Planetary Nebula NGC Research Articles

Rings and arcs around evolved stars – II. The Carbon Star AFGL 3068 and the Planetary Nebulae NGC 6543, NGC 7009, and NGC 7027

ABSTRACT We present a detailed comparative study of the arcs and fragmented ring-like features in the haloes of the planetary nebulae (PNe) NGC 6543, NGC 7009, and NGC 7027 and the spiral pattern around the carbon star AFGL 3068 using high-quality multi-epoch HST images. This comparison allows us to investigate the connection and possible evolution between the regular patterns surrounding AGB stars and the irregular concentric patterns around PNe. The radial proper motion of these features, ≃15 km s−1, are found to be consistent with the AGB wind and their linear sizes and interlapse times (500–1900 yr) also agree with those found around AGB stars, suggesting a common origin. We find evidence using radiative-hydrodynamic simulations that regular patterns produced at the end of the AGB phase become highly distorted by their interactions with the expanding PN and the anisotropic illumination and ionization patterns caused by shadow instabilities. These processes will disrupt the regular (mostly spiral) patterns around AGB stars, plausibly becoming the arcs and fragmented rings observed in the haloes of PNe.

Detection of Vibrational Emissions from the Helium Hydride Ion (HeH+) in the Planetary Nebula NGC 7027

Abstract We report the detection of emission in the v = 1 − 0 P(1) (3.51629 μm) and P(2) (3.60776 μm) rovibrational lines of the helium hydride cation (HeH+) from the planetary nebula NGC 7027. These detections were obtained with the iSHELL spectrograph on NASA’s Infrared Telescope Facility on Maunakea. They confirm the discovery of HeH+ reported recently by Güsten et al., who used the GREAT instrument on the SOFIA airborne observatory to observe its pure rotational J = 1 − 0 transition at 149.137 μm. The flux measured for the HeH+ v = 1 − 0 P(1) line is in good agreement with our model for the formation, destruction, and excitation of HeH+ in NGC 7027. The measured strength of the J = 1 − 0 pure rotational line, however, exceeds the model prediction significantly, as does that of the v = 1 − 0 P(2) line, by factors of 2.9 and 2.3 respectively. Possible causes of these discrepancies are discussed. Our observations of NGC 7027, covering the 3.26–3.93 μm spectral region, have led to the detection of more than 60 spectral lines including nine rovibrational emissions from CH+. The latter are detected for the first time in an astronomical source.

Probing the size and charge of polycyclic aromatic hydrocarbons

ABSTRACT We present a new method to accurately describe the ionization fraction and the size distribution of polycyclic aromatic hydrocarbons (PAHs) within astrophysical sources. To this purpose, we have computed the mid-infrared emission spectra of 308 PAH molecules of varying sizes, symmetries, and compactness, generated in a range of radiation fields. We show that the intensity ratio of the solo CH out-of-plane bending mode in PAH cations and anions (referred to as the ‘11.0’ μm band, falling in the 11.0–11.3 μm region for cations and anions) to their 3.3 μm emission scales with PAH size, similarly to the scaling of the 11.2/3.3 ratio with the number of carbon atoms (NC) for neutral molecules. Among the different PAH emission bands, it is the 3.3 μm band intensity that has the strongest correlation with NC, and drives the reported PAH intensity ratio correlations with NC for both neutral and ionized PAHs. The 6.2/7.7 intensity ratio, previously adopted to track PAH size, shows no evident scaling with NC in our large sample. We define a new diagnostic grid space to probe PAH charge and size, using the (11.2 + 11.0)/7.7 and (11.2 + 11.0)/3.3 PAH intensity ratios, respectively. We demonstrate the application of the (11.2 + 11.0)/7.7–(11.2 + 11.0)/3.3 diagnostic grid for galaxies M82 and NGC 253, for the planetary nebula NGC 7027, and the reflection nebulae NGC 2023 and NGC 7023. Finally, we provide quantitative relations for PAH size determination depending on the ionization fraction of the PAHs and the radiation field they are exposed to.

H2 emission in the low-ionization structures of the planetary nebulae NGC 7009 and NGC 6543

ABSTRACT Despite the many studies in the last decades, the low-ionization structures (LISs) of planetary nebulae (PNe) still hold several mysteries. Recent imaging surveys have demonstrated that LISs are composed of molecular gas. Here, we report H2 emission in the LISs of NGC 7009 and NGC 6543 by means of very deep narrow-band H2 images taken with NIRI@Gemini. The surface brightness of the H2 1-0 S(1) line is estimated to be (0.46–2.9)× 10−4 erg s−1 cm−2 sr−1 in NGC 7009 and (0.29–0.48)× 10−4 erg s−1 cm−2 sr−1 in NGC 6543, with signal-to-noise ratios of 10–42 and 3–4, respectively. These findings provide further confirmation of hidden H2 gas in LISs. The emission is discussed in terms of the recent proposed diagnostic diagram R(H2) = H2 1-0 S(1)/H2 2-1 S(1) versus R(Brγ) = H2 1-0 S(1)/Brγ, which was suggested to trace the mechanism responsible for the H2 excitation. Comparing our observations to shock and ultraviolet (UV) molecular excitation models, as well as a number of observations compiled from the literature showed that we cannot conclude for either UV or shocks as the mechanism behind the molecular emission.

