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On the Origin of Morphological Structures of Planetary Nebulae Kwok, Sun Jun 26, 2018

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galaxiesArticleOn the Origin of Morphological Structures ofPlanetary NebulaeSun Kwok 1,2 ID1 Department of Earth, Ocean, and Atmospheric Sciences, University of British Columbia, Vancouver,BC V6T 1Z4, Canada; skwok@eoas.ubc.ca or sunkwok@hku.hk; Tel.: +1-778-858-57522 Laboratory for Space Research, The University of Hong Kong, Hong Kong, ChinaReceived: 24 May 2018; Accepted: 22 June 2018; Published: 26 June 2018Abstract: We suggest that most of the mass in planetary nebulae (PNe) resides in the equatorialregion and the spherical envelope and the optically bright lobes of PNe are in fact low-density cavitiescleared out by fast outflows and photoionized by UV photons leaked from the torus. The nature ofmulti-polar PNe is discussed under this framework.Keywords: planetary nebulae; asymptotic giant branch stars; mass loss1. IntroductionThe diverse morphological shapes of planetary nebulae (PNe) were recognized since the beginningof PNe research [1]. Does this diversity suggest that every PN is different, or is there an intrinsicstructure common to all PNe? The possibility that there is a uniform 3-D structure of PNe was exploredby models such as an open-ended toroid [2] or an ellipsoidal shell [3] inclined w.r.t. the line of sight.Ionization penetrating to different depths could create the different apparent shapes of PNe [4].A separate question is what is the physical mechanism that creates such non-spherically symmetricstructures? The realization that PNe are formed by the process of interacting winds [5] led tothe suggestion that nebular shaping can be achieved by asymmetry in one or more of the stellarwinds [6].Observations with CCD cameras, space-based observations, and imaging beyond the visiblewavelengths, have greatly expanded our capabilities of mapping the morphological structures ofPN. In this paper, we discuss whether these new observations can lead to a new paradigm in ourunderstanding of the origin of morphological structures of PNe.2. Problems with Morphological ClassificationsOur morphological classifications of PNe are primarily based on the apparent morphologicalshapes of optical images of PNe. These classification schemes suffer from the following problems:(i). Orientation effects (PNe are 3D but images are 2D). A near-pole-on bipolar nebula may appearas a ring because of the prominence of the torus. Many well-known PNe (the Ring, the Helix,the Dumbbell, NGC 7027) turn out to have similar intrinsic bipolar structures [7–10].(ii). Dynamic range: if we go deep enough we see more/different structures. The well-known MessierPN M76 turned out to be bipolar when observed by CCD detectors, when earlier photographicplate images only showed the central torus. The bipolar lobes of Sh 1-89 were only found withnarrow-band CCD imaging [11].(iii). Limited field of view (FoV). We may miss large outer structures because the limited FoV ofobservations (examples: IPHAS PN-1 [12], M1-41 [13]).Galaxies 2018, 6, 66; doi:10.3390/galaxies6030066 www.mdpi.com/journal/galaxiesGalaxies 2018, 6, 66 2 of 5(iv). limited wavelength coverage. Bipolar structures not obvious in visible images may revealthemselves in the infrared [14].(v). Internal dust extinction. Optical morphology of PNe may be affected by effects of circumstellardust extinction [15].3. Multipolar NebulaeAs the result of high dynamic-range imaging, more and more PNe are found to be multipolar [16,17]. HST observations have found that many compact PNe have multi-polar lobes [18]. Interestingly,many of the multi-polar lobes are roughly equal in length [19,20]. Some PNe with prominent bipolarlobes also show secondary multi-polar lobes in other directions [21]. The existence of multi-polarnebulae suggests collimated ejections formed simultaneously or episodically [18]. Precessing bipolar,rotating episodic jets have been proposed to explain the observed morphology [22].What is the fraction of multi-polar PNe among the PNe population? Current statistics suggestsa range between 10–20% [23,24], but these are almost certainly lower limits as high-dynamic-rangeimaging of PNe has only been done for a small fraction of PNe. If a majority of PNe are multi-polar,what are the consequences? We have performed an exercise assuming that PNe have 3 pairs ofidentical-length lobes oriented at random directions and simulate their apparent images when observedfrom different lines of sight to different degrees of sensitivity. We found that many different observedmorphological shapes of PNe can be reproduced with this simulation. For example, tori and doubleinner–outer bipolar lobes are the result of overlapping multi-polar lobes, and ansae and knots maybe the bright tips of undetected lobes [25]. Nearly aligned pairs of lobes can give the appearance ofpoint-symmetric S-shape morphology, which does not necessarily