Elliptical Planetary Nebulae Research Articles

Planetary nebulae of the Large Magellanic Cloud I. A multiwavelength analysis

Planetary nebulae (PNe) have three main components: a central star (CS), ionized gas, and dust in the nebula. Each contains critical chemical fingerprints of the PN's evolution, which serve as tracers of the evolution, nucleosynthesis, and dust production that occurred during the preceding asymptotic giant branch (AGB) phase. We aim to build a bridge to link the PN phase to the evolution of progenitors, to better understand the dust production and mass-loss mechanism during the final AGB phase. Here we present a comprehensive study of nine Large Magellanic Cloud spherical or elliptical PNe whose observations from the UV through the IR are available in the literature. We characterize nebulae and CSs, finding information necessary to reconstruct the evolutionary history of mass-loss and dust production, such as as the amount of gas that makes up the nebula and the dust that surrounds the CS. We compared the observed energy distribution of the selected PNe to that obtained from photoionization modeling, taking the presence of dust into account. The physical and chemical parameters of the CSs were then compared with predictions from the evolutionary tracks. We characterize the source, assigning a progenitor, early-AGB mass to each CS. We estimate the mass of the nebula and the dust-to-gas ratio. For five objects, we find evidence for the presence of a near-IR bump, which would indicate the presence of hot dust.

The [S ii] Spectral Images of the Planetary Nebula NGC 7009. II. Major Axis

We derived position–velocity density distribution diagrams along the major (PA = 77°) axis of the elliptical planetary nebula NGC 7009 with the Keck HIRES [S ii] 6716/6731 Å doublet spectral images. The average densities of the main shell and knots of NGC 7009 derived from the [S ii] 6716/6731 Å fluxes integrated over the line of sight indicate a density range of N e = 103.4 to 103.9 cm−3, while the local densities from the volume fraction resolved in radial velocities along the line of sight show a considerably large range of about 102.8–104.7 cm−3. The derived projection angle of the major axis of the main shell is ψ ∼ 18.3° (±2°). Assuming that the main shell is an ellipsoidal shell with a ≃ 16″ and b ≃ 6″, we found the range of the expansion velocity, radius, and latitude of four knots and a hot bubble. The four knots at points in symmetrical positions are roughly aligned with the same axis of expansion of latitudes ϕ ∼ ±34.5° (±0.6°): one pair expands at about 35 km s−1 close to the main ellipsoidal shell, and the other expands rapidly at about 60 km s−1 at a distance of r ∼ 16″. In the latitude range ϕ = 65°–75°, the hot bubble of a relatively large structure expands rapidly with a velocity of 130–150 km s−1. The four knots and hot bubble points, which expand faster than the main shell, appear to have been formed by two to three eruptions at a different epoch from the primary structure formation.

Common-envelope shaping of planetary nebulae – IV. From protoplanetary to planetary nebula

ABSTRACT We present 2D hydrodynamical simulations of the transition of a protoplanetary nebula (PPN) to a planetary nebula for central stars in binary systems that have undergone a common-envelope event. After 1000 yr of magnetically driven dynamics (PPN phase), a line-driven stellar wind is introduced into the computational domain and the expansion of the nebula is simulated for another 10 000 yr, including the effects of stellar photoionization. In this study we consider central stars with main sequence (final) masses of 1 (0.569) and 2.5 (0.677) M⊙, together with a 0.6-M⊙ main-sequence companion. Extremely bipolar, narrow-waisted PPNe result in bipolar planetary nebulae, while the rest of the shapes mainly evolve into elliptical planetary nebulae. The initial magnetic field’s effects on the collimated structures, such as jets, tend to disappear in most of the cases, leaving behind the remnants of those features in only a few cases. Equatorial zones fragmented mainly by photoionization (1-M⊙progenitors), result in ‘necklace’ structures made of cometary clumps aligned with the radiation field. On the other hand, fragmentation by photoionization and shocked wind (2.5-M⊙progenitors) give rise to the formation of multiple clumps in the latitudinal direction, which remain within the lobes, close to the center, which are immersed and surrounded by hot shocked gas, not necessarily aligned with the radiation field. These results reveal that the fragmentation process has a dependence on the stellar-mass progenitor. This fragmentation is made possible by the distribution of gas in the previous post-common-envelope PPN as sculpted by the action of the jets.