The MUSE view of the planetary nebula NGC 3132

Aims. Two-dimensional spectroscopic data for the whole extent of the NGC 3132 planetary nebula have been obtained. We deliver a reduced data-cube and high-quality maps on a spaxel-by-spaxel basis for the many emission lines falling within the Multi-Unit Spectroscopic Explorer (MUSE) spectral coverage over a range in surface brightness >1000. Physical diagnostics derived from the emission line images, opening up a variety of scientific applications, are discussed. Methods. Data were obtained during MUSE commissioning on the European Southern Observatory (ESO) Very Large Telescope and reduced with the standard ESO pipeline. Emission lines were fitted by Gaussian profiles. The dust extinction, electron densities, and temperatures of the ionised gas and abundances were determined using Python and PyNeb routines. Results. The delivered datacube has a spatial size of ~63′′× 123′′, corresponding to ~0.26 × 0.51 pc2 for the adopted distance, and a contiguous wavelength coverage of 4750–9300 Å at a spectral sampling of 1.25 Å pix−1. The nebula presents a complex reddening structure with high values (c(Hβ) ~ 0.4) at the rim. Density maps are compatible with an inner high-ionisation plasma at moderate high density (~1000 cm−3), while the low-ionisation plasma presents a structure in density peaking at the rim with values ~700 cm−3. Median Te, using different diagnostics, decreases according to the sequence [N II], [S II] →[S III] → [O I] → He I → Paschen Jump. Likewise, the range of temperatures covered by recombination lines is much larger than those obtained from collisionally excited lines (CELs), with large spatial variations within the nebula. If these differences were due to the existence of high density clumps, as previously suggested, these spatial variations would suggest changes in the properties and/or distribution of the clumps within the nebula. We determined a median helium abundance He/H = 0.124, with slightly higher values at the rim and outer shell. The range of measured ionic abundances for light elements are compatible with literature values. Our kinematic analysis nicely illustrates the power of 2D kinematic information in many emission lines, which sheds light on the intrinsic structure of the nebula. Specifically, our derived velocity maps support a geometry for the nebula that is similar to the diabolo-like model previously proposed, but oriented with its major axis roughly at PA ~ −22°. We identified two low-surface brightness arc-like structures towards the northern and southern tips of the nebula, with high extinction, high helium abundance, and strong low-ionisation emission lines. They are spatially coincident with some extended low-surface brightness mid-infrared emission. The characteristics of the features suggest that they could be the consequence of precessing jets caused by the binary star system. A simple 1D Cloudy model is able to reproduce the strong lines in the integrated spectrum of the whole nebula with an accuracy of ~15%. Conclusions. Together with similar work with MUSE on NGC 7009, the present study illustrates the enormous potential of wide field integral field spectrographs for the study of Galactic planetary nebulae.

A Carbon-rich Hot Bubble in the Planetary Nebula NGC 5189

Abstract We present the discovery of extended X-ray emission from the planetary nebula (PN) NGC 5189 around the [WO1]-type WD 1330−657 with XMM-Newton. The X-ray-emitting gas fills the cavities detected in the Hubble Space Telescope [O iii] narrowband image and presents a limb-brightened morphology toward the outer edges of the east and west lobes. The bulk of the X-ray emission is detected in the soft (0.3–0.7 keV) band with the XMM-Newton EPIC spectra dominated by the C vi Lyα line at 0.37 keV (=33.7 Å). Spectral analysis resulted in carbon and neon abundances 38 and 6 times their solar values, with a plasma temperature of kT = 0.14 ± 0.01 keV (T = 1.6 × 106 K) and X-ray luminosity of L X = (2.8 ± 0.8) × 1032 erg s−1. NGC 5189 is an evolved and extended PN (≲0.70 pc in radius), thus, we suggest that the origin of its X-ray emission is consistent with the born-again scenario in which the central star becomes carbon-rich through an eruptive very late thermal pulse, subsequently developing a fast, carbon-rich wind powering the X-ray emission as suggested for A 30 and A 78.

