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Classification parameters for the emission-line spectra of extragalactic objects
An investigation is made of the merits of various emission-line intensity ratios for classifying the spectra of extragalactic objects. It is shown empirically that several combinations of
Physical conditions in the Orion Nebula and an assessment of its helium abundance
New long-slit spectrophotometric observations were obtained to redetermine the helium abundance of the Orion Nebula. The ionic ratio He(+)/H(+) is found to remain nearly constant at 0.088 {plus
An Atlas of Computed Equivalent Widths of Quasar Broad Emission Lines
We present graphically the results of several thousand photoionization calculations of broad emission-line clouds in quasars, spanning 7 orders of magnitude in hydrogen ionizing flux and particle
Locally Optimally Emitting Clouds and the Origin of Quasar Emission Lines
The similarity of quasar line spectra has been taken as an indication that the emission line clouds have preferred parameters, suggesting that the environment is subject to a fine-tuning process. We
The Origin of Fe II Emission in Active Galactic Nuclei
We used a very large set of models of broad emission line region (BELR) clouds in active galactic nuclei to investigate the formation of the observed Fe II emission lines. We show that photoionized
Locally Optimally Emitting Clouds and the Narrow Emission Lines in Seyfert Galaxies
The narrow emission line spectra of active galactic nuclei are not accurately described by simple photoionization models of single clouds. Recent Hubble Space Telescope images of Seyfert 2 galaxies
The s-process should occur in all but the lower mass progenitor stars of planetary nebulae, and this should be reflected in the chemical composition of the gas that is expelled to create the current