Observing Planetary and Pre-Planetary Nebulae with the James Webb Space Telescope

Abstract

Most stars in the Universe that leave the main sequence in a Hubble time will end their lives evolving through the Planetary Nebula (PN) evolutionary phase. The heavy mass loss which occurs during the preceding Asymptotic Giant Branch (AGB) phase is important across astrophysics, dramatically changing the course of stellar evolution, contributing to the dust content of the interstellar medium, and influencing its chemical composition. The evolution from the AGB phase to the PN phases remains poorly understood, especially the dramatic transformation that occurs in the morphology of the mass-ejecta as AGB stars enter the post-AGB phase and their round circumstellar envelopes evolve into pre-PNe (PPNe) and then to PNe. The majority of PPNe and PNe deviate strongly from spherical symmetry. Strong binary interactions most likely play a fundamental role in influencing this evolutionary phase, but the details of these interactions remain shrouded in mystery. Thus, understanding the formation and evolution of these objects is of wide astrophysical importance. PNe have long been known to emit across a very large span of wavelengths, from the radio to X-rays. Extensive use of space-based observatories at X-ray (Chandra/ XMM-Newton), optical (HST) and mid- to far-infrared (Spitzer, Herschel) wavelengths in recent years has produced significant new advances in our knowledge of these objects. Given the expected advent of the James Webb Space Telescope (JWST) in the near future, we focus on future high-angular-resolution, high-sensitivity observations at near and mid-IR wavelengths with JWST that can help in addressing the major unsolved problems in the study of PNe and their progenitors.

1. Introduction: The Extraordinary Deaths of Ordinary Stars

The late evolution and deaths of a very significant percentage (∼60–80%) of stars in the Universe are likely to be fundamentally affected by strong binary interactions1. Binary interactions in particular affect our understanding of stars in general, since such interactions affect important diagnostic observables of stars and stellar populations (e.g., mass-loss and evolutionary time-scales). A striking example of the effect of binary interactions on stellar evolution are planetary nebulae (PNe) (e.g., see [2]), which represent the bright end-stage of most stars in the Universe that evolve in a Hubble time (i.e., those with main-sequence masses of 1–8 $M_{\odot }$). The progenitors of PNe—Asymptotic Giant Branch (AGB) stars—lose almost all of their stellar envelopes via extensive mass-loss (with rates as high as $\sim {10}^{-4}$ $M_{\odot }$ yr${}^{-1}$). This is followed by the formation of pre-PNe (PPNe) and PNe during the post-AGB stellar phase. Intriguingly, all PPNe and the majority (i.e., ∼80%) of PNe, unlike the generally roughly round circumstellar envelopes around most AGB stars2, display a spectacular array of morphologies and geometrical complexities. These include elliptical, bipolar and multipolar shapes, nested geometrical structures (e.g., hourglass within hourglass structures), equatorial density enhancements (including rotating disks, torii), low-latitude jets, and central stars offset from the geometrical centers of the nebulae (e.g., [7] (ST98), [8,9]), etc. In addition to pre-PNe and PNe, post-AGB objects include the so-called disk-prominent post-AGB or dpAGB objects3 [10], that harbor large, possibly rotating disks, and little or no extended nebulosities. There are many possible ways a companion can generate aspherical PPNe, PNe and dpAGB objects—for example, via common envelope evolution (CEE), grazing envelope evolution (GEE), or focusing of mass-loss toward the equatorial plane (Bondi–Hoyle or Roche–Lobe Overflow), and the associated formation of circum-companion or circumbinary disks and jets (ST98, [11,12,13]). Although single-star models to explain PNe, invoking fast rotation and/or magnetic fields ([14]) have been proposed in the past, these are not likely ([15,16]).

The world’s most powerful astronomical observatories such as HST, Chandra and XMM-Newton from space, and the IRAM 30 m, ALMA, SMA, PdBI and the VLA, from the ground (e.g., Astronomy and Astrophysics Decadal Survey Astro 2020: [17,18]), have been used extensively in order to characterize the physical properties of aspherical PPNe, PNe and/or dpAGB objects, in the quest to understand how such objects result from AGB stars. In this paper, we discuss how the next large telescope in space for astronomy, the James Webb Space Telescope (JWST), can be used to provide key observational diagnostics for understanding the mysterious deaths of low- and intermediate-mass stars.

