Excitation in the ionized diffuse interstellar medium

Details Excitation in the ionized diffuse interstellar medium Excitation in the ionized diffuse interstellar medium

Apr 1986

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1986a%26a...158..279s&link_type=abstract

Astronomy and Astrophysics (ISSN 0004-6361), vol. 158, no. 1-2, April 1986, p. 279-287.

Astronomy and Astrophysics

Astrophysics

32

H Alpha Line, Interstellar Gas, Ionized Gases, Stellar Spectrophotometry, Diffusion, Galaxies, H Ii Regions, Magellanic Clouds, Supernova Remnants

Scientific paper

Large-scale spectra have been obtained in the diffuse, ionized background of the Sagittarius-Carina arm and in the large complex of loops and filaments located in Orion and Eridanus. The intensity ratios of the emission lines of O III forbidden line, H-beta, H-alpha, N II forbidden line and S II forbidden line have been derived from these spectra, and are analyzed using models of H II regions in ionization equilibrium at very low densities, down to 0.01/cu cm. The confrontation of the observed ratios with the predictions of the models, which have been calibrated against observations of classical H II regions, shows that the S II forbidden line (6717 + 6731)/H-alpha ratio is too large to arise in a gas submitted only to a stellar flux with which it comes into ionization equilibrium, whatever the dilution of the matter. Contribution of shock excitation seems a natural explanation, as shocks are likely to occur considering the chaotic morphology of the studied regions. Some alternative explanations are also suggested. However, this medium is principally ionized by radiation, and it is shown that the forbidden line O III/H-beta ratios are well accounted for by the known population of O stars within the expected uncertainties, while ionization by white dwarfs or by B stars suggested by previous authors are excluded. The mean effective temperature for ionizing stars (less than 35,000 K) is lower than that of stars exciting classical H II regions.

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