imply precession. It is remarkablethat morphology of very diverse PNe can be simulated by a single, very simple 3-D model whenorientation and sensitivity effects are taken into account.In the past, we have relied on slit spectroscopy to infer the kinematic structure of PNe. The bestway to test the multipolar hypothesis is through integral field spectroscopy. The velocity maps fromsuch observations can be compared with 3-D spatial-kinematic models to determine the true intrinsicstructure of PNe.4. Unseen “Dark” Matter in Planetary NebulaeMany bipolar PNe (e.g., NGC 6302, NGC 2346) have very tight waists, suggesting that they arebeing confined by unseen external material. Other bipolar PNe (e.g., IC 4406, Hen 3-401, Hen 2-320)show sharp, well-defined lobe boundaries, suggesting that the optical lobes are dynamically confinedby external media (Figure 1). This unseen matter is probably in the form of molecular gas and can betraced by molecular-line imaging. Mapping of the 2.12 µm H2 line requires the molecule to be highlyexcited by shocks or UV radiation so this line can map only dynamically interacting regions but isnot a good tracer of cold molecular gas. The H2 ground-state rotational transition (para S(0) J = 2–0)requires less (500 K) excitation but this 28 µm line cannot be observed from the ground [26].Galaxies 2018, 6, x FOR PEER REVIEW  2 of 5  (iv) limited wavelength coverage. Bipolar structures not obvious in visible images may reveal themselves in the infrared [14]. (v) Internal dust extinction. Optical morphology of PNe may be affected by effects of circumstellar dust extinction [15]. 3. Multipolar Nebulae As the result of high dynamic-range imaging, more and more PNe are found to be multipolar [16,17]. HST observations have found that many c mpact PNe have m lti-p lar lobes [18]. In ere tingly, many of the multi-polar lobes are roughly equal in length [19,20]. Some PNe with prominent bipolar lobes also show secondary multi-polar lobes in other directions [21]. The existence of multi-polar nebulae suggests collimated ejections formed simultaneously or episodically [18]. Precessing bipolar, rotating episodic jets have been proposed to explain the observed morphology [22].  What is the fraction of multi-polar PNe among the PNe population? Current statistics suggests a range between 10–20% [23,24], but these are almost certainly lower limits as high-dynamic-range imaging of PNe has only bee  done for a small fraction of PNe. If a majority of PNe are multi-polar, what are the consequences? We have performed an exercise assuming that PNe have 3 pairs of identical-length lobes oriented at random directions and simulate their apparent images when observed from different lines of sight to different degrees of sensitivity. We found that many different observed morphological shapes of PNe can be reproduced with this simulation. For example, tori and double inner–outer bipolar lobes are the result of overlapping multi-polar lobes, and ansae and knots may be the bright tips of undetected lobes [25]. Nearly aligned pairs of lobes can give the appearance of point-symmetric S-shape morphology, which does not necessarily imply precession. It is remarkable that morphology of very diverse PNe can be simulated by a single, very simple 3-D model when orientation and sensitivity effects are taken into account. In the past, we have relied on slit spectroscopy to infer the kinematic structure of PNe. The best way to test the multipolar hypothesis is through integral field spectroscopy. The velocity maps from such observations can be compared with 3-D spatial-kinematic models to determine the true intrinsic structure of PN . 4. Unseen “Dark” Matter in Planetary ebulae Many bipolar PNe (e.g., NGC 6302, NGC 2346) have very tight waists, suggesting that they are being confined by unseen external material. Other bipolar PNe (e.g., IC 4406, Hen 3-401, Hen 2-320) show sharp, well-defined lobe boundaries, suggesting that the optical lobes are dynamically confined by external media (Figure 1). This unseen matter is probably in the form of molecular gas and can e traced by molecular-line imaging. Mapping of the 2.12 µm H2 line requires the molecule to be highly excited by shocks or UV radiation so this line can map only dynamically interacting regions but is not a good tracer of cold molecular gas. The H2 ground-state rotational transition (para S(0) J = 2–0) requires less (500 K) excitation but this 28 µm line cannot be observed from the ground [26].  