Double Shells of the Planetary Nebula NGC 7009 Minor Axis

We analyzed the minor-axis spectra of the elliptical planetary nebula (PN) NGC 7009 observed with Keck HIRES with a 0.″862 × 10″ slit placed at about 7.″5 and 10″ away from the center and a 0.″862 × 14″ slit at the center. The mean densities derived from the integrated [S ii] 6716/6731 Å fluxes along the Keck HIRES slit length indicate a density range of 103.7–104.1 cm−3, while the local densities derived from the slit spectral images show a large local density variation of about 102.8–104.6 cm−3: local densities vary more substantially than values integrated over the line of sight. The expansion rates of the main and outer shells obtained by [S ii] are about 21.7 and 30.0 km s−1, respectively. The kinematic results of the [S ii] spectral lines correspond to the outermost regions of the two shells and are not representative of the whole PN but are closely related to the other emission lines observed in the shell gas. We conclude that the density contrast leads to the formation of the inner shell, while the change in ionization state leads to the formation of the outer shell. We suggest that the inner main and outer shells result from two successive major ejections. The physical conditions of the central star must have been different when these shells first formed.

Rectangular core-collapse supernova remnants: application to Puppis A

ABSTRACT Core-collapse supernova remnants are the gaseous nebulae of galactic interstellar media (ISM) formed after the explosive death of massive stars. Their morphology and emission properties depend both on the surrounding circumstellar structure shaped by the stellar wind–ISM interaction of the progenitor star and on the local conditions of the ambient medium. In the warm phase of the Galactic plane ($n\approx 1\, \rm cm^{-3}$, $T\approx 8000\, \rm K$), an organized magnetic field of strength $7\, \mu \rm G$ has profound consequences on the morphology of the wind bubble of massive stars at rest. In this paper, we show through 2.5D magnetohydrodynamical simulations, in the context of a Wolf–Rayet-evolving $35\, \rm M_{\odot }$ star, that it affects the development of its supernova remnant. When the supernova remnant reaches its middle age ($15\!-\!20\, \rm kyr$), it adopts a tubular shape that results from the interaction between the isotropic supernova ejecta and the anisotropic, magnetized, shocked stellar progenitor bubble into which the supernova blast wave expands. Our calculations for non-thermal emission, i.e. radio synchrotron and inverse-Compton radiation, reveal that such supernova remnants can, due to projection effects, appear as rectangular objects in certain cases. This mechanism for shaping a supernova remnant is similar to the bipolar and elliptical planetary nebula production by wind–wind interaction in the low-mass regime of stellar evolution. If such a rectangular core-collapse supernova remnant is created, the progenitor star must not have been a runaway star. We propose that such a mechanism is at work in the shaping of the asymmetric core-collapse supernova remnant Puppis A.

Fallback in bipolar planetary nebulae?

The subclass of bipolar Planetary Nebulae (PNe) exhibits well-defined low-power outflows and some shows shock-related equatorial spiderweb structures and hourglass structures surrounding these outflows. These structures are distinctly different from the phenomena associated with spherical and elliptical PNe and suggest a non-standard way to simultaneously energise both kinds of structures. This paper presents evidence from the published literature on bipolar PN Hb 12 and other sources in support of an alternative scenario for energising these structures by means of accretion from material shells deposited during earlier post-AGB and pre-PNe evolutionary stages. In addition to energising the bipolar outflow, a sub-Eddington accretion scenario could hydrodynamically explain the spiderweb and outer hourglass structures as oblique shockwaves for guiding the accreting material into the equatorial region of the source. Estimates of the accretion rate resulting from fallback-related spherical accretion could indeed help to drive a low-power outflow and contribute to the total luminosity of these sources.

