The excitation of the Network nebulae.

Abstract

view Abstract Citations References Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS The excitation of the Network nebulae. Chamberlain, Joseph W. Abstract It has been proposed that the Network nebulae in Cygnus derive their energy of radiation from an expanding nova shell1 or, as recent work indicates,2 from a shock wave set up by a nova explosion that is moving through the interstellar medium. Presumably, the shock front would generate very high temperatures in the interstellar gas, so that the kinetic energy of the electrons would be sufficient to excite neutral hydrogen atoms by inelastic collisions. Neglecting the effect of any stellar radiation, but considering electron-hydrogen collisions, we may set up the equations of statistical equilibrium for hydrogen in an optically thin nebula. These equations have been solved in a manner analogous to that used by Menzel and Baker for the case of excitation by stellar radiation.3 The solution gives us values of b (the factor inserted in the combined excitation-ionization equation to take account of departures from thermodynamic equilibrium) for all levels n > I. The bn's and the Balmer decrement have thus been determined for Te = 10,0000; 20,0000; and 40,0000K. Collisional cross sections obtained from the Born approximation were used in the calculations. A number of direct photographs of the Network nebulae have been made with the ~-inch, f3.5 Schmidt telescope at the University of Michigan for the purpose of determining surface brightness in the filaments. Three combinations of filters and emulsion types were used in order to isolate the emission lines of IJot, the green nebular lines of EOiii , and the EOIIJ doublet at X3727. Photometric comparisons were made with M 32. Since the sizes of the luminous filaments may be estimated, the measures of surface brightness could be used to obtain approximate values of the emission in ergs cm 3 sec- in each of the three emission lines. The data, as interpreted by the collisional theory, seem to give the most consistent picture at Te 20,0000K. Although the O7H ratio appears to be too small by a factor of 5.5, a considerable portion of the oxygen may be in ionization stages not observed. A hydrogen density of 500 atomscm3 is indicated, and hydrogen would be about 95 per cent ionized at 20,0000 K. A similar interpretation is provided by radiative- excitation theory (Menzel and Baker's3 case A2) for Te = io,ooo0K, which is about the maximum temperature allowed under excitation by stellar radiation. Therefore, the final decision between the excitation mechanisms cannot be made with the present data. Much will depend on data obtainable only from photometrically calibrated slit spectra made with a large reflector. Since the computed collisional decrement is considerably steeper than the case A2 decrement, measures of the relative intensities of the hydrogen lines would be extremely valuable. Moreover, the electron temperature has not yet been satisfactorily determined but could probably be estimated fairly precisely from the relative intensities of the EOIII lines. Details of the computations and the observations will be published elsewhere. I. J. H. Qort, M. N. io6, 359, 3946. 2. J. M. Burgers, Private communications. 3. Ap. J. 86, 70, 3937; 88, 52, 1938. Geophysics Research Division, Air Force Cambridge Research Center, Cambridge, Mass. Publication: The Astronomical Journal Pub Date: October 1952 DOI: 10.1086/106733 Bibcode: 1952AJ.....57..158C full text sources ADS |