2. Using the James Webb Space Telescope (JWST) to Study PPNe and PNe

JWST is a large infrared telescope with a 6.5-m wide, segmented, primary mirror that will be launched on an Ariane 5 rocket from French Guiana on 30 March 2021. JWST is an international collaboration between NASA, the European Space Agency (ESA), and the Canadian Space Agency (CSA). The primary, consisting of 18 hexagonal segments (with a clear aperture of 25 m${}^2$), has to be unfolded and the segments co-phased in space. The telescope optics has to be kept shielded from the Sun, the Earth and the Moon—this will be achieved by a giant sunshield (approximately 22 × 14 m${}^2$), and putting JWST into orbit at ∼1.5 × ${10}^6$ km from the Earth at the L2 Lagrangian point. JWST’s operational lifetime is limited by the propellant for station-keeping to be about 10 years (with margin); the JWST project will perform mission assurance testing of the flight system to guarantee five years of scientific operations starting at the end of the commissioning period six months after launch. Its wavelength coverage is 0.6–28.6 $\mathsf{\mu }$m. JWST is diffraction limited at wavelengths $\stackrel{>}{\sim }2$ $\mathsf{\mu }$m, with a PSF FWHM of 0$.{}^{″}$075 at 2 $\mathsf{\mu }$m.

The sunshield allows the telescope to cool down to a temperature $<50$ K by passively radiating its heat into space. Its three near-infrared instruments (NIRCam, NIRSpec, FGS/NIRISS: see below) will work at about 39 K through a passive cooling system. The mid-infrared instrument (MIRI: see below) will work at a temperature of 7 K using a cryocooler system.

2.1. Instrument Summary

JWST’s suite of four instruments are listed below, together with their wavelength coverage (the organizations/institutions responsible for their construction) and a short summary of the science observing modes offered by each instrument. A visual representation of these capabilities is given in Figure 1:

2.1.1. NIRCam (Near Infrared Camera): 0.6–5 $\mathsf{\mu }$m (U. Arizona)

NIRCam offers imaging, coronagraphy, and grism slitless spectroscopy. NIRCam has 2 modules pointing to adjacent fields of view. Each module uses a dichroic to observe simultaneously in a short wavelength channel (0.6–2.3 $\mathsf{\mu }$m) and a long wavelength channel (2.4–5 $\mathsf{\mu }$m). NIRCam has 5 observing modes for science: (i) imaging of two $2.{}^{\prime }2\times 2.{}^{\prime }2$ fields separated by 44${}^{″}$ covering 9.7 arcmin${}^2$ in total (ii) coronagraphic imaging at multiple wavelengths (see Section 2.1.5), (iii) wide field slitless spectroscopy (2.4–5 $\mathsf{\mu }$m) using grisms with resolving power $R=\lambda /\Delta \lambda \sim 1600$ at 4 $\mathsf{\mu }$m, (iv) time-series imaging (photometric monitoring), and (v) grism time series (spectroscopic monitoring), with $R=1600$ at 4 $\mathsf{\mu }$m. The pixel size is $0.{}^{″}031$ (0.6–2.3 $\mathsf{\mu }$m) and $0.{}^{″}063$ (2.8–5.0 $\mathsf{\mu }$m) for modes i, ii, and iv, and $0.{}^{″}063$ for modes iii and v. NIRCam also serves as a wavefront sensor for aligning and phasing JWST’s primary mirror.

2.1.2. NIRSpec (Near Infrared Spectrograph): $<5$ $\mathsf{\mu }$m (ESA, and NASA/GSFC)

NIRSpec has 4 observing modes for near-infrared multi-object spectroscopy (MOS), integral field unit (IFU) spectroscopy, fixed slits (FS) spectroscopy, and bright object time-series (BOTS) spectroscopy, from 0.6–5.3 $\mathsf{\mu }$m, as follows: (i) the MOS obtains simultaneous spectroscopy of multiple science sources within a $3.6{}^{\prime }\times 3.4{}^{\prime }$ field of view (FOV) using the micro-shutter assembly (MSA)4, (ii) IFU spectroscopy uses the integral field unit to provide spatially resolved spectroscopy over a $3{}^{″}\times 3{}^{″}$ FOV sampled with 900 pixels of size $0.{}^{″}1\times 0.{}^{″}1$, (iii) FS spectroscopy provides 4 fixed slits (+1 for redundancy) for obtaining high sensitivity single object spectroscopy, and (iv) bright object time-series (BOTS) spectroscopy uses a $1.{}^{″}6\times 1.{}^{″}6$ fixed slit aperture for observations of bright sources that require stable, high throughput time-resolved spectroscopy. The resolving power of NIRSpec varies from ∼100 using a prism, to ∼1000 (2000)5 using medium- (high-) resolution gratings.

