Carbon Abundances in Compact Galactic Planetary Nebulae: An Ultraviolet
  spectroscopic study with the Space Telescope Imaging Spectrograph (STIS)
  We surveyed a sample of compact Galactic planetary nebulae (PNe) with the
Space Telescope Imaging Spectrograph on the Hubble Space Telescope (HST/STIS)
to determine their gas-phase carbon abundances. Carbon abundances in PNe
constrain the nature of their asymptotic giant branch (AGB) progenitors, as
well as cosmic recycling. We measured carbon abundances, or limits thereof, of
11 compact Galactic PNe, notably increasing the sample of Galactic PNe whose
carbon abundance based on HST ultraviolet spectra is available. Dust content of
most targets has been studied elsewhere from Spitzer spectroscopy; given the
compact nature of the nebulae, both UV and IR spectra can be directly compared
to study gas- and dust-phase carbon. We found that carbon-poor (C/O<1) compact
Galactic PNe have oxygen-rich dust type (ORD), while their carbon-enhanced
counterparts (C/O>1) have carbon-rich dust (CRD), confirming the correlation
between gas- and dust-phase carbon content which was known for Magellanic Cloud
PNe. Based on models of expected final yields from AGB evolution we interpret
the majority of the carbon-poor PNe in this study as the progeny of ~1.1-1.2
M$_{\odot}$ stars that experienced some extra-mixing on the red giant branch
(RGB), they went through the AGB but did not go through the carbon star phase.
Most PNe in this group have bipolar morphology, possibly due to the presence of
a sub-solar companion. Carbon-enhanced PNe in our sample could be the progeny
of stars in the ~1.5-2.5 M$_{\odot}$ range, depending on their original
metallicity.
