352.0-04.6 | 1.53$\pm$0.07 | 11.14$\pm$0.02 | 8.88${}_{-0.04}^{+0.03}$ | 9.02$\pm$0.04 | 8.42$\pm$0.04 | 7.30${}_{-0.06}^{+0.05}$ | 6.92${}_{-0.04}^{+0.03}$ | 5.43${}_{-0.04}^{+0.03}$
352.1+05.1 | 1.49${}_{-0.01}^{+0.02}$ | 11.10$\pm$0.01 | 8.56$\pm$0.02 | 8.76$\pm$0.01 | 8.32$\pm$0.01 | 7.01${}_{-0.05}^{+0.04}$ | 6.62$\pm$0.02 | 5.37$\pm$0.01
352.6+03.0 | 2.58$\pm$0.06 | 11.18$\pm$0.01 | 8.66$\pm$0.02 | 8.84$\pm$0.02 | 8.49$\pm$0.01 | 7.08$\pm$0.08 | 6.74${}_{-0.04}^{+0.03}$ | 5.50$\pm$0.02
⋆353.2-05.2 | 0.80$\pm$0.06 | 11.24$\pm$0.01 | 8.50${}_{-0.04}^{+0.03}$ | 8.69$\pm$0.03 | 8.09$\pm$0.05 | 6.64${}_{-0.06}^{+0.05}$ | 6.41${}_{-0.04}^{+0.03}$ | 4.47$\pm$0.05
353.3+06.3 | 0.91$\pm$0.05 | 10.96$\pm$0.02 | 7.72${}_{-0.09}^{+0.08}$ | 8.46${}_{-0.05}^{+0.04}$ | 7.94${}_{-0.08}^{+0.05}$ | 6.56${}_{-0.08}^{+0.07}$ | 6.00${}_{-0.05}^{+0.04}$ | 4.97${}_{-0.06}^{+0.05}$
353.7+06.3 | 0.81$\pm$0.07 | 11.05$\pm$0.02 | 8.10$\pm$0.05 | 8.81$\pm$0.04 | 8.02$\pm$0.10 | 6.88${}_{-0.06}^{+0.05}$ | 6.38$\pm$0.04 | 5.24$\pm$0.03
354.5+03.3 | 3.48$\pm$0.07 | 10.96$\pm$0.02 | 8.56$\pm$0.03 | 8.70$\pm$0.02 | 8.22$\pm$0.02 | 6.87$\pm$0.04 | 6.48$\pm$0.04 | 5.08${}_{-0.04}^{+0.03}$
354.9+03.5 | 2.33$\pm$0.07 | 11.08$\pm$0.01 | 8.23$\pm$0.05 | 8.62${}_{-0.07}^{+0.06}$ | 8.35${}_{-0.08}^{+0.07}$ | 6.90${}_{-0.09}^{+0.08}$ | 6.41${}_{-0.06}^{+0.05}$ | 5.15${}_{-0.05}^{+0.04}$
355.4-02.4 | 1.98$\pm$0.06 | 11.13$\pm$0.01 | 8.79$\pm$0.03 | 8.81$\pm$0.03 | 8.30$\pm$0.03 | 7.12$\pm$0.06 | 6.74${}_{-0.06}^{+0.05}$ | 5.45$\pm$0.03
355.9+03.6 | 1.47$\pm$0.07 | 10.71$\pm$0.02 | 7.13${}_{-0.14}^{+0.05}$ | 8.04${}_{-0.20}^{+0.07}$ | 7.60${}_{-0.20}^{+0.06}$ | 6.53${}_{-0.06}^{+0.05}$ | 5.68${}_{-0.07}^{+0.05}$ | 4.46${}_{-0.12}^{+0.07}$
355.9-04.2 | 1.19$\pm$0.03 | 11.17$\pm$0.02 | 8.58$\pm$0.02 | 9.11${}_{-0.15}^{+0.09}$ | 7.94${}_{-0.04}^{+0.03}$ | 7.39$\pm$0.11 | 6.85$\pm$0.05 | 5.76${}_{-0.04}^{+0.03}$
⋆356.1-03.3 | 1.20$\pm$0.05 | 11.14$\pm$0.01 | 8.55$\pm$0.02 | 8.90$\pm$0.04 | 8.20$\pm$0.04 | 7.00$\pm$0.04 | 6.57$\pm$0.04 | 5.50${}_{-0.06}^{+0.05}$
356.3-06.2 | 0.43$\pm$0.05 | 11.11$\pm$0.01 | 8.49${}_{-0.08}^{+0.05}$ | 8.70${}_{-0.07}^{+0.06}$ | 7.83${}_{-0.04}^{+0.03}$ | 6.84${}_{-0.06}^{+0.05}$ | 6.40${}_{-0.04}^{+0.03}$ | 5.07$\pm$0.04
⋆356.5-03.6 | 2.46$\pm$0.10 | 11.13$\pm$0.02 | 8.47${}_{-0.07}^{+0.06}$ | 8.78${}_{-0.05}^{+0.04}$ | 7.78${}_{-0.07}^{+0.06}$ | 7.05${}_{-0.07}^{+0.06}$ | 6.63${}_{-0.05}^{+0.04}$ | 5.44$\pm$0.04
⋆356.8+03.3 | 2.24$\pm$0.06 | 10.82$\pm$0.01 | 8.36$\pm$0.04 | 8.54${}_{-0.07}^{+0.06}$ | - | 6.65${}_{-0.11}^{+0.10}$ | 6.39${}_{-0.06}^{+0.05}$ | 5.45$\pm$0.08
356.8-05.4 | 0.84$\pm$0.06 | 11.12$\pm$0.01 | 8.14$\pm$0.03 | 8.86$\pm$0.04 | 7.87$\pm$0.04 | 6.84$\pm$0.04 | 6.49${}_{-0.04}^{+0.03}$ | 5.03$\pm$0.03
356.9+04.4 | 1.94$\pm$0.05 | 10.94$\pm$0.01 | 8.44${}_{-0.06}^{+0.05}$ | 8.67${}_{-0.06}^{+0.05}$ | 8.01${}_{-0.04}^{+0.03}$ | 6.99${}_{-0.07}^{+0.06}$ | 6.55$\pm$0.04 | 5.07$\pm$0.04
357.0+02.4 | 2.32$\pm$0.06 | 11.12$\pm$0.01 | 8.55${}_{-0.04}^{+0.03}$ | 8.88$\pm$0.03 | 8.23$\pm$0.04 | 7.23$\pm$0.06 | 6.78${}_{-0.04}^{+0.03}$ | 5.55${}_{-0.05}^{+0.04}$
357.1+03.6 | 0.99$\pm$0.05 | 11.00$\pm$0.01 | 8.06${}_{-0.05}^{+0.04}$ | 8.73$\pm$0.04 | 8.40$\pm$0.04 | 7.17${}_{-0.07}^{+0.06}$ | 6.58$\pm$0.04 | 5.03${}_{-0.06}^{+0.05}$
⋆357.1+04.4 | 1.81${}_{-0.05}^{+0.06}$ | 11.16$\pm$0.01 | 7.36${}_{-0.09}^{+0.07}$ | 9.03${}_{-0.08}^{+0.07}$ | 8.51${}_{-0.11}^{+0.08}$ | 6.69${}_{-0.15}^{+0.10}$ | 6.54${}_{-0.07}^{+0.06}$ | 4.90${}_{-0.07}^{+0.06}$
357.1-04.7 | 1.02${}_{-0.05}^{+0.04}$ | 10.21$\pm$0.02 | 8.50${}_{-0.08}^{+0.06}$ | 8.50${}_{-0.17}^{+0.11}$ | - | 6.86${}_{-0.08}^{+0.07}$ | - | 5.64${}_{-0.10}^{+0.07}$
⋆357.2+02.0 | 2.34$\pm$0.07 | 10.98$\pm$0.01 | 7.87$\pm$0.04 | 8.83$\pm$0.04 | 8.16$\pm$0.03 | 6.65$\pm$0.05 | 6.30$\pm$0.03 | 4.88$\pm$0.04
⋆357.3+04.0 | 1.65$\pm$0.05 | 11.05$\pm$0.01 | 8.18${}_{-0.07}^{+0.06}$ | 8.81$\pm$0.03 | 8.26$\pm$0.03 | 7.05${}_{-0.07}^{+0.06}$ | 6.55$\pm$0.03 | 4.98${}_{-0.04}^{+0.03}$
⋆357.5+03.1 | 1.98$\pm$0.07 | - | 6.98${}_{-0.08}^{+0.07}$ | 7.09${}_{-0.20}^{+0.13}$ | - | 5.94${}_{-0.07}^{+0.06}$ | - | 4.89${}_{-0.14}^{+0.11}$
357.5+03.2 | 2.30$\pm$0.08 | 11.11$\pm$0.01 | 8.86$\pm$0.05 | 8.93$\pm$0.04 | 8.47${}_{-0.06}^{+0.05}$ | 7.37${}_{-0.06}^{+0.05}$ | 6.78${}_{-0.05}^{+0.04}$ | 5.43${}_{-0.08}^{+0.07}$
357.6-03.3 | 1.59$\pm$0.06 | 11.16$\pm$0.01 | 7.95${}_{-0.05}^{+0.04}$ | 8.53${}_{-0.11}^{+0.09}$ | 8.43${}_{-0.17}^{+0.12}$ | 6.51$\pm$0.04 | 6.09${}_{-0.07}^{+0.06}$ | -
