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Optical studies of gamma-ray burst fields. 3: GRB 790613 and GRB 781124
An optical photometric and proper motion study of the fields of thegamma-ray bursts GRB 790613 and GRB 781124 is presented. No objects werevisible on our Charge Coupled Devices (CCD) images inside the GRB 790613error box. About 15 objects are visible on our CCD images inside the GRB781124 error box. Two of these objects were found to be faint galaxies.Two stars with sizeable proper motions were identified near, but outsideof the GRB 781124 error box. No large proper motion objects were foundin the area surrounding the GRB 790613 error box.
| Excitation in the ionized diffuse interstellar medium
Large-scale spectra have been obtained in the diffuse, ionizedbackground of the Sagittarius-Carina arm and in the large complex ofloops and filaments located in Orion and Eridanus. The intensity ratiosof the emission lines of O III forbidden line, H-beta, H-alpha, N IIforbidden line and S II forbidden line have been derived from thesespectra, and are analyzed using models of H II regions in ionizationequilibrium at very low densities, down to 0.01/cu cm. The confrontationof the observed ratios with the predictions of the models, which havebeen calibrated against observations of classical H II regions, showsthat the S II forbidden line (6717 + 6731)/H-alpha ratio is too large toarise in a gas submitted only to a stellar flux with which it comes intoionization equilibrium, whatever the dilution of the matter.Contribution of shock excitation seems a natural explanation, as shocksare likely to occur considering the chaotic morphology of the studiedregions. Some alternative explanations are also suggested. However, thismedium is principally ionized by radiation, and it is shown that theforbidden line O III/H-beta ratios are well accounted for by the knownpopulation of O stars within the expected uncertainties, whileionization by white dwarfs or by B stars suggested by previous authorsare excluded. The mean effective temperature for ionizing stars (lessthan 35,000 K) is lower than that of stars exciting classical H IIregions.
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