# Morphologies and Central Stars of Planetary Nebulae in the Galactic bulge
from VLT, _HST_ and Pan-STARRS imaging

Shuyu Tan,1,2 Quentin A. Parker1,2 Albert Zijlstra3,2,1 Andreas Ritter1,2
1Department of Physics, Chong Yuet Ming Physics Building, The University of
Hong Kong, Pokfulam Road, Hong Kong
2The Laboratory for Space Research, The University of Hong Kong, Cyberport 4,
Hong Kong
3Jodrell Bank Centre for Astrophysics, The University of Manchester, Oxford
Road, M13 9PL, Manchester, UK E-mail<EMAIL_ADDRESS><EMAIL_ADDRESS>
(Accepted XXX. Received YYY; in original form ZZZ)

###### Abstract

This is the first in a series of papers that present sets of different results
for 136 compact, known planetary nebulae within a 10 $\times$ 10 degree region
of the Galactic bulge. We use a large, previously unpublished sample of our
own extensive ESO 8 m VLT deep imaging and spectroscopic data. This is
combined with archival deep _HST_ imaging where available to provide a
detailed morphological classification and study. The influence of angular
resolution and sensitivity when assigning a morphology is discussed. A large
fraction (68%) of the sample are shown to be bipolars and the implications for
this in the context of planetary nebulae progenitors are explored. Four new
planetary nebula central stars are also identified which are not in _Gaia_.
This is based on both VLT and deep archival Pan-STARRS broad-band imagery.
Some 11 putative central stars previously reported, based on _Gaia_ positions,
are also not the true central star. In other cases the _Gaia_ central stars
reported in the literature are actually based on the overall centroid position
of a very compact planetary nebula rather than the actual central star within
it. _Gaia_ parallax distances and kinematic ages for PNe in this sample are
provided where possible based on fresh angular size measures from the new VLT
imagery and _Gaia_ distances and literature expansion velocities when
available. All these results are discussed within the context of the overall
characteristics of the Galactic bulge and its planetary nebula population.

###### keywords:

planetary nebulae: general – Galaxy: bulge – Galaxy: centre

††pubyear: 2015††pagerange: Morphologies and Central Stars of Planetary
Nebulae in the Galactic bulge from VLT, _HST_ and Pan-STARRS imaging–B

##  1 Introduction

Planetary nebulae (PNe) are bright, gaseous emission envelopes ejected by low-
to intermediate-mass stars ($M_{\star}\sim 0.8-8M_{\odot}$) during their post-
AGB stage as the residual cores evolve to the white dwarf (WD) phase. Most PNe
with hotter ($>$ 45,000K) central stars (CSPN) are optically thin to Lyman
continuum photons (e.g. Schönberner et al., 2007). As such, outermost
electrons of multiply-charged ions are ejected, giving PNe their predominate
and strong emission lines. This makes PNe detectable even at large
Galactocentric distances and in external galaxies like the Magellanic clouds
(Reid & Parker, 2006) and other members of the local group (e.g. Jacoby, 1980;
Merrett et al., 2006). For an excellent, recent, general PNe review see
Kwitter & Henry (2022) and for a more specific PNe discovery and selection
techniques review see Parker (2022a).

PNe exhibit a diversity of shapes (morphologies hereafter) that nevertheless
can be allocated to a few basic underlying forms: elliptical (E), round (R),
bipolar or butterfly (B), in rare cases irregular (I), asymmetric (A) and,
when extremely compact, (S) for point-source like (see, e.g. Greig, 1971; Chu
et al., 1987; Manchado et al., 2000; Stanghellini et al., 2002a; Corradi et
al., 2003; Parker et al., 2006; Sahai et al., 2011a).

Current morphological classifications assigned to all known Galactic PNe are
contained in the "Gold Standard" HASH database111HASH: available online
http://www.hashpn.space Parker et al. (2016). HASH consolidates and federates
available multi-wavelength imaging, spectroscopic and other data for both
Galactic and Magellanic Cloud PNe. The assigned morphologies adopt the
ERBIAS/sparm classification scheme introduced by Parker et al. (2006) and used
exclusively in HASH. The underlying physical causes for the observed
morphological variety and their connection to their central stars (CSPN
hereafter) have been extensively explored (e.g. Stanghellini et al., 2002a),
but mysteries concerning the definitive shaping mechanisms remain.

From an observational perspective, the interpretation of PN shapes has been
largely dependent on the morphological characterisation of 2-D projections of
largely optical emission distributions from what are obviously 3-D nebula
sources. The conversion from 2-D to a true 3-D representation of a PN depends
on detailed kinematic sampling from high dispersion Integral Field Unit (IFU)
spectroscopic observations (e.g. Danehkar et al., 2013) and associated 3-D
modelling with software such as SHAPE (Steffen & López, 2006; Clyne et al.,
2015). While IFU observations are now becoming more common, e.g Ali et al.
(2016); Clairmont et al. (2022), they remain, for now, the exception. This is
at least partly due to the competition for access to limited, suitable IFU
instrumentation (i.e. those with sufficient areal coverage, and angular and
kinematic resolution). Consequently, morphological classifications and more
fundamentally an understanding of them, is based on PNe 2-D imagery that will
remain the default for the foreseeable future.

Modern, higher sensitivity and higher angular resolution observations, using
dedicated ground-based and space-based telescopes and surveys, have further
revealed a wealth of additional small-scale and large-scale features beyond
the basic forms. These have further complicated this basic classification
scheme. They include point-symmetric structures, jets, ansae, minor bipolar
and multi-polar lobes, internal nebular striations, external, concentric rings
and extensive "AGB" haloes and shells, (e.g. Górny et al., 1999; Sahai et al.,
2011b; Sabin et al., 2014; Akras & Gonçalves, 2016; Stanghellini et al., 2016;
Sabin et al., 2021). These secondary morphological features are classified by
the "sparm" system used by HASH (see later).

Some background to the importance of PNe morphologies is given in Section 2.
The observations yielding the PN imaging data and details of the morphological
and CSPN analysis are described in Section 3 and Section 4 respectively. The
various measurement results with the imaging data are listed and discussed in
Section 5. In Section 6, we briefly summarise current ideas about PN shaping
mechanisms and compare the properties of PNe in our sample with theoretical
predictions. Finally, Section 7 contains a summary of our main results with an
emphasis on the prevalence of bipolar PNe uncovered.

##  2 Why are PNe Morphologies Important?

PNe morphologies are a useful means to describe how gaseous ejected shells of
dying stars are manifested. The most accurately determined morphologies from
the best available (usually optical) imagery can permit detailed investigation
as a function of different parameters. This is especially when combined with
the statistical power from HASH classifications of large numbers of PNe. Such
parameters include Galactic location and environment, angular/physical size
(so evolution), CSPN properties when available (including if in a binary
system), nebular abundances and kinematic ages etc. Such studies can provide
vital clues into the actual shaping mechanisms at play and their relative
importance.

Indeed, PNe morphologies, as a key component of multi-wavelength and
spectroscopic observation and analysis, can deliver powerful insights into
their chemical and physical conditions, central star and nebula evolutionary
stages and other key properties. PNe morphologies can reveal effects from
interactions with the local ISM, e.g. Wareing et al. (2007), the influence of
possible external or internal magnetic fields, e.g. Sabin et al. (2007), the
results of fast winds interacting with previously ejected material at an
earlier stage of evolution, e.g. Kastner et al. (2022) and, as often invoked,
the influence of CSPN binarity and common envelope evolution as perhaps the
key driver of PNe shaping, e.g. De Marco (2009a).

Being so diverse, PNe shapes are clearly telling us something fundamental
about how and why PNe are formed and why most are not just a canonical,
spherical gaseous envelope ejected from a single star. The observed diversity
may result, at least in part, from different types of progenitor stars where
the basic 3 main bipolar, elliptical and round PNe morphologies are suggested
to emerge from stars that have slightly different mass distributions (Górny et
al., 1997) and where bipolar PNe in particular are thought produced by more
massive progenitors (Corradi & Schwarz, 1995; Mellema, 1997; Zhang & Kwok,
1998; Phillips, 2001). This is especially given their preferred location at
lower Galactic scale-heights in the mid-plane (Parker et al., 2006). Here, any
PNe found in such assumed younger environments should have derived from higher
mass stars to be going through the PN phase now. Also see Stanghellini et al.
(1993); Amnuel (1995); Greig (1971); Górny et al. (1997); Stanghellini et al.
(2002b) for further detailed comment and see Balick & Frank (2002) for a
review. Indeed, non-spherical features observed in the majority of PNe
($\sim$80% - as determined from HASH) have long been attributed to stellar or
sub-stellar companions. See De Marco (2009a) for a detailed case and review of
the "binary hypotheses" of PNe formation and the supposition that most bipolar
PNe arise from binaries (Soker, 1998). This counterpoint to the high mass,
young progenitor bipolar origin scenario was addressed by this latter work.
Here, it is suggested that the observed correlation of more massive
progenitors with bipolar PNe is due to larger ratios of the physical radii of
red giant branch (RGB) stars compared to AGB stars that is exhibited by lower-
mass stars compared to their higher mass equivalents. The larger radii of such
stars on the RGB cause most stellar binary companions to interact with their
lower-mass primaries that are already on the RGB. These could have formed
bipolar PNe if the primary had been on the AGB.

###  2.1 The importance of PNe in the Galactic bulge

The Galactic bulge has been considered as an almost separate entity to the
rest of the Galaxy consisting largely of older stars ($\sim$10 Gyrs) and low
in gas and dust. For a good review see Nataf (2016). Previously, the bulge has
been considered like an elliptical galaxy in overall colour and form. This
simple picture has changed considerably over the last 20 years and the long-
held view that the bulge is shaped like a tri-axial spheroid composed of old
stars (Dwek et al., 1995), resembling an elliptical Galaxy shorn of its disk
and spiral arms, is no longer valid. The bulge is now known to host an
elongated central bar (Blitz & Spergel, 1991; Stanek et al., 1997). A detailed
3-D map using red clump stars as distance indicators (by assuming they have a
constant absolute magnitude) also showed the bulge to be X-shaped (Saito et
al., 2011; Saito et al., 2012a; Wegg & Gerhard, 2013), i.e. a boxy peanut-
shaped structure that becomes a bar in the inner bulge (e.g. Athanassoula,
2005; Zoccali et al., 2014) which has implications for feasible formation
scenarios. Bulge dynamics are different to the plane and in the inner region
more chaotic.

How well the currently available bulge PNe sample represents the underlying
stellar population and its evolutionary history remains an open question.
However, PNe represent a highly visible "touchstone" population from which to
investigate certain bulge characteristics (e.g. Peimbert, 1978; Peimbert &
Torres-Peimbert, 1983; Corradi & Schwarz, 1995). Their range of inherent
morphologies may play a significant role in aiding understanding late stage
stellar evolution mass loss processes and, via spectroscopy of the ejecta of
different PNe types, large-scale chemical evolution and trends (Bensby et al.,
2013, 2017). This provides a key motivation for our work. The reliable
morphological classification for compact Galactic bulge PNe, identification of
their CSPN and estimation of their kinematic ages are the main focus of this
first paper.

##  3 Sample Selection: New Observations and Archival Data

In this work we present results from our new ESO VLT narrow-band imagery for
PNe located in the inner 10 $\times$ 10 degree region of the Galactic bulge.
All the targets observed by the ESO VLT and in the _HST_ samples were selected
to be highly likely physical members of the bulge. This is based on the
following criteria as detailed in Rees & Zijlstra (2013) and designed to
reject contamination from bulge stars and foreground objects.

  1. 1.

The PN lies within the inner 10 degrees of the Galactic Centre.

  2. 2.

The PN has a measured angular size of $>$ 2 arcseconds and less than 35
arcseconds; e.g. see Acker et al. (2006) that was further restricted to $\leq$
10 arcseconds (see below)

  3. 3.

When a 5 GHz PN radio flux is available it must lie within the interval (4.2
mJy, 59.1 mJy). (Siódmiak & Tylenda, 2001; Acker et al., 1992)

Whether a bulge PNe is compact or extended is irrelevant to their ages given
the short overall PNe lifetimes c.f. that of their host stars. However, it is
germane to their nebular evolutionary state and ease of observation. The
further restriction of the above selection criteria to angular extents $\leq$
10 arcseconds was applied to: i) yield young PNe that are usually of higher
surface brightness; ii) be less affected by any seeing variations while iii)
producing a sample size for the VLT that has realistic prospects of full
completion. The restricted angular size also makes the sample more amenable to
deep spectroscopy for detailed abundance studies - a key aim of this overall
project. Their compactness, though, does make their morphologies harder to
discern from existing ground based imagery. Here we re-examine both existing
_Hubble Space Telescope_ (_HST_) and our new VLT imagery to provide
significantly improved classifications.

The above final sample selection was based on vetting of all confirmed PNe
from HASH in the bulge region that satisfied the above original criteria back
in 2015. This produced 136 sources. However, HASH is an evolving database and
in the interim many PNe entries have been updated, including some angular size
measurements. Indeed, we provide fresh angular size measurements for all PNe
in this sample (refer section 5.1) based largely on the new VLT narrow-band
imagery. As a result many PNe would no longer satisfy the strict $\leq$ 10
arcsecond diameter criterion. Many are now a little larger in these fresh
estimates with an average value of 9.5 arcseconds with $\sigma$ = 4.2, but the
integrity of the sample as compact PNe in the Galactic bulge remains.

All 136 original targets were spectroscopically observed with the FORS2
instrument mounted on UT1 (Antu) of the European Southern Observatory (ESO)
8.2 m Very Large Telescope (VLT) (Appenzeller et al., 1998). The observations
took place between 2015 and 2019 under program IDs 095.D-0270(A),
097.D-0024(A), 099.D-0163(A), and 0101.D-0192(A) for PIs Zijlstra and Parker.
FORS2 narrow-band pre-imaging was obtained for all 136 sources. This imagery
was supplemented by a further 40 objects previously observed using the Wide
Field and Planetary Camera 2 Instrument (WFPC2) of the _HST_ in 2002, 2003 and
2009 through _HST_ -SNAP-8345 (PI: Sahai), _HST_ -SNAP-9356 (PI: Zijlstra) and
_HST_ -GO-11185 (PI: Rubin). _HST_ imagery was used for morphological
classification whenever available as it is obviously far superior.

The main point of the VLT observations was to obtain deep, high quality PNe
spectroscopy for abundance studies (the subject of 3 further papers in this
series). The generally excellent narrow-band VLT imaging obtained for most was
a mere by-product of the FORS2 spectroscopic observational process that often
requires pre-imaging of the PNe in order to determine where best to place the
spectroscopic slit. The final observed VLT sample here comprises 136 PNe,
representing about $\sim$20% of the currently known bulge PNe population in
HASH.

Our completed ESO VLT project was designated as a filler programme with no
photometric quality requirement (and one of the reasons for choosing the
target sample to be $\leq$ 10 arcseconds across). These could proceed even
under modest conditions, e.g. some clouds and/or high humidity and/or
relatively poor seeing. This gave increased likelihood of our observations
being obtained but as a result care must be taken when attempting any reliable
photometric assessment. Fortunately, for the purposes of this particular
paper, the adverse effects on discernable morphological detail and point
source (CSPN) detection were modest over the range of observing conditions
achieved in practice.

Our VLT imagery and a careful assessment of the existing _HST_ imagery now
enables us to better resolve both macro- and microscopic structures for most
of the objects not seen in previous studies and so present fresh, detailed
morphological classification and analysis of this sample. These data are
combined with the excellent resolution and deep Pan-STARRS broad band imagery
(Kaiser et al., 2010), when available. This includes the key blue ’g-band’ and
has allowed more accurate measurements and improvement to the position and
information of CSPN where they are clearly identifiable, including 4 new CSPN
discoveries not in _Gaia_.

###  3.1 The ESO VLT instrumental configuration

The FORS2 instrument is a multi-mode optical instrument mounted on the ESO VLT
UT1 telescope at the Cassegrain focus. It can perform imaging and long slit
spectroscopy but also has a multi-object spectroscopy capability and even
polarimetry (Nicklas et al., 1997) that can cover a wavelength range from the
atmospheric cut-off in the blue out to $\sim 1\mu$m in the red.