⋆357.9-03.8 | 1.56$\pm$0.05 | 11.25$\pm$0.01 | 6.78${}_{-0.10}^{+0.08}$ | 7.63$\pm$0.04 | 7.27$\pm$0.04 | 5.31${}_{-0.07}^{+0.06}$ | 5.82$\pm$0.04 | 3.96${}_{-0.05}^{+0.04}$
357.9-05.1 | 1.32$\pm$0.07 | 11.07$\pm$0.01 | 8.61${}_{-0.03}^{+0.02}$ | 8.78$\pm$0.04 | 8.23$\pm$0.03 | 6.97${}_{-0.04}^{+0.03}$ | 6.47$\pm$0.03 | 5.16$\pm$0.03
⋆358.0+09.3 | 0.63${}_{-0.11}^{+0.12}$ | 11.11$\pm$0.02 | 7.40${}_{-0.10}^{+0.08}$ | 8.51${}_{-0.05}^{+0.04}$ | 7.94$\pm$0.04 | 6.11${}_{-0.07}^{+0.06}$ | 6.12${}_{-0.05}^{+0.04}$ | 4.63${}_{-0.08}^{+0.07}$
358.2+03.5 | 2.69${}_{-0.11}^{+0.10}$ | 10.96$\pm$0.02 | 7.91${}_{-0.07}^{+0.06}$ | 8.47${}_{-0.06}^{+0.05}$ | 7.61$\pm$0.04 | 6.56${}_{-0.09}^{+0.07}$ | 5.97${}_{-0.05}^{+0.04}$ | 4.85$\pm$0.05
358.2+04.2 | 2.15$\pm$0.06 | 11.09$\pm$0.01 | 8.26${}_{-0.04}^{+0.03}$ | 8.75$\pm$0.03 | 8.32$\pm$0.03 | 6.95${}_{-0.10}^{+0.09}$ | 6.65${}_{-0.04}^{+0.03}$ | 5.40$\pm$0.04
⋆358.5+02.9 | 2.09$\pm$0.06 | 10.95$\pm$0.02 | 7.34${}_{-0.04}^{+0.03}$ | 8.37$\pm$0.04 | 7.78$\pm$0.03 | 6.31${}_{-0.06}^{+0.05}$ | 5.85$\pm$0.03 | 4.57$\pm$0.04
358.5-04.2 | 1.52$\pm$0.05 | 10.96$\pm$0.03 | 7.66$\pm$0.04 | 8.34${}_{-0.04}^{+0.03}$ | 7.75${}_{-0.04}^{+0.03}$ | 6.77${}_{-0.06}^{+0.05}$ | 6.15${}_{-0.04}^{+0.03}$ | 4.81${}_{-0.04}^{+0.03}$
⋆358.6+07.8 | 0.98$\pm$0.05 | 11.04$\pm$0.01 | 8.07$\pm$0.04 | 8.72$\pm$0.03 | 8.13${}_{-0.03}^{+0.02}$ | 7.09$\pm$0.05 | 6.36$\pm$0.03 | 5.42${}_{-0.04}^{+0.03}$
358.6-05.5 | 0.80$\pm$0.06 | 11.18$\pm$0.01 | 8.40${}_{-0.04}^{+0.03}$ | 8.51${}_{-0.03}^{+0.02}$ | 8.01$\pm$0.03 | 6.71${}_{-0.07}^{+0.06}$ | 6.39$\pm$0.04 | 4.73$\pm$0.06
358.7+05.2 | 2.57$\pm$0.10 | 10.26$\pm$0.02 | 8.32${}_{-0.06}^{+0.05}$ | 8.51${}_{-0.11}^{+0.09}$ | - | 6.90${}_{-0.08}^{+0.06}$ | 6.28${}_{-0.08}^{+0.07}$ | 5.96${}_{-0.18}^{+0.15}$
358.8+03.0 | 1.89$\pm$0.06 | 11.11$\pm$0.01 | 8.45$\pm$0.03 | 8.88$\pm$0.03 | 8.19$\pm$0.03 | 7.14$\pm$0.04 | 6.71$\pm$0.03 | 5.25${}_{-0.04}^{+0.03}$
358.9+03.4 | 2.54$\pm$0.07 | 11.11$\pm$0.01 | 8.83$\pm$0.02 | 8.91$\pm$0.02 | 8.48$\pm$0.02 | 7.57${}_{-0.06}^{+0.05}$ | 6.97${}_{-0.04}^{+0.03}$ | 5.77${}_{-0.02}^{+0.03}$
359.0-04.1 | 1.16$\pm$0.07 | 11.14$\pm$0.01 | 8.71$\pm$0.03 | 8.88$\pm$0.04 | 8.31${}_{-0.05}^{+0.04}$ | 7.14$\pm$0.05 | 6.69$\pm$0.04 | 5.25${}_{-0.04}^{+0.03}$
359.1-02.9 | 1.31$\pm$0.06 | 11.13$\pm$0.01 | 8.73${}_{-0.04}^{+0.03}$ | 8.85$\pm$0.04 | 8.24${}_{-0.05}^{+0.04}$ | 6.85${}_{-0.08}^{+0.06}$ | 6.54$\pm$0.04 | 5.26${}_{-0.10}^{+0.09}$
⋆359.2+04.7 | 2.10$\pm$0.07 | 11.24$\pm$0.02 | 7.43${}_{-0.09}^{+0.05}$ | 8.28${}_{-0.12}^{+0.07}$ | - | 5.87${}_{-0.17}^{+0.08}$ | 6.43${}_{-0.08}^{+0.05}$ | 5.54${}_{-0.07}^{+0.05}$
359.3-01.8 | 3.28$\pm$0.05 | 10.20$\pm$0.02 | 8.30${}_{-0.12}^{+0.09}$ | 8.54${}_{-0.21}^{+0.13}$ | - | 6.95${}_{-0.15}^{+0.09}$ | 5.99${}_{-0.09}^{+0.08}$ | 5.64${}_{-0.18}^{+0.11}$
359.6-04.8 | 0.95$\pm$0.04 | 11.28$\pm$0.01 | 7.33$\pm$0.06 | 8.13$\pm$0.03 | 7.65$\pm$0.03 | 6.35${}_{-0.07}^{+0.06}$ | 6.13$\pm$0.03 | 4.64${}_{-0.05}^{+0.04}$
Table 11: continued PN G | c(H$\beta$) | He/H | N/H | O/H | Ne/H | S/H | Ar/H | Cl/H
---|---|---|---|---|---|---|---|---
359.7-01.8 | 2.47$\pm$0.13 | 10.97$\pm$0.02 | 7.47${}_{-0.43}^{+0.17}$ | 8.59${}_{-0.43}^{+0.09}$ | 7.48${}_{-0.05}^{+0.04}$ | 6.40${}_{-0.43}^{+0.13}$ | 6.06${}_{-0.05}^{+0.04}$ | 4.84${}_{-0.07}^{+0.06}$
⋆359.8+02.4 | 2.85$\pm$0.08 | 10.79$\pm$0.01 | 8.44${}_{-0.05}^{+0.04}$ | 8.78${}_{-0.07}^{+0.06}$ | - | 6.96${}_{-0.08}^{+0.07}$ | 6.43${}_{-0.07}^{+0.06}$ | 5.51${}_{-0.06}^{+0.05}$
359.8+03.7 | 2.32$\pm$0.09 | 10.98$\pm$0.02 | 7.63${}_{-0.07}^{+0.06}$ | 8.53${}_{-0.06}^{+0.05}$ | - | 6.63${}_{-0.08}^{+0.07}$ | 6.16${}_{-0.06}^{+0.05}$ | 5.08${}_{-0.06}^{+0.05}$
⋆359.8+05.2 | 1.33$\pm$0.04 | 11.03$\pm$0.01 | 7.99${}_{-0.05}^{+0.04}$ | 8.37${}_{-0.12}^{+0.09}$ | - | 6.72$\pm$0.04 | 6.07${}_{-0.06}^{+0.05}$ | 6.20${}_{-0.13}^{+0.11}$
359.8+05.6 | 0.72${}_{-0.08}^{+0.07}$ | 10.11$\pm$0.03 | 8.26${}_{-0.09}^{+0.07}$ | 8.66${}_{-0.17}^{+0.11}$ | - | 7.03${}_{-0.15}^{+0.11}$ | 6.45${}_{-0.09}^{+0.07}$ | 5.86${}_{-0.14}^{+0.11}$
359.8+06.9 | 1.42${}_{-0.07}^{+0.06}$ | 11.02$\pm$0.02 | 8.57$\pm$0.04 | 8.78${}_{-0.03}^{+0.02}$ | 8.23$\pm$0.03 | 7.02${}_{-0.05}^{+0.04}$ | 6.48$\pm$0.04 | 5.15$\pm$0.03
359.8-07.2 | 0.67${}_{-0.09}^{+0.10}$ | 10.95$\pm$0.02 | 7.48${}_{-0.09}^{+0.07}$ | 8.44$\pm$0.03 | 7.83$\pm$0.02 | 6.27$\pm$0.05 | 5.88$\pm$0.03 | 5.05${}_{-0.07}^{+0.06}$
359.9-04.5 | 1.97${}_{-0.10}^{+0.09}$ | 11.16$\pm$0.02 | 8.76$\pm$0.03 | 8.78$\pm$0.03 | 8.36$\pm$0.02 | 7.21${}_{-0.09}^{+0.08}$ | 6.78${}_{-0.05}^{+0.04}$ | 5.55${}_{-0.04}^{+0.15}$

