11256(  1) - 03/30/07 11:11  - [  1]

                  HUBBLE SPACE TELESCOPE OBSERVING PROGRAM 11256

Version: 1    Check-in Time: 30-Mar-2007 15:10:55       STScI Edit Number: 0

Title
Neon Abundance in Hot Central Stars of Planetary Nebulae: A New Clue to Late Stellar
Evolution
------------------------------------------------------------------------------------
Type             Cycle     Parallel Pointing Tolerance
AR               16        <none>
------------------------------------------------------------------------------------
Investigators
                                                                                     Contact?
    PI: Dr. James Herald                  The Johns Hopkins University
   CoI: Dr. Luciana C. Bianchi            The Johns Hopkins University                  N

------------------------------------------------------------------------------------
Abstract

We propose to analyze archival HST spectra of a selected sample of central stars of
planetary nebulae (CSPN) for which FUSE spectra are also available. The primary goal
is to derive the neon abundance in their stellar atmospheres. While the abundances
of most "hydrogen-rich" CSPN are well explained by evolutionary models, those of
"hydrogen-deficient" CSPN are not. One explanation for the abundances of the latter
is the "born-again" scenario, which predicts enhancements in carbon, oxygen, and
neon resulting from a late helium shell flash. From recent modeling of far-UV and UV
spectra with a revised version of the stellar atmosphere code CMFGEN which included
high-ionization stages of neon, we discovered that in very hot (Teff > 80, 000 K)
CSPN, several transitions from high ionized neon may be visible at UV and far-UV
wavelengths (Herald, Bianchi & Hillier, 2005). Our modeling shows that these
features are highly sensitive to the neon abundance. We will model archival HST
spectra concurrently with FUSE spectra to determine the stellar parameters such as
effective temperature, and also the neon abundance. Our sample of hot CSPN spans a
range of evolutionary phases. The neon abundance is an important clue to the
chemical processing and subsequent dredge-up and mixing in the stellar atmosphere as
predicted by the born-again scenario. Also, neon is a key component to understanding
how CSPN contribute to the chemical enrichment of the surrounding ISM. This study
will probe the late stages of stellar evolution with unprecedented diagnostics.
------------------------------------------------------------------------------------