2.1.3. FGS/NIRISS (Fine Guidance Sensor/Near-Infrared Imager and Slitless Spectrograph): 0.6–5 $\mathsf{\mu }$m (CSA)

The FGS provides data for science attitude determination, fine pointing, and attitude stabilization using guide stars in the JWST focal plane. NIRISS provides 4 science observing modes that include slitless spectroscopy, high-contrast interferometric imaging, and standard imaging, covering a $2{}^{\prime }2\times 2{}^{\prime }2$ FOV with a pixel size of $0.{}^{″}065$ These modes are: (i) wide field slitless spectroscopy (WFSS) with $R=150$ at 1.4 $\mathsf{\mu }$m over the entire field of view, using one or both of a pair of identical grisms and a selection of blocking filters to isolate specific wavelength intervals between 0.8–2.2 $\mathsf{\mu }$m6, (ii) single object slitless spectroscopy (SOSS) with a cross-dispersed grism with $R=700$ at 1.4 $\mathsf{\mu }$m that is designed to deliver broad wavelength coverage and spectro-photometric stability, optimized for time-series observations (TSOs), (iii) aperture masking interferometry (AMI) through specific filters that is enabled by a mask with seven sub-apertures (see Section 2.1.5), and (iv) standard imaging, that can be done in 7 wide- and 5 medium-band filters that are closely matched to the NIRCam filter set between 0.9–5.0 $\mathsf{\mu }$m.

2.1.4. MIRI (Mid Infrared Instrument): ∼5–30 $\mathsf{\mu }$m (ESA and JPL)

MIRI provides 4 observing modes for science: (i) imaging with 9 photometric bands (∼5–12 $\mathsf{\mu }$m) covering a $74{}^{″}\times 113{}^{″}$ FOV, (ii) coronagraphic imaging with four different filters (see Section 2.1.5), (iii) low-resolution spectroscopy (∼5–12 $\mathsf{\mu }$m) with a slit (size $0.{}^{″}51\times 4.{}^{″}7$ or slitless configuration, and (iv) medium-resolution spectroscopy (∼4.9–28.8 $\mathsf{\mu }$m) with four different IFUs, R ∼1550–3250. The pixel size for modes i to iii is $0.{}^{″}11$, and $0.{}^{″}196-0.{}^{″}273$ for mode iv. MIRI can also be used effectively for parallel observations with other instruments.

2.1.5. JWST High-Contrast Imaging

JWST can perform coronagraphic observations with each of its 3 major science instruments:

(1)

NIRCam, using (i) a round coronagraphic mask for observations at 2–4.4 $\mathsf{\mu }$m, with an Inner Working Angle [IWA] of $0.{}^{″}4-0.{}^{″}81$, and (ii) a bar for observations at 2–3.7 $\mathsf{\mu }$m, with an IWA of $0.{}^{″}13-0.{}^{″}88$.

(2)

MIRI, using (i) a Lyot stop for observations at 23 $\mathsf{\mu }$m, with an IWA of $2.{}^{″}16$ and a $30{}^{″}\times 30{}^{″}$ FOV, and (ii) a 4-quadrant phase mask (4QPM) and filters at 10.65, 11.4, 15.5 $\mathsf{\mu }$m, with an IWA of $0.{}^{″}33\times 0.{}^{″}49$.

(3)

NIRISS, which admits light through seven holes or sub-apertures in an otherwise opaque pupil mask that interferes to produce an interferogram on the detector in the AMI, effectively making the full aperture of JWST into an interferometric array. The interferogram created by the aperture mask has a sharper core than is provided by normal “direct” imaging. The AMI mode allows the detection and characterization of a faint source within $\sim 0.{}^{″}07-0.{}^{″}4$ of a bright one and up to ∼9 magnitudes fainter. It can also be used to reconstruct high resolution maps of extended sources. The AMI mode uses one of 4 filters (three medium-band, one wideband) covering the 2.8–4.8 $\mathsf{\mu }$m range.

2.2. JWST Strengths and Limitations for PPNe and PNe Studies

JWST’s major strengths for PNe and PPNe research are its ability to provide precise mid-IR imaging (due to its stable point-spread-function or PSF) with very high sensitivities. Although mid-IR observations can be done from the ground, the ambient thermal emission is a huge limiting factor, even at the short end of the mid-IR range: for example, for Keck, in the Lp band (3 $\mathsf{\mu }$m), the sky saturates the detector in a >0.18 s exposure.

JWST’s major limitations, for PPNe and PNe studies, are:

(1)

Saturation (especially for Galactic targets), however mitigation strategies to deal with this to some extent are available and have been investigated by the Solar System science community (e.g., for enabling observations of Mars and the gas giants). Note though that none of the detectors can be damaged by bright light.