We compared our data with results of PNe in the Galactic bulge given in
Chiappini et al. (2009) CGS09, which combined the data sets from Górny et al.
(2004) which is itself a merger of PNe abundance measures from Cuisinier et
al. (1999), Escudero & Costa (2001), and Escudero et al. (2004), Górny et al.
(2009) and Wang & Liu (2007). We limited our comparison to high-quality
measurements with reported uncertainties of less than 0.3 dex (removing of 25%
of their data set), as done in their analysis. The median, 25th and 75th
percentile values of He/H, N/H, O/H, Ar/H, Ne/H, S/H, Cl/H derived in this
work and in CGS09 are presented in Table.12 together with the number of PNe
with each measurement available.

Table 12: Statistics of chemical abundances for all the Bulge PNe measured in this work and in CGS09. Columns 2-5 provide information about the median, 25th and 75th percentile values (in parentheses in columns 2-3), and the number of PNe measurements used for the calculation (in brackets in column 5). Columns 6-9 present results obtained from the combined literature data sets in CGS09, expressed in the same manner. Column 10 displays solar abundances taken from Asplund et al. (2009). The abundances are given on a log scale where $\log$(H)$=12$ | This work |  | CGS09 | Sun
---|---|---|---|---
He/H | 11.06 | [10.97, 11.14] | (123) |  | 11.11 | [11.05, 11.19] | (144) | 10.93$\pm$0.01
N/H | 8.27 | [7.88, 8.49] | (122) |  | 8.11 | [7.76, 8.50] | (123) | 7.83$\pm$0.05
O/H | 8.70 | [8.53, 8.83] | (124) |  | 8.57 | [8.40, 8.72] | (117) | 8.69$\pm$0.05
Ne/H | 8.14 | [7.92, 8.29] | (105) |  | 7.93 | [7.71, 8.17] | (77) | 7.93$\pm$0.10
S/H | 6.90 | [6.63, 7.08] | (124) |  | 6.79 | [6.54, 7.04] | (94) | 7.12$\pm$0.03
Ar/H | 6.44 | [6.14, 6.61] | (122) |  | 6.34 | [6.05, 6.56] | (120) | 6.40$\pm$0.13
Cl/H | 5.14 | [4.89, 5.41] | (119) |  | 6.22 | [6.00, 6.50] | (47) | 5.50$\pm$0.30

In comparison to the CGS09 findings, our results show higher median abundances
for N and the alpha elements (O, Ne, S, and Ar), with differences ranging from
0.10-0.30 dex. Notably, the median O/H, Ne/H, Ar/H values in our data are
slightly super-solar. However, the median abundance for S, other $\alpha$
elements, remains sub-solar due to the well-known ’sulphur anomaly’ (Henry et
al., 2004, 2012). Our results also show a slightly lower median abundance for
He, with a tail towards lower values, by approximately 0.05 dex. In terms of
chlorine, our median abundance value is lower than the literature value by
1.09 dex. This discrepancy is primarily due to the CGS09 study, where most of
the Cl/H values are derived from GCS09. We discuss the reasons for this
discrepancy in Section 5.1. The Cl/H results in CGS09 are mostly extremely
super-solar, which is also contrary to the behaviour of other alpha elements
in their results. In contrast, 90% of the Cl/H values in our results are sub-
solar, and the difference between the PN Cl abundances and solar abundance
becomes smaller. Thus, we believe that our Cl/H estimation is more reliable.

