HST Observations of Orion -- Probing the Origin of Abundance Anomalies in H II Regions

Details HST Observations of Orion -- Probing the Origin of Abundance Anomalies in H II Regions HST Observations of Orion -- Probing the Origin of Abundance Anomalies in H II Regions

Dec 1997

adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1997hst..prop.7514r&link_type=abstract

HST Proposal ID #7514

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Hst Proposal Id #7514 Interstellar Matter

Scientific paper

We propose a set of observations of emission-line diagnostics of the Orion Nebula, the brightest and best-resolved H II region. The major goal is to provide reliable elemental abundances in this benchmark of the ISM. Currently, Orion is one of a number of nebulae for which large differences in heavy element abundances are deduced from the bright forbidden lines and the much fainter recombination lines. The existence of large temperature fluctuations is one possible reason for this discrepancy; other less explored reasons include density and ionization fluctuations. Our long-slit observations with STIS cross prominent arcs associated with an HH object, a proplyd, and the Bar -- an ionization front viewed edge-on. These are the places where the effects of mechanical energy deposition into the H II region would be easiest to detect. The combination of spectral lines studied is designed to measure density, temperature, and ionic structure and their variations, and then to determine what influence they have on the inference of abundances. Analysis depends on accurate removal of the effects of foreground extinction. Proposed Palpha imagery combined with our existing H Beta images is required to put this on a secure footing. Other major goals of this program are confirmation of a serious problem discovered in our earlier HST spectra concerning the theoretical decrement of the ultraviolet He 1 lines and validation of the analysis methods commonly used on the fainter extragalactic H II regions.

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