arXiv:1001.0027v1  [astro-ph.GA]  30 Dec 2009New candidate Planetary Nebulae in the IPHAS survey: the cas e of
PNe with ISM interaction.
Laurence SabinA, Albert A. ZijlstraA, Christopher WareingB, Romano L.M.
CorradiC, Antonio MampasoC, Kerttu ViironenC, Nicholas J. WrightDand
Quentin A. ParkerE
AJodrell Bank Center for Astrophysics, School of Physics and Astronomy, University of Manchester,
Manchester M13 9PL, UK
BDepartment of Applied Mathematics, University of Leeds, Le eds, LS2 9JT, UK
CInstituto de Astrofisica de Canarias, Tenerife, Spain
DHarvard-Smithsonian Center for Astrophysics, 60 Garden St reet, Cambridge, MA, 02138, USA
EMacquarie University/Anglo-Australian Observatory, Dep artment of Physics, North Ryde, Sydney
NSW 2190, AUSTRALIA
AEmail: laurence.sabin@manchester.ac.uk
Abstract: We present the results of the search for candidate Planetary Nebulae interacting with
the interstellar medium (PN-ISM) in the framework of the INT Photometric H αSurvey (IPHAS)
and located in the right ascension range 18h-20h. The detect ion capability of this new Northern
survey, in terms of depth and imaging resolution, has allowe d us to overcome the detection problem
generally associated to the low surface brightness inheren t to PNe-ISM. We discuss the detection of
21 IPHAS PN-ISM candidates. Thus, different stages of intera ction were observed, implying various
morphologies i.e. from the unaffected to totally disrupted s hapes. The majority of the sources belong
to the so-called WZO2 stage which main characteristic is a br ightening of the nebula’s shell in the
direction of motion. The new findings are encouraging as they would be a first step into the reduction
of the scarcity of observational data and they would provide new insights into the physical processes
occurring in the rather evolved PNe.
Keywords: Planetary nebulae, ISM interaction, survey.
1 Introduction
Large Hαsurveys have so far allowed the detection of
∼3000 planetary nebulae (PNe) in the Galaxy. The
data can be principally found in the Strasbourg-ESO
Catalogue (Acker et al.1992)andtherecentMacquarie-
AAO-StrasbourgH αPlanetaryNebulaCatalogues: MASH
IandII(Parker et al.(2006)andMiszalski et al(2008)).
Unfortunatelyalimitation inour understandingofthis
short and rather complex phase of stellar evolution lies
either in the deepness of the detections realised or the
type of PNe investigated. Indeed, although enormous
progress has been made over the years in terms of ob-
servations, the well-studied PNe are generally bright
and often young. This hampers the study of:
•PNe hidden by the interstellar medium, partic-
ularly those located at low galactic height.
•PNe with (very)low surface brightness where we
find the group of old PNe.
•Very distant PNe which appear as unresolved
and not recognisable as nebulae.•PNe located in crowded areas such as the galac-
tic plane.
Moreover, excluding these objects from global studies
(morphology, abundances,luminosityfunction...etc)may
bias our understanding of planetary nebulae. As an il-
lustration, few PNe are described in the literature as
“PNe with ISM interaction”, which is the step before
the complete dilution of the nebulae in the interstel-
lar medium (Borkowski et al. (1990), Ali et al. (2000),
Xilouris et al. (1996) and Tweedy et al. (1996)). The
study of the interaction process would give new in-
sights intoseveral aspects of the PNevolution. Indeed,
the density difference between ISM and PNe will affect
their shape. This is expected to be observable in old
objects where the nebular density declines sufficiently
to be overcome by the ISM density. Other phenom-
ena like the flux and brightness enhancement following
the compression of the external shell, the increase of
the recombination rate in the PN Rauch et al. (2000),
the occurrence of turbulent Rayleigh-Taylor instabili-
ties and the implication of magnetic fields Dgani et al.
(1998) are among the physical processes which need
to be addressed not only from a theoretical but also
observational point of view.
12 Publications of the Astronomical Society of Australia
The low surface brightness generally associated to
PNe-ISM has for a long time prevented any deeper ob-
servation and good statistical study of these interac-
tions, where only the interacting rim is well seen. New
generations of H αsurveys have overcome this prob-
lem. A perfect example is the discovery of PFP 1 by
Pierce et al. (2004)intheframeworkoftheAAO/UKST
SuperCOSMOS H αsurvey (SHS) (Parker et al. 2005).
