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First Results from a Panchromatic HST/WFC3 Imaging Study of the Young, Rapidly Evolving Planetary Nebulae NGC 7027 and NGC 6302

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We present the first results from comprehensive, near-UV-to-near-IR Hubble Space Telescope Wide Field Camera 3 (WFC3) emission-line imaging studies of two young planetary nebulae (PNe), NGC 7027 and NGC 6302. These two objects represent key sources for purposes of understanding PNe shaping processes. Both nebulae feature axisymmetric and point-symmetric (bipolar) structures and, despite hot central stars and high nebular excitation states, both harbor large masses of molecular gas and dust. The sweeping wavelength coverage of our Cycle 27 Hubble Space Telescope (HST)/WFC3 imaging surveys targeting these two rapidly evolving PNe will provide a battery of essential tests for theories describing the structural and chemical evolution of evolved star ejecta. Here, we present initial color overlays for selected images, and we highlight some of the first results gleaned from the surveys.
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galaxies
Article
First Results from a Panchromatic HST/WFC3
Imaging Study of the Young, Rapidly Evolving
Planetary Nebulae NGC 7027 and NGC 6302
Joel H. Kastner 1,2,* , Jesse Bublitz 2, Bruce Balick 3, Rodolfo Montez, Jr. 4, Adam Frank 5and
Eric Blackman 5
1Center for Imaging Science and Laboratory for Multiwavelength Astrophysics, Rochester Institute of
Technology, Rochester, NY 14623, USA
2School of Physics & Astronomy, Rochester Institute of Technology, Rochester, NY 14623, USA;
jtb1435@rit.edu
3Department of Astronomy, University of Washington, Seattle, WA 98195, USA; balick@uw.edu
4Center for Astrophysics|Harvard & Smithsonian, Cambridge, MA 02138, USA;
rodolfo.montez@cfa.harvard.edu
5Department of Physics & Astronomy, University of Rochester, Rochester, NY 14627, USA;
afrank@pas.rochester.edu (A.F.); blackman@pas.rochester.edu (E.B.)
*Correspondence: jhk@cis.rit.edu
Received: 2 March 2020; Accepted: 10 May 2020; Published: 15 June 2020


Abstract:
We present the first results from comprehensive, near-UV-to-near-IR Hubble Space
Telescope Wide Field Camera 3 (WFC3) emission-line imaging studies of two young planetary
nebulae (PNe), NGC 7027 and NGC 6302. These two objects represent key sources for purposes of
understanding PNe shaping processes. Both nebulae feature axisymmetric and point-symmetric
(bipolar) structures and, despite hot central stars and high nebular excitation states, both harbor large
masses of molecular gas and dust. The sweeping wavelength coverage of our Cycle 27 Hubble Space
Telescope (HST)/WFC3 imaging surveys targeting these two rapidly evolving PNe will provide a
battery of essential tests for theories describing the structural and chemical evolution of evolved star
ejecta. Here, we present initial color overlays for selected images, and we highlight some of the first
results gleaned from the surveys.
Keywords: stars—evolution; stars—winds and outflows; planetary nebulae
1. Introduction
The transformation from slow (
10–20 km s
1
), quasi-spherical asymptotic giant branch (AGB)
star wind to fast (
100–300 km s
1
), collimated outflows during post-AGB and early planetary nebula
(PN) evolutionary stages remains one of the most intriguing yet poorly understood aspects of the
deaths of intermediate-mass stars. The community of PN researchers has reached a broad consensus
that the influence of a companion to the mass-losing central star is responsible for this transformation
(e.g., [
1
3
]), and that the interaction of collimated post-AGB outflows with previously ejected AGB
wind material can explain the shapes of PNe spanning a wide range of morphologies and dynamical
ages (e.g., [
4
,
5
]). As discussions at the WorkPlaNS II meeting made apparent, however, fundamental
problems raised by this general binary-driven PNe shaping scenario remain to be resolved. Among the
most vexing challenges is to specify the nature of the binary interactions underlying the late-breaking
(post-AGB, pre-PNe) collimated flows: are such outflows generated via onset of a common envelope
(CE) configuration, jets associated with a companion star’s accretion disk, or some combination of
these (or other) potential mass ejection and collimation mechanisms (see, e.g., [6])?
Galaxies 2020,8, 49; doi:10.3390/galaxies8020049 www.mdpi.com/journal/galaxies
Galaxies 2020,8, 49 2 of 6
Our understanding of this PNe shaping process has benefitted enormously from Hubble Space
Telescope (HST) imaging of PNe and pre-PNe in a plethora of optical and near-IR emission lines
(e.g.,
[4,710]
). Yet—even as HST passed a quarter century of unparalleled achievements in the realm
of subarcsecond imaging of astrophysical sources—the tremendous potential of its Wide Field Camera 3
(WFC3) to obtain a comprehensive set of emission-line images extending over its full wavelength
range, from near-UV through optical through near-IR, had yet to be exploited for any individual PN.