The future influence of six exoplanets on the envelope properties of their parent stars on the giant branches

ABSTRACT We study the evolution of six exoplanetary systems with the stellar evolutionary code Modules for Experiments in Stellar Astrophysics(mesa) and conclude that they will likely spin-up the envelope of their parent stars on the red giant branch (RGB) or later on the asymptotic giant branch (AGB) to the degree that the mass-loss process might become non-spherical. We choose six observed exoplanetary systems where the semimajor axis is $a_i \simeq 1\small{-}2 {~\rm au}$, and use the binary mode of mesa to follow the evolution of the systems. In four systems, the star engulfs the planet on the RGB, and in two systems on the AGB, and the systems enter a common envelope evolution (CEE). In two systems where the exoplanet masses are Mp ≃ 10MJ, where MJ is Jupiter mass, the planet spins up the envelope to about $10{{\ \rm per\ cent}}$ of the break-up velocity. Such envelopes are likely to have significant non-spherical mass-loss geometry. In the other four systems where Mp ≃ MJ the planet spins up the envelope to values of $1{-}2 {{\ \rm per\ cent}}$ of break-up velocity. Magnetic activity in the envelope that influences dust formation might lead to a small departure from spherical mass-loss even in these cases. In the two cases of CEE on the AGB, the planet deposits energy to the envelope that amounts to ${\gt}10{{\ \rm per\ cent}}$ of the envelope binding energy. We expect this to cause a non-spherical mass-loss that will shape an elliptical planetary nebula in each case.

On the role of reduced wind mass-loss rate in enabling exoplanets to shape planetary nebulae

ABSTRACT We use the stellar evolution code MESA–binary and follow the evolution of three exoplanets and two brown dwarfs (BDs) to determine their potential role in the future evolution of their parent star on the red giant branch (RGB) and on the asymptotic giant branch (AGB). We limit this study to exoplanets and BDs with orbits that have semimajor axis of $1 {~\rm au}\lesssim a_0 \lesssim 20 {~\rm au}$, a high eccentricity, $e_0 \gtrsim 0.25$, and having a parent star of mass M*,0 ≥ 1 M⊙. We find that the star HIP 75 458 will engulf its planet HIP 75 458b during its RGB phase. The planet will remove the envelope and terminate the RGB evolution, leaving a bare helium core of mass 0.4 M⊙ that will evolve to form a helium white dwarf. Only in one system out of five, the planet beta Pic c will enter the envelope of its parent star during the AGB phase. For that to occur, we have to reduce the wind mass-loss rate by a factor of about four from its commonly used value. This strengthens an early conclusion, which was based on exoplanets with circular orbits, which states that to have a non-negligible fraction of AGB stars that engulf planets we should consider lower wind mass-loss rates of isolated AGB stars (before they are spun-up by a companion). Such an engulfed planet might lead to the shaping of the AGB mass-loss geometry to form an elliptical planetary nebula.

A Preferred Orientation Angle for Bipolar Planetary Nebulae

We present preliminary results from measuring Galactic orientation angles of 766 elliptical and bipolar Planetary Nebulae (PNe) in the Hong Kong/Australian Astronomical Observatory/Strasbourg Observatory H-alpha Planetary Nebula research platform and database (HASH DB). For elliptical PNe the distribution of orientation angles is found to be more or less uniform. However, for bipolar PNe there is statistically significant evidence for preferred orientation angles (as tentatively reported before with smaller samples) across the whole Galaxy.

Jsolated Stars of Low Metallicity

We study the effects of a reduced mass-loss rate on the evolution of low metallicity Jsolated stars, following our earlier classification for angular momentum (J) isolated stars. By using the stellar evolution code MESA we study the evolution with different mass-loss rate efficiencies for stars with low metallicities of Z = 0.001 and Z = 0.004 , and compare with the evolution with solar metallicity, Z = 0.02 . We further study the possibility for late asymptomatic giant branch (AGB)—planet interaction and its possible effects on the properties of the planetary nebula (PN). We find for all metallicities that only with a reduced mass-loss rate an interaction with a low mass companion might take place during the AGB phase of the star. The interaction will most likely shape an elliptical PN. The maximum post-AGB luminosities obtained, both for solar metallicity and low metallicities, reach high values corresponding to the enigmatic finding of the PN luminosity function.

Planets, Planetary Nebulae, and Intermediate Luminosity Optical Transients (ILOTs)

I review some aspects related to the influence of planets on the evolution of stars before and beyond the main sequence. Some processes include the tidal destruction of a planet on to a very young main sequence star, on to a low-mass main sequence star, and on to a brown dwarf. This process releases gravitational energy that might be observed as a faint intermediate luminosity optical transient (ILOT) event. I then summarize the view that some elliptical planetary nebulae are shaped by planets. When the planet interacts with a low-mass, upper asymptotic giant branch (AGB) star, it both enhances the mass-loss rate and shapes the wind to form an elliptical planetary nebula, mainly by spinning up the envelope and by exciting waves in the envelope. If no interaction with a companion, stellar or substellar, takes place beyond the main sequence, the star is termed a Jsolated star, and its mass-loss rates on the giant branches are likely to be much lower than what is traditionally assumed.