FORS2 is equipped with a mosaic of two MIT CCD detectors, each a 4096 $\times$
2048 pixel array with an image scale of 0.126 arcsec/pixel (see FORS2
manual222http://www.eso.org/sci/facilities/paranal/instruments/fors/inst/pola.html).
So called "pre-imaging" observations were performed under the direct imaging
mode (IMG) using two narrow-band filters, H$\alpha$ (FWHM of 61Å at 6563Å,
transmits both H$\alpha$ and [N II] $\lambda\lambda$ 6548, 6584 lines) and [O
III] (FWHM of 57Å at 5001Å). Among the 136 confirmed PNe, 83 PNe have images
with both H$\alpha$ and [O III] filters. A further 26 PNe have an image with
the H$\alpha$ filter only and 27 have an image with the [O III] filter only.
The integration times vary from 2 seconds for the high surface brightness PNe
to 240 seconds for those of lower surface brightness. Despite the filler
program status the recorded seeing was typically better than 1.5 arcseconds,
underlining the excellent nature of the ESO VLT site. A summary log of the VLT
observations is provided in Table 9 in Appendix A. All images were bias-
subtracted and flat-fielded, where the associated raw calibration files are
available, using the standard ESO Reflex Data Reduction Pipeline (Freudling et
al., 2013).

###  3.2 The _HST_ imagery

For the 40 _HST_ sub-sample of bulge PNe, the WFPC2/_HST_ has 800 $\times$ 800
pixel silicon CCD with a pixel size of 0.046 arcseconds. The narrow-band
filter F656N (22Å wide at 6564Å) was utilised for all observations. Exposure
times were mostly 80 or 120 seconds but a few were of 0.5, 40, 160, 200, 240,
320 and 400 seconds. The F502N filter (covering [O III] 5007Å) was used for 31
objects in our sample. Here, the exposure time ranged from 60 to 600 seconds.
The summary log of the _HST_ observations is given in Table 10 in Appendix A.
Due to the higher spatial resolution of the _HST_ , these imaging data were
used wherever possible for morphological classifications and for CSPN hunting
and verification in this study.

###  3.3 Pan-STARRS imagery as a useful adjunct

For some PNe the narrow-band VLT imagery was less sharp and indeed less deep
for continuum sources than the available broad-band Pan-STARRS imagery (Kaiser
et al., 2010) due to occasional poorer seeing for our VLT filler program. Pan-
STARRS is itself superior and better than the default main $\sim$2 arcsecond
resolution HASH imagery from the SuperCOSMOS H$\alpha$ survey (Parker et al.,
2005) that was originally used for morphological classification and that does
not work well for the compact PNe sample here. Pan-STARRS is also particularly
useful for CSPN verification as the broad multi-band photometry is quite deep.
The typical 5$\sigma$ limiting magnitudes for point sources in the g, r, i, z
and y bands are 23.3, 23.2, 23.1, 22.3 and 21.4 respectively (Chambers et al.,
2016). This is much deeper than _Gaia_ , especially for the g-band, and also
allows CSPN candidate blue colours to be seen. This helps to confirm the CSPN
identifications made here.

In Fig. 1 we provide two examples showing a comparison of the VLT (left) and
Pan-STARRS (right) imagery. The image quality of the narrow-band VLT imagery
is usually superior to that of Pan-STARRS. The upper panel present the images
of PNG 007.5+07.4. The VLT image clearly shows a well defined ring structure
and a faint AGB halo but not the CSPN which is only visible in the broad-band
Pan-STARRS red image shown. It is also clear that Pan-STARRS goes deeper (in
this example) for continuum sources than the VLT equivalent but with a
slightly inferior stellar PSF. The lower panel contrasts images of PN
007.6+06.9 where the VLT image (left) exhibits a ring structure with an
enhancement in brightness in the NW direction, as well as hints of an outer
shell. The Pan-STARRS image (right) is superior to the VLT both in terms of
detail and again the CSPN is only visible in the Pan-STARRS broad-band red
image.

Although the two Pan-STARRS telescopes are only 1.8 m in diameter they benefit
from generally excellent seeing conditions being situated at Haleakala
Observatory, Hawaii. This yielded the high quality broad band filter images
used here for both checking CSPN candidates and uncovering CSPN not evident in
the VLT imaging. Often the VLT imaging does not go deep enough to detect faint
CSPN in the narrow-band filters. Pan-STARRS data was only incorporated into
HASH after original morphological classifications were assigned.

(a) PNG 007.5+07.4

(b) PNG 007.6+06.9

Figure 1: Two comparison examples between the VLT (left) and Pan-STARRS
(right) imagery. The upper panel is of PNG 007.5+07.4. The CSPN is only
visible in the broad-band red Pan-STARRS image. Pan-STARRS goes deeper in this
example for continuum sources than the VLT equivalent. The lower panel
contrasts images of PNG 007.6+06.9. The Pan-STARRS image (right) in this case
is superior to the VLT both in terms of detail and depth. Again the CSPN is
only visible in the Pan-STARRS broad-band red image. Figure 2: Distribution of
the 136 bulge PNe sample observed with the VLT in a Galactic latitude versus
Galactic longitude plot. The blue crosses mark the objects with no _HST_ data
so the best images are usually from the VLT observations. The orange crosses
represent objects imaged with the _HST_ (even if they also have VLT pre-
imaging observations).

##  4 Morphological Classification, CSPN and other Measurements

Of the 136 PNe in our sample, imaging data of 96 PNe are taken from the VLT
observations while observations of 40 are from the _HST_ and used for
morphological classification. The Galactic distribution of this sample is
shown in Fig. 2. All spectroscopic data are used later in this series of
papers for accurate abundance determinations and analysis (Tan et al., in
preparation). The full set of the best available optical imagery for each of
the 136 PNe is presented in Appendix B.

Below we provide details of the updated morphological classifications obtained
from the _HST_ and VLT imagery and the detection and positional measurements
of any plausible CSPN seen in these and the supplementary Pan-STARRS images.

###  4.1 Morphological classification

As mentioned, the "ERBIAS sparm" morphological classification scheme, first
described in Parker et al. (2006), was adopted. The upper case designator
first indicates the basic class as Elliptical (E), Round (R), Bipolar (B),
Irregular (I), Asymmetric (A) and the unresolved quasi-stellar PNe (S). To
describe the secondary structural features evident in the nebulae, the lower-
case ‘sparm’ sub-classifiers were used immediately following the main class.
These are used to describe an object that also exhibits obvious internal
structure with details such as filaments, knots, striations etc ‘s’, possesses
some point symmetry ‘p’, has an enhancement or one-sided brightness/asymmetry
‘a’, has an evident, even if fractured, ring-like structure or annulus ‘r’ or
shows multiple shells or external structure ‘m’. A scheme schematic of the
main and sub-classifiers is presented in Fig. 3. PNe may have none, several or
even all of these sub-classifiers.

This classification scheme was then applied to the best available multi-
wavelength or single, narrow-band image data for all PNe in our sample,
whether it be from the _HST_ or VLT. Of course this is inevitably a somewhat
subjective judgement but the team is very experienced.

(a) ERBIAS (b) sparm

Figure 3: A schematic of the main classifiers ‘ERBIAS’ and the sub-classifiers
‘sparm’ for indicating the PN morphology taken from (Parker et al., 2006) and
(Parker et al., 2016). The dotted circles indicating the centre of the nebula
are used as a reference to infer the secondary PN morphological features.

In Table 1 we present the updated morphological classifications for all 136
PNe in the selected bulge sample. Provided are the unique HASH ID number, the
PNG designation, remeasured PNe centroid positions in J2000 RA and DEC, our
updated morphological classifications and new angular diameter estimates (in
arcseconds) measured from the VLT, _HST_ and, in three cases, Pan-STARRS
imagery. The telescope used is indicated in the last entry for each PN.