This PN, starting to interact with the ISM at the
rim, is very large (radius = 1.5 ±0.6 pc) and very
faint (logarithm of the H αsurface brightness equal
to -6.05 ergcm−2.s−1.sr−1). In order to unveil and
study this “missing PN population” in the Northern
hemisphere we need surveys providing the necessary
observing depth: the Isaac Newton Telescope (INT)
Photometric H αSurvey (IPHAS) is one of them and
will complete the work done in the South by the SHS.
2 IPHAS contribution
IPHAS is a new fully photometric CCD survey of the
Northern Galactic Plane, started in 2003 (Drew et al.
(2005), Gonzalez-Solares et al (2008)) and which has
now been completed1. Using the 2.5m Isaac Newton
Telescope (INT)in LaPalma (Canary Islands, SPAIN)
and the Wide Field Camera (WFC) offering a field of
view of 34.2 ×34.2 arcmin2, IPHAS targets the Galac-
tic plane in the Northern hemisphere, at a latitude
range of -5◦<b<5◦and covers 1800 deg2. This
international survey is conducted not only in H αbut
also makes use of two continuum filters, respectively
the Sloan r’ and i’. IPHAS is viewed as an enhance-
ment to former narrow-band surveys, first due to the
use of CCD and the particularly small pixel scale al-
lowed bytheWFCwith0.33 arcsec pix−1butalso (and
mainly) due to the depth reached for point sources de-
tection. Thus sources with a r’ magnitude between 13
and 19.5-20 could be detected with a very good pho-
tometric accuracy. The most interesting characteristic
for our purpose is the ability to detect resolved ex-
tended emissions with an H αsurface brightness down
to 2×10−17erg cm−2s−1arcsec−2.
In this paper we will focus on extended (candi-
date) PNe (i.e. objects with a size greater than 5 arc-
sec). They were searched for via a visual inspection of
2 deg2Hα-r(continuum removal) mosaics made from
the different IPHAS observations. And in order to al-
low the detection of objects of multiple size and bright-
ness level, the mosaics were binned at respectively 15
pixels×15 pixels (5 arcsec) and 5 pixels ×5 pixels (1.7
arcsec). The first binning level, which is of particu-
lar interest to us, helps to detect resolved, low surface
brightness objects (down to the IPHAS limit) and to
accentuate the contours/shape of the nebulae (this is
particularly useful to see, for example, the full extent
of an outflow or a tail). The second set, is used to de-
tect intermediate size nebulae i.e. smaller than ∼15-20
arcsec in diameter.
1http://www.iphas.orgThe first area that has been fully investigated is the re-
gion between RA=18h and RA=20h. We detected 233
candidate PNe among which other nebulosities may be
found e.g. small HII regions (Sabin, PhD thesis, to be
published). Around 20% of this sample have been so
far spectroscopically confirmed as PNe (Sabin et al.,
in preparation). If we look at the particularities of the
PNe and candidate PNe uncovered, we observe that
from thepointofviewofthesize, large objects (greater
than 20 arcsec) constitute the main new group (Fig.
1). As large objects are generally considered as more
evolved, we are confident in finding in this group new
old PNe and byextension new cases of PNe interacting
with the surrounding ISM (PNe/ISM).
Figure 1: Galactic distribution of the IPHAS neb-
ulae according to their size.
3 Candidate PNewith ISMin-
teraction
Fundamental in PN development, the interaction with
the ISM does not only concern old PNe, as may be
commonly thought. Indeed, the PN-ISM interaction
has mainly been detected in a rather small number of
nebulae, which are generally bright objects (“young”
and“mid-age”PNe). Rauch et al.(2000)andWareing et al.
(2007) showed that different stages of interaction are
exhibited during the PNe life. The low surface bright-
ness, generally associated with nebulae mixing with
the ISM and “old” PNe, has for a long time prevented
any deeper observation and good statistical study of
these interactions. Although faint objects will still re-
main difficult to detect, the IPHAS survey provides
a noticeable improvement. Nevertheless, a caveat is
the difficulty to visually separate PNe-ISM from other
faint and extended structures like old HII regions, Su-
pernovae (SNRs) or diffuse H αstructures. As an ex-
ample, faint bow shocks generally characteristics ofwww.publish.csiro.au/journals/pasa 3
PNemixingwiththeISMcanalsobefilamentarystruc-
tures from old SNRs. A spectroscopic analysis is the
only way to have a clear identification.
