Physics
Scientific paper
Jul 1996
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=1996hst..prop.6780j&link_type=abstract
HST Proposal ID #6780
Physics
Hst Proposal Id #6780 Interstellar Medium
Scientific paper
We found the third {and probably last} PN to be associated with a Galactic globular cluster in NGC 6441. Optical spectra indicate an unexpectedly severe oxygen depletion {O/Fe - 0.8}. Ionization models {CLOUDY} indicate that oxygen at higher ionization stages may account for half of the depletion. HST observations are requested to {1} obtain UV spectra of the nebula to constrain models further and to determine directly the contributions from unseen higher stages of ionization of oxygen, while measuring carbon and nitrogen abundances, and {2} image the nebula at high resolution to ascertain its degree of disruption by the ISM. Neither the UV spectra nor the high resolution imaging can be done with ground-based telescopes. The optically derived O/Fe is 30-40* smaller than seen in the M15 PN or in field stars. This ratio impacts the ages derived for clusters where the field star assumption is used. Stellar dredge-up theory does not account for a depletion of oxygen this extreme. There is a small chance that the NGC 6441 PN is NOT a member of the cluster; its velocity differs from the cluster by 20 km/s. This difference could be due to an interaction with the ISM. Direct images of the nebula are requested to examine the morphology. An interaction will be evident by the presence of a bow shock or by displacement of the ``central'' star from the nebula center.
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