(2)

Surveys of object classes that require slews from target-to-target will be expensive due to the long slew time (∼30 min)7.

(3)

At the short-end of the NIR range, JWST faces competition from ground-based Large Telescopes with extreme Adaptive Optics (AO) capabilities. However, it is important to note that the Strehl ratio8 for a ground-based telescope with AO depends on the seeing conditions, which can vary during the course of an observation, so observations of very faint extended circumstellar stricture around bright central sources can only be done with JWST.

3. Selected Science Investigations

Summarized below are several representative projects to study PNe and PPNe that exploit JWST’s unique capabilities.

(a)

Photometric and spectroscopic search for binary companions to, and/or disks around, the central stars of PNe (CSPNe), post-AGB, and post-RGB9 stars.

(b)

Imaging faint and obscured circumstellar structures in PNe and post-AGB objects.

(c)

Studying the composition and evolution of dust in galaxies.

(d)

Studying the physics and chemistry of UV-irradiated molecular clumps in PNe.

3.1. Binary Companions and Disks in Post-AGB and Post-RGB Objects

If the formation and shaping of most PNe is a result of binary evolution (e.g., see review [12]), then the majority of CSPNe should have binary companions. Searches for such companions have been made using photometric (and radial velocity) variability over many decades starting with the early work by [19] that revealed companions in 10–15% of a sample of ∼100 CSPNe with periods, P $\stackrel{<}{\sim }$ 3 d. Recent searches have now considerably increased the sample size—with a dearth of objects at long periods ($P>3$ d, [20,21,22]). The small period (thus small orbital separation) of the binaries suggests that these objects must have undergone common-envelope (CE) evolution at an earlier phase of their evolution. Newer searches have used the infrared-excess as a signature of a cool (M-type, thus low-mass) companion—for example, the authors of [23] carried out I-band (and some J-band photometry) in 27 CSPNe and found 8 detections, but with low significance (<2$\sigma$); the authors of [24] observed 34 CSPNe, with a detection rate 32 ± 16% (52 ± 24%) in the I-band (J-band).

JWST can measure IR-excesses in the mid-IR with great precision, and can thus, compared to ground-based observations, find lower-mass companions in CSPNe and with a much greater success rate. For example, using MIRI imaging with different filters in the 5.6–28 $\mathsf{\mu }$m range and a very modest integration time of 1000 s (that provides a $10\sigma$ sensitivity of 0.63 (2.2) $\mathsf{\mu }$Jy at 5.6 $\mathsf{\mu }$m (10 $\mathsf{\mu }$m)10), and/or its low- and medium spectral resolution spectrographs, we can detect companions with masses as low as $0.2$ $M_{\odot }$, in CSPNe that are still very luminous (e.g., $L\sim {10}^{3.4-3.78}$ $L_{\odot }$, i.e., when they are at/near the knee of the WD cooling tracks [25]).

Table 1. Mid-infrared (IR) excess of a main-sequence (MS) companion ${}^1$ in a central star of PNe (CSPN).

WD	${\mathit{M}}_{\mathit{f}}{}^2=0.54$${\mathit{M}}_{\odot }$	${\mathit{T}}_{\mathit{eff}}={10}^5$ K	$\mathit{L}={10}^{3.4 }$${\mathit{L}}_{\odot }$
$\mathit{\lambda }$ ($\mathsf{\mu }$m)	$\mathit{R}$ (ms/wd)	F${}_{\mathit{WD}}$ ($\mathsf{\mu }$Jy)	F${}_{\mathit{MS}}$ ($\mathsf{\mu }$Jy)
0.8	0.0017	1938	3.2
5.6	0.12	42.8	5.1
10	0.155	13.4	2.1
WD	${\mathit{M}}_{\mathit{f}}{}^{\mathbf{2}}=\mathbf{0}.\mathbf{58}$${\mathit{M}}_{\odot }$	${\mathit{T}}_{\mathit{eff}}={\mathbf{10}}^{\mathbf{4}.\mathbf{7}}$ K	$\mathit{L}={\mathbf{10}}^{\mathbf{3}.\mathbf{78} }$${\mathit{L}}_{\odot }$
$\mathit{\lambda }$ ($\mathsf{\mu }$m)	$\mathit{R}$ (ms/wd)	F${}_{\mathit{WD}}$ ($\mathsf{\mu }$Jy)	F${}_{\mathit{MS}}$ ($\mathsf{\mu }$Jy)
0.8	$9.4\times {10}^{-5}$	33,880	3.2
5.6	0.0063	810	5.1
10	0.0082	257	2.1
WD	${\mathit{M}}_{\mathit{f}}{}^{\mathbf{2}}=\mathbf{0}.\mathbf{58}$${\mathit{M}}_{\odot }$	${\mathit{T}}_{\mathit{eff}}={\mathbf{10}}^{\mathbf{5}.\mathbf{2}}$ K	$L={10}^{3.45 }$$L_{\odot }$
$\mathit{\lambda }$ ($\mathsf{\mu }$m)	$\mathit{R}$ (ms/wd)	F${}_{\mathit{WD}}$ ($\mathsf{\mu }$Jy)	F${}_{\mathit{MS}}$ ($\mathsf{\mu }$Jy)
0.8	0.006	549	3.2
5.6	0.44	11.7	5.1
10	0.56	3.7	2.1