Table 1: Listing of HASH & PNG IDs, J2000 RA/DEC positions, morphological classifications, new, angular size measurements (arcseconds) for our PNe sample as measured from the VLT, _HST_ imagery and, in 3 cases, from Pan-STARRS. The telescope data used is indicated as the last entry for each PN. HASH ID | PN G | PN Centroid Coords. | Morph. |  Ang. Diam. |  Tele. |  HASH ID | PN G | PN Centroid Coords. | Morph. |  Ang. Diam. | Tele.
---|---|---|---|---|---|---|---|---|---|---|---
RA(J2000) | DEC(J2000) | RA(J2000) | DEC(J2000)
12 | 000.1$+$02.6 | 17:35:35.44 | -27:24:05.37 | Ars | 12.6 |  VLT | 171 | 005.8$-$06.1 | 18:22:54.20 | -26:49:17.00 | Bms | 9.4 | VLT
16 | 000.1$+$04.3 | 17:29:23.41 | -26:26:04.38 | B | 12.0 |  VLT | 4108 | 006.1$+$08.3 | 17:28:57.61 | -19:15:53.95 | Ems | 2.4 | _HST_
17 | 000.1$-$02.3 | 17:55:20.53 | -29:57:36.78 | Rars | 10.2 |  VLT | 179 | 006.3$+$03.3 | 17:47:33.95 | -21:47:23.71 | Bams | 9.0 | PS
19 | 000.2$-$01.9 | 17:53:45.64 | -29:43:46.98 | Brs | 19.4 |  VLT | 180 | 006.3$+$04.4 | 17:43:28.75 | -21:09:51.77 | Bamps | 5.4 | _HST_
20 | 000.2$-$04.6 | 18:04:44.09 | -31:02:48.88 | Bs | 8.0 |  VLT | 182 | 006.4$+$02.0 | 17:52:41.44 | -22:21:57.18 | Bmps | 7.0 | _HST_
24 | 000.3$+$06.9 | 17:20:22.04 | -24:51:52.24 | Brs | 11.0 |  VLT | 181 | 006.4$-$04.6 | 18:18:13.33 | -25:38:07.95 | Eas | 9.2 | VLT
22 | 000.3$-$04.6 | 18:05:02.68 | -30:58:18.03 | Bms | 11.8 |  VLT | 189 | 006.8$+$02.3 | 17:52:22.62 | -21:51:13.51 | Bms | 5.2 | VLT
26 | 000.4$-$01.9 | 17:54:25.37 | -29:36:08.89 | Brs | 8.4 |  VLT | 188 | 006.8$-$03.4 | 18:14:28.74 | -24:43:38.12 | Bms | 7.0 | VLT
27 | 000.4$-$02.9 | 17:58:19.40 | -30:00:39.36 | Bamrs | 8.8 |  VLT | 193 | 007.0$+$06.3 | 17:38:11.54 | -19:37:35.83 | Bps | 7.2 | VLT
40 | 000.7$+$03.2 | 17:34:54.72 | -26:35:57.36 | Bprs | 17.8 |  VLT | 192 | 007.0$-$06.8 | 18:27:59.62 | -26:06:48.11 | Bms | 12.4 | VLT
37 | 000.7$-$02.7 | 17:58:09.59 | -29:44:19.98 | Bps | 8.2 |  VLT | 197 | 007.5$+$07.4 | 17:35:10.24 | -18:34:20.71 | Eamrs | 18.2 | PS
38 | 000.7$-$03.7 | 18:02:19.26 | -30:14:25.94 | Bmrs | 11.4 |  VLT | 198 | 007.6$+$06.9 | 17:37:21.99 | -18:46:42.00 | Bamrs | 16.6 | PS
42 | 000.7$-$07.4 | 18:17:37.20 | -31:56:46.68 | Ears | 11.8 |  VLT | 200 | 007.8$-$03.7 | 18:17:16.01 | -23:58:55.07 | Bams | 17.0 | VLT
50 | 000.9$-$02.0 | 17:56:02.79 | -29:11:16.69 | Bps | 5.2 |  VLT | 201 | 007.8$-$04.4 | 18:20:08.84 | -24:15:05.39 | Eamrs | 7.2 | _HST_
48 | 000.9$-$04.8 | 18:07:06.16 | -30:34:17.39 | Eamrs | 15.4 |  VLT | 4111 | 008.2$+$06.8 | 17:38:57.37 | -18:17:35.92 | Bms | 4.8 | _HST_
54 | 001.1$-$01.6 | 17:54:52.07 | -28:48:55.79 | Eam | 7.8 |  VLT | 208 | 008.4$-$03.6 | 18:18:23.81 | -23:24:57.50 | Bmprs | 25.2 | VLT
58 | 001.2$+$02.1 | 17:40:12.83 | -26:44:21.83 | Bamrs | 5.2 |  _HST_ | 210 | 008.6$-$02.6 | 18:14:50.91 | -22:43:55.52 | Bams | 4.2 | _HST_
60 | 001.2$-$03.0 | 18:00:37.62 | -29:21:50.78 | E | 6.6 |  VLT | 217 | 009.4$-$09.8 | 18:44:43.15 | -25:21:34.06 | Eamrs | 8.4 | VLT
62 | 001.3$-$01.2 | 17:53:47.16 | -28:27:17.84 | Bmrs | 6.2 |  VLT | 223 | 009.8$-$04.6 | 18:25:05.00 | -22:34:52.64 | Bamrs | 11.0 | VLT
64 | 001.4$+$05.3 | 17:28:37.63 | -24:51:07.16 | Baps | 8.6 |  VLT | 1147 | 350.5$-$05.0 | 17:42:54.08 | -39:36:24.29 | Emrs | 9.2 | VLT
70 | 001.6$-$01.3 | 17:54:34.89 | -28:12:43.62 | Brs | 5.4 |  VLT | 1151 | 351.1$+$04.8 | 17:03:46.84 | -33:29:44.64 | Emrs | 6.0 | _HST_
73 | 001.7$+$05.7 | 17:28:01.74 | -24:25:23.72 | Eamrs | 12.2 |  VLT | 1152 | 351.2$+$05.2 | 17:02:19.10 | -33:10:05.02 | Bm | 14.4 | VLT
74 | 001.7$-$04.4 | 18:07:14.56 | -29:41:24.67 | Bamrs | 4.6 |  _HST_ | 1156 | 351.6$-$06.2 | 17:50:44.60 | -39:17:25.98 | Bamprs | 15.0 | VLT
81 | 002.0$-$06.2 | 18:15:06.50 | -30:15:32.90 | Eams | 6.4 |  VLT | 1159 | 351.9$+$09.0 | 16:50:17.08 | -30:19:55.39 | Bmprs | 15.4 | VLT
83 | 002.1$-$02.2 | 17:59:19.33 | -28:13:48.31 | Bmrs | 5.2 |  VLT | 1160 | 351.9$-$01.9 | 17:33:00.68 | -36:43:52.94 | Bs | 4.8 | _HST_
82 | 002.1$-$04.2 | 18:07:07.28 | -29:13:06.07 | Bmps | 8.2 |  _HST_ | 1161 | 352.0$-$04.6 | 17:45:06.80 | -38:08:49.49 | Bmp | 7.4 | VLT
90 | 002.2$-$09.4 | 18:29:11.64 | -31:29:59.24 | Bmprs | 13.8 |  _HST_ | 1163 | 352.1$+$05.1 | 17:05:30.72 | -32:32:08.22 | Bs | 18.0 | VLT
91 | 002.3$+$02.2 | 17:42:30.02 | -25:45:28.84 | Eamrs | 6.6 |  VLT | 1164 | 352.6$+$03.0 | 17:14:42.93 | -33:24:47.61 | Bamprs | 5.2 | _HST_
93 | 002.3$-$03.4 | 18:04:28.81 | -28:37:38.29 | Bmrs | 5.8 |  _HST_ | 1171 | 353.2$-$05.2 | 17:50:45.20 | -37:23:53.09 | Bmrs | 16.8 | VLT
98 | 002.5$-$01.7 | 17:58:31.20 | -27:37:05.37 | Ears | 5.6 |  VLT | 1172 | 353.3$+$06.3 | 17:04:18.33 | -30:53:29.08 | Bp | 8.2 | VLT
4326 | 002.6$+$02.1 | 17:43:39.48 | -25:36:42.87 | Bs | 18.4 |  VLT | 1178 | 353.7$+$06.3 | 17:05:13.91 | -30:32:19.53 | Bmprs | 10.0 | VLT
105 | 002.7$-$04.8 | 18:11:04.99 | -28:58:59.22 | Bamprs | 18.2 |  _HST_ | 1185 | 354.5$+$03.3 | 17:18:51.94 | -31:39:06.52 | Bas | 2.6 | _HST_
106 | 002.8$+$01.7 | 17:45:39.80 | -25:40:00.52 | Bmars | 4.0 |  _HST_ | 1189 | 354.9$+$03.5 | 17:19:20.24 | -31:12:40.95 | Bas | 3.6 | _HST_
107 | 002.8$+$01.8 | 17:45:28.34 | -25:38:11.89 | Bars | 9.4 |  VLT | 1191 | 355.1$-$06.9 | 18:02:32.32 | -36:39:11.22 | Em | 9.8 | VLT
109 | 002.9$-$03.9 | 18:08:05.76 | -28:26:10.65 | Bamrs | 6.6 |  _HST_ | 1199 | 355.4$-$02.4 | 17:44:20.61 | -34:06:40.96 | Bms | 10.6 | _HST_
114 | 003.1$+$03.4 | 17:40:07.42 | -24:25:42.77 | Eamrs | 4.0 |  _HST_ | 1202 | 355.6$-$02.7 | 17:46:06.30 | -34:03:45.54 | E | 8.0 | VLT
116 | 003.2$-$06.2 | 18:17:41.44 | -29:08:19.88 | Bps | 11.0 |  VLT | 1209 | 355.9$+$03.6 | 17:21:31.91 | -30:20:48.75 | Eam | 3.8 | _HST_
125 | 003.6$+$03.1 | 17:41:57.26 | -24:11:16.33 | Baps | 5.8 |  _HST_ | 1207 | 355.9$-$04.2 | 17:52:58.94 | -34:38:22.99 | Bmps | 8.2 | _HST_
126 | 003.6$-$02.3 | 18:03:11.87 | -26:58:32.76 | Bars | 10.0 |  VLT | 1212 | 356.1$-$03.3 | 17:49:50.90 | -34:00:31.31 | Bas | 5.2 | _HST_
128 | 003.7$+$07.9 | 17:24:45.28 | -21:33:34.88 | Bars | 11.2 |  VLT | 1217 | 356.3$-$06.2 | 18:02:32.00 | -35:13:13.69 | Ers | 11.6 | VLT
129 | 003.7$-$04.6 | 18:12:34.39 | -27:58:10.70 | Ears | 6.8 |  VLT | 1219 | 356.5$-$03.6 | 17:51:50.60 | -33:47:35.59 | Bps | 11.4 | _HST_
131 | 003.8$-$04.3 | 18:11:29.26 | -27:46:15.92 | Baps | 5.4 |  _HST_ | 1230 | 356.8$+$03.3 | 17:25:06.10 | -29:45:16.88 | Bp | 3.6 | _HST_
135 | 003.9$+$01.6 | 17:48:28.47 | -24:41:23.79 | Bars | 8.6 |  VLT | 1227 | 356.8$-$05.4 | 18:00:18.23 | -34:27:40.73 | Bmrs | 14.4 | VLT
133 | 003.9$-$02.3 | 18:03:39.30 | -26:43:33.87 | Bas | 7.6 |  VLT | 1232 | 356.9$+$04.4 | 17:21:04.46 | -29:02:59.76 | Bmps | 5.8 | _HST_
134 | 003.9$-$03.1 | 18:06:50.03 | -27:06:19.58 | Ears | 7.8 |  _HST_ | 1234 | 357.0$+$02.4 | 17:28:50.35 | -30:07:44.99 | Ears | 8.6 | VLT
139 | 004.0$-$03.0 | 18:06:40.91 | -26:54:56.28 | Bamrs | 6.6 |  _HST_ | 1235 | 357.1$+$03.6 | 17:24:34.43 | -29:24:19.59 | Bas | 8.4 | VLT
140 | 004.1$-$03.8 | 18:10:12.28 | -27:16:35.41 | Bms | 3.4 |  _HST_ | 4139 | 357.1$+$04.4 | 17:21:37.98 | -28:55:14.63 | Ers | 10.8 | VLT
142 | 004.2$-$03.2 | 18:08:01.38 | -26:54:02.29 | Bs | 6.4 |  VLT | 1239 | 357.1$-$04.7 | 17:58:14.45 | -33:47:37.45 | Bprs | 3.6 | _HST_
141 | 004.2$-$04.3 | 18:12:25.12 | -27:29:13.19 | Bps | 6.6 |  VLT | 1242 | 357.2$+$02.0 | 17:31:08.11 | -30:10:28.15 | Rmrs | 4.8 | _HST_
143 | 004.2$-$05.9 | 18:18:38.34 | -28:07:59.00 | Bs | 9.0 |  VLT | 1246 | 357.3$+$04.0 | 17:23:24.94 | -28:59:06.07 | Ea | 8.6 | VLT
4315 | 004.3$+$01.8 | 17:48:36.54 | -24:16:34.24 | Bs | 8.8 |  VLT | 1252 | 357.5$+$03.1 | 17:27:24.36 | -29:21:14.62 | Ea | 6.0 | VLT
148 | 004.6$+$06.0 | 17:33:37.58 | -21:46:24.80 | Emrs | 8.0 |  VLT | 1253 | 357.5$+$03.2 | 17:26:59.83 | -29:15:31.87 | Bas | 10.20 | VLT
151 | 004.8$+$02.0 | 17:49:00.51 | -23:42:54.90 | Eas | 4.0 |  _HST_ | 1256 | 357.6$-$03.3 | 17:53:16.81 | -32:40:38.58 | Bs | 12.6 | VLT
150 | 004.8$-$05.0 | 18:16:11.43 | -27:14:57.98 | Bars | 11.4 |  VLT | 1259 | 357.9$-$03.8 | 17:56:13.93 | -32:37:22.21 | Bmrs | 13.0 | VLT
156 | 005.0$-$03.9 | 18:12:22.99 | -26:32:54.50 | Eamrs | 12.8 |  VLT | 4140 | 357.9$-$05.1 | 18:01:22.20 | -33:17:43.08 | Bmps | 26.4 | VLT
162 | 005.2$+$05.6 | 17:36:22.64 | -21:31:12.37 | Ers | 9.0 |  VLT | 1258 | 358.0$+$09.3 | 17:05:44.60 | -25:25:01.49 | Bamprs | 14.8 | VLT
4106 | 005.5$+$06.1 | 17:35:21.32 | -20:57:20.52 | Bs | 6.0 |  VLT | 1263 | 358.2$+$03.5 | 17:27:32.87 | -28:31:06.94 | E | 6.8 | VLT
165 | 005.5$-$04.0 | 18:13:40.62 | -26:08:39.43 | Ears | 12.4 |  VLT | 1266 | 358.2$+$04.2 | 17:24:52.10 | -28:05:54.60 | Em | 14.0 | VLT
Table 2: continued HASH ID | PN G | PN Centroid Coords. | Morph. |  Ang. Diam. |  Tele. |  HASH ID | PN G | PN Centroid Coords. | Morph. |  Ang. Diam. | Tele.
---|---|---|---|---|---|---|---|---|---|---|---
RA(J2000) | DEC(J2000) | RA(J2000) | DEC(J2000)
1275 | 358.5$+$02.9 | 17:30:30.43 | -28:35:54.90 | Bas | 3.8 |  _HST_ | 1312 | 359.3$-$01.8 | 17:51:18.93 | -30:23:53.23 | Bp | 6.8 | VLT
1276 | 358.5$-$04.2 | 17:59:02.51 | -32:21:43.63 | Bmps | 7.8 |  _HST_ | 1319 | 359.6$-$04.8 | 18:04:07.75 | -31:39:10.74 | Brs | 16.6 | VLT
1280 | 358.6$+$07.8 | 17:12:39.17 | -25:43:37.50 | Bs | 13.4 |  VLT | 1322 | 359.7$-$01.8 | 17:52:05.97 | -30:05:14.12 | Ramrs | 8.0 | VLT
1281 | 358.6$-$05.5 | 18:04:56.21 | -32:54:01.25 | Bamps | 16.8 |  VLT | 1327 | 359.8$+$02.4 | 17:35:48.12 | -27:43:20.40 | Em | 5.2 | VLT
1286 | 358.7$+$05.2 | 17:22:28.29 | -27:08:42.51 | Eamrs | 3.6 |  _HST_ | 1324 | 359.8$+$03.7 | 17:30:46.74 | -27:05:59.76 | Bp | 7.2 | VLT
1293 | 358.8$+$03.0 | 17:31:09.28 | -28:14:50.17 | Ears | 8.8 |  VLT | 1328 | 359.8$+$05.2 | 17:25:23.63 | -26:11:53.00 | Bprs | 21.4 | VLT
1295 | 358.9$+$03.4 | 17:30:02.55 | -27:59:18.19 | Bamps | 3.2 |  _HST_ | 1329 | 359.8$+$05.6 | 17:24:01.48 | -25:59:23.39 | B | 7.0 | VLT
1298 | 359.0$-$04.1 | 17:59:56.74 | -31:54:27.90 | Bms | 9.2 |  VLT | 1326 | 359.8$+$06.9 | 17:19:13.39 | -25:17:17.32 | Bmps | 31.8 | VLT
1302 | 359.1$-$02.9 | 17:55:05.70 | -31:12:16.85 | Bs | 10.0 |  VLT | 1330 | 359.8$-$07.2 | 18:14:50.60 | -32:36:55.30 | Em | 11.6 | VLT
1308 | 359.2$+$04.7 | 17:25:44.08 | -26:57:48.04 | Ears | 2.6 |  _HST_ | 1334 | 359.9$-$04.5 | 18:03:52.62 | -31:17:46.79 | E | 9.0 | VLT

###  4.2 Astrometric integrity of the _HST_ , Pan-STARRS and VLT imagery

Prior to detection and measurement of any CSPN evident in our data and
comparison with _Gaia_ it is necessary to establish the astrometric integrity
across the various image data sets used. The applied astrometric solution for
the _HST_ (Anderson & King, 2000) and Pan-STARRS data (Magnier et al., 2020)
are reported as highly accurate. However, for the VLT imagery the astrometric
fits provided from the ESO pipelines are not sufficiently accurate. It was
necessary to boot-strap the VLT data to the equivalent _Gaia_ data co-ordinate
system before CSPN position comparisons. Small-scale translation and rotations
were needed with typical offsets found to be 1 to a few VLT image pixels. This
task was accomplished using Python scripts after identifying 6-10 stars in
common widely spread across the images and _Gaia_ data.

We also noticed a small systematic $\sim$1 pixel offset to the North and the
West between the _Gaia_ and Pan-STARRS frames and so a similar exercise was
done here. Once completed we could then confidently assess the CSPN detected
in the VLT, _HST_ and Pan-STARRS imagery with the co-ordinates for the CSPN
reported from _Gaia_ data, e.g. Chornay & Walton (2021) and González-
Santamaría et al. (2021) (see later).

###  4.3 Identification of CSPN

CSPN exhibit a wide range of heterogeneous properties and range from weak
emission line stars (wels) to PG1159 stars, Wolf-Rayet type stars of the [WO],
[WC] and even [WN] sequence (where the square bracket demarcates them from
their population I high mass equivalents), different kinds of white dwarfs
(DA, DAO, DO) and early and late O(H) and Of(H) stars etc, e.g. see Weidmann
et al. (2018). How these stars have influenced the characteristics of the
surrounding PNe is a key question and it depends on the timescales for the
CSPN evolution and the visibility lifetime of the PN itself as it expands to
ultimate dissolution in the ambient ISM e.g. see Fragkou et al. (2022).

In Table 3 we present the results of our careful investigation into the CSPN
that we have independently identified in the _HST_ , and astrometrically
corrected VLT and Pan-STARRS imagery for all 136 PNe in our sample.

A total of 78 PNe have high confidence CSPN detected in the combined imaging
data for a 57.4% detection rate. In most cases (58/78) the _HST_ and VLT
imagery provided the best data to identify the CSPN. However, in 18 cases the
Pan-STARRS data was the best (though in 2 cases the CSPN was seen only in the
Vista Variables in the Via Lactea (VVV) Near Infrared survey data (Saito et
al., 2012b) due to dust extinction. This is because not only is the Pan-STARRS
data deeper in many cases than that from the VLT, but for some sources the
resolution is also better (due to the filler program nature of the VLT
observations). Also, because Pan-STARRS used broad-band filters, the CSPN (and
their colours) are easier to see.

These candidates are selected as being almost exclusively at the exact
geometric centres of their respective host, compact PNe. Of these, 22 are
already listed in the putative CSPN catalogue of Weidmann et al. (2020) that
reports not the actual accurate CSPN co-ordinates in many cases, but rather
the centroid positions of these high surface brightness, compact PNe. These
are quoted only to be accurate to 0.1 seconds of time for Right Ascension and
0.1 seconds of arc for Declination. This is insufficiently accurate for
reporting the positions of the true CSPN visible in the _HST_ and VLT imagery.
The offsets of 0.20 to 0.5 arcseconds typical in both co-ordinates in Weidmann
et al. (2020) can miss the CSPN completely in the high resolution imagery. The
spectral types of 22 CSPN reported in Weidmann et al. (2020) are also shown in
the last column. They largely comprise O-type stars, _wels_ and [WR] stars.

Of our 78 CSPN, 74 also have _Gaia_ EDR3 counterparts included in Table 3. We
also provide _Gaia_ CSPN G magnitudes, proper motions and distance estimates
where available. An assessment of the reported _Gaia_ CSPN identifications is
given below.

