Compact Planetary Nebulae in the Galactic Disk: Analysis of the Central
  Stars
  We have obtained multi-wavelength observations of compact Galactic planetary
nebulae (PNe) to probe post-Asymptotic Giant Branch (AGB) evolution from the
onset of nebular ejection. We analyze new observations from HST to derive the
masses and evolutionary status of their central stars (CSs) in order to better
understand the relationship between the CS properties and those of the
surrounding nebulae. We also compare this sample with others we obtained using
the same technique in different metallicity environments: the Large and Small
Magellanic Clouds. We work with HST/WFC3 images of 51 targets obtained in a
snapshot survey (GO-11657). The high spatial resolution of HST allows us to
resolve these compact PNe and distinguish the CS emission from that of their
surrounding PNe. The targets were imaged through the filters F200LP, F350LP,
and F814W from which we derive Johnson V and I magnitudes. We derive CS
bolometric luminosities and effective temperatures using the Zanstra technique,
from a combination of HST photometry and ground-based spectroscopic data. We
present new unique photometric measurements of 50 CSs, and derived effective
temperatures and luminosities for most of them. Central star masses for 23
targets were derived by placing the stars on a temperature-luminosity diagram
and compare their location with the best available single star post-AGB
evolutionary tracks, the remaining masses were indeterminate most likely
because of underestimates of the stellar temperature, or because of substantial
errors in the adopted statistical distances to these objects. The distribution
of CS masses in the sample of compact PNe is different than sample in the LMC
and SMC, but with a median mass of 0.59 solar masses it is similar to other
Galactic samples. We conclude that the compact nature of many of the PNe is a
result of their large distance, rather than their physical dimension.