The work presented here is based on the classification
from Wareing et al. (2007) (WZO 1-4 called after the
authors’ names) and will allow us to establish the de-
gree of interaction for each nebula. Their classifica-
tion is the result of the first extensive investigation
of the applicable parameter space, varying stellar pa-
rameters, relative velocities through the ISM and ISM
densities.
The depth reached by the IPHAS survey combined
with the binning detection method allowed us to iden-
tify 21 cases of interacting candidate PNe.
3.1 WZO1 type
The first group of PNe/ISM concerns those where the
main PN is still unaffected and which may display a
distant bow shock. In our area of study (18h-20h), the
majority of candidates answer the first condition, but
none show the outer bow shock. Outside this area, the
nicknamed“EarNebula”orIPHASXJ205013.7+465518
with a 6 arcmin size may be coincident with a WZO1
description as this object is a confirmed bipolar PN
(Fig. 3) surrounded by a shell which may be an AGB
remnant shell or would indicate a multiple shell nebula
(Fig. 2).
3.2 WZO2 type
This category concerns PNe showing a bright rim in
the direction of motion. This is the most common fea-
ture found in our sample and 17 objects out of 21 fall
under this classification. Fig. 4 presents 3 examples
with different angular sizes, although they all display
a diameter on the order of a few arcmin (we consid-
ered the assumed full extent of the round nebulae).
We point out in Fig. 4-Top the difficulty to determine
the true direction of motion regarding the CS position
and off-axis bow shock. Such a geometry could be ex-
plained by an ISM gradient from high on the left to
low on the right. We also notice a particularly low
observed surface brightness (SB) which may explain
previous non detections.
3.3 WZO3 type
This type is exemplified by PNe whose geometric cen-
tres are shifted away from the central star (CS): both
are no longer coincident. An example, is the ancient
PN Sh 2-188 around which IPHAS has uncovered an
extended structure (Wareing et al. 2006). We identi-
fied 3 candidate PNe coincident with this description.
The most probing WZO3 type in our sample is pre-
sented in figure 5 and corresponds, according to hy-
drodynamical models, to a PN with a CS velocity of
about 100 km/s.3.4 WZO4 type
The WZO4 corresponds to the most difficult types of
PN to be detected: the CS has left the vicinity of
the now totally disrupted PN, leaving an amorphous
structure. The challenge does not lie in the detection
ability (it enters in the IPHAS range of detection) but
more intheselection oftheobjects as possible PNedue
to the total lack of symmetry or axi-symmetry. This
type of interaction is also discussed in more detail by
Wareing et al. in these proceedings.
We identified 1 candidate PN which could fit the
given description. Fig. 6 presents the selected can-
didate in the top panel. We suggest the the nebular
material has been moved from the front to the rear
leaving a remnant “wall of material”. We also notice
that some features may be linked to turbulence effects.
The comparison with the hydrodynamical model (bot-
tom panel) seems to support this hypothesis. Nev-
ertheless a spectroscopic confirmation of the nebula’s
nature will be needed. The model implies a velocity
relative to the ISM of 100 Km/s and an evolution in
the post-AGB phase of 10 000 years.
3.5 Distribution of the candidates
Fig. 7-top shows that the majority of the WZO2 nebu-
lae typesare locatedinzones ofrelatively lowISMden-
sity (compared to the Galactic Centre). The low stress
exerted on the nebulae may explain why they still keep
their quasi circular shape. The ISM is more dense in
the Galactic Plane than in the zone towards the anti-
centreor thezoneaboveaheightof100pc(from obser-
vation of neutral hydrogen gas, Dickey et al. (1990)).
We therefore expected a greater influence of the in-
teraction process in this area. Indeed, we observed
that the most advanced stages of interaction, namely
WZO3 and WZO4, are detected in areas of high ISM
density, where PN are more likely to be affected by
such densities.