${}^1$ Spectral type M5V, mass $0.2$ $M_{\odot }$, $T_{eff}=2800$ K, $L={10}^{-2.1}$ $L_{\odot }$. ${}^2$ post-AGB mass of the CSPN.

shows the companion to primary CSPN flux ratio, $R(ms/wd)$, at $\lambda$ = 0.8, 5.6 and 10 $\mathsf{\mu }$m for 3 CSPNe at a distance of 3 kpc, spanning a representative range of effective temperatures (e.g., [26] find, for their list of 30 CSPNe in the LMC, that the majority have $T_{eff}$ values of ${10}^{4.8-5}$ K, whereas about 20% have $T_{eff}>{10}^5$ K). Including the source noise for the brightest WD at 10 $\mathsf{\mu }$m in the above table ($T_{eff}={10}^{4.7}$ K), we find, using the JWST Exposure Time Calculator11, that we can detect the companion with $7\sigma$ sensitivity. The strong increase in $R(ms/wd)$ with wavelength clearly demonstrates the advantage of going to longer wavelengths for detecting the companion (the fluxes have been computed using black-body approximations for stellar spectral energy distributions in the mid-IR range.) An additional important advantage of working in the mid-IR range compared to the near-IR, is the significantly smaller effect of nebular/interstellar reddening.

Mid-IR excesses towards central stars may also arise from dusty disk structures, in addition to those due to the presence of a low-mass companion. Small disks (<0.01 AU), likely resulting from tidally disrupted asteroids, have been inferred from mid-IR excesses around many old, cool (<24,000 K) WDs (e.g., [27,28,29]). Larger disks (∼40–100 AU, i.e., Kuiper Belt size), perhaps resulting from collisions amongst dynamically rejuvenated Kuiper Belt Objects, have been found around central WD in the Helix Nebula, from Spitzer 24 and 70 $\mathsf{\mu }$m observations [30]. Spitzer observations of PNe ([31,32]) have revealed that 24/84 CSPNe visible in IRAC/MIPS images have IR-excesses (Figure 2). These excesses have been inferred to result from a (i) companion (2 objects), (ii) free-free and line emission from winds in [WC]-type central stars (7 objects), and (iii) dusty disk (13 objects). JWST, with a 7X smaller PSF and $\stackrel{>}{\sim }$50X higher sensitivity than Spitzer, will reduce nebular contamination and significantly increase the sample size that can be studied. By measuring the spectral-energy-distribution (SED) using MIRI imaging photometry and spectroscopy, together with NIRCam/NIRSpec observations in the near-IR, we will be able to search for the presence of both companions and disks. A strong correlation between the presence of companions and disks would imply that disk formation is related to the presence of the companion.

An exciting new evolutionary channel for producing PPNe or dpAGB objects that requires binary interaction, but which occurs when the primary star is still on the Red Giant Branch (or RGB) has been highlighted in an ALMA study of the PPN, the Boomerang Nebula ([33] (Setal17)). The mass-loss that stripped the primary star of its envelope was driven at an extreme rate (∼10${}^{-3}$ $M_{\odot }$ yr${}^{-1}$) and expansion velocity (165 km s${}^{-1}$), much higher than found for any AGB star ([34]), and over a relatively short period (3500 yr), but the luminosity of the Boomerang’s central star is much lower (L ∼300 $L_{\odot }$) than possible for a post-AGB object. Setal17 found that the kinetic energy of the ejecta is extremely high, >$4.8\times {10}^{47}$ erg; thus mass-loss cannot be the result of the radiatively-driven dusty wind that characterizes the AGB phase, but is a result of a common-envelope (CE) ejection event which occurred when the primary was an RGB or early-AGB (E-AGB) star, thus explaining the low luminosity of the Boomerang’s central star. A class of post-RGB objects similar to the Boomerang (i.e., with $L\lesssim 1000$ $L_{\odot }$) have recently been identified in the Large and Small Magellanic Clouds (LMC and SMC), using optical spectroscopy and Spitzer12 photometry ([36,37,38]) photometry ([36,37,38]). Like the galactic post-AGB objects (i.e., PPNe and dpAGB), these post-RGB objects can be divided into shell-prominent sources (similar to PPNe) and disk-prominent sources (similar to dpAGB objects). Existing theoretical models of strong binary interactions for producing post-RGB objects such as CEE or GEE are still in their infancy (e.g., [39,40,41]), and one of the main reasons is the lack of observational data on the ejecta resulting from these interactions. Important diagnostics include the mass of material ejected in the equatorial plane (forming a disk), compared to that in a shell, and the chemical composition of the ejecta.