This gaping hole in HST’s legacy extended to even the most intensively studied, structurally rich PNe
that serve as touchstones to understand PNe shaping, photoionization, and shock processes.
With this as motivation, we have undertaken the first such comprehensive NUV-to-NIR
HST/WFC3 emission-line studies of two seminal, young, and (evidently) rapidly evolving PNe:
(1) the enigmatic NGC 7027(
D
0.9 kpc [
11
], dynamical age
1100–1600 yr [
9
]), which for decades
has served as the “industry standard” for astrophysical emission-line and continuum studies from
radio through X-ray regimes (e.g., [
12
,
13
]); and (2) the extreme bipolar nebula NGC 6302 (
D1.2 kpc
,
dynamical age
2200 yr; [
14
]), which was among the most photogenic subjects for early science
observations with WFC3 upon its installation in HST in 2009. These two structurally complex PNe
have been proposed as representative of the two distinct classes of interacting-binary-derived PNe
shaping process noted earlier, i.e., CE (NGC 7027; e.g., [
13
]) and companion accretion disk (NGC
6302; e.g., [
15
]). Both PNe are (likely) descended from relatively high-mass (
3–5
M
) progenitors;
both display among the highest known states of ionization and excitation among PNe, indicative of
exceedingly hot central stars (
Tef f
200 kK; [
10
,
16
]). Both have axisymmetric and point-symmetric
(bipolar) structures and, despite their high nebular excitation and high-
Tef f
central stars, harbor large
masses of molecular gas and dust (e.g., [
12
,
15
,
17
] and refs. therein). Indeed, given the enormous
estimated mass of its molecular torus (
1
M
) [
18
], it is also possible that the central star of NGC
6302 has emerged from a CE event that took place within the past few hundred years (given the
dynamical age of the nebula). On the other hand, in contrast to NGC 7027, NGC 6302 does not display
X-ray emission from shocks [
19
]. Thus, NGC 6302 may represent a snapshot of PNe evolution in
which nebula-shaping “blowouts,” such as characterize present-day NGC 7027 [
20
], have very recently
expanded and cooled below (X-ray) detectability.
In this paper—summarizing a poster presented during the WORKPLANS II meeting [
21
]—we
highlight some of the first results gleaned from our HST/WFC3 imaging surveys of NGC 7027 and
NGC 6302.
2. Observations
2.1. Survey Strategy and Goals
In Table 1, we summarize our HST/WFC3 imaging program targeting NGC 7027 and NGC
6302 (Program ID 15953; PI: J. Kastner) in terms of overarching science goals, filters used, and the
specific lines, atomic/ionic species, and bands targeted. In the following, we briefly describe the main
components of this observing strategy.
Table 1.
Hubble Space Telescope (HST)/Wide Field Camera 3 (WFC3) Imaging Survey of NGC 7027
and NGC 6302: Overview.
Science Goal λRegime: WFC3 Filters aLines/Bands Targeted
photoionization vs. heat conduction near-UV: FQ243N, F343N [Ne IV], [Ne V]
shocks vs. photoionization visible: F487N, F502N, F656N, F673N Hβ, [O III], Hα, [S I I]
shocks vs. photoionization near-IR: F128N, F130N, F164N, F167N Paβ, [Fe II] (+ adj. continuum)
scattered light dust imaging near-IR: F110W, F160W ‘J’, ‘H’ band continuum
Note: (a) For NGC 6302, we also obtained an image in filter F658N ([N II]).
Mapping variations in PNe ionization state, via imaging in lines of highly ionized Ne. Both nebulae were
imaged using narrow-band near-UV filters that isolate emission from forbidden transitions of adjacent
Galaxies 2020,8, 49 3 of 6
ionization states of Ne (specifically, [Ne IV]
λ
2425 and [Ne V]
λ
3426). The resulting Ne ionization state
maps can be used to establish the illumination patterns and penetration depths of nebular gas by EUV
photons from the unusually hot central stars within NGC 7027 and NGC 6302. In the case of NGC
7027, the [Ne IV] and [Ne V] imaging was also aimed at the possible detection of conduction fronts at
the interfaces between
10
4
K nebular gas and
10
6
K, X-ray-emitting plasma generated by highly
energetic shocks [20].
Establishing the domain of shocks and dust, via near-IR imaging. The [Fe II] emission line at 1.64
µ
m
is a well-established tracer of moderate-strength (
100 km s
1
) astrophysical shocks (e.g., [
22
]).
In PNe, such emission typically arises where stellar wind streamlines slam into the the walls of
bipolar cavities, creating an inner (reverse) shock that should vary with polar angle (since only
the perpendicular velocity component shocks the gas). The H
2
emission that is a ubiquitous
feature of bipolar PNe like NGC 7027 and NGC 6302 [
23
] can arise from slower (
20 km s
1
),
leading shocks; indeed, the combination of [Fe II] and H
2
emission has been vital in diagnosing
shocks in YSO outflows (e.g., [
24
]). The dust structures in both nebulae, and the potential influence of
these structures on shock-induced nebular shaping, will be further explored via near-IR continuum
(scattered-light) imaging.