Table 3: A total of 78 PNe with CSPN detected in the combined imaging data of our sample. Cols. 1-2 list HASH IDs and PN G numbers of each nebula, cols 3-4 refer to the CSPN coordinates measured in this work, telescopes used for the CSPN detection are given in col. 5, cols. 6-7 are coordinates of _Gaia_ CSPN identifications. The 63 we consider to have been correctly identified in _Gaia_ EDR3 are listed with "Y" in the "True?" column (col. 8). The angular separations between true CSPNe and _Gaia_ identifications are listed in col. 9. Cols. 10-14 are the apparent magnitudes in _Gaia_ G band, parallaxes and uncertainties in parallaxes respectively. The spectral types of 22 CSPN that were reported in Weidmann et al. (2020) are also shown in the last column. There are 3 CSPN spectra types that are indicated as being in binary systems by ’+?’ where the nature of the binary companion is unknown. HASH | PN G | This Work | Tele. | Gaia EDR3 | True? | ${\Delta r_{{\star}}}$ | _G_ | $\omega$ | $\sigma_{\omega}$ | $D$ | Spec. Type
---|---|---|---|---|---|---|---|---|---|---|---
ID | RAJ2000 | DECJ2000 | RAJ2000 | DECJ2000 | (arcsec) | (mag) | (mas) | (mas) | (kpc)
12 | 000.1+02.6 | 17:35:35.44 | -27:24:05.37 | VLT | 17:35:35.45 | -27:24:05.40 | Y | 0.07 | 17.9 |  |  |  |
17 | 000.1-02.3 | 17:55:20.55 | -29:57:36.40 | VLT |  |  | New |  |  |  |  |  |
19 | 000.2-01.9 | 17:53:45.64 | -29:43:46.98 | VLT | 17:53:45.65 | -29:43:46.94 | Y | 0.13 | 16.9 | -0.164 | 0.112 | 6.49${}_{-1.16}^{+1.27}$ | O(H)6-8 III-V+?
27 | 000.4-02.9 | 17:58:19.40 | -30:00:39.36 | VLT | 17:58:19.39 | -30:00:39.36 | Y | 0.10 | 18.3 | 0.367 | 0.189 | 4.63${}_{-1.38}^{+1.5}$ |
40 | 000.7+03.2 | 17:34:54.73 | -26:35:57.28 | PS | 17:34:54.72 | -26:35:57.33 | Y | 0.14 | 20.9 |  |  |  |
38∙ | 000.7-03.7 | 18:02:19.26 | -30:14:25.94 | VLT | 18:02:19.26 | -30:14:25.89 | Y? | 0.05 | 18.4 | 1.391 | 0.299 | 0.98${}_{-0.28}^{+2.18}$ |
42⋄ | 000.7-07.4 | 18:17:37.20 | -31:56:46.87 | VVV | 18:17:37.20 | -31:56:46.99 | N | 0.12 |  |  |  |  |
58 | 001.2+02.1 | 17:40:12.83 | -26:44:21.83 | _HST_ | 17:40:12.84 | -26:44:21.75 | Y | 0.11 | 18.7 | -0.116 | 0.404 | 5.92${}_{-1.71}^{+1.78}$ |
60 | 001.2-03.0 | 18:00:37.62 | -29:21:50.67 | PS | 18:00:37.62 | -29:21:50.73 | Y | 0.08 | 15.7 | 0.091 | 0.052 | 8.92${}_{-1.82}^{+2.11}$ | [WC 11]?
62 | 001.3-01.2 | 17:53:47.17 | -28:27:17.99 | VLT | 17:53:47.17 | -28:27:18.09 | Y | 0.12 | 18.0 | 0.751 | 0.297 | 4.84${}_{-2.84}^{+3.77}$ |
73 | 001.7+05.7 | 17:28:01.74 | -24:25:23.72 | VLT | 17:28:01.75 | -24:25:23.43 | N | 0.32 |  |  |  |  |
74∗ | 001.7-04.4 | 18:07:14.56 | -29:41:24.67 | _HST_ | 18:07:14.56 | -29:41:24.61 | N | 0.07 |  |  |  |  | [WC 11]
90 | 002.2-09.4 | 18:29:11.64 | -31:29:59.24 | _HST_ | 18:29:11.65 | -31:29:59.19 | Y | 0.09 | 15.2 | 0.276 | 0.086 | 4.57${}_{-1.03}^{+1.24}$ | [WO 4]pec
91 | 002.3+02.2 | 17:42:30.02 | -25:45:28.84 | VLT | 17:42:29.90 | -25:45:27 | N | 2.42 |  |  |  |  |
93 | 002.3-03.4 | 18:04:28.81 | -28:37:38.29 | _HST_ | 18:04:28.81 | -28:37:38.29 | Y | 0.05 | 18.9 |  |  |  |
98 | 002.5-01.7 | 17:58:31.20 | -27:37:05.37 | VLT | 17:58:31.22 | -27:37:04.60 | N | 0.82 |  |  |  |  |
4326 | 002.6+02.1 | 17:43:39.48 | -25:36:42.87 | VLT | 17:43:39.55 | -25:36:42.84 | N | 0.97 |  |  |  |  |
105 | 002.7-04.8 | 18:11:04.99 | -28:58:59.22 | _HST_ | 18:11:04.99 | -28:58:59.09 | Y | 0.13 | 18.3 | -1.359 | 0.524 | 4.31${}_{-1.03}^{+1.1}$ | cont.
106 | 002.8+01.7 | 17:45:39.80 | -25:40:00.52 | _HST_ | 17:45:39.80 | -25:40:00.50 | Y | 0.02 | 17.8 | -0.148 | 0.16 | 11.85${}_{-3.31}^{+3.59}$ |
109 | 002.9-03.9 | 18:08:05.76 | -28:26:10.32 | _HST_ | 18:08:05.77 | -28:26:10.81 | N | 0.51 |  |  |  |  |
114 | 003.1+03.4 | 17:40:07.42 | -24:25:42.77 | _HST_ | 17:40:07.42 | -24:25:42.67 | Y | 0.11 | 16.5 | 0.083 | 0.062 | 7.88${}_{-1.53}^{+1.73}$ |
125 | 003.6+03.1 | 17:41:57.26 | -24:11:16.33 | _HST_ | 17:41:57.26 | -24:11:16.24 | Y | 0.10 | 17.6 | -0.45 | 0.581 | 8.43${}_{-2.31}^{+2.35}$ | _wels_
129 | 003.7-04.6 | 18:12:34.39 | -27:58:10.70 | PS | 18:12:34.39 | -27:58:10.65 | Y | 0.06 | 16.9 |  |  |  | _wels_
131 | 003.8-04.3 | 18:11:29.26 | -27:46:15.92 | _HST_ | 18:11:29.31 | -27:46:16.16 | N | 0.69 |  |  |  |  |
134 | 003.9-03.1 | 18:06:50.03 | -27:06:19.58 | _HST_ | 18:06:50.03 | -27:06:19.51 | Y | 0.08 | 19.4 |  |  |  |
139 | 004.0-03.0 | 18:06:40.91 | -26:54:56.28 | _HST_ | 18:06:40.91 | -26:54:56.38 | Y | 0.11 | 14.4 | 0.053 | 0.038 | 9.69${}_{-1.55}^{+1.74}$ | O(H)f+?
142 | 004.2-03.2 | 18:08:01.39 | -26:54:02.14 | PS | 18:08:01.39 | -26:54:02.18 | Y | 0.04 | 17.2 | -0.104 | 0.109 |  |
141 | 004.2-04.3 | 18:12:25.11 | -27:29:13.18 | PS | 18:12:25.11 | -27:29:13.20 | Y | 0.04 | 16.4 | 0.283 | 0.082 | 6.10${}_{-2.11}^{+3.21}$ | _wels_
143 | 004.2-05.9 | 18:18:38.35 | -28:07:58.53 | VLT | 18:18:38.35 | -28:07:58.60 | Y | 0.07 | 17.3 | 0.286 | 0.151 |  |
4315 | 004.3+01.8 | 17:48:36.54 | -24:16:34.24 | VLT | 17:48:36.54 | -24:16:34.23 | Y | 0.05 | 16.2 | -0.024 | 0.142 | 5.73${}_{-1.18}^{+1.28}$ |
148 | 004.6+06.0 | 17:33:37.58 | -21:46:24.80 | VLT | 17:33:37.58 | -21:46:24.88 | Y | 0.08 | 17.6 | 0.048 | 0.15 | 6.41${}_{-1.52}^{+1.65}$ | _wels_
151 | 004.8+02.0 | 17:49:00.51 | -23:42:54.90 | _HST_ | 17:49:00.51 | -23:42:54.84 | Y | 0.06 | 16.1 | 0.169 | 0.052 | 8.86${}_{-2.64}^{+3.73}$ |
150 | 004.8-05.0 | 18:16:11.44 | -27:14:57.83 | PS | 18:16:11.44 | -27:14:57.86 | Y | 0.04 | 17.8 | -0.194 | 0.151 | 7.55${}_{-1.65}^{+1.79}$ |
156 | 005.0-03.9 | 18:12:22.99 | -26:32:54.50 | VLT | 18:12:22.98 | -26:32:54.52 | Y | 0.09 | 19.0 |  |  |  |
162 | 005.2+05.6 | 17:36:22.65 | -21:31:12.60 | VLT | 17:36:22.64 | -21:31:12.39 | N | 0.25 |  |  |  |  |
4106 | 005.5+06.1 | 17:35:21.43 | -20:57:23.47 | PS | 17:35:21.43 | -20:57:23.45 | Y | 0.02 | 16.6 | 0.014 | 0.095 |  |
165 | 005.5-04.0 | 18:13:40.60 | -26:08:39.53 | PS | 18:13:40.60 | -26:08:39.60 | Y | 0.07 | 19.3 | 0.608 | 0.429 |  |
179 | 006.3+03.3 | 17:47:33.95 | -21:47:23.29 | VLT | 17:47:33.94 | -21:47:23.29 | Y | 0.11 | 18.5 | -0.022 | 0.246 |  |
180 | 006.3+04.4 | 17:43:28.75 | -21:09:51.77 | _HST_ | 17:43:28.76 | -21:09:51.68 | Y | 0.12 | 18.1 | 0.018 | 0.177 |  | Of?
182 | 006.4+02.0 | 17:52:41.44 | -22:21:57.18 | _HST_ | 17:52:41.45 | -22:21:57.14 | Y | 0.10 | 17.6 |  |  |  | _wels_
193 | 007.0+06.3 | 17:38:11.59 | -19:37:37.62 | PS | 17:38:11.60 | -19:37:37.75 | Y | 0.21 | 15.7 | 0.177 | 0.041 |  |
197 | 007.5+07.4 | 17:35:10.22 | -18:34:20.76 | PS | 17:35:10.22 | -18:34:20.70 | Y | 0.06 | 19.3 | 0.886 | 0.335 |  |
198 | 007.6+06.9 | 17:37:21.99 | -18:46:41.76 | PS | 17:37:21.99 | -18:46:41.75 | Y | 0.03 | 18.6 | -0.119 | 0.257 |  |
200 | 007.8-03.7 | 18:17:16.03 | -23:58:54.89 | PS | 18:17:16.03 | -23:58:54.84 | Y | 0.06 | 17.1 |  |  |  | [WC]
201 | 007.8-04.4 | 18:20:08.84 | -24:15:05.39 | _HST_ | 18:20:08.84 | -24:15:05.36 | Y | 0.05 | 14.5 | 0.033 | 0.032 | 9.93${}_{-1.32}^{+1.46}$ | O(H)8-9 I
4111 | 008.2+06.8 | 17:38:57.37 | -18:17:35.92 | _HST_ | 17:38:57.37 | -18:17:35.80 | Y | 0.12 | 13.6 | 0.05 | 0.029 |  | O(H)7-8
208 | 008.4-03.6 | 18:18:23.83 | -23:24:57.50 | PS | 18:18:23.83 | -23:24:57.57 | Y | 0.07 | 17.7 | 0.231 | 0.114 |  |
210 | 008.6-02.6 | 18:14:50.91 | -22:43:55.52 | _HST_ | 18:14:50.91 | -22:43:55.51 | Y | 0.06 | 17.9 | 0.089 | 0.188 |  |
217 | 009.4-09.8 | 18:44:43.16 | -25:21:33.80 | PS | 18:44:43.15 | -25:21:33.89 | Y | 0.14 | 17.5 | -0.089 | 0.146 | 8.10${}_{-1.79}^{+1.91}$ |
223∙ | 009.8-04.6 | 18:25:04.97 | -22:34:52.48 | PS | 18:25:04.96 | -22:34:52.48 | Y? | 0.09 | 18.7 | 3.371 | 0.551 | 0.32${}_{-0.05}^{+0.08}$ | [WO 2]
Table 4: continued HASH | PN G | This Work | Tele. | Gaia EDR3 | True? | ${\Delta r_{{\star}}}$ | _G_ | $\omega$ | $\sigma_{\omega}$ | $D$ | Spec. Type
---|---|---|---|---|---|---|---|---|---|---|---
ID | RAJ2000 | DECJ2000 | RAJ2000 | DECJ2000 | (arcsec) | (mag) | (mas) | (mas) | (kpc)
1151 | 351.1+04.8 | 17:03:46.84 | -33:29:44.64 | _HST_ | 17:03:46.84 | -33:29:44.52 | Y | 0.12 | 16.1 | 0.102 | 0.071 | 7.08${}_{-1.4}^{+1.57}$ | wels?
1156⋄ | 351.6-06.2 | 17:50:44.62 | -39:17:26.80 | VVV | 17:50:44.60 | -39:17:26.64 | N | 0.32 |  |  |  |  |
1159 | 351.9+09.0 | 16:50:17.08 | -30:19:55.39 | VLT | 16:50:17.08 | -30:19:55.28 | Y | 0.12 | 16.6 | 0.122 | 0.082 | 7.49${}_{-1.66}^{+1.84}$ |
1160 | 351.9-01.9 | 17:33:00.68 | -36:43:52.94 | _HST_ | 17:33:00.68 | -36:43:52.90 | Y | 0.05 | 18.5 |  |  |  |
1164 | 352.6+03.0 | 17:14:42.93 | -33:24:47.71 | _HST_ |  |  | New |  |  |  |  |  |
1178 | 353.7+06.3 | 17:05:13.91 | -30:32:19.53 | VLT | 17:05:13.91 | -30:32:19.60 | Y | 0.08 | 17.2 |  |  |  | O
1189 | 354.9+03.5 | 17:19:20.24 | -31:12:40.95 | _HST_ | 17:19:20.24 | -31:12:40.96 | Y | 0.02 | 19.4 | 0.438 | 0.447 |  |
1199# | 355.4-02.4 | 17:44:20.61 | -34:06:40.96 | _HST_ | 17:44:20.64 | -34:06:40.80 | N | 0.34 |  |  |  |  |
1207 | 355.9-04.2 | 17:52:58.94 | -34:38:22.99 | _HST_ | 17:52:58.94 | -34:38:22.85 | Y | 0.14 | 16.3 | 0.197 | 0.08 | 6.28${}_{-1.53}^{+1.81}$ | wels
1235 | 357.1+03.6 | 17:24:34.45 | -29:24:19.80 | PS | 17:24:34.45 | -29:24:19.79 | Y | 0.02 | 16.2 | 0.172 | 0.054 | 5.67${}_{-0.99}^{+1.17}$ | wels
1242 | 357.2+02.0 | 17:31:08.11 | -30:10:28.08 | _HST_ |  |  | New |  |  |  |  |  |
1246 | 357.3+04.0 | 17:23:24.93 | -28:59:05.91 | PS | 17:23:24.93 | -28:59:06.03 | Y | 0.13 | 18.7 | 0.007 | 0.254 | 7.49${}_{-2.13}^{+2.25}$ |
1252 | 357.5+03.1 | 17:27:24.36 | -29:21:14.63 | PS | 17:27:24.36 | -29:21:14.68 | Y | 0.06 | 15.2 | 0.093 | 0.037 |  |
1256 | 357.6-03.3 | 17:53:16.82 | -32:40:38.50 | VLT | 17:53:16.82 | -32:40:38.58 | Y | 0.08 | 19.1 | 0.953 | 0.447 | 5.37${}_{-2.92}^{+2.99}$ | O(H)+?
1259 | 357.9-03.8 | 17:56:13.93 | -32:37:22.21 | VLT | 17:56:13.93 | -32:37:22.20 | Y | 0.05 | 18.7 | -2.383 | 0.341 | 8.31${}_{-1.66}^{+1.75}$ |
1258 | 358.0+09.3 | 17:05:44.60 | -25:25:01.49 | VLT | 17:05:44.60 | -25:25:01.52 | Y | 0.07 | 17.2 | 0.086 | 0.098 |  |
1263 | 358.2+03.5 | 17:27:32.87 | -28:31:06.86 | PS | 17:27:32.86 | -28:31:06.93 | Y | 0.11 | 19.1 | -1.589 | 0.486 | 8.98${}_{-2.29}^{+2.39}$ |
1275 | 358.5+02.9 | 17:30:30.43 | -28:35:54.90 | _HST_ | 17:30:30.44 | -28:35:54.92 | Y | 0.13 | 19.0 | 1.294 | 0.397 | 10.8${}_{-7.25}^{+5.02}$ |
1276 | 358.5-04.2 | 17:59:02.51 | -32:21:43.63 | _HST_ | 17:59:02.51 | -32:21:43.50 | Y | 0.13 | 15.1 | -0.447 | 0.242 | 7.90${}_{-1.81}^{+1.91}$ |
1280 | 358.6+07.8 | 17:12:39.17 | -25:43:37.50 | VLT | 17:12:39.16 | -25:43:37.56 | Y | 0.11 | 16.8 | -0.032 | 0.073 |  |
1281 | 358.6-05.5 | 18:04:56.22 | -32:54:01.05 | VLT | 18:04:56.23 | -32:54:01.08 | Y | 0.11 | 18.6 | 0.36 | 0.206 | 5.33${}_{-1.79}^{+1.95}$ |
1286 | 358.7+05.2 | 17:22:28.29 | -27:08:42.51 | _HST_ | 17:22:28.29 | -27:08:42.46 | Y | 0.05 | 16.9 | 0.152 | 0.084 |  |
1293 | 358.8+03.0 | 17:31:09.28 | -28:14:50.17 | VLT |  |  | New |  |  |  |  |  |
1308 | 359.2+04.7 | 17:25:44.08 | -26:57:48.04 | _HST_ | 17:25:44.08 | -26:57:48.01 | Y | 0.04 | 15.3 | 0.064 | 0.037 |  |
1312 | 359.3-01.8 | 17:51:18.93 | -30:23:53.23 | VLT | 17:51:18.92 | -30:23:53.18 | Y | 0.10 | 16.0 | 0.083 | 0.062 | 6.92${}_{-1.3}^{+1.51}$ | [WC 11]
1322 | 359.7-01.8 | 17:52:05.97 | -30:05:14.12 | VLT | 17:52:05.96 | -30:05:14.15 | Y | 0.10 | 17.7 | 0.135 | 0.126 | 5.16${}_{-1.25}^{+1.46}$ |
1324 | 359.8+03.7 | 17:30:46.73 | -27:05:59.91 | PS | 17:30:46.73 | -27:05:59.86 | Y | 0.05 | 18.1 | 0.172 | 0.22 | 8.09${}_{-2.42}^{+2.51}$ |
1328 | 359.8+05.2 | 17:25:23.63 | -26:11:53.00 | VLT | 17:25:23.62 | -26:11:53.00 | Y | 0.09 | 19.0 | -1.284 | 0.353 |  |
∙ These two PNe gave suspiciously low _Gaia_ distances which give unfeasibly
short kinematic lifetimes. See main text for caveats.
∗ _HST_ shows two stars within the PN envelope $\sim$0.2 arcseconds apart.
_Gaia_ cannot resolve these. The reported astrometric solution is of low
quality
${}^{\ }$ (Goodness of fit statistic $>$ 100).
# _Gaia_ has selected a compact nebula knot $\sim$0.3 arcseconds from the
actual CSPN evident in the short exposure broad-band 457M filter _HST_ imagery
⋄ Two examples where the VVV NIR imagery has revealed a faint CSPN within an
inner oval NIR nebula ring

##  5 Results

Here we present results from our work on morphological classifications, CSPN
identification and measurement and kinematic age estimates. PNe centroid
positions, morphological classifications, updated angular size estimates, CSPN
coordinates have been determined (where available) from the combined _HST_ ,
VLT, Pan-STARRS and in two cases, near infrared J, H and Ks data from the VVV
(Saito et al., 2012b). These measurements are summarised in Table 1 and Table
3. All centroid coordinates of PNe in this sample were remeasured with this
unique set of combined high angular resolution and high sensitivity imaging
data. Currently none of the VLT images have yet been ingested into HASH.

