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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.,
[4,7–10]
). 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 (
D∼1.2 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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