Mathematics – Logic
Scientific paper
Jul 2000
adsabs.harvard.edu/cgi-bin/nph-data_query?bibcode=2000hst..prop.8714p&link_type=abstract
HST Proposal ID #8714
Mathematics
Logic
Hst Proposal Id #8714 Ism And Circumstellar Matter
Scientific paper
We propose to carry out high quality spectroscopic observations of ^12C and ^13C in the ionized gas of the planetary nebula {PN} NGC 3242 by using the C iii multiplet near < m 1908 Angstrom. Our goal is to determine the ^12C/^13C ratio in this object, following the method successfully pionereed by Clegg et al. {1997}. NGC 3242 is the only PN with a measured abundance of ^3He, an isotope of cosmological interest. The observed ^3He abundance is in agreement with the predictions of standard stellar evolution models for a 1 M_odot star. However, low-mass stars cannot produce ^3He at the level indicated by NGC 3242 and by standard models otherwise the resulting ^3He abundance in the Galaxy would be a factor 20-100 higher than observed in the local ISM, in H ii regions, and in meteorites. The only solution to this problem appears to rely on a non-standard mixing process that preferentially destroys ^3He during the RGB/AGB phases. A consequence of such mixing mechanism is the reduction by a factor 5 of the ^12C/^13C ratio in the stellar envelope. Thus, an independent measurement of the ^12C/^13C ratio, combined with the available information on the ^3He abundance, will make possible to perform a crucial test on theories of low-mass stellar nucleosynthesis. Only STIS on HST can perform the required UV observations of the C iii multiplet and measure the carbon isotopic ratio in this unique PN.
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