Astronomy and Astrophysics – Astrophysics
Scientific paper
Jul 2007
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2007hst..prop11185r&link_type=abstract
HST Proposal ID #11185. Cycle 16
Astronomy and Astrophysics
Astrophysics
Scientific paper
Our recent abundance survey of a large sample of Galactic planetary nebulae PNe has led to the discovery of a group of super-metal-rich nebulae whose spectra are characterized by prominent optical recombination lines ORLs from C, N, O, & Ne ions and a large Balmer discontinuity jump. The heavy element abundances derived from ORLs for several PNe are more than an order of magnitude higher than those derived from the traditional method based on collisionally excited lines CELs, while the Balmer jump yields electron temperatures Te significantly lower than values derived from the [O III] 5007/4363 CEL line ratio. A proposition that aspires to explain both the nebular abundance and Te problems is one according to which these nebulae contain at least two distinct emission regions - one of "normal" Te ~ 10000 K and chemical composition ~solar and another of very low Te that is H-deficient, thus having high helium and metal abundances relative to hydrogen. The latter component emits strong He and heavy element ORLs but essentially no CELs. The consistent picture that emerges from fitting a 2-component photoionization model to the spectroscopic data is that the H-poor component is in high-density inclusions, which provide only a minor fraction of the total nebular mass. We propose to directly detect these inclusions in the planetary nebula M 1-42 using WFPC2 PC to make a high spatial resolution image in the He I 5876 A ORL and ratio it to Halpha. With NICMOS NIC1, we plan to observe the He I 10830 A line, which is substantially collisionally excited, along with Palpha 18760 A. The ratio image of He I 10830 to Palpha is expected to be less likely to show the inclusions, thus serving as an important control to the optical imaging. M 1-42 is one of the most extreme cases of the abundance and Te problem; it is reasonably bright and compact. This program has the potential to resolve a serious challenge to our current understanding of nebular astrophysics.;
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