Accounting for planet-shaped planetary nebulae

By following the evolution of several observed exoplanetary systems we show that by lowering the mass loss rate of single solar-like stars during their two giant branches, these stars will swallow their planets at the tip of their asymptotic giant branch (AGB) phase. This will most likely lead the stars to form elliptical planetary nebulae (PNe). Under the traditional mass loss rate these stars will hardly form observable PNe. Stars with a lower mass loss rate as we propose, about 15 per cent of the traditional mass loss rate of single stars, leave the AGB with much higher luminosities than what traditional evolution produces. Hence, the assumed lower mass loss rate might also account for the presence of bright PNe in old stellar populations. We present the evolution of four exoplanetary systems that represent stellar masses in the range of 0.9-1.3 Mo. The justification for this low mass loss rate is our assumption that the stellar samples that were used to derive the traditional average single-star mass loss rate were contaminated by stars that suffer binary interaction.

H2in low-ionization structures of planetary nebulae

We report the detection of near-IR H$_2$ emission from the low-ionization structures (knots) in two planetary nebulae. The deepest ever, high-angular resolution H$_2$ 1-0 S(1) at 2.122$\mu$, H$_2$ 2-1 S(1) at 2.248$\mu$ and Br$\gamma$ images of K 4-47 and NGC 7662, obtained using the Near InfraRed Imager and Spectrometer (NIRI) at Gemini-North, are analyzed here. K 4-47 reveals a remarkable highly collimated bipolar structure not only in the optical but also in the molecular hydrogen emission. The H$_2$ emission emanates from the walls of the bipolar outflows and also from the pair of knots at the tip of the outflows. The H$_2$ 1-0 S(1)/2-1 S(1) line ratio ranges from ~7 to ~10 suggesting the presence of shock interactions. Our findings can be explained by the interaction of a jet/bullet ejected from the central star with the surrounding asymptotic giant branch material. The strongest H$_2$ line, v=1-0 S(1) is also detected in several low-ionization knots located at the periphery of the elliptical planetary nebula NGC 7662, but only four of these knots are detected in the H$_2$ v=2-1 S(1) line. These four knots exhibit an H$_2$ line ratio between 2 and 3.5, which suggests that the emission is caused by the UV ionizing flux of the central star. Our data confirms the presence of H$_2$ gas in both fast- and slow-moving low-ionization knots, which has only been confirmed before in the nearby Helix nebula and Hu 1-2. Overall, the low-ionization structures of planetary nebulae are found to share similar traits to photodissociation regions.

Modelling SNR G1.9+0.3 as a Supernova inside a Planetary Nebula

Using 3D numerical hydrodynamical simulations we show that a Type Ia supernova (SN Ia) explosion inside a planetary nebula (PN) can explain the observed shape of the G1.9+0.3 supernova remnant (SNR) and its X-ray morphology. The SNR G1.9+0.3 morphology can be generally described as a sphere with two small and incomplete lobes protruding on opposite sides of the SNR, termed ‘ears’, a structure resembling many elliptical PNe. Observations show the synchrotron X-ray emission to be much stronger inside the two ears than in the rest of the SNR. We numerically show that a spherical SN Ia explosion into a circumstellar matter (CSM) with the structure of an elliptical PN with ears and clumps embedded in the ears can explain the X-ray properties of SNR G1.9+0.3. While the ejecta has already collided with the PN shell in most of the SNR and its forward shock has been slowed down, the ejecta is still advancing inside the ears. The fast forward shock inside the ears explains the stronger X-ray emission there. SN Ia inside PNe (SNIPs) seem to comprise a non-negligible fraction of resolved SN Ia remnants.

Weak magnetic fields in central stars of planetary nebulae?