Connecting UV/NIR-based and optical-based photoionization and shock diagnostics. Maps of various line
ratios constructed from images of [O III] 5007 Å, [S II] 6716, 6731 Å, H
α
, and H
β
emission, when placed
in the context of model predictions, have long served to disentangle shock- vs. UV-driven excitation
and ionization (e.g., [
25
] and refs. therein). In obtaining a suite of images in these bright emission
lines, we can further exploit the aforementioned combination of [Ne V]/[Ne IV] and [Fe II] imaging to
cross-calibrate these (near-UV and near-IR) diagnostics of shock-induced ionization vs. photoionization
against (far more widely used) optical emission-line diagnostics.
2.2. Image Acquisition and Processing
Data for our HST/WFC3 imaging surveys of NGC 7027 and NGC 6302 were obtained early in
Cycle 27 (2019 September and October) during the course of 11 orbits
1
. Typical total exposure times
were
1200 s. Images presented in this overview paper have thus far only been subject to standard
WFC3 pipeline processing. Refinement of processing parameters, in particular cosmic ray removal
and optimizing astrometric accuracy and precision (absolute reference frame and filter-to-filter image
registration), is now underway.
3. First Results
Color overlays of selected narrow-band WFC3 images of NGC 7027 and NGC 6302 spanning the
near-UV through near-IR wavelength ranges are presented in Figures 1and 2, respectively. Below,
we briefly highlight some of the first results obtained from these and the other WFC3 images of
each nebula.
3.1. NGC 7027
In Figure 1, NGC 7027 displays the now-familiar juxtaposition of structures previously seen
in HST imaging [
9
,
10
]: a bright, well-defined elliptical shell of semimajor axis
6
00
(
8
×
10
16
cm)
surrounded by a set of concentric circular rings extending to at least
15
00
(
2
×
10
17
cm) in radius;
a narrow equatorial dust ring or torus skirting the elliptical shell’s minor axis; and a complex set of
multipolar “blowouts,” oriented more or less along the elliptical shell’s major axis, that appear to
puncture the shell and project well out into the concentric ring system. The dust rings are seen in
reflection and, because they are illuminated by NGC 7027’s inner, highly ionized shell—which is among
1
Initial observations of NGC 6302 in filters F502N and F673N failed, and so were repeated during 2 orbits in 2020 March,
with the addition of filter F658N.
Galaxies 2020,8, 49 4 of 6
the brightest [O III] sources in the sky—they appear greenish in Figure 1(i.e., they are brightest in the
F502N image). The new WFC3 images also reveal, in unprecedented detail, the delicate, filamentary
dust structures that are superimposed on the elliptical shell. These dust filaments, which appear to
be associated (perhaps entrained within) the multipolar blowout structures, appear red in Figure 1
(i.e., they are brightest in the F164N image) largely as a consequence of their large local extinction,
not because they are regions of bright [Fe II] line emission. The same is true of the dusty equatorial ring.
A preliminary F164N
F167N difference image (not shown) reveals that, in fact, the [Fe II] emission
traces the southeast-northwest-oriented collimated outflows that also produce the X-ray-emitting
shocks imaged by Chandra [
20
]. The possible presence of bright [Ne V] emission in the northwest
blowout (the bluest region of Figure 1) could be due to heat conduction from the X-ray-emitting
shocked wind plasma to the cooler, photoionized nebular gas. However, this and other extended blue
regions of Figure 1may instead be due to bright, interior nebular line emission that is scattered off dust
in NGC 7027’s extended, cool (predominantly neutral/molecular) envelope.
Finally, we note
that the
central star is detected in most of our new WFC3 images, such that we should now be able to construct
(and attempt to deredden) the star’s spectral energy distribution from 243 nm to 1.6 µm.
Figure 1.
Color overlay of Cycle 27 HST/WFC3 narrow-band images of NGC 7027. Filter F343N
([Ne V]) is blue, F502N ([O III]) is green, and F164N ([Fe II]) is red. The field of view is
38
00 ×
38
00
;
north is up and east is to the left.
3.2. NGC 6302
In Figure 2, NGC 6302 displays its classical, pinched-waist bipolar morphology. As in the images
previously obtained by HST [
18
,
26
], the new WFC3 images clearly define the thick, dusty toroidal
equatorial structure (central dark lane) that bisects the polar lobes, and reveal fine structures (knots
and filaments) within the lobes. The most striking aspect of this new WFC3 color image overlay is the
bright, S-shaped [Fe II] emission (red channel, i.e., F164N filter, in Figure 2) that traces the southern
interior of the east lobe rim and the northern interior of the west lobe rim, in point-symmetric fashion.