In summary, our data offers a highly accurate and precise representation of
the general abundance pattern of PNe in the Galactic bulge, thanks to the
consistently reduced and analysed high s/n VLT spectra, the availability of
emission lines for a broader range of ionic species, and the incorporation of
updated atomic data and the ICF scheme introduced in DMS14. Our results reveal
a systematic increase in median values for elemental abundances across the
majority of elements. The median abundances of alpha elements, O, Ne, and Ar,
are found to be super-solar in our assessment.

##  7 Discussion

It is clear from this work that accurate PN physical conditions and abundance
measures, even determined from excellent, high quality s/n spectra is an
inherently difficult process as is the comparison with previous work. There
are many reasons for this. Prime among these is the assumption that chemical
abundances determined for any individual PN are representative of the entire
nebula whereas we know that internal abundance variations do exist, albeit
with small variations (e.g., Manchado et al. (1988); Danehkar et al. (2013)).
Nevertheless, for the comparisons made here we use a single, overall abundance
estimate as measured from an integrated 1-D spectrum from our compact PNe that
are all $\leq$10 arcseconds across. Hence decent central, representative
fractions of all our PNe are always sampled by the spectrograph slit. We also
know that PNe are complex, resolved 3-D objects with internal condensations,
density, temperature and ionisation variations etc, e.g. Monteiro et al.
(2004); Akras et al. (2016). An important secondary factor, especially when
comparing with other work, are issues such as the chosen telescope and
spectrograph, the slit widths and positions in terms of orientation on the
projected PN image and offsets from any CSPN and of course the observing
conditions (seeing, transparency, airmass etc.) all of which vary between data
sets.

The future of PNe abundance determinations is with deep IFU work such as in
Danehkar et al. (2013); García-Rojas et al. (2022). IFUs have already shown
how physical conditions and chemical abundances can vary across a given PN.
However, until direct point-to-point mapping of physical parameters and
chemical abundances based on IFU observations are available for large samples
of PNe from different studies, this type of research remains essential.
Nevertheless, the currently best plasma diagnostics used to determine the same
property for some individual PNe do not show the levels of agreement hoped for
and for all the reasons given above. Nevertheless, we believe we have produced
the most internally self-consistent and robust data set to date for PNe
abundance studies.

###  7.1 Correlations between the abundances $\alpha$-element

The $\alpha$-elements, including O, Ne, S, and Ar, are produced via the alpha
processes during the evolution of massive stars, where nuclear fusion
transforms helium into elements heavier than carbon. As these elements share
common synthesis sites in massive stars (e.g. Henry et al., 2008; Esteban et
al., 2020), their elemental abundances are expected to vary in lockstep.
Therefore, in low- to- intermediate-mass stars, including the PN progenitors,
their abundance ratios should remain constant. Similarly, Cl is produced by
single particle (proton or neutron) capture by an S or Ar isotope, and
therefore, indicating that it should exhibit similar behaviour (Esteban et
al., 2015). Thus, generally, the abundances of alpha elements observed in PNe
reflect the amounts that were initially present in the star-forming gas of the
progenitor star, which enables PNe to serve as a tool for establishing the
metallicity of the ISM during the formation of PN progenitor stars (Kwitter &
Henry, 2022).

Among these elements, O is commonly used as a proxy for metallicity in ionised
gases around stars, and a lockstep behaviour between $\alpha$ elements has
been demonstrated in H ii regions and blue compact galaxies (BCG), with a
strong correlation between elemental abundances of Ne, Ar and S, and O/H and a
slope close to unity on a log scale. However, previous studies on PN have
observed a slightly weaker correlation. To investigate the lockstep behaviour
with PN abundances compiled in this work, we compared the abundances of
$\alpha$-elements Ne and Ar, along with Cl, with that of oxygen in Fig. 12.
The plot showing the relationship between S/H and O/H will be presented in the
next paper in the series (Tan & Parker, submitted), where we specifically
address the well-known ‘sulfur anomaly’ observed in PNe. Thus, it is not
included in the current paper.

Each element-versus-oxygen plot was fitted with a straight line using the
least squares fitting method. The slopes, intercepts of best-fit lines, and
correlation coefficients are summarised in Table 13. For Ne and Ar, the dashed
line represents the best-fit to a collection of published H ii region and BCG
abundances (H2BCG) that clearly demonstrating the lockstep behaviour of
$\alpha$ elements, with abundances of H ii regions from Kennicutt Jr et al.
(2003) and BCGs compiled by Izotov & Thuan (1999). The analogous fit for Cl
was inferred from a study of Galactic disk H ii regions by Arellano-Córdova et
al. (2020). The red asterisks in the figure denote solar abundances.

Previous studies on PN abundances (Henry et al., 2004; Milingo et al., 2010,
e.g.) have observed the lockstep behaviour with correlation coefficients of
around 0.7 of Ne and Ar with a slope around 0.9 (Kwitter & Henry, 2022).
However, this study obtained a much stronger correlation than those in the
literature of around 0.85. The best-fit line for Ar/h versus O/H is highly
consistent with the H2BCG results, suggesting that the previous less apparent
lockstep behaviour observed in PNe could largely be attributed to
observational and measurement errors. However, the Ne/H results from PNe do
not agree with the best-fit of H2BCG results at extremely sub-solar
abundances, which could be due to uncertainties in ICFs used to account for
unobserved ionization stages more dominant than Ne2+. As a result, the slope
derived for Ne/H versus O/H is smaller than unity.

As previously mentioned in Sec. 9, the determination of Cl/H is typically
subject to uncertainty. The Cl/H versus O/H plot appears messier, exhibiting
large scatters and increased measurement uncertainty at extremely sub-solar
and super-solar abundances. Among all elements, the correlation between Cl/H
and O/H is the weakest, with a correlation coefficient of $r=0.59$.

Figure 12: Abundances for $\log$(Ne/H), $\log$(Ar/H) and $\log$(Cl/H) for all
PNe in our sample that provide an estimate plotted against $\log$(O/H), with
error bars are from our measurement uncertainties. The black solid line in
each panel represents the best-fit to the data. The best-fit parameters and
associated uncertainties are presented in Table.13. For Ne and Ar, the dashed
orange lines show the fit to abundances for H ii regions and BCGs from the
literature. The orange dashed line for Cl presents an analogous fit based on
abundances of Galactic disk H ii regions from Arellano-Córdova et al. (2020).
The red asterisks denote the solar abundances.