Thesizedistribution, Fig. 7-bottom, indicatesthat
althoughmostofthedetectedcandidatePNearelarge2,
i.e with a size greater than 100 arcsec, or of medium
size i.e. between 20 and 100 arcsec, small nebulae
also show signs of interaction. This confirms that the
ISM interaction process does not “a priori” only im-
ply “old” nebulae. We also observe that large objects
mainly lie at higher latitudes than smaller nebulae but
it is also interesting to notice that we detect large ob-
jects inzones ofhighextinction; large PNeseemtosur-
vive at relatively low latitudes. They would undergo
strong alteration by the ISM and would display more
advanced stages of interaction. Those disruptions tend
to affect them more than smaller size nebulae at the
same latitude range.
4 Conclusion and Perspectives
In the first fully analysed area of the Galactic plane,
RA=18h to RA=20h, the new H αphotometric survey
2The sizes here are defined in terms of angular sizes, so
the physical correspondence will depend on the distance.4 Publications of the Astronomical Society of Australia
IPHASappearstobeanexcellenttooltostudyPNein-
teracting with the ISM. Indeed the survey contributes
to the detection of nebulae so far hidden mainly due to
their faintness. Thus, 21 objects have been identified
aspossible planetarynebulaeinteractingwiththeISM.
They show diverse sizes (although the majority display
a diameter greater than 100 arcsec) and morphologies
corresponding to the four different cases of interaction
commonly defined going from the unaffected to the to-
tally disrupted nebula. The most common stage is the
WZO2correspondingtonebulaeshowingabrightening
of their rim in the direction of motion. This is coinci-
dent with the observations made by Wareing and al (in
these proceedings) crossing different H αsurveys. We
were also able to reach those targets at low latitudes
and found that some could survive in those environ-
ments although they would be strongly affected by the
ISM. The total lack of PNe/ISM at the highest point
of ISM density (b= ±0.5 deg and 30 deg <l<50 deg)
can either be due to the limitation of IPHAS or be-
cause they have been totally destroyed by the effects
of ISM interaction.
The next logical step is the spectroscopic identification
of these sources, their central star study and physical
size determination. The low surface brightness implies
the use of particular means such as integral field spec-
troscopy to be able to retrieve the maximum infor-
mation. Therefore a new programme of IPHAS PN
candidate follow-up spectroscopy led by Q. Parker, A.
Zijlstra and R. Corradi is now underway.
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Direction of motionThick outer
Filamentsshell: rim
+ Bipolar outflowBright edges of the bipolar
Sharp structures
Faint opposite edge
Figure 2: An example of WZO1 type: The “Ear Nebula” IPHAS PN. Nort h on the top and East on the
left.
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/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0/0
/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1/1erg/cm2/s/A erg/cm2/s/A
Figure 3: WHT spectra of the “EarNebula” using the R300Band R158 R gratings. This nebula, for which
we show some of the “strongest” emission lines useful for an identifi cation, presents a clear [NII] over-
intensity and it has been confirmed as true PN using the revised diagn ostic diagram from Riesgo et al.
(2006) (particularly the log [H α/[SII]]vslog [Hα/[NII]] diagram).6 Publications of the Astronomical Society of Australia
Interacting rimDirection of motion
CS candidate
Direction of motion
Geometric centerBow shock CS candidates
Direction of motionCS candidateBright rim
Figure 4: Examples of WZO2 types. Top: Size=1.2 arcmin and SB=3.4e−17erg cm−2s−1arcsec−2.
Middle: Size= 8.5 arcmin and SB=1.1e−16erg cm−2s−1arcsec−2, Bottom: 4.3 arcmin and SB=2.7e−16
erg cm−2s−1arcsec−2. North on the top and East on the left.
 Direction of motion
Most probable CS
Bright rim
Figure 5: WZO3 type of ISM interaction in a IPHAS candidate PNe. Nor th on the top and East on the
left.www.publish.csiro.au/journals/pasa 7
 
Direction of motion
Dense "wall" of nebular material gas and dust
or
turbulences Traces ofFaint frontal bow shock
Figure 6: A possible example of WZO4 ISM interaction in one IPHAS PN ca ndidate (top: North on the
top and East on the left) with the corresponding hydrodynamical m odel (bottom) [reproduction of figure
5(d) from Wareing et al. (2007)].8 Publications of the Astronomical Society of Australia
Figure 7: Galactic distribution of the candidate PNe/ISM according t o their stage of interaction and
their size.
Figure 8: Example of candidates with sizes greater than 100 arcsec (respectively 7.7 and 2.9 arcmin) and
located at b= ±1 deg. These objects present a WZO2 stage of interaction and only their (very) faint
interacting rim are seen.