Given that [Fe II] emission is typically a marker of fast shocks, the strict confinement of the emission to
these point-symmetric lobe structures strongly suggests that these particular surfaces of the PNe lobes
Galaxies 2020,8, 49 5 of 6
are being actively shaped by collimated winds emanating from the immediate vicinity of its central
star. This surprising misalignment of the central engine’s present collimated fast wind direction and
the nebula’s main axis of symmetry presents an especially daunting challenge for models of the origin
and evolution of NGC 6302’s bipolar structure. The central star itself appears to be directly detected,
within the dark lane, in our new near-IR (F128N and F164N) WFC3 images. However, we find this
star is offset by
0.1–0.2
00
from the point-like source that was identified as the PN’s central star in the
first-epoch (2009) WFC3 images [
26
]. Followup astrometry is underway, to confirm whether these
putative central star identifications are in fact discrepant.
Figure 2.
Color overlay of Cycle 27 HST/WFC3 narrow-band images of NGC 6302. Filter F343N
([Ne V]) is blue, F128N (Pa
β
) is green, and F164N ([Fe II]) is red. The field of view is
120
00 ×
70
00
;
north is up and east is to the left.
Author Contributions:
Conceptualization, J.H.K., B.B., J.B., R.M.J.; methodology, J.H.K., B.B., R.M.J., A.F., and E.B.;
formal analysis, J.H.K., B.B., J.B., and R.M.J.; investigation, J.H.K. and B.B.; data curation, J.H.K., J.B., and R.M.J.;
writing—original draft preparation, J.H.K.; writing—review and editing, J.H.K., B.B., J.B., and E.B.; visualization,
J.H.K.; supervision, J.H.K.; project administration, J.H.K.; funding acquisition, J.H.K. All authors have read and
agreed to the published version of the manuscript.
Funding: This research is funded by Space Telescope Science Institute grant number HST-GO-15953.001.
Conflicts of Interest: The authors declare no conflicts of interest.
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c
2020 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access
article distributed under the terms and conditions of the Creative Commons Attribution
(CC BY) license (http://creativecommons.org/licenses/by/4.0/).
... Hubble Space Telescope (HST) emission-line imaging studies of the most recently formed, and hence most rapidly evolving, PNe provide particularly effective means to understand PN ionization and shaping processes (e.g., Sahai & Trauger 1998). In HST's Cycle 27, NGC 7027 was one of the first two PNe to be targeted for a comprehensive, contemporaneous set of emission-line images, from near-UV through optical to near-IR, with the Wide Field Camera 3 (WFC3) (Kastner et al. 2020). We selected NGC 7027 for such a study because it is among the youngest and most rapidly evolving PN within ∼1 kpc of the Sun (D = 0.89 kpc; Masson 1989). ...
... First results from our Cycle 27 HST/WFC3 imaging programs targeting NGC 7027 and a second, well-studied PN -the similarly young (expansion age ∼2000 yr), nearby (D ∼ 1.0 kpc) bipolar PN NGC 6302-were presented in Kastner et al. (2020). In Kastner et al. (2022), we presented a detailed overview of the HST/WFC3 imaging survey of NGC 6302. ...
Article
Full-text available
The iconic planetary nebula (PN) NGC 7027 is bright, nearby ( D ∼ 1 kpc), highly ionized, intricately structured, and well observed. This nebula is hence an ideal case study for understanding PN shaping and evolution processes. Accordingly, we have conducted a comprehensive imaging survey of NGC 7027 comprised of 12 HST Wide Field Camera 3 images in narrow-band and continuum filters spanning the wavelength range 0.243–1.67 μ m. The resulting panchromatic image suite reveals the spatial distributions of emission lines covering low-ionization species such as singly ionized Fe, N, and Si, through H recombination lines, to more highly ionized O and Ne. These images, combined with available X-ray and radio data, provide the most extensive view of the structure of NGC 7027 obtained to date. Among other findings, we have traced the ionization structure and dust extinction within the nebula in subarcsecond detail; uncovered multipolar structures actively driven by collimated winds that protrude through and beyond the PN’s bright inner core; compared the ionization patterns in the WFC3 images to X-ray and radio images of its interior hot gas and to its molecular outflows; pinpointed the loci of thin, shocked interfaces deep inside the nebula; and more precisely characterized the central star. We use these results to describe the recent history of this young and rapidly evolving PN in terms of a series of shaping events. This evolutionary sequence involves both thermal and ram pressures, and is far more complex than predicted by extant models of UV photoionization or winds from a single central progenitor star, thereby highlighting the likely influence of an unseen binary companion.
... Hubble Space Telescope (HST) emission-line imaging studies of the most recently formed, and hence most rapidly evolving, PNe provide particularly effective means to understand PN ionization and shaping processes (e.g., Sahai & Trauger 1998). In HST's Cycle 27, NGC 7027 was one of the first two PNe to be targeted for a comprehensive, contemporaneous set of emission-line images, from near-UV through optical to near-IR, with the Wide Field Camera 3 (WFC3) (Kastner et al. 2020). We selected NGC 7027 for such a study because it is among the youngest and most rapidly evolving PN within ∼1 kpc of the Sun (D = 0.89 kpc; Masson 1989). ...