X/H vs. O/H | Slope | y-Intercept | r | # of PNe
---|---|---|---|---
Ne | $0.90\pm 0.05$ | $\ \ 0.32\pm 0.46$ | 0.86 | 105
Ar | $0.95\pm 0.06$ | $-1.86\pm 0.48$ | 0.84 | 122
Cl | $0.76\pm 0.10$ | $-1.44\pm 0.83$ | 0.59 | 119
Table 13: Least squares fit parameters and Pearson’s r values for
$\alpha$-element correlations. The straight line was fit to $12+\log$(O/H) and
$12+\log$(X/H). The number of PNe used is shown in the last column.

To conclude, the expected lockstep behaviour for alpha elements is more
prominent in our data compared to previous PN abundance studies. We observe a
strong correlation between O/H and both Ar/H and Ne/H on a log scale, as
reported in the literature. The lockstep association between Cl and O is not
clearly evident in our data, which we attribute to large measurement
uncertainties.

###  7.2 He and N abundances

He and N are elements that could be produced in more massive PN progenitors as
suggested by Kaler (1978) and Kaler & Jacoby (1990). We find 104 out of 124
PNe exhibit He/H measurements greater than solar (i.e. $>10.93$) while with
several objects with measured $12+\log$(He/H) lower than 10.3. The derived N
enrichment, $\log$(N/O), range from $-1.67$ to 0.43, with a median value of
$-0.48_{-0.37}^{+0.29}$ dex. This is 0.09 and 0.15 dex lower than the results
compiled in KB94 for southern PNe and in CGS09 for the bulge PNe,
respectively. We remark there that the KB94 ICF scheme we adopted for N
abundance in this study uses N/O$=$N+/O+ and we observe large discrepancies in
N/H when comparing with the literature. This discrepancy could be due to the
subtraction of the recombination contribution of [N ii] and [O ii] auroral
lines. In this case, ignoring the recombination correction could lead to
significant systematic uncertainties.

Fig. 13 presents the relationship between N/O and He/H for all objects with
measurable abundances. A significant fractions of objects in our sample
exhibit N enrichment above the solar value. In addition to a few objects show
extremely sub-solar He/H values, which are presumably underestimated, as well
as a few objects with low N/O ratios in which the large uncertainties might
present as discussed in Wesson et al. (2018), the overall pattern suggests
that high N/O values are associated with high He/H. We will further use the
measurements of N enrichment and He/H to investigate the central star
properties of PNe in Paper V in this series of papers.

Figure 13: The N enrichment relative to O, $\log$(N/O), versus the he
abundances $12+\log$(He/H), of 121 PNe in this sample. The horizontal black
dotted line indicates the solar $12+\log$(He/H) values of 10.93 while the
vertical line represents the solar nitrogen enrichment of $-0.85$. The red
asterisks also denote the solar values.

###  7.3 Oxygen-poor PNe in this bulge sample

Three PNe, PNG 004.0-03.0, PNG 005.0-03.9 and PNG 357.5+03.1 are measured with
extremely sub-solar oxygen abundances, with $12+\log$(O/H) $<7.5$ dex ( 15.5
times lower than the solar abundance). Their $12+\log$(O/H) estimates are
$6.95\pm 0.02$, $7.41^{+0.04}_{-0.05}$ and $7.09^{+0.13}_{-0.20}$ dex,
respectively. Despite large uncertainties in O/H for PNG 357.5+03.1, where the
lower error is 46%, the O/H values measured for PNG 004.0-03.0 and PNG
005.0-03.9 have good precision, with uncertainties below 0.1 dex, confirming
their oxygen poor status based on our spectroscopic observations. The
extremely sub-solar oxygen abundance of PNG 004.0-03.0 has been noticed in
previous studies, such as Peña et al. (1991) and Exter et al. (2004). Torres-
Peimbert et al. (1997) and Miszalski et al. (2009) found that this anomalously
low abundance is primarily due to low spatial resolution ground-based
observations averaging over the strong spatial variation of [O iii
$\lambda$4363/$\lambda$5007\. Torres-Peimbert et al. (1997) measured an
$12+\log$(O/H) value of 8.5 dex from _HST_ spectroscopy of the inner nebula,
while Miszalski et al. (2011) calculated an $12+\log$(O/H) of 8.3 dex in the
outer nebula using VLT/FLAMES IFU observations. PNG 005.0-03.9 has a complex
structure classified as Eamrs according to Tan et al. (2023a), all the
temperature diagnostic lines, including [O ii], [N ii] and [O iii], give an
estimate of electron temperatures greater than 15,000 K, which is similar to
PNG 004.0-03.0. The low oxygen abundance measured with the VLT spectroscopy
could be due to the low spatial resolution as for PNG 004.0-03.0.

PNG 357.5+03.1 is a very compact and low-excitation nebula with no [O iii]
$\lambda$4363 line detected in its spectra. Only the weak [O iii]
$\lambda$5007 lines of O2+ were well-detected with low s/n.

##  8 Conclusions

We have presented logarithmic extinction coefficients, plasma diagnostics of
$n_{\mathrm{e}}$ and $T_{\mathrm{e}}$ and elemental abundances of He, N, O,
Ne, S, Ar and Cl for a well-defined sample of 124 Galactic bulge PNe with
high-quality, wide-wavelength coverage optical spectra obtained using the ESO
VLT/FORS2 with consistent instrumental configurations and observing
strategies. We believe this represents the most self-consistent and well
characterised sample of PNe available with high s/n spectra, from which well-
determined abundances are compiled. For 34 PNe, the abundance determinations
are presented for the first time, significantly adding ($\sim 14$%) to the
Bulge PNe population abundances. Moreover, for an additional 6 Bulge PNe, we
provide the first reliable abundances. Comparing our work with the best
previous Bulge PNe abundances in the literature for PNe overlapping with our
sample demonstrates the robustness and overall reliability and consistency of
this sample.

The physical parameters in PNe exhibit a wider range in this study. The low-
to-medium resolution spectroscopic observation from the VLT exhibit excellent
consistency with previous high-resolution spectra with 2-m class telescope,
demonstrating the reliability of weak line detection, the line deblending
methods and our line flux measurement, particularly for weak recombination
lines. Our abundance compilation, which adopted updated atomic data and ICF
schemes introduced in DMS14, results in overall higher abundances of alpha
elements than solar compared to the general abundance pattern in the
literature. The lockstep behaviour of alpha elements, which was less evident
in previous studies, is clearly observed in our results.

The further analysis and investigation of these abundances form the basis of
the following paper in the series.

## Acknowledgements

We are very grateful to the anonymous referee for his/her insightful comments
that have significantly contributed to the improvement of the final version of
this paper. ST thanks HKU and QAP for provision of an MPhil scholarship and a
research assistant position. QAP thanks the Hong Kong Research Grants Council
for GRF research support under grants 17326116 and 17300417. AAZ acknowledges
support from STFC under grant ST/T000414/1.