It is not yet clear whether magnetic fields play an essential role in shaping planetary nebulae (PNe), or whether stellar rotation alone and/or a close binary companion can account for the variety of the observed nebular morphologies. In a quest for empirical evidence verifying or disproving the role of magnetic fields in shaping PNe, we follow up on previous attempts to measure the magnetic field in a representative sample of PN central stars. We obtained low-resolution polarimetric spectra with FORS 2 at VLT for a sample of twelve bright central stars of PNe with different morphology, including two round nebulae, seven elliptical nebulae, and three bipolar nebulae. Two targets are Wolf-Rayet type central stars. For the majority of the observed central stars, we do not find any significant evidence for the existence of surface magnetic fields. However, our measurements may indicate the presence of weak mean longitudinal magnetic fields of the order of 100 Gauss in the central star of the young elliptical planetary nebula IC 418, as well as in the Wolf-Rayet type central star of the bipolar nebula Hen2-113 and the weak emission line central star of the elliptical nebula Hen2-131. A clear detection of a 250 G mean longitudinal field is achieved for the A-type companion of the central star of NGC 1514. Some of the central stars show a moderate night-to-night spectrum variability, which may be the signature of a variable stellar wind and/or rotational modulation due to magnetic features. We conclude that strong magnetic fields of the order of kG are not widespread among PNe central stars. Nevertheless, simple estimates based on a theoretical model of magnetized wind bubbles suggest that even weak magnetic fields below the current detection limit of the order of 100 G may well be sufficient to contribute to the shaping of PNe throughout their evolution.

A DETAILED MORPHO-KINEMATIC MODEL OF THE ESKIMO, NGC 2392: A UNIFYING VIEW WITH THE CAT'S EYE AND SATURN PLANETARY NEBULAE

The 3-D and kinematic structure of the Eskimo nebula, NGC 2392, has been notoriously difficult to interpret in detail given its complex morphology, multiple kinematic components and its nearly pole-on orientation along the line of sight. We present a comprehensive, spatially resolved, high resolution, long-slit spectroscopic mapping of the Eskimo planetary nebula. The data consist of 21 spatially resolved, long-slit echelle spectra tightly spaced over the Eskimo and along its bipolar jets. This data set allows us to construct a velocity-resolved [NII] channel map of the nebula with a resolution of 10 km/s that disentangles the different kinematic components of the nebula. The spectroscopic information is combined with HST images to construct a detailed three dimensional morpho-kinematic model of the Eskimo using the code SHAPE. With this model we demonstrate that the Eskimo is a close analog to the Saturn and the Cat's Eye nebulae, but rotated 90 degrees to the line of sight. Furthermore, we show that the main characteristics of our model apply to the general properties of the group of elliptical planetary nebulae with ansae or FLIERS, once the orientation is considered. We conclude that these kind of nebulae belongs to a class with a complex common evolutionary sequence of events.

A Cat's Eye view of the Eskimo from Saturn

AbstractThe 3-D and kinematic structure of the Eskimo nebula, NGC 2392, has been notoriously difficult to interpret given its complex morphology, multiple kinematic components and its nearly pole-on orientation along the line of sight. Here we present the most comprehensive high resolution spectroscopic mapping of the Eskimo planetary nebula to date. The data consist of 21 spatially resolved, long-slit echelle spectra tightly spaced over the Eskimo and along its bipolar jets. This data set allowed us to construct a velocity-resolved [NII] channel map of the nebula with a resolution of 10 km/s that disentagles the differente kinematic components of the nebula and reveals clearly for the first time the kinematic expansion pattern for each of the components. The spectroscopic information is combined with a HST image to construct the first detailed three dimensional model of the Eskimo with the code SHAPE. With this model we demostrate that the Eskimo is nearly a twin to the Saturn nebula, but rotated 90° to the line sight. Furthermore, we show that the main characteristics of our model apply to the general properties of the group of elliptical planetary nebulae with ansae, once the orientation is considered.

