  We present new HST optical imagery as well as new UV and IR spectroscopic
data obtained with the Hubble and Spitzer Space Telescopes, respectively, of
the halo planetary nebula DdDm-1. For the first time we present a resolved
image of this object which indicates that the morphology of DdDm-1 can be
described as two orthogonal elliptical components in the central part
surrounded by an extended halo. The extent of the emission is somewhat larger
than was previously reported in the literature. We combine the spectral data
with our own previously published optical measurements to derive nebular
abundances of He, C, N, O, Ne, Si, S, Cl, Ar, and Fe. Our abundance
determinations include the use of the newly developed program ELSA for
obtaining abundances directly from emission line strengths along with detailed
photoionization models to render a robust set of abundances for this object.
The metallicity, as gauged by oxygen, is found to be 0.46 dex below the solar
value, confirming DdDm-1's status as a halo PN. In addition, we find that Si
and Fe are markedly underabundant, suggesting their depletion onto dust. The
very low (but uncertain) C/O ratio suggests that the chemistry of the nebula
should be consistent with an oxygen-rich environment. We find that the sulfur
abundance of DdDm-1 is only slightly below the value expected based upon the
normal lockstep behavior between S and O observed in H II regions and blue
compact galaxies. The central star effective temperature and luminosity are
estimated to be 55,000 K and 1000 solar luminosities, respectively, implying an
initial progenitor mass of <1 solar masses. Finally, we report on a new radial
velocity determination from echelle observations.