... First results from our Cycle 27 HST/WFC3 imaging programs targeting NGC 7027 and a second, well-studied PN -the similarly young (expansion age ∼2000 yr), nearby (D ∼ 1.0 kpc) bipolar PN NGC 6302 -were presented in Kastner et al. (2020). In Kastner et al. (2022), we presented a detailed overview of the HST/WFC3 imaging survey of NGC 6302. ...
Preprint
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The iconic planetary nebula (PN) NGC 7027 is bright, nearby (D ~ 1 kpc), highly ionized, intricately structured, and well observed. This nebula is hence an ideal case study for understanding PN shaping and evolution processes. Accordingly, we have conducted a comprehensive imaging survey of NGC 7027 comprised of twelve HST Wide Field Camera 3 images in narrow-band and continuum filters spanning the wavelength range 0.243--1.67 microns. The resulting panchromatic image suite reveals the spatial distributions of emission lines covering low-ionization species such as singly ionized Fe, N, and Si, through H recombination lines, to more highly ionized O and Ne. These images, combined with available X-ray and radio data, provide the most extensive view of the structure of NGC 7027 obtained to date. Among other findings, we have traced the ionization structure and dust extinction within the nebula in sub-arcsecond detail; uncovered multipolar structures actively driven by collimated winds that protrude through and beyond the PN's bright inner core; compared the ionization patterns in the WFC3 images to X-ray and radio images of its interior hot gas and to its molecular outflows; pinpointed the loci of thin, shocked interfaces deep inside the nebula; and more precisely characterized the central star. We use these results to describe the recent history of this young and rapidly evolving PN in terms of a series of shaping events. This evolutionary sequence involves both thermal and ram pressures, and is far more complex than predicted by extant models of UV photoionization or winds from a single central progenitor star, thereby highlighting the likely influence of an unseen binary companion.
... If these structures have been strongly affected, they would not represent accurately the properties of the orbital parameters. For example, the case of NGC 7027 where multiple disconnected arc-like features in addition to jets are detected surrounding its main cavity (e.g., Kastner et al. 2020;Moraga Baez et al. 2023). We note that similar structures have been unveiled by the unprecedented images of NGC 3132 obtained by the James Webb Space Telescope (De Marco et al. 2022) ...
Preprint
We present the first 3D radiation-hydrodynamic simulations of the formation of planetary nebulae (PNe) emerging from 3D spiral patterns. We use the GUACHO code to create 3D spiral structures as a consequence of the distortions on the geometry of the intrinsically isotropic wind of an asymptotic giant branch (AGB) star produced by a companion star in a circular orbit. We found that the orbital period of the binary producing the 3D spiral pattern has consequences on the formation and shaping of the PN itself. Stellar systems with longer period create less entwined 3D spirals, producing PNe with rounder inner cavities, and prevent the expansion of jet towards the polar directions. The spiral fitting procedure used in the literature to predict the binary's orbital period may be misleading in the case of proto-PNe and PNe as spiral patterns are diluted by their own thermal expansion down to the average AGB density profile within a few hundred years and are further disrupted by the action of jets. By adopting a phase of jet ejections between the AGB and post-AGB stages, we are able to recover the morphologies of proto-PNe and PNe that exhibit ring-like structures in their halos.
... As HST neared the end of its third decade in operation, however, the community of PNe researchers had yet to take full advantage of the potential of HST's most capable imaging instrument, the Wide Field Camera 3 (WFC3). This changed in HST Cycle 27, when we used HST/WFC3 to obtain the first comprehensive, contemporaneous sets of near-UV through near-IR (243 nm to 1.6 μm) emission-line images of two especially structurally rich PNe, NGC 7027 and NGC 6302 (see Kastner et al. 2020, for an initial overview of these HST/WFC3 imaging surveys). As we demonstrate in this paper, the broad, contemporaneous wavelength coverage of these WFC3 image suites yields fullnebula emission line image overlays and line ratio maps at ∼0 1 resolution that are free from distortions and artifacts caused by nebular proper motions or cross-instrument calibration uncertainties (among other potential pitfalls). ...
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We present the results of a comprehensive, near-UV-to-near-IR Hubble Space Telescope (HST) Wide Field Camera 3 (WFC3) imaging study of the young planetary nebula (PN) NGC 6302, the archetype of the class of extreme bilobed, pinched-waist PNe that are rich in dust and molecular gas. The new WFC3 emission-line image suite clearly defines the dusty toroidal equatorial structure that bisects NGC 6302's polar lobes, and the fine structures (clumps, knots, and filaments) within the lobes. The most striking aspect of the new WFC3 image suite is the bright, S-shaped 1.64 μ m [Fe ii ] emission that traces the southern interior of the east lobe rim and the northern interior of the west lobe rim, in point-symmetric fashion. We interpret this [Fe ii ] emitting region as a zone of shocks caused by ongoing, fast (∼100 km s ⁻¹ ), collimated, off-axis winds from NGC 6302's central star(s). The [Fe ii ] emission and a zone of dusty, N- and S-rich clumps near the nebular symmetry axis form wedge-shaped structures on opposite sides of the core, with boundaries marked by sharp azimuthal ionization gradients. Comparison of our new images with earlier HST/WFC3 imaging reveals that the object previously identified as NGC 6302's central star is a foreground field star. Shell-like inner lobe features may instead pinpoint the obscured central star’s actual position within the nebula’s dusty central torus. The juxtaposition of structures revealed in this HST/WFC3 imaging study of NGC 6302 presents a daunting challenge for models of the origin and evolution of bipolar PNe.
