TRACING ABUNDANCES IN GALAXIES WITH THE SPITZER ...

distances to more of our objects than other studies, such as van de Steene & Zijl-
stra (1995) and Cahn et al. (1992). An accurate statistical distance scale for PNe
is difficult to obtain, and controversies exist as to which statistical distance scales
are the best: for example Bensby & Lundström (2001) find that Zhang’s distance
scale is good, while Ciardullo et al. (1999) find that it is not as good. However,
regardless of which statistical distances we adopt, the conclusions of the paper will
remain unchanged because all of the PNe in our sample are constrained to be in
the Bulge by other criteria and we include the large uncertainties that go with
these statistical distances in the data analysis.
We adopt the distance from the Sun to the Galactic Center, Ro, from Reid
(1993) who determines the best estimate of this distance by taking a weighted
average of the various determinations of Ro from different methods. Reid (1993)
finds that Ro = 8.0 ± 0.5 kpc, and this value seems to agree with more current
estimates of this distance (e.g. López-Corredoira et al., 2000; Eisenhauer et al.,
2005; Groenewegen et al., 2008). Galactocentric distances (RGC) are then calcu-
lated assuming this Ro, and uncertainties on RGC are calculated using standard
error propagation and assuming an uncertainty of 40% on the heliocentric distance
(Zhang 1995 estimates the accuracy of the PN distance scale as 35-50% on aver-
age). If the distance to a PN is unknown, we assume it is within 4 kpc of the
Galactic Center. Table 3.2 lists the heliocentric and galactocentric distances for
each object.
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