using different line intensity ratios. We derive the He+, C++, O+, and O++ ionic abundances as well as—for the first time in a nebular object—the total O abundance from recombination lines; these nebular values are independent of the temperature structure of the nebula. We have also derived abundances from collisionally excited lines for a large number of ions and elements; these abundances do depend on the temperature structure. Accurate t2 values have been derived by comparing the C++, O+, and O++ ionic abundances obtained making use of both collisionally excited lines and recombination lines. A comparison of the solar, Orion Nebula, and M8 chemical abundances is made.