###  5.1 Updated angular size measurements

The new VLT imagery provides an opportunity to improve the angular size
estimates currently in HASH as previously estimated from the SHS imagery. The
generally better seeing in the VLT imagery c.f. the 2 arcseconds typical for
the SHS, and the better VLT resolution allows improved estimation of PNe
angular size. This is an important parameter when estimating kinematic age.
Angular size measures are taken from the main body of the PNe and not from any
very faint outer halos that likely reflect earlier mass loss off the AGB. All
new angular size values are provided in Table 1 and were consistently taken
from the 98% intensity plots from the VLT and _HST_ fits images with histogram
equalized scaling. We believe this choice better reflects the true extent of
the ejected material that constitutes these PNe.

Estimated errors in angular size measurements are 4 image pixels which is 0.5
and 0.2 arcseconds for the VLT and _HST_ images respectively. Comparison with
current HASH values show a mean difference of 3.4 arcseconds. The average
angular size of this re-measured $\leq$ 10 arcsecond PNe bulge sample is 9.5
with $\sigma$ = 5.0. Interestingly, 47 PNe now exceed (mostly marginally) the
original $\leq$ 10 arcsecond major diameter HASH based angular diameter
selection criterion.

###  5.2 Morphological classification

Below we present the results from our re-evaluated morphological
classifications for all 136 PNe in the bulge sample. We examine the effect of
image quality, show the new distribution of morphological classes, reveal the
surprising prevalence of bipolars and the incidence of point-symmetric and
non-symmetric structures. This discussion supplements the PN by PN listing
given in Table 1. Improved image resolution for many compact bulge PNe from
the VLT (and in some cases from Pan-STARRS) together with a fresh assessment
of the _HST_ imagery has revealed additional morphological structures for many
PNe in this sample. All PNe _HST_ imagery is already in HASH and was usually
used for the original HASH morphological classifications. In some cases even
the true basic PNe form, not apparent from the lower resolution imagery
previously used, has been revealed for the first time.

####  5.2.1 The effect of image quality

It is clear from this work that assigned PNe morphology, both in terms of
basic classes and also sub-classifications is, unsurprisingly, a strong
function of the resolution, depth and sensitivity of the available imagery
especially when the PN is compact. In Fig. 4 we show two examples that
demonstrate the image quality comparison between the original SHS imagery and
the VLT and _HST_ imagery selected from the 40 PNe where both observational
data sets are available. This demonstrates the effect in going from lower to
intermediate to high image resolution and how this can alter assigned
morphological classifications. This is only really an issue for compact PNe in
HASH with diameters $<$ 10 arcseconds, as for our bulge sample.

In the top panel PNG 352.6+03.0 is presented. It shows a main morphology shift
from class from ‘E’ to ‘B’ when going from the SHS to the VLT image. The final
_HST_ images at right reveal more structural details that allow for additional
‘sparm’ classifiers. The lower panel show images of PNG 008.6-02.6. The main
morphological class has been changed from ‘R’ in the SHS to ‘E’ in the VLT
while the _HST_ image shows the PN is actually a bipolar with outer structures
and internal over densities. The CSPN is now also visible. Both examples
demonstrate the importance of image quality and resolution in PNe
morphological classification. We note 32 of the 40 objects classified from the
VLT imagery have morphological results somewhat inconsistent with their _HST_
counterparts which are used in preference. This demonstrates that using images
where small-scale internal and outer PN features are not well-resolved or seen
can lead to basic classification errors and differences in the sub-class
assignments.

Figure 4: Two cases that contrast the difference in PN morphological classification with varying image quality. PNG 352.6+03.0 (upper panel) shows a change in morphological class assignment from ‘E’ $\rightarrow$ ‘B’ when going from the SHS to the VLT image. The final _HST_ image reveals more structural details, including the bipolar lobes and central ring structure allowing for additional ‘sparm’ classifiers to be added - better seen in the expanded, final image. The lower panel show images of PNG 008.6-02.6. The main morphological class has been changed from ‘R’ in the SHS to ‘E’ in the VLT while the _HST_ image shows the PN is actually a bipolar with outer structures and internal over densities. In both cases the CSPN are also now visible thanks to _HST_. Morphology | Change | Telescope | Total
---|---|---|---
VLT | _HST_ | PS
Main Class | E $\rightarrow$ B | 28 | 2 | - | 30
| R $\rightarrow$ B | 4 | - | - | 4
| S $\rightarrow$ B | 4 | - | - | 4
| R $\rightarrow$ E | 6 | - | 1 | 7
| S $\rightarrow$ E | 7 | 1 | - | 8
| E $\rightarrow$ A | 1 | - | - | 1
Total |  |  |  |  | 54
Sub-class | s | 61 | 3 | 2 | 66
| p | 16 | 2 | - | 18
| a | 25 | 5 | 1 | 31
| r | 27 | 3 | 1 | 31
| m | 34 | 6 | 2 | 42
Total |  |  |  |  | 188
Table 5: Improvement in PN morphological classifications possible mainly from
the combined _HST_ and VLT imagery. The integers show the numbers of PNe that
have undergone a classification change in this study. A total of 56 PNe have
had their major class changed with the bulk (38) being reclassified to bipolar
for a total change of 54/136 or 70%. The bottom part of the table illustrates
detection of the newly resolved sub-structures. The combined imagery allows to
add 188 sub-classifiers for the whole sample (1.4 new ‘sparm’ sub-classifiers
per nebula on average).

####  5.2.2 An enrichment in bipolar morphology revealed

The revised morphological classification of the bulge PNe were compared with
those recorded in the HASH database. Changes in the main and sub-classifiers
are summarised in Table 5. PNe of the morphological class I and A are rare:
only one was found in this sample. In HASH overall they represent a mere 0.7%
and 0.4% of all Galactic PNe.

Among the 96 of 136 PNe observed with the VLT and where the pre-imaging
provides the best available data, 5 now reveal an elliptical nebulosity with
no resolved internal structure, resulting in a new ‘E’ class designation. They
were previously classified as ‘S’ for point-like objects in HASH from the
previous lower resolution optical imagery. For the remaining 91 PNe, the main
"ERBIAS" morphology of 45 PNe have changed in this new assessment from their
current HASH values. Some 36 PNe classified as E, R or S actually exhibit
clear bipolar features in the VLT observations. For 76 PNe, we have also made
changes to the secondary "sparm" classification.

All _HST_ observations are accessible in HASH via a clickable icon on the main
database page for each PNe. Based on assessment of the _HST_ imagery we now
re-classify PNG 355.9+03.6 from ‘S’ to ‘E’, PNG 001.2+02.1 and PNG 004.0-03.0
from ‘E’ to ‘B’ (the old HASH morphologies for these two were out of date). A
further 11 PNe with _HST_ imagery have had their ‘sparm’ sub-classifiers
upgraded after careful examination.

We present the fraction of objects in the different main morphological classes
in the histogram in Fig. 5. We include the re-assessed morphologies of this
PNe sample as well as for all Galactic "True" PNe in the HASH database plotted
for comparison. From Table 5, 70% of the changes made to main HASH
classifications are for newly-resolved bipolar PNe. As a result, evident
bipolar PN now represent the majority (68%) of bulge PNe in our sample.
Interestingly, even with the old HASH classifications, the fraction of bipolar
PNe in the Galactic bulge exceeds that for general Galactic disk PNe
population and accounts for the largest proportion of this population. This
reveals an underestimation of the actual fraction of bipolar PNe compared to
what lower resolution and poorer sensitivity imaging had indicated. For
example, when bipolar lobes are not seen (too faint) or resolved (too
compact), the morphology can appear to belong to class E or even S.

This 68% fraction must be a lower limit as inclination of the bipolar to the
line of sight can also hide the bipolar nature of some PNe (Pollacco & Bell,
1997). Indeed, the thick nebular annular ring of PNG 357.2+02.0 shown in the
upper right middle panel in Fig. 6 is probably one such case. It is a very
close analogue to that modelled for a bipolar inclined only 10 degrees to the
line of sight as in Fig.2 of Pollacco & Bell (1997). It is also reminiscent of
the example for PN Sp 1 (PNG 329.0+01.9) presented by Mitchell et al. (2006)
where additional kinematic evidence indicate we are seeing a bipolar PN pole
on.

Figure 5: The fractional populations of different, basic PNe morphological
classes. The blue histogram shows the sample of 136 PNe in the combined VLT
and _HST_ observations. The dark grey histogram indicates the previous
fraction of types of this sample in the HASH database. The light grey
histogram presents the fractional morphological results for all 2676 ’True"
Galactic PNe with a morphological classification in HASH.

####  5.2.3 Resolved point-symmetric features in bipolar PNe

The fraction of PNe with a specific secondary "sparm" morphology among each
main class is summarised in Table 6. With the combined VLT, _HST_ and
occasional Pan-STARSS observations, the number of PNe with internal structure
resolved (sub-class s) doubled. The fraction of bipolar PNe in the sub-class
‘s’ is larger than that of the elliptical PNe, which supports the idea that
determination of bipolar morphology requires better-resolved imaging. Three
class ‘E’ PNe, PNG 003.6-02.3, PNG 008.4-03.6 and PNG 353.7+06.3, and one
class ‘R’ PN, PNG 002.5-01.7, were observed with point-symmetric features in
HASH. However, all PNe in sub-class ‘p’ in this work have bipolar morphology.
There is strong observational evidence that PNe with such point-symmetric
features house short-period binaries (Miszalski et al., 2009b). In our sample
such features were also seen in PNG 002.5-01.7, PNG 003.6-02.3 and PNG
008.4-03.6 in their H$\alpha$ outer shells in catalogued images in HASH. High-
resolution images of PNG 002.5-01.7 and PNG 003.6-02.3 in this combined data
show no point-symmetric structure while our VLT image of PNG 008.4-03.6
exhibits clearly lobes and the equatorial concentration of matter expected for
bipolar PNe, although VLT H$\alpha$ images of PNG 002.5-01.7 and PNG
003.6-02.3 are not available.

Class |  | - | s | p | r | a | m | Total
---|---|---|---|---|---|---|---|---
E | HASH | 19 [0.35] | 13 [0.24] |  3 [0.05] | 15 [0.27] | 22 [0.40] | 14 [0.25] | 55
This Work |  4 [0.10] | 27 [0.69] | - | 24 [0.62] | 23 [0.59] | 20 [0.51] | 39
R | HASH |  8 [0.57] |  3 [0.21] |  1 [0.07] |  5 [0.36] |  2 [0.14] |  1 [0.07] | 14
This Work | - |  3 [1.00] | - |  2 [0.67] |  3 [1.00] |  2 [0.67] | 3
B | HASH | 14 [0.25] | 34 [0.62] | 16 [0.29] | 17 [0.31] |  8 [0.15] | 19 [0.35] | 55
This Work |  2 [0.02] | 85 [0.91] | 35 [0.38] | 33 [0.35] | 36 [0.39] | 47 [0.51] | 93
Table 6: Number of objects in a given main and sub-class for the sample of
bulge PN in this work. The fraction of the total PNe of each main class is
indicated in the parentheses. The upper and the lower rows compared the
results in HASH and in this work.

####  5.2.4 The incidence of non-symmetric structures

All sub-classifiers, except for sparm sub-classifier ‘a’ which describes a
one-side enhancement, refer to either axisymmetrical or spherical or point-
symmetrical features about the geometric centroid centre of the nebulae. The
majority of PNe in this sample show morphological structures with one or more
of these symmetries. However, 5 PNe, PNG 002.1-02.2, PNG 008.6-02.6, PNG
354.9+03.5, PNG 356.1-03.3 and PNG 357.5+03.2, show certain structural
features that are neither axisymmetrical nor point-symmetrical. Unlike a one-
side protrusion or asymmetry (PNe with sub-class a) or unequal lobes that
could be explained by binary interactions, such features that show a departure
from axisymmetry and point-symmetry simultaneously may have resulted from the
presence of a triple stellar system (e.g. Jones, 2016). We do not explore this
issue further in this paper.

###  5.3 Summary of findings for CSPN

Below we provide summary details of the main findings from our search for and
identification of CSPN in our complete bulge sample.

####  5.3.1 Existing catalogues of CSPN and _Gaia_

Searching for CSPN in the overall Galactic PNe population from _Gaia_ DR2 data
was first conducted by González-Santamaría et al. (2019), Stanghellini et al.
(2020) through a point-based matching approach, and in Chornay & Walton (2020)
through an automated matching process that considered both _Gaia_ relative
positions and colour information. The results were updated with the latest
Gaia EDR3 data in Chornay & Walton (2021) and González-Santamaría et al.
(2021), hereafter CW21 and GSM21 respectively. A preliminary analysis by
Parker et al. (2022), developed further at the WD 2022 conference in Tubingen
in August 2022 by co-author Parker, indicates that the Chornay & Walton (2021)
catalogue is the more reliable, but that both suffer problems of correct CSPN
identification (see later). This is at least in part because many true CSPN
are actually fainter than the _Gaia_ limits and the bluest, most central
remaining candidate gets selected instead. We compare our detected CSPN sample
with those reported in CW21 and GSM21. The full CW21 sample used the entire
HASH Galactic PN catalogue as the input list.

####  5.3.2 Comparison with existing _Gaia_ CSPN listings

A quick comparison revealed that 70 of 78 CSPN recovered from our work have a
nominal identification in CW21 while 71 are identified in GSM21 and 67 CSPNe
appear in results of both studies for a combined total of 74 _Gaia_ CSPN. Four
CSPN observed, those of PNG 000.1-02.3, PNG 352.6+03.0, PNG 357.2+02.0,and PNG
358.8+03.0, were not in either of these CSPN catalogues, nor have they a
_Gaia_ identification, so they are newly discovered CSPN in this work. These
are shown in the upper four panels of Fig. 6.