This paper is based on observations made with the ESO VLT under programme IDs
095.D-0270(A), 097.D-0024(A), 099.D-0163(A), and 0101.D-0192(A) (PI: Rees).
This research made use of NASA’s Astrophysics Data System; the SIMBAD
database, operated at CDS, Strasbourg, France; alfa: an automated line fitting
algorithm (Wesson, 2016); neat: nebular empirical analysis tool (Wesson et
al., 2012); APLpy, an open-source plotting package for Python hosted at
http://aplpy.github.io; Astropy, a community-developed core Python package for
Astronomy (Robitaille et al., 2013); MATPLOTLIB, a Python library for
publication quality graphics (Barrett et al., 2005); chianti, a collaborative
project involving George Mason University, the University of Michigan (USA),
and the University of Cambridge (UK).

## Data Availability

The raw VLT/FORS2 spectroscopic data can be accessed through the ESO archive
facility at http://archive.eso.org/. The codes for the analysis, as well as
intermediate data products are available from ST under reasonable request.

## References

  * Acker et al. (1991) Acker A., Raytchev B., Koeppen J., Stenholm B., 1991, Astronomy and Astrophysics Supplement Series, 89, 237
  * Acker et al. (1992) Acker A., Marcout J., Ochsenbein F., Stenholm B., Tylenda R., Schohn C., 1992, The Strasbourg-ESO Catalogue of Galactic Planetary Nebulae. Parts I, II.. European Southern Observatory
  * Acker et al. (2006) Acker A., Peyaud A. E., Parker Q., 2006, Proceedings of the International Astronomical Union, 2, 355
  * Akras et al. (2016) Akras S., Clyne N., Boumis P., Monteiro H., Gonçalves D., Redman M., Williams S., 2016, Monthly Notices of the Royal Astronomical Society, 457, 3409
  * Aller (1984) Aller L. H., 1984, Physics of Thermal Gaseous Nebulae
  * Appenzeller et al. (1998) Appenzeller I., et al., 1998, The messenger, 94
  * Arellano-Córdova et al. (2020) Arellano-Córdova K. Z., Esteban C., García-Rojas J., Méndez-Delgado J. E., 2020, Monthly Notices of the Royal Astronomical Society, 496, 1051
  * Arnaboldi (2014) Arnaboldi M., 2014, Formation and Evolution of the Galactic Bulge, p. 10
  * Asplund et al. (2009) Asplund M., Grevesse N., Sauval A. J., Scott P., 2009, Annual review of astronomy and astrophysics, 47, 481
  * Athanassoula (2005) Athanassoula E., 2005, Monthly Notices of the Royal Astronomical Society, 358, 1477
  * Babusiaux et al. (2010) Babusiaux C., et al., 2010, Astronomy & Astrophysics, 519, A77
  * Barbuy et al. (2018) Barbuy B., Chiappini C., Gerhard O., 2018, Annual Review of Astronomy and Astrophysics, 56, 223
  * Barrett et al. (2005) Barrett P., Hunter J., Miller J. T., Hsu J.-C., Greenfield P., 2005, in Astronomical data analysis software and systems XIV. p. 91
  * Bensby et al. (2011a) Bensby T., et al., 2011a, Astronomy & Astrophysics, 533, A134
  * Bensby et al. (2011b) Bensby T., Alves-Brito A., Oey M., Yong D., Melendez J., 2011b, The Astrophysical Journal Letters, 735, L46
  * Bensby et al. (2013) Bensby T., et al., 2013, Astronomy & Astrophysics, 549, A147
  * Bensby et al. (2017) Bensby T., et al., 2017, Astronomy & Astrophysics, 605, A89
  * Blagrave et al. (2007) Blagrave K., Martin P., Rubin R., Dufour R., Baldwin J., Hester J., Walter D., 2007, The Astrophysical Journal, 655, 299
  * Boffin et al. (2018) Boffin H. M., et al., 2018, Astronomy & Astrophysics, 619, A84
  * Brocklehurst & Seaton (1972) Brocklehurst M., Seaton M., 1972, Monthly Notices of the Royal Astronomical Society, 157, 179
  * Brooks & Christensen (2016) Brooks A., Christensen C., 2016, Galactic Bulges, pp 317–353
  * Cavichia et al. (2010) Cavichia O., Costa R., Maciel W., 2010, Revista mexicana de astronomía y astrofísica, 46, 159
  * Chiappini et al. (2003) Chiappini C., Romano D., Matteucci F., 2003, Monthly Notices of the Royal Astronomical Society, 339, 63
  * Chiappini et al. (2009) Chiappini C., Górny S., Stasińska G., Barbuy B., 2009, Astronomy & Astrophysics, 494, 591
  * Combes & Elmegreen (1993) Combes F., Elmegreen B., 1993, Astronomy and Astrophysics, 271, 391
  * Combes & Sanders (1981) Combes F., Sanders R., 1981, Astronomy & Astrophysics, 96, 164
  * Cuisinier et al. (1999) Cuisinier F., Maciel W. J., Köppen J., Acker A., Stenholm B., 1999, arXiv preprint astro-ph/9910521
  * Danehkar (2021) Danehkar A., 2021, The Astrophysical Journal Supplement Series, 257, 58
  * Danehkar et al. (2013) Danehkar A., Parker Q. A., Ercolano B., 2013, MNRAS, 434, 1513
  * Debattista et al. (2004) Debattista V. P., Carollo C. M., Mayer L., Moore B., 2004, The Astrophysical Journal, 604, L93
  * Del Zanna & Storey (2022) Del Zanna G., Storey P., 2022, Monthly Notices of the Royal Astronomical Society, 513, 1198
  * Delgado-Inglada et al. (2014) Delgado-Inglada G., Morisset C., Stasińska G., 2014, Monthly Notices of the Royal Astronomical Society, 440, 536
  * Delgado-Inglada et al. (2015) Delgado-Inglada G., Rodríguez M., Peimbert M., Stasińska G., Morisset C., 2015, Monthly Notices of the Royal Astronomical Society, 449, 1797
  * Dere et al. (1997) Dere K., Landi E., Mason H., Fossi B. M., Young P., 1997, Astronomy and Astrophysics Supplement Series, 125, 149
  * Dere et al. (2019) Dere K. P., Del Zanna G., Young P. R., Landi E., Sutherland R. S., 2019, The Astrophysical Journal Supplement Series, 241, 22
  * Di Matteo et al. (2014) Di Matteo P., et al., 2014, Astronomy & Astrophysics, 567, A122
  * Dopita & Meatheringham (1990) Dopita M., Meatheringham S., 1990, The Astrophysical Journal, 357, 140
  * Durand et al. (1998) Durand S., Acker A., Zijlstra A., 1998, Astronomy and Astrophysics Supplement Series, 132, 13
  * Escalante & Morisset (2005) Escalante V., Morisset C., 2005, Monthly Notices of the Royal Astronomical Society, 361, 813
  * Escalante et al. (2012) Escalante V., Morisset C., Georgiev L., 2012, Monthly Notices of the Royal Astronomical Society, 426, 2318
