Faint emission lines and temperature fluctuations in M8

We present echelle spectroscopy in the 3500-10300 Angstrom 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 Astronomico Nacional in San Pedro Martir, Baja California. We have measured the intensities of 274 emission lines, in particular 88 permitted lines of C+, N-0, N+, O-0, O+, Ne-0, S-0, S+, Si-0, 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 t(2) 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.