A further 11 CSPN we have identified have relatively large offsets from the
stars listed as the _Gaia_ CSPN in CW21/GSM21 or have other issues where we
believe _Gaia_ has not actually seen the true CSPN (e.g. PNG 355.4-02.4).
These are false _Gaia_ CSPN identifications. They are identified with the
letter ’N’ in Table 3 under the column heading "True?" and comprise 14.9% of
the reported _Gaia_ CSPN in this sample. Four examples are shown in the lower
panels of Fig. 6. In the figure the centre of the open red crosses represents
the new CSPN positions while green triangles show the corresponding _Gaia_
CSPN reported in CW21/CSM21. PNG 002.3+02.2 (left panel) and PNG 002.5-01.7
(left middle panel) are examples from VLT imagery where the reported _Gaia_
CSPN is just part of a bright nebula region at the rim. PNG 002.9-03.9 (right
middle panel) and PNG 003.8-04.3 (right panel) are examples from _HST_ imagery
where a brighter _Gaia_ star other than the actual CSPN (which is too faint
for _Gaia_) was selected. The true CSPN are fainter and bluer and both are
located at the nebula’s geometric centroid. There are finally two examples of
CSPN identified from the VVV survey (Saito et al., 2012b) for PNG 000.7-07.4
and PNG 351.6-06.2. There is no evidence in their _HST_ imagery of any visible
CSPN but instead an obscured zone. We believe the reported _Gaia_ CSPN merely
reflects fits to the overall compact PNe and not to any true CSPN which we
consider obscured by dust but visible in the NIR. The VVV imagery of both is
similar \- fractured oval rings surrounding a clear, faint CSPN at the centre.
The optical nebula spectra of both PNe are high excitation with He II detected
so there is clearly a hot star present, even if obscured. There are a further
two PNe, PNG 000.7-03.7 and PNG 009.8-04.6 whose _Gaia_ distances are
suspiciously low. They have been assigned a Y? entry in Table 3 and are
discussed in Section 5.3.4 in this paper.

(a) Images of the 4 PNe with newly discovered CSPN: PNG 000.1-02.3, PNG
352.6+03.0, PNG 357.2+02.0, and PNG 358.8+03.0. The CSPN are indicated as the
centres with open red crosses. The origin of the discovery image as VLT, _HST_
or Pan-STARRS is given at the top with the PNG designation

(b) Examples of false _Gaia_ CSPN identifications from CW21/CSM21. The open
red crosses are the new CSPN positions while green triangles show the
corresponding _Gaia_ putative CSPN detections reported in CW21/CSM21. PNG
002.3-02.2 (left panel) and PNG 002.5-01.7 (left middle panel) are examples
from VLT imagery where the reported _Gaia_ CSPN is just part of a bright
nebula region at the rim. PNG 002.9-03.9 (right middle panel) and PNG
003.8-04.3 (right panel) are examples (which is too faint for _Gaia_) was
selected. The true CSPN are fainter and bluer and both located at the nebula’s
geometric centroid.

Figure 6: Upper panel: Identifications of 4 new CSPN discovered in this work.
Lower panel: Examples of 4 PNe showing that the previously reported _Gaia_
CSPN detections (green triangles) are false with the actual CSPN indicated via
open red crosses.

####  5.3.3 Issues with _Gaia_ as a resource for CSPN

The _Gaia_ satellite telescopes have relatively small rectangular mirrors of
1.45 $\times$ 0.5 m yielding a typical angular resolution never smaller than
$\sim$0.18 arcseconds Lindegren et al. (2021a). In practice very few _Gaia_
stars are found with separations $<$ 0.6 arcseconds due to other limitations.
Furthermore, as can be seen via the typical ground based imagery for these
Bulge PNe in HASH (see Fig. 3), that without the benefit of high resolution
_HST_ and VLT and even Pan-STARRS image data, these largely high-surface
brightness PNe appear very compact. Consequently, it is very difficult to both
clearly assess the PNe morphology or actually see the CSPN, as Weidmann et al.
(2020) notes. Hence, for some of the putative CSPN identified in _Gaia_ for
very compact PNe (e.g. for PNG 001.7-04.4), they actually likely reflect the
fitted position of the compact PNe itself. This is rather than the actual
resolved position and identification of the true CSPN, which in some cases are
below the _Gaia_ magnitude limits (Parker et al., 2022). This is even if the
reported positions are sufficiently accurate as they are based on the centroid
estimates of a very compact PNe.

In CW21 CSPN are attributed with a reliability factor (certainty of the CSPN
identification) of between 0 and 1, and a quality label A, B or C in GSM21.
These are not listed in Table 3. The reliability factor depends on the angular
separation between the match and the position of the PN relative to the size
of the nebulae and the colour. For 61 PNe with _Gaia_ identifications
consistent with the observed CSPN, 33 have a reliability factor $>$ 0.85 and
26 are concluded as less-reliable with a reliability $<$ 0.85 in CW21. When
considering the reliability groups in GSM21, 11 of 15 objects with a quality
label A in GSM21 show a decent match between the _Gaia_ identification and
with what we consider to be the actual CSPN.

The angular separations between our independently measured positions from our
high resolution VLT and archival _HST_ images and the reported _Gaia_ CSPN
coordinates in CW21 are listed in Table 3 as parameter ${\Delta r_{{\star}}}$.
The values of ${\Delta r_{{\star}}}$ are generally less than 10% of the major
radii of the nebulae in HASH. This is followed, when available, by the
astrometric measurements of parallaxes and the distance measurement in CW21.

####  5.3.4 CSPN parallax distances from _Gaia_

The parallax distances of 33 of the _Gaia_ sources that match our CSPN
observations are available. For most we consider the CSPN well determined.

For PNG 000.7-03.7 and PNG 009.8-04.6 their distances are
$0.98^{+1.27}_{-0.28}$ kpc and $0.32^{+0.08}_{-0.05}$ kpc respectively. If
these CSPN are correctly identified then these two PNe are in the near
foreground to the bulge. However, we believe these distances are anomalous.
For PNG 000.7-03.7 (HASH ID 38) the identified CSPN has a reasonable "Goodness
of Fit" (GoF) statistic value (Lindegren et al., 2021b) of $\sim 4$ but is
0.25 arcsec from the PN’s geometric centre which is unexpected for such a
compact PN (angular diameter is $\sim$13 arcseconds). The PN is high
excitation with He II $\lambda$ 4686 $\sim$ H$\beta$. In this case neither
size nor spectrum are compatible with the CSPNs resultant kinematic age of
800${}_{-300}^{+1800}$ years. PNG 009.8-04.6 (HASH ID 223) has a _Gaia_ GoF
value of 18. Values greater than 3 are considered poor fits (Lindegren et al.,
2021b). The putative CSPN is also not at the PN’s geometric centre. It could
even be just a dense nebula knot as it appears somewhat indistinct in the VLT
imagery. The PN itself is well resolved in Pan-STARRS and has a high
excitation spectrum, neither of which fits with a very low kinematic age of
only 200$\pm$10 years based on the identified CSPN.

If we adopt the canonical bulge distance of 8 kpc for these two PNe then the
kinematic ages are 5200 and 4300 years for PNG 000.7-03.7 and PNG 009.8-04.6
respectively. This fits with expectations and our other results.

For five further PNe, PNG 002.7-04.8, PNG 002.2-09.4, PNG 000.4-02.9, PNG
357.1+03.6 and PNG 359.7-01.8, the _Gaia_ sources are at
$4.31^{+1.1}_{-1.03}$, $4.57^{+1.24}_{-1.03}$, $4.63^{+1.5}_{-1.38}$,
$5.67^{+1.17}_{-0.99}$ and $5.16^{+1.46}_{-1.25}$ kpc, these distances are
3.4$\sigma$, 2.8$\sigma$, 2.2$\sigma$ and 1.9$\sigma$ smaller than 8 kpc
respectively, so these five sources could possibly be PNe in the middle
foreground to the bulge. Finally PNG 358.5+02.9 and PNG 002.8+01.7 have
parallax distances of $10.8^{+5.02}_{-7.25}$ and $11.85^{+3.59}_{-3.31}$ kpc
respectively, but still in the bulge to within the large errors. All other
distances for the sample are compatible with being in the bulge.

We provide new, accurate CSPN positions for many compact bulge CSPN in this
work. Our new results show that $\sim$14.9% (11 out of 74 with a further two
doubtful) CSPN identifications based on _Gaia_ EDR3 data presented in CW21
(and GSM21) do not identify the correct CSPN nor reflect the actual CSPN
positions properly. As reported by Parker et al. (2022) this is a broader
issue where for many PNe the true CSPN can be extremely faint and may fall
below the Gaia limits. This may lead to the bluest star closest to the PNe
geometric centroid being selected. As we are working in the dense bulge region
there is usually such an alternative star choice available. Our preliminary,
independent assessment of Gaia CSPN lists from GSM21 and CW21 for 430 checked
HASH CSPN across the general Galaxy (Parker et al., in preparation), show 28%
and 12% mis-identifications respectively. Correct astronomical and photometric
measurements of true CSPN is vital to help understand the resultant PNe and to
help ascertain their actual bulge membership when _Gaia_ distances are
available so care should be taken when using these compilations.

Figure 7: Distribution of available _Gaia_ "Goodness of Fit" (GoF) parameter
values for PNe in our sample. The red dotted vertical line is at GoF = 7, a
value which includes the apparent Gaussian distribution of all but 14 of the
74 PNe with available GoF measures.

###  5.4 Kinematic Age Calculations

In Table 7 we present results for kinematic ages for all PNe in the sample
using the best available data. This includes _Gaia_ parallax distances and
literature expansion velocity estimates. For 24 PNe kinematic ages are derived
from the updated angular size measurements in arcseconds (from the HST, VLT
and Pan-STARRS imagery) converted to a physical size in parsecs from the
distances from _Gaia_ data (34 cases) and literature expansion velocities in
km s-1 (77 cases) where both are available. These are assigned quality class
A. When only one of either the _Gaia_ distance or literature expansion
velocity are available these are assigned quality class B where a canonical
bulge distance of 8 kpc is assumed if the distance is missing or an average
expansion velocity of 20 km s-1 $\sigma$ = 8 is adopted if no literature value
is available. This accounts for 64 PNe in Table 7 part (b). Quality class C is
where the canonical bulge distance and average expansion velocity (as
calculated from all the literature expansion velocity values available in
class A and B) is adopted and this covers 48 PNe. According to Gesicki et al.
(2014), a correction factor of 1.4 was applied to the mass-averaged expansion
velocities in the literature to account for the mean propagation speed. The
errors on the kinematic ages were calculated using Monte Carlo simulations
with 10,000 realizations. Lower and upper uncertainties take the 16 and 84
percentile values of the simulation estimates. The average kinematic age
obtained here is 6400 $\pm~{}$3800 years. The results for HASH PNe 38 and 223
are excluded from the calculations as there are serious doubts about their
very low _Gaia_ distances (see earlier discussion). This work represents the
most accurate estimations ever provided for the kinematic ages of PNe in the
Galactic bulge.

The histogram of kinematic ages as a function of morphological type is shown
in Fig. 8. The histogram bin size is about the same as the typical calculated
error on the individual kinematic ages. The 7 ostensibly oldest PNe are all
bipolars and they completely dominate after 10000 years with 14/16 cases. Over
the range 1500 to 10000 years the fraction of ellipticals to bipolars remains
approximately constant.

Figure 8: Distribution of kinematic ages determined for this bulge PN sample.
Histograms are color-coded according to the main morphological classes. Table
7: Kinematic age estimates for all PNe in the sample. For 23 PNe these are
derived from the updated angular size measurements in arcseconds converted to
a physical size in parsecs from the distances from _Gaia_ data and literature
expansion velocities in km s-1. These are assigned quality class (Q) of "A".
When only one of either _Gaia_ distance or literature expansion velocity are
available these are Q of "B" (65 PNe). A Q of "C" is where a canonical bulge
distance of 8 kpc is assumed in lieu if a _Gaia_ distance and an average
expansion velocity of 20 km s-1 assumed as calculated from all the literature
values available in Q of A and B (48 PNe).

(a) HASH ID | PNG | Q |  $D$ (kpc) |  $R_{\mathrm{out}}$ (pc) |  $V_{\mathrm{exp}}$ (km s-1) |  $t_{\mathrm{kin}}$ (yr) |  HASH ID | PNG | Q |  $D$ (kpc) |  $R_{\mathrm{out}}$ (pc) |  $V_{\mathrm{exp}}$ (km s-1) |  $t_{\mathrm{kin}}$ (yr)
---|---|---|---|---|---|---|---|---|---|---|---|---|---
19 | 000.2-01.9 | A | 6.49${}_{-1.16}^{+1.27}$ | 0.31 | 10[2] |  21600${}_{-4400}^{+4700}$ | 217 | 009.4-09.8 | A | 8.10${}_{-1.79}^{+1.91}$ | 0.16 | 23[3] | 4800${}_{-1100}^{+1300}$
38 | 000.7-03.7 | A | 0.98${}_{-0.28}^{+2.18}$ | 0.03 | 26[2] |  800${}_{-200}^{+1600}$ | 223 | 009.8-04.6 | A | 0.32${}_{-0.05}^{+0.08}$ | 0.01 | 30[3] | 200$\pm$000
90 | 002.2-09.4 | A | 4.57${}_{-1.03}^{+1.24}$ | 0.15 | 28[3] |  3700${}_{-800}^{+1000}$ | 1151 | 351.1+04.8 | A | 7.08${}_{-1.4}^{+1.57}$ | 0.10 | 23[1] | 3000${}_{-600}^{+700}$
105 | 002.7-04.8 | A | 4.31${}_{-1.03}^{+1.1}$ | 0.19 | 19[3] |  6900${}_{-1700}^{+1800}$ | 1159 | 351.9+09.0 | A | 7.49${}_{-1.66}^{+1.84}$ | 0.28 | 29[5] | 6700${}_{-1500}^{+1600}$
106 | 002.8+01.7 | A | 11.85${}_{-3.31}^{+3.59}$ | 0.11 | 29[1] |  2600${}_{-800}^{+700}$ | 1207 | 355.9-04.2 | A | 6.28${}_{-1.53}^{+1.81}$ | 0.13 | 15[2] | 6000${}_{-1600}^{+1400}$
114 | 003.1+03.4 | A | 7.88${}_{-1.53}^{+1.73}$ | 0.07 | 24[1] |  2000${}_{-400}^{+500}$ | 1235 | 357.1+03.6 | A | 5.67${}_{-0.99}^{+1.17}$ | 0.11 | 23[4] | 3300${}_{-600}^{+700}$
125 | 003.6+03.1 | A | 8.43${}_{-2.31}^{+2.35}$ | 0.12 | 16[1] |  5200$\pm$1400 | 1256 | 357.6-03.3 | A | 5.37${}_{-2.92}^{+2.99}$ | 0.16 | 15[2] | 7400${}_{-3900}^{+4300}$
139 | 004.0-03.0 | A | 9.69${}_{-1.55}^{+1.74}$ | 0.16 | 12[3] |  9300${}_{-1600}^{+1900}$ | 1263 | 358.2+03.5 | A | 8.98${}_{-2.29}^{+2.39}$ | 0.15 | 22[3] | 4700$\pm$1200
141 | 004.2-04.3 | A | 6.10${}_{-2.11}^{+3.21}$ | 0.10 | 18[3] |  3800${}_{-1400}^{+1900}$ | 1275 | 358.5+02.9 | A | 10.8${}_{-7.25}^{+5.02}$ | 0.10 | 28[1] | 2400${}_{-1600}^{+1000}$
148 | 004.6+06.0 | A | 6.41${}_{-1.52}^{+1.65}$ | 0.12 | 14[2] |  5900${}_{-1300}^{+1900}$ | 1276 | 358.5-04.2 | A | 7.90${}_{-1.81}^{+1.91}$ | 0.15 | 14[1] | 7400${}_{-1600}^{+1900}$
151 | 004.8+02.0 | A | 8.86${}_{-2.64}^{+3.73}$ | 0.08 | 18[1] |  3100${}_{-900}^{+1300}$ | 1322 | 359.7-01.8 | A | 5.16${}_{-1.25}^{+1.46}$ | 0.10 | 23[3] | 3000${}_{-800}^{+900}$
150 | 004.8-05.0 | A | 7.55${}_{-1.65}^{+1.79}$ | 0.21 | 32[3] |  4500$\pm$1000 |  |  |  |  |  |  |