  * Escudero & Costa (2001) Escudero A., Costa R., 2001, Astronomy & Astrophysics, 380, 300
  * Escudero et al. (2004) Escudero A., Costa R., Maciel W., 2004, Astronomy & Astrophysics, 414, 211
  * Esteban et al. (2015) Esteban C., García-Rojas J., Pérez-Mesa V., 2015, Monthly Notices of the Royal Astronomical Society, 452, 1553
  * Esteban et al. (2020) Esteban C., Bresolin F., García-Rojas J., Toribio San Cipriano L., 2020, Monthly Notices of the Royal Astronomical Society, 491, 2137
  * Exter et al. (2004) Exter K., Barlow M., Walton N., 2004, Monthly Notices of the Royal Astronomical Society, 349, 1291
  * Feltzing & Gilmore (1999) Feltzing S., Gilmore G., 1999, in Galaxy Evolution: Connecting the Distant Universe with the Local Fossil Record: Proceedings of a Colloquium held at the Observatoire de Paris-Meudon from 21–25 September, 1998. pp 337–340
  * Fisher & Drory (2016) Fisher D. B., Drory N., 2016, Galactic Bulges, pp 41–75
  * Fitzpatrick (1999) Fitzpatrick E. L., 1999, Publications of the Astronomical Society of the Pacific, 111, 63
  * Freeman et al. (2013) Freeman K., et al., 2013, MNRAS, 428, 3660
  * Freudling et al. (2013) Freudling W., Romaniello M., Bramich D., Ballester P., Forchi V., García-Dabló C., Moehler S., Neeser M., 2013, Astronomy & Astrophysics, 559, A96
  * García-Rojas et al. (2022) García-Rojas J., Morisset C., Jones D., Wesson R., Boffin H. M., Monteiro H., Corradi R. L., Rodríguez-Gil P., 2022, Monthly Notices of the Royal Astronomical Society, 510, 5444
  * Gesicki et al. (2014) Gesicki K., Zijlstra A., Hajduk M., Szyszka C., 2014, Astronomy & Astrophysics, 566, A48
  * Girard et al. (2007) Girard P., Köppen J., Acker A., 2007, Astronomy & Astrophysics, 463, 265
  * Gonzalez et al. (2011) Gonzalez O., Rejkuba M., Minniti D., Zoccali M., Valenti E., Saito R., 2011, Astronomy & Astrophysics, 534, L14
  * Gonzalez et al. (2013) Gonzalez O., Rejkuba M., Zoccali M., Valent E., Minniti D., Tobar R., 2013, Astronomy & Astrophysics, 552, A110
  * Górny et al. (2004) Górny S., Stasińska G., Escudero A., Costa R., 2004, Astronomy & Astrophysics, 427, 231
  * Górny et al. (2009) Górny S., Chiappini C., Stasińska G., Cuisinier F., 2009, Astronomy & Astrophysics, 500, 1089
  * Gutenkunst et al. (2008) Gutenkunst S., Bernard-Salas J., Pottasch S., Sloan G., Houck J., 2008, The Astrophysical Journal, 680, 1206
  * Henry et al. (2004) Henry R. B., Kwitter K., Balick B., 2004, The Astronomical Journal, 127, 2284
  * Henry et al. (2008) Henry R., Kwitter K., Dufour R., Skinner J., 2008, The Astrophysical Journal, 680, 1162
  * Henry et al. (2012) Henry R. B., Speck A., Karakas A. I., Ferland G. J., Maguire M., 2012, The Astrophysical Journal, 749, 61
  * Izotov & Thuan (1999) Izotov Y. I., Thuan T. X., 1999, The Astrophysical Journal, 511, 639
  * Johnson et al. (2011) Johnson C. I., Rich R., Fulbright J., Valenti E., McWilliam A., 2011, The Astrophysical Journal, 732, 108
  * Johnson et al. (2014) Johnson C. I., Rich R. M., Kobayashi C., Kunder A., Koch A., 2014, The Astronomical Journal, 148, 67
  * Jones et al. (2016) Jones D., Wesson R., García-Rojas J., Corradi R. L., Boffin H. M., 2016, Monthly Notices of the Royal Astronomical Society, 455, 3263
  * Juan de Dios & Rodríguez (2021) Juan de Dios L., Rodríguez M., 2021, Monthly Notices of the Royal Astronomical Society, 507, 5331
  * Kaler (1978) Kaler J. B., 1978, The Astrophysical Journal, 226, 947
  * Kaler (1986) Kaler J. B., 1986, The Astrophysical Journal, 308, 322
  * Kaler & Jacoby (1990) Kaler J. B., Jacoby G. H., 1990, The Astrophysical Journal, 362, 491
  * Kennicutt Jr et al. (2003) Kennicutt Jr R. C., Bresolin F., Garnett D. R., 2003, The Astrophysical Journal, 591, 801
  * Kingsburgh & Barlow (1994) Kingsburgh R. L., Barlow M., 1994, Monthly Notices of the Royal Astronomical Society, 271, 257
  * Kisielius et al. (2009) Kisielius R., Storey P., Ferland G. J., Keenan F., 2009, Monthly Notices of the Royal Astronomical Society, 397, 903
  * Kormendy & Kennicutt Jr (2004) Kormendy J., Kennicutt Jr R. C., 2004, Annu. Rev. Astron. Astrophys., 42, 603
  * Kwitter & Henry (2022) Kwitter K. B., Henry R., 2022, Publications of the Astronomical Society of the Pacific, 134, 022001
  * Liu (2006) Liu X.-w., 2006, Proceedings of the International Astronomical Union, 2, 219
  * Liu et al. (2000) Liu X.-W., Storey P., Barlow M., Danziger I., Cohen M., Bryce M., 2000, Monthly Notices of the Royal Astronomical Society, 312, 585
  * Liu et al. (2006) Liu X., Barlow M., Zhang Y., Bastin R., Storey P., 2006, Monthly Notices of the Royal Astronomical Society, 368, 1959
  * Loon et al. (2003) Loon J. T. v., et al., 2003, Monthly Notices of the Royal Astronomical Society, 338, 857
  * Magrini et al. (2009) Magrini L., Stanghellini L., Villaver E., 2009, The Astrophysical Journal, 696, 729
  * Manchado et al. (1988) Manchado A., Pottasch S. R., Mampaso A., 1988, A&A, 191, 128
  * Mari et al. (2023) Mari M. B., Akras S., Gonçalves D. R., 2023, Monthly Notices of the Royal Astronomical Society, 525, 1998
  * Martinez-Valpuesta & Gerhard (2013) Martinez-Valpuesta I., Gerhard O., 2013, The Astrophysical Journal Letters, 766, L3
  * Méndez-Delgado et al. (2023) Méndez-Delgado J., et al., 2023, Monthly Notices of the Royal Astronomical Society, 523, 2952
  * Mendoza & Zeippen (1982) Mendoza C., Zeippen C., 1982, Monthly Notices of the Royal Astronomical Society, 199, 1025
  * Mendoza & Zeippen (1983) Mendoza C., Zeippen C., 1983, Monthly Notices of the Royal Astronomical Society, 202, 981
  * Milingo et al. (2010) Milingo J., Kwitter K., Henry R., Souza S., 2010, The Astrophysical Journal, 711, 619
  * Miszalski et al. (2009) Miszalski B., Acker A., Moffat A. F., Parker Q. A., Udalski A., 2009, Astronomy & Astrophysics, 496, 813