The Role of Planets in Shaping Planetary Nebulae

ABSTRACTIn 1997 Soker laid out a framework for understanding the formation and shaping of planetary nebulae (PN). Starting from the assumption that nonspherical PN cannot be formed by single stars, he linked PN morphologies to the binary mechanisms that may have formed them, basing these connections almost entirely on observational arguments. In light of the last decade of discovery in the field of PN, we revise this framework, which, although simplistic, can still serve as a benchmark against which to test theories of PN origin and shaping. Within the framework, we revisit the role of planets in shaping PN. Soker invoked a planetary role in shaping PN because there are not enough close binaries to shape the large fraction of nonspherical PN. In this article we adopt a model whereby only ∼20% of all 1–8 M⊙ stars make a PN. This reduces the need for planetary shaping. Through a propagation of percentages argument, and starting from the assumption that planets can only shape mildly elliptical PN, we conclude that ∼20% of all PN were shaped via planetary and other substellar interactions, but we add that this corresponds to only ∼5% of all 1–8 M⊙ stars. This may be in line with findings of planets around main-sequence stars. PN shaping by planets is made plausible by the recent discovery of planets that have survived interactions with red giant branch (RGB) stars. Finally, we conclude that of the ∼80% of 1–8 M⊙ stars that do not make a PN, about one-quarter do not even ascend the AGB due to interactions with stellar and substellar companions, while three-quarters ascend the AGB but do not make a PN. Once these stars leave the AGB they evolve normally and can be confused with post-RGB, extreme horizontal branch stars. We propose tests to identify them.

Mapping and spectroscopy of the planetary nebula NGC 7009 in the visual and infrared

NGC 7009 is a fascinating example of a high-excitation, elliptical planetary nebula (PN) containing circumnebular rings, and FLIERs and jets along the major axis. We present visual spectroscopy along multiple position angles through the nucleus, taken with the Observatorio Astronomico Nacional (Mexico); mid-infrared (MIR) spectroscopy and imaging acquired using the Infrared Space Observatory (ISO) and Spitzer Space Telescope (SST), and narrow-band imaging obtained using the Hubble Space Telescope (HST). The data show that the mid-infrared (MIR) continuum is dominated by a broad ≈100 K continuum, and a strong excess attributable to crystalline silicate emission. The primary peaks in this excess are similar to those observed in Forsterite and clino- and ortho-enstatite. The MIR images, by contrast, appear to be dominated by ionic transitions, with the 8.991 μm transition of [Ar iii] being important in the 8.0-μm band. The morphology and size of the envelope are found to vary with wavelength, with the largest dimensions occurring at 8.0 μm – a trend which is also reflected in an increase in the 8.0 μm/4.5 μm and 5.8 μm/4.5 μm ratios with distance from the nucleus. The visual spectroscopy permits us to map density and temperature throughout the shell, and confirm that the lowest values of ne are located close to the ansae, where densities appear to be of the order of 900–2600 cm−3. We provide mean line intensities for 116 transitions in six regions of the shell, and use mapping to confirm a systematic increase in excitation in the outer portions of the envelope. We finally use the ground-based spectroscopy, and ratioing of HST images to investigate the presence of shocks in the ansae and interior envelope. It is concluded that line ratios in the ansae may be partially consistent with shock excitation, although these features are primarily dominated by photoionization. We also note evidence for shock excitation at the limits of the interior elliptical shell, and for multiple bow-shock structures centred upon the ansae. The orientations of the easterly bow-shocks may have varied over time, indicating precession of the collimating engine at a rate of deg yr−1, whilst the outward splaying of the westerly ‘jet’ appears consistent with shock refraction modelling. We finally note that HST observations of the halo rings show them to have widths of the order of ∼1–3 arcsec, and steep changes in surface brightness consistent with local shock activity.

High-Resolution Spectroscopy of the Planetary Nebulae PM 1–242, PM 1–318 and PM 1–322

AbstractWe have recently confirmed the planetary nebula (PN) nature of PM 1–242, PM 1–318 and PM 1–322. Here we present high-resolution long-slit spectra of these three PNe in order to analyze their internal kinematics and to investigate their physical structure. PM 1–242 is a tilted ring and not an elliptical PN as suggested by direct images. The object is probably related to ring-like PNe and shows an unusual pointsymmetric brightness distribution in the ring. PM 1–318 is a pole-on elliptical PN, instead of a circular one as suggested by direct images. PM 1–322 is spatially unresolved and its spectrum shows large differences between the forbidden lines and Hα profiles, with the latter showing a double-peaked profile and relatively extended wings (FWZI ~325 km s−1). These properties are found in other PNe that are suspected to host a symbiotic central star.