... This changed in HST Cycle 27, when we used HST/WFC3 to obtain the first comprehensive, contemporaneous sets of near-UV through near-IR (243 nm to 1.6 µm) emission-line images of two especially structurally rich PNe, NGC 7027 and NGC 6302. In Kastner et al. (2020) we provided an initial, high-level overview of these HST/WFC3 imaging surveys. Here, we present the full suite of HST/WFC3 images of NGC 6302, along with various line ratio images and a detailed examination of the key results gleaned from these images thus far. ...
Preprint
We present the results of a comprehensive, near-UV-to-near-IR Hubble Space Telescope WFC3 imaging study of the young planetary nebula (PN) NGC 6302, the archetype of the class of extreme bi-lobed, pinched-waist PNe that are rich in dust and molecular gas. The new WFC3 emission-line image suite clearly defines the dusty toroidal equatorial structure that bisects NGC 6302's polar lobes, and the fine structures (clumps, knots, and filaments) within the lobes. The most striking aspect of the new WFC3 image suite is the bright, S-shaped 1.64 micron [Fe II] emission that traces the southern interior of the east lobe rim and the northern interior of the west lobe rim, in point-symmetric fashion. We interpret this [Fe II] emitting region as a zone of shocks caused by ongoing, fast (~100 km/s), collimated, off-axis winds from NGC 6302's central star(s). The [Fe II] emission and a zone of dusty, N- and S-rich clumps near the nebular symmetry axis form wedge-shaped structures on opposite sides of the core, with boundaries marked by sharp azimuthal ionization gradients. Comparison of our new images with earlier HST/WFC3 imaging reveals that the object previously identified as NGC 6302's central star is a foreground field star. Shell-like inner lobe features may instead pinpoint the obscured central star's actual position within the nebula's dusty central torus. The juxtaposition of structures revealed in this HST/WFC3 imaging study of NGC 6302 presents a daunting challenge for models of the origin and evolution of bipolar PNe.
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We present the first 3D radiation-hydrodynamic simulations of the formation of planetary nebulae (PNe) emerging from 3D spiral patterns. We use the guacho code to create 3D spiral structures as a consequence of the distortions on the geometry of the intrinsically isotropic wind of an asymptotic giant branch (AGB) star produced by a companion star in a circular orbit. We found that the orbital period of the binary producing the 3D spiral pattern has consequences on the formation and shaping of the PN itself. Stellar systems with longer period create less entwined 3D spirals, producing PNe with rounder inner cavities, and prevent the expansion of jet towards the polar directions. The spiral fitting procedure used in the literature to predict the binary’s orbital period may be misleading in the case of proto-PNe and PNe as spiral patterns are diluted by their own thermal expansion down to the average AGB density profile within a few hundred years and are further disrupted by the action of jets. By adopting a phase of jet ejections between the AGB and post-AGB stages, we are able to recover the morphologies of proto-PNe and PNe that exhibit ring-like structures in their haloes.
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In this review/tutorial we explore planetary nebulae as a stage in the evolution of low-to-intermediate-mass stars, as major contributors to the mass and chemical enrichment of the interstellar medium, and as astrophysical laboratories. We discuss many observed properties of planetary nebulae, placing particular emphasis on element abundance determinations and comparisons with theoretical predictions. Dust and molecules associated with planetary nebulae are considered as well. We then examine distances, binarity, and planetary nebula morphology and evolution. We end with mention of some of the advances that will be enabled by future observing capabilities.
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I argue that the high percentage of planetary nebulae (PNe) that are shaped by jets show that main sequence stars in binary systems can accrete mass at a high rate from an accretion disk and launch jets. Not only does this allow jets to shape PNe, but this also points to the importance of jets in other types of binary systems and in other processes. These processes include the grazing envelope evolution (GEE), the common envelope evolution (CEE), and the efficient conversion of kinetic energy to radiation in outflows. Additionally, the jets point to the possibility that many systems launch jets as they enter the CEE, possibly through a GEE phase. The other binary systems in which jets might play significant roles include intermediate-luminosity optical transients (ILOTs), supernova impostors (including pre-explosion outbursts), post-CEE binary systems, post-GEE binary systems, and progenitors of neutron star binary systems and black hole binary systems. One of the immediate consequences is that the outflow of these systems is highly-non-spherical, including bipolar lobes, jets, and rings.