  * Miszalski et al. (2011) Miszalski B., et al., 2011, Astronomy & Astrophysics, 528, A39
  * Monteiro et al. (2004) Monteiro H., Schwarz H. E., Gruenwald R., Heathcote S., 2004, The Astrophysical Journal, 609, 194
  * Ness et al. (2013) Ness M., et al., 2013, Monthly Notices of the Royal Astronomical Society, 430, 836
  * Osterbrock & Ferland (2006) Osterbrock D. E., Ferland G. J., 2006, Astrophysics Of Gas Nebulae and Active Galactic Nuclei. University science books
  * Parker (2022) Parker Q. A., 2022, Frontiers in Astronomy and Space Sciences, 9, 895287
  * Parker et al. (2006) Parker Q. A., et al., 2006, MNRAS, 373, 79
  * Parker et al. (2016) Parker Q. A., Bojičić I. S., Frew D. J., 2016, in Journal of Physics Conference Series. p. 032008 (arXiv:1603.07042), doi:10.1088/1742-6596/728/3/032008
  * Peimbert et al. (2017) Peimbert M., Peimbert A., Delgado-Inglada G., 2017, Publications of the Astronomical Society of the Pacific, 129, 082001
  * Peña et al. (1991) Peña M., Torres-Peimbert S., Ruiz M., 1991, Publications of the Astronomical Society of the Pacific, 103, 865
  * Porter et al. (2005) Porter R., Bauman R., Ferland G. J., MacAdam K., 2005, The Astrophysical Journal, 622, L73
  * Pottasch & Bernard-Salas (2015) Pottasch S., Bernard-Salas J., 2015, Astronomy & Astrophysics, 583, A71
  * Pradhan (1976) Pradhan A., 1976, Monthly Notices of the Royal Astronomical Society, 177, 31
  * Proxauf et al. (2014) Proxauf B., Oettl S., Kimeswenger S., 2014, Astronomy & Astrophysics, 561, A10
  * Raha et al. (1991) Raha N., Sellwood J., James R., Kahn F., 1991, nature, 352, 411
  * Ratag et al. (1992) Ratag M. A., Pottasch S. R., Dennefeld M., Menzies J. W., 1992, Astronomy and Astrophysics, 255, 255
  * Rees (2011) Rees B., 2011, A study of planetary nebulae in and towards the Galactic Bulge. The University of Manchester (United Kingdom)
  * Rees & Zijlstra (2013) Rees B., Zijlstra A., 2013, Monthly Notices of the Royal Astronomical Society, 435, 975
  * Reid & Parker (2010) Reid W. A., Parker Q. A., 2010, Publications of the Astronomical Society of Australia, 27, 187
  * Robitaille et al. (2013) Robitaille T. P., et al., 2013, Astronomy & Astrophysics, 558, A33
  * Rodríguez (2020) Rodríguez M., 2020, Monthly Notices of the Royal Astronomical Society, 495, 1016
  * Samland et al. (1992) Samland M., Köppen J., Acker A., Stenholm B., 1992, Astronomy and Astrophysics, 264, 184
  * Sellwood (2014) Sellwood J., 2014, Reviews of Modern Physics, 86, 1
  * Shen et al. (2010) Shen J., Rich R. M., Kormendy J., et al. 2010, The Astrophysical Journal, 720, L72
  * Shortridge et al. (2014) Shortridge K., Meyerdierks H., Currie M. J., Davenhall C., Jenness T., Clayton M., 2014, Astrophysics Source Code Library, pp ascl–1411
  * Siódmiak & Tylenda (2001) Siódmiak N., Tylenda R., 2001, Astronomy & Astrophysics, 373, 1032
  * Smith et al. (2014) Smith C. L., Zijlstra A. A., Dinerstein H. L., 2014, Monthly Notices of the Royal Astronomical Society, 441, 3161
  * Smith et al. (2017) Smith C. L., Zijlstra A. A., Gesicki K. M., Dinerstein H. L., 2017, Monthly Notices of the Royal Astronomical Society, 471, 3008
  * Sowicka et al. (2017) Sowicka P., Jones D., Corradi R. L., Wesson R., García-Rojas J., Santander-García M., Boffin H. M., Rodríguez-Gil P., 2017, Monthly Notices of the Royal Astronomical Society, 471, 3529
  * Stanghellini & Haywood (2018) Stanghellini L., Haywood M., 2018, The Astrophysical Journal, 862, 45
  * Stasinska & Tylenda (1994) Stasinska G., Tylenda R., 1994, Astronomy and Astrophysics, 289, 225
  * Storey & Zeippen (2000) Storey P., Zeippen C., 2000, Monthly Notices of the Royal Astronomical Society, 312, 813
  * Tan et al. (2023a) Tan S., Parker Q. A., Zijlstra A., Ritter A., 2023a, Monthly Notices of the Royal Astronomical Society, 519, 1049
  * Tan et al. (2023b) Tan S., Parker Q. A., Zijlstra A. A., Ritter A., Rees B., 2023b, The Astrophysical Journal Letters, 951, L44
  * Torres-Peimbert et al. (1997) Torres-Peimbert S., Dufour R., Peimbert M., Pena M., 1997, in Planetary Nebulae. p. 281
  * Uttenthaler et al. (2012) Uttenthaler S., Schultheis M., Nataf D. M., Robin A. C., Lebzelter T., Chen B., 2012, Astronomy & Astrophysics, 546, A57
  * Van Dokkum (2001) Van Dokkum P. G., 2001, Publications of the Astronomical Society of the Pacific, 113, 1420
  * Viegas & Clegg (1994) Viegas S. M., Clegg R. E., 1994, Monthly Notices of the Royal Astronomical Society, 271, 993
  * Wang & Liu (2007) Wang W., Liu X.-W., 2007, Monthly Notices of the Royal Astronomical Society, 381, 669
  * Warren-Smith et al. (2014) Warren-Smith R. F., Draper P. W., Taylor M., Allan A., 2014, Astrophysics Source Code Library, pp ascl–1403
  * Wesson (2016) Wesson R., 2016, Monthly Notices of the Royal Astronomical Society, 456, 3774
  * Wesson et al. (2012) Wesson R., Stock D., Scicluna P., 2012, Monthly Notices of the Royal Astronomical Society, 422, 3516
  * Wesson et al. (2018) Wesson R., Jones D., García-Rojas J., Boffin H., Corradi R., 2018, Monthly Notices of the Royal Astronomical Society, 480, 4589
  * Wiese et al. (1996) Wiese W. L., Fuhr J., Deters T., 1996, Atomic transition probabilities of carbon
  * Wyse et al. (1997) Wyse R., Gilmore G., Franx M., 1997, Annual Review of Astronomy and Astrophysics, 35, 637
  * Yuan et al. (2011) Yuan H.-B., Liu X.-W., Péquignot D., Rubin R., Ercolano B., Zhang Y., 2011, Monthly Notices of the Royal Astronomical Society, 411, 1035
  * Zeippen (1982) Zeippen C., 1982, Monthly Notices of the Royal Astronomical Society, 198, 111
  * Zijlstra (1990) Zijlstra A. A., 1990, Astronomy and Astrophysics, 234, 387
  * Zoccali et al. (2008) Zoccali M., Hill V., Lecureur A., Barbuy B., Renzini A., Minniti D., Gómez A., Ortolani S., 2008, Astronomy & Astrophysics, 486, 177