The Spectrophotometry of the Orion Nebula

Citation

Méndez, Manuel E. (1964) The Spectrophotometry of the Orion Nebula. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/SPX8-CV53. https://resolver.caltech.edu/CaltechETD:etd-10182002-085141

Abstract

NOTE: Text or symbols not renderable in plain ASCII are indicated by [...]. Abstract is included in .pdf document. The spectral range from [lamda]3600 to [lamda]11 000 has been observed in the spectra of the Orion nebula. The large amount of dust in the nebula effects the intensity of the lines, according to their wavelengths. The correction, which takes into account this effect, has been obtained using the ratio of the Paschen to the Balmer lines, which arise from the same upper level. The unreddened spectrum-emission lines plus continuum has been analyzed. The self-absorption of the Balmer lines has been proven to be non-existent. The Balmer decrement observed is almost equal to that produced by pure recombination. The helium lines indicate small departures from the prediction of the recombination theory. The metastability of the 2[superscript 3]S level has been investigated, including the ionization of the neutral atom, from this level, by Ly-[alpha] quanta. It has been demonstrated that the idea of the exciting star being in the center of the nebula, is consistent with the strengths of the [lamda]3889 lines observed in absorption. Since the self-absorption of the triplet lines of HeI is significant, the H/He ratio has been redetermined. The result indicates a ratio of 11 to 1 (by number). Since the recombination theory has been found to explain the Balmer decrement, the electron density distribution was obtained from surface brightness determinations and compared with the densities indicated by the [lamda]3726-29 [0II] lines obtained by Osterbrock in the past. A direct comparison of those two sets of observations has shown that the density fluctuations grow with distance from the center of the nebula. The mass of the nebula is about 30M[...] with a probable dust content of about 0.15M[...]. The latter quantity derived with the assumption that the radius of the solid particles can be represented by an average size of 0.16[microns]. With the use of the [0III] forbidden lines, the electron temperature was determined at several points in the nebula. The distribution obtained clearly indicates that the inner regions are colder than the outer. Since the lines of various elements were observed, an abundance determination was undertaken. The ratio, by number, of He to heavier elements is close to 50, which agrees with the abundance found in the solar cosmic rays. Then by mass, the chemical composition of the nebula can be represented by the following three numbers: X = 0.72, Y = 0.26,, and Z = 0.02. The collision strengths for SII are not well known. Therefore an empirical determination of these parameters is also presented. The ratio of the nebular to auroral lines of [0II] also indicates the existence of density variations. The ratios observed in the nebula have been used to determine the extent of those fluctuations.

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