Using MIRI/MRS on JWST, we can obtain ∼5–28 $\mathsf{\mu }$m spectra of a select sample of post-RGB stars in the LMC and SMC, and thus determine the chemical composition of the dust in the post-RGB objects. Post-RGB objects are expected to show O-rich chemistry, because He-shell burning and the third dredge-up (responsible for making C-rich stars) have supposedly not occured in RGB stars. However, recently, [42], from an analysis of FORCAST/GRISM spectra of a sample of RV Tauri stars (these belong to the class of dpAGB objects), find both carbon-rich and oxygen-rich dust in all objects in their sample, and conclude that the most likely scenario for this is CEE—the dual-chemistry mineralogy has been produced in a CE environment. The MIRI spectra will also constrain the abundance of crystalline silicates—these are generally lacking in the sample studies by [42], except for AC Her, where these have a relatively high abundance [43]. These differences may be related to the specific evolutionary channel (e.g., AC Her is believed to be a product of Roche-lobe overflow). It will be interesting to compare the chemical composition of dust in the post-RGB with post-AGB objects, since the bulk of the material in the extended (“shell”) component in a post-AGB object is the result of a slow wind that operates over $\sim {10}^5$ yr, and the dust formation conditions are very different than for post-RGB objects, where the CEE produces an outflow over very short timescales (few ∼$\times {10}^3$ yr).

Using a simple 1D modeling approach, with a maximum of two components (as in [44])—an extended shell and a compact inner component13—SS20 have fitted the broad-band photometry of a small sample of post-RGB stars and find that rough constraints can be set on the physical properties of the emitting component(s) (e.g., temperature distribution, dust masses, and the sizes of components that contain the emitting masses) (Figure 3). We have also made a preliminary 2D model with astronomical silicate grains of sizes 0.01–0.3 $\mathsf{\mu }$m to fit the broad-band photometry of the Boomerang, and found that at 10–25 $\mathsf{\mu }$m, more than 99% of the total flux comes from an inner region $2.{}^{″}5$ in diameter, consistent with the imaging data, which shows a warm, bright compact central disk structure (Setal17). These studies demonstrate that the MRS 5–28 $\mathsf{\mu }$m spectrum (together with broad-band photometry, as necessary) will provide specific information about the dust composition, and enable one to constrain 2D models.

3.2. Imaging Faint and Obscured Circumstellar Structures in Post-AGB Objects

JWST’s coronagraphic capabilities with NIRCam and MIRI, due to their relatively large IWAs, limit how close one can probe circumstellar matter around a bright central star of a PPN or a PN. However, Aperture Masking Interferometry (AMI) using NIRISS ([46]), in which most of the primary aperture is masked, leaving a set of holes that produce a set of 21 non-redundant baselines, can be used to detect faint structures of simple geometry close to bright sources—the resulting IWA is $0.{}^{″}054-0.{}^{″}094$. Since this mode is designed to detect the thermal emission of young massive planets around bright stars, it is potentially sensitive to detecting expected faint structures (e.g., equatorial rings, disks) as well as unexpected ones, around the central stars of PPNe and PNe—simulations by [47] demonstrate NIRISS’s capability for various sky geometries (Figure 4).

Bipolar and multipolar PPNe show extended lobe structures and harbor dense, dusty, equatorial disks or torii. Their properties (physical structure, mass, kinematics) are intimately linked to the physical process(es) that terminate AGB evolution. The dusty disks/torii are generally optically thick at optical and near-IR wavelengths, and at least for a few objects with very high mass-loss rates, even the lobes can be optically thick at $\lambda \stackrel{<}{\sim }2$ $\mathsf{\mu }$m—observations at ∼10 $\mathsf{\mu }$m are necessary to view the critical structures hidden within the lobes.