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This workshop is the second of the WORKPLANS series, which we started in 2016. The main goal of WORKPLANS is to build up a network of planetary nebulae (PNe) experts to address the main open questions in the field of PNe research. The specific aims of the WORKPLANS workshop series are (i) to discuss and prioritize the most important topics to be investigated by the PN community in the following years; (ii) to establish a network of excellent researchers with complementary expertise; (iii) to formulate ambitious observing proposals for the most advanced telescopes and instrumentation presently available (ALMA, SOFIA, VLT, GTC, HST, etc.), addressing those topics; and (iv) to develop strategies for major proposals to future observatories (JWST, ELT, SPICA, Athena, etc.). To achieve these goals, WORKPLANS II brought together experts in all key sub-areas of the PNe research field, namely: analysis and interpretation of PNe observational data; theoretical modeling of gas and dust emission; evolution from Asymptotic Giant Branch stars (PNe progenitors) to PNe; and the instrumentation and technical characteristics of the relevant observatories.
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We develop a physical framework for interpreting high-resolution images and kinematics of pre-planetary nebulae (“prePNe”). We use hydrodynamical models to infer the historical properties of fast collimated nuclear flows (“jet”) that successfully form hollow, candle-shaped lobes over ≈10 ³ yr, including the density, momenta, and geometry of the jet and its environment. Next we vary the most influential parameters of this “baseline” model to investigate how changes in the flow parameters affect the model outcomes after 900 yr. Several generic conclusions emerge, such as the injected flows that create the hollow lobes must be light, “tapered,” and injected considerably faster than the lobe expansion speed. Multipolar and starfish prePNe probably evolve from wide-angle flows in which thin-shell instabilities corrugate their leading edges. We show how the common linear correlation of Doppler shift and position along the lobe is a robust outcome of the interaction of tapered diverging streamlines with the lobes’ curved walls. Finally, we probe how modest toroidal magnetic fields added to the fast flow affect the outcome of the baseline model. We conclude that the light, field-free, tapered baseline flow model is not only a successful and universal paradigm for unraveling the histories of lobe formation in prePNe, but also provides a comprehensive, inclusive framework for understanding the details of the shapes, sizes, and internal kinematics of their edges.
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Certain planetary nebulae (PNe) contain shells, filaments, or globules of cold gas and dust whose heating and chemistry are likely driven by UV and X-ray emission from their central stars and from wind-collision-generated shocks. We present the results of a survey of molecular line emission in the 88–236 GHz range from nine nearby (<1.5 kpc) planetary nebulae spanning a range of UV and X-ray luminosities, using the 30 m telescope of the Institut de Radioastronomie Millimétrique. Rotational transitions of thirteen molecules, including CO isotopologues and chemically important trace species, were observed and the results compared with and augmented by previous studies of molecular gas in PNe. Lines of the molecules HCO ⁺ , HNC, HCN, and CN, which were detected in most objects, represent new detections for four planetary nebulae in our study. Specifically, we present the first detections of ¹³ CO (1–0, 2–1), HCO ⁺ , CN, HCN, and HNC in NGC 6445; HCO ⁺ in BD+30°3639; ¹³ CO (2–1), CN, HCN, and HNC in NGC 6853; and ¹³ CO (2–1) and CN in NGC 6772. Flux ratios were analyzed to identify correlations between the central star and/or nebular UV and X-ray luminosities and the molecular chemistries of the nebulae. This analysis reveals a surprisingly robust dependence of the HNC/HCN line ratio on PN central star UV luminosity. There exists no such clear correlation between PN X-rays and various diagnostics of PN molecular chemistry. The correlation between HNC/HCN ratio and central star UV luminosity demonstrates the potential of molecular emission line studies of PNe for improving our understanding of the role that high-energy radiation plays in the heating and chemistry of photodissociation regions.
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The morphologies of planetary nebula have long been believed to be due to wind shaping processes in which a “fast wind” from the central star impacts a previously ejected envelope. It is assumed that asymmetries existing in the “slow wind” envelope would lead to inertial confinement, shaping the resulting interacting wind flow. We present new results demonstrating the effectiveness of Common Envelope Evolution (CEE) at producing aspherical envelopes which, when impinged upon by a spherical fast stellar wind, produce highly bipolar, jet-like outflows. We have run two simple cases using the output of a single PHANTOM SPH CEE simulation. Our work uses the Adaptive Mesh Refinement code AstroBEAR to track the interaction of the fast wind and CEE ejecta allows us to follow the morphological evolution of the outflow lobes at high resolution in 3-D. Our two models bracket low and high momentum output fast winds. We find the interaction leads to highly collimated bipolar outflows. In addition, the bipolar morphology depends on the fast wind momentum injection rate. With this dependence comes the initiation of significant symmetry breaking between the top and bottom bipolar lobes. Our simulations, though simplified, confirm the long-standing belief that CEE can plan a major role in PPN and PN shaping. These simulations are intended as an initial exploration of the post-CE/PPN flow patterns that can be expected from central source outflows and CE ejecta.