(b) HASH ID | PNG |  Q |  $D$ (kpc) |  $R_{\mathrm{out}}$ (pc) |  $V_{\mathrm{exp}}$ (km s-1) |  $t_{\mathrm{kin}}$ (yr) |  HASH ID | PNG |  Q |  $D$ (kpc) |  $R_{\mathrm{out}}$ (pc) |  $V_{\mathrm{exp}}$ (km s-1) |  $t_{\mathrm{kin}}$ (yr)
---|---|---|---|---|---|---|---|---|---|---|---|---|---
16 | 000.1+04.3 | B | 8.0$\pm$1.0 | 0.23 | 20[3] |  8000${}_{-1000}^{+1200}$ | 210 | 008.6-02.6 | B | 8.0$\pm$1.0 | 0.08 | 28[1] | 1900${}_{-200}^{+300}$
26 | 000.4-01.9 | B | 8.0$\pm$1.0 | 0.16 | 24[3] |  4600${}_{-600}^{+800}$ | 1160 | 351.9-01.9 | B | 8.0$\pm$1.0 | 0.09 | 13[1] | 4800${}_{-600}^{+900}$
27 | 000.4-02.9 | B | 4.63${}_{-1.38}^{+1.5}$ | 0.10 | 20a |  3400${}_{-1300}^{+2500}$ | 1161 | 352.0-04.6 | B | 8.0$\pm$1.0 | 0.14 | 17[3] | 5700${}_{-800}^{+1100}$
40 | 000.7+03.2 | B | 8.0$\pm$1.0 | 0.35 | 23[3] |  10600${}_{-1300}^{+1400}$ | 1163 | 352.1+05.1 | B | 8.0$\pm$1.0 | 0.35 | 17[3] | 14300$\pm$2000
37 | 000.7-02.7 | B | 8.0$\pm$1.0 | 0.16 | 27[3] |  4100$\pm$600 | 1164 | 352.6+03.0 | B | 8.0$\pm$1.0 | 0.10 | 23[1] | 3000$\pm$400
50 | 000.9-02.0 | B | 8.0$\pm$1.0 | 0.10 | 16[3] |  4300$\pm$800 | 1172 | 353.3+06.3 | B | 8.0$\pm$1.0 | 0.16 | 22[5] | 5000${}_{-700}^{+800}$
48 | 000.9-04.8 | B | 8.0$\pm$1.0 | 0.30 | 30[2] |  6900$\pm$900 | 1178 | 353.7+06.3 | B | 8.0$\pm$1.0 | 0.19 | 14[2] | 9400${}_{-1300}^{+1600}$
58 | 001.2+02.1 | B | 5.92${}_{-1.71}^{+1.78}$ | 0.08 | 20a |  2700${}_{-1000}^{+2000}$ | 1185 | 354.5+03.3 | B | 8.0$\pm$1.0 | 0.05 | 19[1] | 1800${}_{-200}^{+300}$
60 | 001.2-03.0 | B | 8.92${}_{-1.82}^{+2.11}$ | 0.14 | 20a |  4800${}_{-1700}^{+3400}$ | 1189 | 354.9+03.5 | B | 8.0$\pm$1.0 | 0.07 | 16[1] | 3000$\pm$400
62 | 001.3-01.2 | B | 4.84${}_{-2.84}^{+3.77}$ | 0.07 | 20a |  2400${}_{-1600}^{+2900}$ | 1191 | 355.1-06.9 | B | 8.0$\pm$1.0 | 0.19 | 15[3] | 8800$\pm$1300
73 | 001.7+05.7 | B | 8.0$\pm$1.0 | 0.24 | 39[3] |  4200${}_{-500}^{+600}$ | 1199 | 355.4-02.4 | B | 8.0$\pm$1.0 | 0.21 | 23[3] | 6300${}_{-800}^{+600}$
74 | 001.7-04.4 | B | 8.0$\pm$1.0 | 0.09 | 26[1] |  2400${}_{-300}^{+200}$ | 1202 | 355.6-02.7 | B | 8.0$\pm$1.0 | 0.15 | 11[3] | 9500${}_{-1600}^{+1800}$
81 | 002.0-06.2 | B | 8.0$\pm$1.0 | 0.13 | 7[3] |  12900${}_{-2800}^{+2500}$ | 1209 | 355.9+03.6 | B | 8.0$\pm$1.0 | 0.07 | 21[1] | 2300$\pm$300
83 | 002.1-02.2 | B | 8.0$\pm$1.0 | 0.10 | 22[3] |  3100${}_{-600}^{+400}$ | 1212 | 356.1-03.3 | B | 8.0$\pm$1.0 | 0.10 | 20[1] | 3400${}_{-400}^{+500}$
82 | 002.1-04.2 | B | 8.0$\pm$1.0 | 0.16 | 15[3] |  7400${}_{-900}^{+1000}$ | 1219 | 356.5-03.6 | B | 8.0$\pm$1.0 | 0.22 | 25[1] | 6100$\pm$800
91 | 002.3+02.2 | B | 8.0$\pm$1.0 | 0.13 | 17[3] |  5300${}_{-800}^{+900}$ | 1230 | 356.8+03.3 | B | 8.0$\pm$1.0 | 0.07 | 9[1] | 5400${}_{-700}^{+800}$
93 | 002.3-03.4 | B | 8.0$\pm$1.0 | 0.11 | 29[1] |  2600$\pm$300 | 1232 | 356.9+04.4 | B | 8.0$\pm$1.0 | 0.11 | 9[1] | 8500${}_{-1100}^{+1700}$
4326 | 002.6+02.1 | B | 8.0$\pm$1.0 | 0.36 | 41[3] |  6100${}_{-700}^{+800}$ | 1239 | 357.1-04.7 | B | 8.0$\pm$1.0 | 0.07 | 27[1] | 1800$\pm$200
109 | 002.9-03.9 | B | 8.0$\pm$1.0 | 0.13 | 39[1] |  2300${}_{-200}^{+300}$ | 1242 | 357.2+02.0 | B | 8.0$\pm$1.0 | 0.09 | 8[1] | 7800${}_{-800}^{+1700}$
116 | 003.2-06.2 | B | 8.0$\pm$1.0 | 0.21 | 22[5] |  6600${}_{-900}^{+1100}$ | 1246 | 357.3+04.0 | B | 7.49${}_{-2.13}^{+2.25}$ | 0.15 | 20a | 5200${}_{-2000}^{+3900}$
126 | 003.6-02.3 | B | 8.0$\pm$1.0 | 0.20 | 28[3] |  4900${}_{-600}^{+700}$ | 1253 | 357.5+03.2 | B | 8.0$\pm$1.0 | 0.20 | 27[3] | 5100$\pm$700
129 | 003.7-04.6 | B | 8.0$\pm$1.0 | 0.13 | 24[3] |  3700$\pm$500 | 1259 | 357.9-03.8 | B | 8.31${}_{-1.66}^{+1.75}$ | 0.26 | 20a | 9000${}_{-3000}^{+6400}$
131 | 003.8-04.3 | B | 8.0$\pm$1.0 | 0.10 | 29[3] |  2400$\pm$300 | 1281 | 358.6-05.5 | B | 5.33${}_{-1.79}^{+1.95}$ | 0.22 | 20a | 7600${}_{-3100}^{+5900}$
133 | 003.9-02.3 | B | 8.0$\pm$1.0 | 0.15 | 20[3] |  5200${}_{-800}^{+600}$ | 1286 | 358.7+05.2 | B | 8.0$\pm$1.0 | 0.07 | 22[1] | 2200$\pm$300
140 | 004.1-03.8 | B | 8.0$\pm$1.0 | 0.06 | 22[1] |  1900${}_{-200}^{+300}$ | 1293 | 358.8+03.0 | B | 8.0$\pm$1.0 | 0.17 | 20[2] | 5900${}_{-700}^{+1000}$
4315 | 004.3+01.8 | B | 5.73${}_{-1.18}^{+1.28}$ | 0.12 | 20a |  4100${}_{-1400}^{+2700}$ | 1295 | 358.9+03.4 | B | 8.0$\pm$1.0 | 0.06 | 17[1] | 2400$\pm$400
165 | 005.5-04.0 | B | 8.0$\pm$1.0 | 0.24 | 32[2] |  5200${}_{-600}^{+800}$ | 1308 | 359.2+04.7 | B | 8.0$\pm$1.0 | 0.05 | 15[1] | 2300$\pm$400
4108 | 006.1+08.3 | B | 8.0$\pm$1.0 | 0.05 | 16[1] |  2100${}_{-200}^{+300}$ | 1312 | 359.3-01.8 | B | 6.92${}_{-1.3}^{+1.51}$ | 0.12 | 20a | 4100${}_{-1300}^{+2900}$
180 | 006.3+04.4 | B | 8.0$\pm$1.0 | 0.11 | 46[1] |  1600${}_{-200}^{+100}$ | 1324 | 359.8+03.7 | B | 8.09${}_{-2.42}^{+2.51}$ | 0.14 | 20a | 4800${}_{-1900}^{+3600}$
182 | 006.4+02 | B | 8.0$\pm$1.0 | 0.14 | 19[1] |  5100${}_{-600}^{+700}$ | 1329 | 359.8+05.6 | B | 8.0$\pm$1.0 | 0.14 | 5[2] | 19500${}_{-4000}^{+5200}$
181 | 006.4-04.6 | B | 8.0$\pm$1.0 | 0.18 | 26[2] |  4800${}_{-700}^{+600}$ | 1330 | 359.8-07.2 | B | 8.0$\pm$1.0 | 0.22 | 29[5] | 5200$\pm$600
201 | 007.8-04.4 | B | 9.93${}_{-1.32}^{+1.46}$ | 0.17 | 20a |  5900${}_{-1800}^{+4100}$ | 1334 | 359.9-04.5 | B | 8.0$\pm$1.0 | 0.17 | 17[3] | 6900${}_{-800}^{+1300}$
4111 | 008.2+06.8 | B | 8.0$\pm$1.0 | 0.09 | 12[1] |  5200${}_{-700}^{+1000}$ |  |  |  |  |  |  |

Table 8: continued (c) HASH ID | PNG |  Q |  $D$ (kpc) |  $R_{\mathrm{out}}$ (pc) |  $V_{\mathrm{exp}}$ (km s-1) |  $t_{\mathrm{kin}}$ (yr) |  HASH ID | PNG |  Q |  $D$ (kpc) |  $R_{\mathrm{out}}$ (pc) |  $V_{exp}$ (km s-1) |  $t_{\mathrm{kin}}$ (yr)
---|---|---|---|---|---|---|---|---|---|---|---|---|---
12 | 000.1+02.6 | C | 8.0$\pm$1.0 | 0.24 | 20a |  8300${}_{-2600}^{+5500}$ | 192 | 007.0-06.8 | C | 8.0$\pm$1.0 | 0.24 | 20a | 8300${}_{-2500}^{+5500}$
17 | 000.1-02.3 | C | 8.0$\pm$1.0 | 0.20 | 20a |  6900${}_{-2200}^{+4300}$ | 197 | 007.5+07.4 | C | 8.0$\pm$1.0 | 0.35 | 20a | 12200${}_{-3700}^{+8000}$
20 | 000.2-04.6 | C | 8.0$\pm$1.0 | 0.15 | 20a |  5200${}_{-1600}^{+3500}$ | 198 | 007.6+06.9 | C | 8.0$\pm$1.0 | 0.32 | 20a | 11100${}_{-3400}^{+7500}$
24 | 000.3+06.9 | C | 8.0$\pm$1.0 | 0.21 | 20a |  7300${}_{-2200}^{+5000}$ | 200 | 007.8-03.7 | C | 8.0$\pm$1.0 | 0.33 | 20a | 11500${}_{-3400}^{+7600}$
22 | 000.3-04.6 | C | 8.0$\pm$1.0 | 0.23 | 20a |  8000${}_{-2500}^{+5400}$ | 208 | 008.4-03.6 | C | 8.0$\pm$1.0 | 0.49 | 20a | 17100${}_{-5200}^{+10900}$
42 | 000.7-07.4 | C | 8.0$\pm$1.0 | 0.23 | 20a |  8000${}_{-2400}^{+5100}$ | 1147 | 350.5-05.0 | C | 8.0$\pm$1.0 | 0.18 | 20a | 6200${}_{-1900}^{+3900}$
54 | 001.1-01.6 | C | 8.0$\pm$1.0 | 0.15 | 20a |  5200${}_{-1600}^{+3400}$ | 1152 | 351.2+05.2 | C | 8.0$\pm$1.0 | 0.28 | 20a | 9700${}_{-3000}^{+6500}$
64 | 001.4+05.3 | C | 8.0$\pm$1.0 | 0.17 | 20a |  5900${}_{-1800}^{+3700}$ | 1156 | 351.6-06.2 | C | 8.0$\pm$1.0 | 0.29 | 20a | 10100${}_{-3100}^{+6700}$
70 | 001.6-01.3 | C | 8.0$\pm$1.0 | 0.11 | 20a |  3800${}_{-1100}^{+2600}$ | 1171 | 353.2-05.2 | C | 8.0$\pm$1.0 | 0.33 | 20a | 11500${}_{-3400}^{+7500}$
98 | 002.5-01.7 | C | 8.0$\pm$1.0 | 0.11 | 20a |  3800${}_{-1100}^{+2600}$ | 1217 | 356.3-06.2 | C | 8.0$\pm$1.0 | 0.22 | 20a | 7600${}_{-2300}^{+5100}$
107 | 002.8+01.8 | C | 8.0$\pm$1.0 | 0.18 | 20a |  6200${}_{-1900}^{+4200}$ | 1227 | 356.8-05.4 | C | 8.0$\pm$1.0 | 0.28 | 20a | 9700${}_{-2900}^{+6400}$
128 | 003.7+07.9 | C | 8.0$\pm$1.0 | 0.22 | 20a |  7600${}_{-2200}^{+4900}$ | 1234 | 357.0+02.4 | C | 8.0$\pm$1.0 | 0.17 | 20a | 5900${}_{-1700}^{+4000}$
135 | 003.9+01.6 | C | 8.0$\pm$1.0 | 0.17 | 20a |  5900${}_{-1700}^{+3900}$ | 4139 | 357.1+04.4 | C | 8.0$\pm$1.0 | 0.21 | 20a | 7300${}_{-2200}^{+4700}$
134 | 003.9-03.1 | C | 8.0$\pm$1.0 | 0.15 | 20a |  5200${}_{-1600}^{+3600}$ | 1252 | 357.5+03.1 | C | 8.0$\pm$1.0 | 0.12 | 20a | 4100${}_{-1200}^{+2900}$
142 | 004.2-03.2 | C | 8.0$\pm$1.0 | 0.12 | 20a |  4100${}_{-1300}^{+2700}$ | 4140 | 357.9-05.1 | C | 8.0$\pm$1.0 | 0.51 | 20a | 17800${}_{-5200}^{+12000}$
143 | 004.2-05.9 | C | 8.0$\pm$1.0 | 0.18 | 20a |  6200${}_{-1800}^{+4000}$ | 1258 | 358.0+09.3 | C | 8.0$\pm$1.0 | 0.29 | 20a | 10100${}_{-2900}^{+6400}$
156 | 005.0-03.9 | C | 8.0$\pm$1.0 | 0.25 | 20a |  8700${}_{-2600}^{+5800}$ | 1266 | 358.2+04.2 | C | 8.0$\pm$1.0 | 0.27 | 20a | 9400${}_{-2900}^{+6200}$
162 | 005.2+05.6 | C | 8.0$\pm$1.0 | 0.17 | 20a |  5900${}_{-1900}^{+4100}$ | 1280 | 358.6+07.8 | C | 8.0$\pm$1.0 | 0.26 | 20a | 9000${}_{-2800}^{+6100}$
171 | 005.8-06.1 | C | 8.0$\pm$1.0 | 0.18 | 20a |  6200${}_{-1800}^{+4400}$ | 1302 | 359.1-02.9 | C | 8.0$\pm$1.0 | 0.19 | 20a | 6600${}_{-2000}^{+4600}$
179 | 006.3+03.3 | C | 8.0$\pm$1.0 | 0.18 | 20a |  6200${}_{-1900}^{+4000}$ | 1319 | 359.6-04.8 | C | 8.0$\pm$1.0 | 0.32 | 20a | 11100${}_{-3500}^{+7600}$
189 | 006.8+02.3 | C | 8.0$\pm$1.0 | 0.10 | 20a |  3400${}_{-1100}^{+2300}$ | 1327 | 359.8+02.4 | C | 8.0$\pm$1.0 | 0.10 | 20a | 3400${}_{-1000}^{+2300}$
188 | 006.8-03.4 | C | 8.0$\pm$1.0 | 0.14 | 20a |  4800${}_{-1300}^{+3100}$ | 1328 | 359.8+05.2 | C | 8.0$\pm$1.0 | 0.41 | 20a | 14300${}_{-4300}^{+9500}$
193 | 007.0+06.3 | C | 8.0$\pm$1.0 | 0.14 | 20a |  4800${}_{-1500}^{+3300}$ | 1326 | 359.8+06.9 | C | 8.0$\pm$1.0 | 0.62 | 20a | 21600${}_{-6300}^{+14000}$
References: [1] Gesicki et al. (2014); [2] Richer et al. (2010); [3] Richer et
al. (2008); [4]Gesicki et al. (2006); [5] Gesicki & Zijlstra (2000).
a Mean expansion velocity: 20 $\pm$ 8 km s-1 used.