For example, in the bipolar “water-fountain” PPN, IRAS 16342−3814, the lobes, which showed smoothly varying brightness distributions in HST optical images, show a limb-brightened appearance in Keck/AO images at $\lambda$∼1.6–3.8 $\mathsf{\mu }$m (Figure 5), and a remarkable corkscrew structure inscribed on the lobe walls is seen in the 3.8 $\mathsf{\mu }$m image [48]. The corkscrew structure has been interpreted to arise from the presence of a precessing jet, actively sculpting the bipolar lobe [48]. JWST’s stable PSF offers a major advantage over current ground-based AO systems in imaging such structures. High signal-to-noise ratio images obtained at >7.7 $\mathsf{\mu }$m using a dither-pattern, where JWST has an oversampled PSF, will allow for robust deconvolution and thus an improvement in the angular resolution beyond the diffraction limit (“super-resolution”) for high S/N images. Ground based mid-infrared surveys of pAGB objects have already revealed the potential for such deconvolution (e.g., [49]). ALMA (interferometric) imaging reveals two collimated high-speed molecular outflows in IRAS 16342-3814, as well as an equatorial expanding ring [50]. Similar ring structures have been found in other objects as well, for example, in the well-studied Butterfly Nebula, M 2-9, [51] find two equatorial rings of cold molecular gas formed during short mass-loss episodes (∼40 yr), separated by ∼500 yr, inferred to result from the presence of a close, low-mass (∼0.1–0.2 $M_{\odot })$ binary companion in a ∼90 yr period orbit.

JWST imaging can reveal the dusty counterparts of such structures with high-fidelity for a select sample of PPNe. It may also find new structures not seen at other wavelengths. For example, Reference [52] find a pair of beautiful dusty rings surrounding the irregular PN, NGC 1514 in a 22 $\mathsf{\mu }$m (WISE) image of this object (Figure 6). Such rings or other extended cool, dusty, structures probably exist around other PNe as well. It is notable that no theory exists as yet to explain these ring structures!

Imaging of a sample of PPNe and PNe with MIRI would thus allow us to better probe the mass-ejection processes that terminate AGB evolution and lead to the dramatic change in morphology that these objects represent. A caveat to such observations is saturation, thus targets have to be chosen carefully, with the aid of dust radiative-transfer modeling that can provide surface brightness estimates. Of course, some objects may contain very small, clumpy regions of high surface brightness (i.e., microstructure) that cannot be predicted a priori (e.g., see HST image of the PN IC 418 (PK 258-00#1) ([9]), and it is possible that these will be partially saturated in the MIRI images.

3.3. The Composition and Evolution of Dust in Galaxies

It is becoming increasingly important to understand the effect of metallicity on the physics and chemistry of the constituent stars and the ISM of galaxies. Different metallicities can result in different compositions for dust (and molecules), changing the spectra of stars and the ISM as well as those of the integrated light of galaxies. Low-metallicity environments are of special interest, as they provide a proxy for the ISM of galaxies in earlier epochs of the Universe. The formation, composition and evolution of dust features has been studied in galactic and extragalactic environments (LMC, SMC) (e.g., [53,54,55,56,57]). Amorphous carbon is thought to be the major constituent of circumstellar dust in carbon-rich stars on the AGB14—these stars are the major contributor of such dust to the ISM. When such stars evolve to the PN phase, we find evidence for the presence of complex organic dust (both aromatic and aliphatic), which is likely produced during the pre-PN phases ([58]) (Figure 7). A well-known series of infrared (IR) emission features at 3.29, 6.2, 7.7–7.9, 8.6, 11.3, and 12.7 $\mathsf{\mu }$m dominate the dust emission, and are believed to be produced by PAHs (Figure 7). Many of the other mid-IR features have been identified with different kinds of dust such as fullerenes, SiC, and MgS (Figure 7). Oxygen-rich objects show the presence of amorphous and crystalline silicates and ices. Some prominent narrow forbidden lines from ionized gas are also seen in the mid-IR spectra of PNe. With MIRI, one can carry out (a) more extensive spectroscopic surveys in the LMC and SMC than was possible with Spitzer, and (b) new spectroscopic surveys in nearby, metal-poor galaxies such as the Fornax dwarf spheroidal at 140 pc, of pAGB objects (PPNe, dpAGB and PNe).