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The increase in discovered close binary central stars of planetary nebulae is leading to a sufficiently large sample to begin to make broader conclusions about the effect of close binary stars on common envelope evolution and planetary nebula formation. Herein I review some of the recent results and conclusions specifically relating close binary central stars to nebular shaping, common envelope evolution off the red giant branch, and the total binary fraction and double degenerate fraction of central stars. Finally, I use parameters of known binary central stars to explore the relationship between the proto-planetary nebula and planetary nebula stages, demonstrating that the known proto-planetary nebulae are not the precursors of planetary nebulae with close binary central stars.
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We present an analysis of a second observation of the young planetary nebula (PN) NGC 7027 by the Chandra X-ray Observatory. This latest 59.2 ks exposure with ACIS-S was acquired approximately 14 years after the initial 18.2 ks ACIS-S observation, and the improved photon statistics allow us to perform a detailed spatial and spectral analysis of the X-ray emission. Comparison with multiwavelength imaging of NGC 7027 reveals a strong anti-correlation between extinction across the nebula and the soft-band X-ray emission. Dissecting the X-ray emission into low- and high-extinction regions results in more robust characterization of the plasma spectral properties. We determine that the X-ray emitting plasma has a temperature of ~3.6 MK, is deficient in Fe, and has an X-ray luminosity of L X ~ 7 × 10³¹ erg s⁻¹, all of which are generally consistent with the plasma properties found in PN hot bubbles. We find no evidence of evolution in the X-ray surface brightness over the 14 year baseline between CXO observations. Our analysis underscores the importance of accounting for nebular absorption of the X-ray emission in accurately determining plasma properties for hot bubbles within PNe.
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We conduct three-dimensional hydrodynamical simulations of two opposite jets launched from a binary stellar system into a previously ejected shell and show that the interaction can form barrel-like and H-like shapes in the descendant nebula. Such features are observed in planetary nebulae and supernova remnants. Under our assumption the dense shell is formed by a short instability phase of the giant star as it interacts with a stellar companion, and the jets are then launched by the companion as it accretes mass through an accretion disk from the giant star. We find that the H-shaped and barrel-shaped morphological features that the jets form evolve with time, and that there are complicated flow patterns, such as vortices, instabilities, and caps moving ahead along the symmetry axis. We compare our numerical results with images of 12 planetary nebulae, and show that jet-shell interaction that we simulate can account for the barrel-like or H-like morphologies that are observed in these PNe.
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Aims. We aim to determine individual distances to a small number of rather round, quite regularly shaped planetary nebulae by combining their angular expansion in the plane of the sky with a spectroscopically measured expansion along the line of sight. Methods. We combined up to three epochs of Hubble Space Telescope imaging data and determined the angular proper motions of rim and shell edges and of other features. These results are combined with measured expansion speeds to determine individual distances by assuming that line of sight and sky-plane expansions are equal. We employed 1D radiation-hydrodynamics simulations of nebular evolution to correct for the difference between the spectroscopically measured expansion velocities of rim and shell and of their respective shock fronts. Results. Rim and shell are two independently expanding entities, driven by different physical mechanisms, although their model-based expansion timescales are quite similar. We derive good individual distances for 15 objects, and the main results are as follows: (i) distances derived from rim and shell agree well; (ii) comparison with the statistical distances in the literature gives reasonable agreement; (iii) our distances disagree with those derived by spectroscopic methods; (iv) central-star “plateau” luminosities range from about 2000 L ⊙ to well below 10 000 L ⊙ , with a mean value at about 5000 L ⊙ , in excellent agreement with other samples of known distance (Galactic bulge, Magellanic Clouds, and K648 in the globular cluster M 15); (v) the central-star mass range is rather restricted: from about 0.53 to about 0.56 M ⊙ , with a mean value of 0.55 M ⊙ . Conclusions. The expansion measurements of nebular rim and shell edges confirm the predictions of radiation-hydrodynamics simulations and offer a reliable method for the evaluation of distances to suited objects.
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M2-9, or the "Butterfly Nebula", is one of the most iconic outflow sources from an evolved star. In this paper we present a hydrodynamic model of M2-9 in which the nebula is formed and shaped by a steady, low-density, mildly collimated "spray" of gas injected at 200 km s^-1 that interacts with a far denser presumed pre-existing AGB wind. This simple outflow model accounts very nicely for the details of all of the structures within the inner and outer lobes, the knot pairs N3/S3 and N4/S4 at their respective leading edges, and the large-scale gradient of Doppler shifts within 20" of the nucleus of M2-9. We emphasize that the knot pairs are not ejected from the star but formed in situ. In addition, the observed radial outflow speed gradient is only indirectly related to the gas that is steadily injected from the star at constant speed. The model allows us to probe the early history of the wind geometry and lobe formation. We also formulate a new estimate of the nebular distance D = 1.3 kpc. We propose that the physical mechanism that accounts for the linear radial speed gradient in M2-9 applies more generally to many other pre planetary nebulae whose lobes exhibit similar gradients.