Figure 1. Continuum-subtracted H2 2.12 µm line image of IC 4406 obtained with the Canada-France-Hawaii Telescope. The bipolar lobes have sharp boundaries and are clearly confined by an external medium. Figure 1. Co um-subtracted H2 2.12 µm line image of IC 4406 obtai d withthe Canada-France-Hawaii Telescope. The bipolar lobes have sharp boundaries and are clearly confinedby an external medium.Galaxies 2018, 6, 66 3 of 5Current CO line mapping from mm/submm-wave interferometers such as SMA and ALMAmainly reveals the molecular torus around the waist of bipolar nebulae [27,28]. Mapping ofthe extended molecular gas will require higher surface brightness sensitivity observations.The dust mixed with the cold molecular gas is probably too low temperature to be observed bymid-infrared (10–20 µm) imaging. Current mid-infrared imaging of PNe mainly reveals dust in the torusor in the bipolar lobes [29,30]. Although efforts have been made to map the cold dust component bySpitzer [31,32], AKARI [33], and Herschel [34–36], the extended cold dust component be truly revealedwith high angular resolution, high sensitivity, far infrared imaging with facilities such as SOFIA.5. DiscussionMany of the present theories on the origin of morphological structures of PNe are based onthe assumption that the observed bright nebulosity represents ejection of physical matter. However,the visible brightness of PNe is due to recombination lines of H and He and collisionally excited linesof metals, all strong emissions originating from the ionized region of PNe that only represents a smallfraction of the total mass of PNe. We suggest that the bipolar lobes of PNe are not regions of massiveejection, but low-density cavities carved out of the neutral circumstellar envelope [22]. In the AGBstage, a massive circumstellar envelope is created by mass loss via a slow wind. In the post-AGB phase,collimated fast outflows, energetic but not massive, emerge from the central star. These outflowsbreak through less dense regions of the circumstellar torus, creating cavities in the envelope. Dustscattering of visible photons from the central star leads to bipolar and multi-polar nebulosity observedin proto-PNe [37]. In the subsequent PNe phase, UV photons from the now hot central star ionizethe low-density cavity, illuminating the bipolar region. Optical morphology of PN is therefore notdefined by regions of massive matter ejection, but represents holes in the matter distribution wheredensities are low enough for UV to ionize. The observable effects of a collimated fast wind interactingwith clumpy circumstellar envelope have been explored by Steffen et al. [38].As central stars of PNe evolve to higher temperatures, their radii contract and the escape velocitiesincrease. These effects lead to higher power fast winds, hotter shocked bubbles, and higher thermalpressure [39]. The dynamical effects of interacting winds may wash out the multi-polar structuresobserved in younger PNe, leading to a lower frequency of multi-polar structures in evolved PNe.As PNe evolve and densities drop, the ionization front will eventually breakthrough andthe nebulae becomes density bounded. The morphology of the PNe will also become more spherical.The low-surface-brightness spherically-shaped nebulae found in deep Hα surveys [40,41] are probablyexamples of density-bounded, highly evolved PNe. The fraction of PNe that is bipolar is thereforedependent on age.6. ConclusionsRecent high-dynamic-range optical and infrared observations have revealed that many PNepreviously classified as round or elliptical are in fact bipolar. Statistical analysis of morphologicalclasses based on apparent shapes is therefore not reliable and the true 3-D structures PNe can onlybe found by proper modeling of the brightness and kinematic structures. We suggest that multipolarnebulae are much more common than currently believed and more PNe will show multi-polar structurewith deeper imaging. Some of the morphological features such as tori, filaments, knots, and ansaecould be manifestation of multi-polar nature of the objects. The optical lobes of bipolar/multipolar PNeand proto-PNe are not volumes of concentrated mass but low-density cavities ionized or illuminatedby the central star. The true mass distribution of PNe can only be revealed with future far infrared andmm/submm molecular-line imaging.Funding: This work is supported by grants from the Research Grants Council of Hong Kong and the NaturalSciences and Engineering Research Council of Canada.Acknowledgments: S.K. thanks Alberto López for valuable comments on an earlier version of this paper.Galaxies 2018, 6, 66 4 of 5Conflicts of Interest: The author declares no conflict of interest.References1. Curtis, H.D. The Planetary Nebulae. Publ. Lick Obs. 1918, 13, 55–74.2. Khromov, G.S.; Kohoutek, L. Morphological Study of Planetary Nebulae. In Symposium-InternationalAstronomical Union; Cambridge University Press: Cambridge, UK, 1968; Volume 34, p. 227.3. Masson, C.R. On the Structure of Ionization-Bounded Planetary-Nebulae. Astrophys. J. 1990, 348, 580–587.[CrossRef]4. Zhang, C.Y.; Kwok, S. A Morphological Study of Planetary Nebulae. Astrophys. J. Suppl. Ser. 1998, 117, 341.[CrossRef]5. Kwok, S. 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This article is an open accessarticle distributed under the terms and conditions of the Creative Commons Attribution(CC BY) license (http://creativecommons.org/licenses/by/4.0/).

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