PAHS and fullerenes (C60, C70), which were first detected in the mid-IR spectra of PNe ([53,59]), are two very important and widespread constituents of the ISM. PAHs play a key role in the heating, chemistry and ionization of the interstellar gas [60]. The recent landmark association of fullerenes with several diffuse interstellar bands (DIBs, e.g., [61,62]) has provided the first definitive identification of a DIB carrier (a long-standing problem in astrophysics). In a recent study, the authors of [63] suggest that fullerenes may form in a top-down scenario, i.e., along a pathway in which larger molecules (with more than 60 C-atoms, e.g., large PAHs such as C${}_{66}$H${}_{48}^{+}$) transfer into smaller derivatives (including fullerenes) due to UV irradiation and a substantial number of the 500+ unassigned DIBs could be due to these derivatives.

In order to understand the formation of PAHs and fullerenes, it is necessary to map the spectral features from these species at high spatial resolution in PNe, since for these objects, the radiation field and density structure are well-defined. For fullerenes, Spitzer observations have provided a sample of galactic PNe where these features been found [64], such as Tc 1 [53]. In this object, Gemini observations at 10 $\mathsf{\mu }$m already show that the ionized gas, the continuum-emitting dust, and the fullerenes have spatially distinct locations (Figure 8). MIRI/NIRSpec IFU mosaics can trace the emission from the central star to the fullerene-emitting region (a ring in Tc 1) and complementary MIRI images can sample the entire PN. The JWST IFUs unique combination of angular resolution, sensitivity and spectral coverage will allow one to pinpoint the spatial distribution of fullerenes in relation to other species (e.g., PAHs, species responsible for mid-IR plateaus), and derive the local physical conditions (gas temperature and density), enabling tests of proposed fullerene formation mechanisms. For example, the MIRI and NIRSpec IFUs offer the prospect of resolving small-scale structures (such as disks) with potentially different dust emission features and trace the variations in these as a function of location in the nebulae—such data would then provide strong constraints on the formation and evolution of carriers of the different spectral components.

3.4. Study of the Physics and Chemistry of UV-Irradiated Molecular Clumps in PNe

PNe provide a well-defined environment for studying the astrophysical problem of the survival of molecules and small dust grains in clumpy UV-irradiated environments (such as the photodissociation regions of massive star forming regions and AGNs, [65]. In evolved PNe, molecules are found in dense knots located inside the ionized region. The knots in PNe have been revealed via high-angular resolution HST optical, and 2.12 $\mathsf{\mu }$m H${}_2$ imaging. Spitzer observations reveal that the PAH emission at 11.2 $\mathsf{\mu }$m in the well-studied PN, NGC 6720 (the Ring Nebula) is globally associated with H${}_2$ [66], but lack the spatial resolution to confirm whether the PAHs are directly associated with the knots, a necessary first step in testing PAH formation mechanisms. Observations of a few key PPNe with knotty structure such as NGC 6720, NGC 2440 and IC 4406 using MIRI IFU and NIRSpec IFU15 observations at selected positions in the nebulae—including those that focus on individual globules (few arc seconds in diameter) and others that probe the main PDR in these objects (Figure 9)—together with narrow-band imaging of the main nebular region using NIRCam and MIRI, will, for the first time, spatially resolve the globules in the mid-IR, and enable mapping of the ionized, neutral and molecular regions within and around the H${}_2$ globules. JWST will detect multiple emission features from both the irradiated and cooler sides of the globules, including H${}_2$ ro-vibrational lines and PAH emission (in contrast, molecular line observations with ALMA can only detect the coldest, most shielded components of the globules). Emission lines from species such as [S iii], [Fe iii], and [Ne iii,v] will be used to diagnose the physical parameters of (and abundances in) the ionized emitting gas surrounding the globules. The unprecedented sensitivity of JWST can potentially also result in detections of fine structure emission lines from trans-iron neutron-capture elements such as [Kr iii,v], [Se iii,iv], and [Cd iv] in the near-IR, unique probes of s-process nucleosynthesis during AGB evolution, which is responsible for the production of around half of the heavy elements beyond Fe in the cosmos [67].

4. Summary

Most stars in the Universe that leave the main sequence in a Hubble time will end their lives evolving through the Planetary Nebula (PN) evolutionary phase, and the accompanying heavy mass loss during their death throes is important across astrophysics—however the evolution from the red giant to the PN phase remains poorly understood. High-angular-resolution, high-sensitivity spectroscopic and imaging observations with JWST, especially at mid-IR wavelengths, will help in addressing the major unsolved problems in the study of PNe and their progenitors (post-AGB and post-RGB objects). Representative investigations that exploit JWST’s unique capabilities include searches for low-mass binary companions to, and/or disks around, the central stars of PNe, post-RGB and post-AGB stars, the imaging of faint and/or obscured circumstellar structures in post-AGB objects, and studies of the composition and evolution of dust in galaxies.