##  6 Discussion

###  6.1 PN Morphologies and the Shaping mechanism of PNe

Round (spherical) PNe occupy a small fraction ($\sim$10%) in our bulge sample
while the overall fraction in HASH is 21%, with many being at larger Galactic
scale heights. With higher-resolution imaging more aspherical features of PNe
previously classed as ‘R’ are found, by either revealing an elliptical outer
envelope or even faint bipolar lopes. In one case, PNG 357.2+02.0, the round
annular ring likely represents a bipolar PN seen pole on (refer earlier
discussion in Section 5.2.2). Such "adjustments" are generally for compact PNe
where significantly improved imaging resolution and sensitivity make the
difference.

The generalised interacting stellar wind (GISW) model (Balick, 1987), has been
the cornerstone paradigm in explaining the observed aspherical features of
elliptical and bipolar morphologies of PNe. Here the slow wind is enhanced in
an equatorial plane expulsion of material from the progenitor post-AGB star.
Under such a generalised scenario, the fast wind from the CSPN would be
preferentially around an axis orthogonal to the equatorial density
enhancement. A follow-up theoretical study Kahn & West (1985) conjectured that
by varying the pole-to-equator density contrast in the AGB slow wind, a nebula
could have a full-range of morphologies from mildly elliptical to strongly
bipolar. This was substantiated by analytical models in Icke (1988), Balick
(1987) and Icke et al. (1989) and later numerical simulations in Soker & Livio
(1989), Mellema et al. (1991) and Icke et al. (1992).

There are also strong morphological clues as to whether a bipolar PN is likely
to host a binary, e.g. Miszalski et al. (2009b). However, whether the CSPN is
in a wide or close binary system cannot generally be inferred just from the PN
shape (e.g. Soker, 2001; Boffin & Jones, 2019). The formation of bipolar lobes
could be due to aspherical winds or jets and may fade on a timescale an order
of magnitude smaller than the typical visibility time of a PN (Jacob et al.,
2013; Gesicki et al., 2016) but see Parker (2022b).

According to best current understanding, observed PNe bipolar morphologies are
largely due to central star binarity (e.g. Soker & Livio, 1994; Boffin et al.,
2012; De Marco et al., 2015; García-Segura et al., 2018; Akashi & Soker, 2021;
Chamandy et al., 2018), while point-symmetric structures are explained by the
precession of the accretion disk around the secondary (Livio & Pringle, 1996;
Boffin et al., 2012).

Therefore, for cases of PNe where the shape of a bipolar waist is seen while
lobes are faint or unseen, they should fit the bipolar class. Further, whether
the lobes or thin rings formed in common envelope evolution (CEE) appear or
not, depends largely on the kinematic age of the PN, the viewing angle and
whether the internal structures are resolved. Our classification for the
bipolar PNe should be more representative to this population compared with
Rees & Zijlstra (2013).

###  6.2 Implications for the large fraction of bipolar PNe found in the
Galactic bulge

The large fraction of bipolar PNe in our sample highlights not only the likely
importance of binary interaction in PN shaping but raises issues of the bulge
stellar population. It is also interesting to confirm that the point-symmetric
features, including occurrences of multi-lobes (PNe with sub-class m and p),
spiral structures or over-densities in the outer lobes are only seen in
bipolar PNe and these have been strongly argued to be specific morphological
traits of PNe with close binary cores (Miszalski et al., 2009b).

However, the question of why an overall ostensibly very old Galactic bulge
stellar population should have a PN population highly dominated by bipolars,
i.e. 68% of all compact bulge PNe known, is important. As also known there is
compelling evidence that bipolars, from Galactic disk scale height arguments
alone, may also at least partially emerge from higher-mass and therefore
younger progenitors, e.g. Corradi & Schwarz (1995); Parker et al. (2006) which
may or may not host binaries. Another key point is the previous best estimates
for the close binary fraction for PNe in the Galactic bulge is only 15-20%,
where up to 60% have bipolar morphologies if the inclination effects are
considered (Miszalski et al., 2009a). This compares to the lower limit bipolar
fraction of 68% for all compact bulge PNe and indeed a high bipolar fraction
of all bulge PNe. So do all bipolars host both more massive progenitors and
binaries while close binaries in particular have additional morphological
features that give them away? If so why are so many bulge PNe bipolars? If
they are of higher surface brightness and so are easier to detect this does
not seem to be a feature of the general Galactic disk PNe population where the
apparent bipolar fraction is $\sim$20% from HASH. We have shown that many
compact elliptical PNe are actually bipolars in higher resolution imaging so
the current HASH bipolar fraction is an underestimate but even so this cannot
easily explain what we see in the Galactic bulge.

Our results for PN morphologies, therefore, suggest an underestimation of the
fraction of younger and more massive stars and/or binary systems in the
Galactic bulge. With a bin size of 1000 years, the number of observed bipolar
and elliptical PNe start to decrease rapidly when they are kinematically older
than 5000 and 6000 yr respectively in our compact sample. Taking those as
estimations of typical PN life times and combining with the total PN number of
2000 in the Galactic bulge predicted in Gesicki et al. (2014) and our results
of the fractions of PN in these two morphological classes (29% for elliptical
and 68% for bipolar), the birth rates of elliptical and bipolar PNs are
$\sim$0.12 $\mathrm{yr}^{-1}$ and $\sim$0.23 $\mathrm{yr}^{-1}$ respectively.
This indicates a higher, nearly doubled, death rate of progenitors of observed
PNe showing bipolar morphologies.

Models show that stellar death rate decreases after the star burst and
plateaus around 7 Gyr (see Fig. 10 in Gesicki et al. (2014). Thus, if
progenitors of both the elliptical and bipolar PNe are older than 7 Gyr, our
PN sample should trace both populations. The higher birth rate of bipolar PNe
might suggest the existence of a younger population $<$ 7 Gyr assuming our
selection criteria on the PN size leads to no morphological bias.
Alternatively, the true fraction of binary systems may be much higher than
expected. This is suggested by the search of binary CSPNe through the infrared
(IR) excess method in (De Marco et al., 2013, 2015). Also more bipolar PNe may
have been formed from long-period wide orbit binaries but that so far has
limited observational evidence, or PNe might just be preferentially formed in
binary systems and so are over-representing the underlying binary fraction
compared with the general main-sequence population that their progenitors
belong to. This is the idea of the PN Binary Hypothesis in De Marco (2009b).
However, there are as yet no clear answers to this conundrum.

##  7 Conclusions

We have undertaken a detailed study of a carefully selected sample of compact
PNe in the Galactic bulge. We have used our own VLT narrow-band imagery and
archival narrow and broad band _HST_ and occasionally Pan-STARRS broad-band
imagery to provide significantly improved morphological classifications and
angular size measurements for a well defined sample of compact bulge PNe. Many
PNe have been re-classified as bipolars at a level that is likely to be
replicated in the general HASH Galactic disk sample for PNe $\leq$ 10
arcseconds across, assuming deeper and higher resolution imaging eventually
becomes available. The increase in the fraction of bipolar PNe comprising our
bulge sample is very significant for a population ostensibly dominated by old
stars. It is clear the high visibility of compact PNe allows us to find and
study such a key touchstone population of late-stage stellar evolution in the
Galactic bulge. If bipolars can derive from higher mass progenitors, as
inferred by their preponderance at lower Galactic scale heights in the
Galactic disk, as well as being in binaries, then there has to be a
significant, young stellar population in the bulge feeding the observed
population today. They dominate all PNe emerging from the general, several
Giga year old bulge stellar population.

We have also used these data to provide a more complete and accurate listing
of bulge CSPN including 4 new discoveries. We also show that 11 previous
_Gaia_ CSPN identified are incorrect. Many actual CSPN are beyond _Gaia_
limits but visible in the deep Pan-STARRS and VLT data allowing their proper
identification. For others _Gaia_ has selected the wrong star or even a
compact emission region as the CSPN. For some the PNe are so compact that we
believe _Gaia_ has simply taken the centroid of this as the CSPN given there
are no other _Gaia_ stars identified in the immediate region. We find for this
sample that 15% of _Gaia_ CSPN listed in CW21 are incorrect. As such we
emphasise that great care must be taken when using the _Gaia_ based CSPN
compilations of CW21 and CSM21. We strongly recommend HASH (Parker et al.,
2016) for such studies as every CSPN listed has been holistically vetted.

Finally, using the best available data, we provide the most complete and
accurate kinematic age estimates ever compiled for all known compact PNe in
the bulge. This includes _Gaia_ parallax distances for 33, accurate expansion
velocities for 77, improved angular size determinations for all (from the HST,
VLT and Pan-STARRS imagery) and average distance and expansion velocities for
the rest. The expected young age of the PNe sample is confirmed with an
average kinematic age of 6400 $\pm$ 3800 years. Kinematically older PNe are
dominated by bipolars with 14/16 PNe with estimated ages $>$ 10000 years being
bipolars. Even in this compact sample these older PNe are more extended and it
is easier to discern bipolar lobes. Hence a selection effect is likely at
play. It is also likely that some of the very compact PNe currently classified
as elliptical from the VLT imagery may turn out to be bipolars in higher
resolution imagery. This would make the bipolar fraction even larger. The best
current work also puts the short-period binary fraction of bulge PNe at 15-20%
- maximum 60% in bipolars (Miszalski et al., 2009a), giving 9-12% of PNe in
binaries are bipolars. This is a factor of up to 7 lower than the bipolar
fraction we find. A younger stellar population in the bulge, wider period
binaries and the relationship between PN formation and the CSPN binarity may
pay a role here but the issue of the progenitor masses of the bipolar family
remains. Finally, all updated angular sizes and morphological classifications
presented here have also now been ingested into HASH. It is clear more work is
needed to understand the observed bulge PNe morphological distribution and the
associated progenitor stellar populations.

## Acknowledgements

QAP thanks the Hong Kong Research Grants Council for GRF research support
under grants 17326116 and 17300417. ST thanks HKU and QAP for provision of an
MPhil scholarship. AR thanks HKU and QAP for the provision of a postdoctoral
fellowship. AAZ acknowledges support from STFC under grant ST/T000414/1. This
work has made use of data from the European Space Agency (ESA) mission Gaia
(https://www.cosmos.esa.int/gaia), processed by the Gaia Data Processing and
Analysis Consortium (DPAC,
https://www.cosmos.esa.int/web/gaia/dpac/consortium. Funding for the DPAC has
been provided by national institutions, in particular the institutions
participating in the Gaia Multilateral Agreement.

This research made use of Astropy, a community-developed core Python package
for Astronomy (Collaboration et al., 2013) and APLpy, an open-source plotting
package for Python hosted at http://aplpy.github.io. This work used
observations made with the NASA/ESA _Hubble Space Telescope_. Some data were
obtained from the Hubble Legacy Archive, which is a collaboration between the
Space Telescope Science Institute (STScI/NASA), the Space Telescope European
Coordinating Facility (ST-ECF/ESA), and the Canadian Astronomy Data Centre
(CADC/NRC/CSA).

## Data Availability

The data underlying this article are available in the article itself and in
its associated online material freely accessible from the HASH database found
here: http://hashpn.space by simply entering the unique HASH ID number for
each source as provided.

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##  Appendix A Observing logs for the VLT (Table A1) and _HST_ (Table A2)
imagery used

Table 9: Observing log of the VLT PNe imagery. Provided are PNG ID, narrow-band filter used, exposure time in seconds date of observation, official ESO observing program ID and the reported seeing in arecseconds (complete table is available online). PN G | Filter | Exposure | Date | Prog. ID | Seeing
---|---|---|---|---|---
PNG 000.1+02.6 | [O III] | 6.0 | 2015-04-17 | 095.D-0270(A) | 0.62
PNG 000.1+04.3 | [O III] | 10.0 | 2015-04-18 | 095.D-0270(A) | 1.23
PNG 000.1-02.3 | [O III] | 5.0 | 2015-04-17 | 095.D-0270(A) | 1.16
PNG 000.2-01.9 | H$\alpha$ | 5.0 | 2015-04-17 | 095.D-0270(A) | 0.63
PNG 000.2-04.6 | [O III] | 5.0 | 2015-04-17 | 095.D-0270(A) | 0.76
PNG 000.3+06.9 | [O III] | 6.0 | 2015-04-19 | 095.D-0270(A) | 1.06
PNG 000.3-04.6 | [O III] | 5.0 | 2015-04-18 | 095.D-0270(A) | 1.32
PNG 000.4-01.9 | H$\alpha$ | 5.0 | 2015-04-18 | 095.D-0270(A) | 1.07
PNG 000.4-02.9 | H$\alpha$ | 4.0 | 2015-04-19 | 095.D-0270(A) | 0.83
PNG 000.7+03.2 | H$\alpha$ | 5.0 | 2015-04-19 | 095.D-0270(A) | 0.83
.. | .. | .. | .. | .. | ..
.. | .. | .. | .. | .. | ..
Table 10: Observing logs of the _HST_ imagery used.Provided are the PN usual names or PNG IDs, the _HST_ narrow band filters used to make our measurements, the exposure time in seconds, date of observation, program ID and data set (complete table is available online). Objects | Filter | Exposure | Date | Prog. ID | Data Set
---|---|---|---|---|---
M1-42 | F502N | 0.5 | 2009-04-22 | 11185 | ua2o010dm
M1-42 | F656N | 0.5 | 2009-04-22 | 11185 | ua2o010bm
M1-42 | F502N | 600.0 | 2009-04-22 | 11185 | ua2o010am
M1-42 | F656N | 300.0 | 2009-04-22 | 11185 | ua2o0108m
M1-42-REPEAT | F502N | 600.0 | 2009-06-09 | 11185 | ua2o1008m
M1-42-REPEAT | F656N | 300.0 | 2009-06-09 | 11185 | ua2o1006m
PK002-04D1 | F656N | 20.0 | 2000-02-17 | 8345 | u5hh4101r
PK002-04D1 | F656N | 140.0 | 2000-02-17 | 8345 | u5hh4102r
PK002-04D1 | F656N | 400.0 | 2000-02-17 | 8345 | u5hh4103r
PK002-09D1 | F656N | 230.0 | 2000-08-26 | 8345 | u5hh5602r
.. | .. | .. | .. | .. | ..
.. | .. | .. | .. | .. | ..

##  Appendix B The best optical PN images.

Figure 9: The best optical PN images from the VLT or _HST_ (narrow-band) and
in a few cases, Pan-STARRS (broad-band) used in this study. The data origin is
given at the top of each image along with the PNG ID. Images are generally
that obtained with H$\alpha$ filters or otherwise indicated. Pan-STARRS images
are in g-band, where used. Complete set of PN images is available online.

(a) PNe with angular sizes greater than 20 arcsec. Each image has a field of
view of 30" $\times$ 30".

.. ..