The Central Star of NGC 2346 as a Clue to Binary Evolution through the Common Envelope Phase

Abstract We present an analysis of the binary central star of the planetary nebula NGC 2346 based on archival data from the International Ultraviolet Explorer, and new low- and high-resolution optical spectra (3700–7300 Å). By including in the spectral analysis the contribution of both stellar and nebular continuum, we reconciled long-time discrepant UV and optical diagnostics and derive E(B–V) = 0.18 ± 0.01. We reclassified the companion star as A5IV by analyzing the wings of the Balmer absorption lines in the high-resolution (R = 67,000) optical spectra. Using the distance to the nebula of 1400 pc from Gaia DR2, we constructed a photoionization model based on abundances and line intensities derived from the low-resolution optical spectra, and obtained a temperature of T eff = 130,000 K and a luminosity of L = 170 L ⊙ for the ionizing star, consistent with the UV continuum. This analysis allows us to better characterize the binary system’s evolution. We conclude that the progenitor star of NGC 2346 has experienced a common envelope phase, in which the companion star has accreted mass and evolved off the main-sequence.

The Quadratic Programming Method for Extracting Emission Line Maps from Line-blended Narrowband Images

Abstract We present a method to separate blended line emission from a set of narrowband (NB) images whose distinct throughput profiles overlap with each other. This method, based on the principle of quadratic programming (QP), treats line blending as a linearly constrained quadratic optimization problem and finds the solution by imposing a condition that the desired individual line intensities are always positive while allowing measurement uncertainties. We demonstrate the method for the quintessential example of line blending in the Hα complex by the Hα and [N ii] lines using the archived Hubble Space Telescope WFPC2 and WFC3 NB images of the planetary nebula NGC 6720, in comparison with its genuine line maps extracted from a spectral cube obtained by our own long-slit scan mapping observations with the Gemini Multi-Object Spectrograph at Gemini-North. Such line-blended NB images typically permit only qualitative line diagnostics, which can lead to a factor of a few difference in the line flux ratios. This novel QP method, however, can extract individual line emission maps as long as the number of line-blended but well-calibrated NB images of differing throughputs is equal to or grater than the number of emission line maps to be extracted. The proposed QP method provides a viable alternative to the existing line emission calibration methods: a simple but innovative way to enable quantitative fully 2D plasma diagnostics at the pixel scale of input NB images available from various instruments, telescopes, and archives.

Solving the distance discrepancy for the open cluster NGC 2453

Context. The open cluster (OC) NGC 2453 is of particular importance since it has been considered to host the planetary nebula (PN) NGC 2452, however their distances and radial velocities are strongly contested. Aims. In order to obtain a complete picture of the fundamental parameters of the OC NGC 2453, 11 potential members were studied. The results allowed us to resolve the PN NGC 2452 membership debate. Methods. Radial velocities for the 11 stars in NGC 2453 and the PN were measured and matched with Gaia data release 2 (DR2) to estimate the cluster distance. In addition, we used deep multi-band UBVRI photometry to get fundamental parameters of the cluster via isochrone fitting on the most likely cluster members, reducing inaccuracies due to field stars. Results. The distance of the OC NGC 2453 (4.7 ± 0.2 kpc) was obtained with an independent method solving the discrepancy reported in the literature. This result is in good agreement with an isochrone fitting of 40–50 Myr. On the other hand, the radial velocity of NGC 2453 (78 ± 3 km s−1) disagrees with the velocity of NGC 2452 (62 ± 2 km s−1). Our results show that the PN is a foreground object in the line of sight. Conclusions. Due to the discrepancies found in the parameters studied, we conclude that the PN NGC 2452 is not a member of the OC NGC 2453.

Discovery of the first molecular ion in the Universe

After 30 years of searching, the helium hydride ion, the first chemical bond that was formed in the Universe, has finally been detected outside the laboratory, in the interstellar medium. It was seen in planetary nebula NGC 7027 using the GREAT spectrometer aboard the Stratospheric Observatory for Infrared Astronomy.

Hidden IR structures in NGC 40: signpost of an ancient born-again event

Abstract We present the analysis of infrared (IR) observations of the planetary nebula NGC 40 together with spectral analysis of its [WC]-type central star HD 826. Spitzer IRS observations were used to produce spectral maps centred at polycyclic aromatic hydrocarbons (PAH) bands and ionic transitions to compare their spatial distribution. The ionic lines show a clumpy distribution of material around the main cavity of NGC 40, with the emission from [Ar ii] being the most extended, whilst the PAHs show a rather smooth spatial distribution. Analysis of ratio maps shows the presence of a toroidal structure mainly seen in PAH emission, but also detected in a Herschel PACS 70 $\mu$m image. We argue that the toroidal structure absorbs the UV flux from HD 826, preventing the nebula to exhibit lines of high-excitation levels as suggested by previous authors. We discuss the origin of this structure and the results from the spectral analysis of HD 826 under the scenario of a late thermal pulse.

The post-common-envelope X-ray binary nucleus of the planetary nebula NGC 2392

Abstract The Chandra X-ray Observatory has detected relatively hard X-ray emission from the central stars of several planetary nebulae (PNe). A subset has no known late-type companions, making it very difficult to isolate which of several competing mechanisms may be producing the X-ray emission. The central star of NGC 2392 is one of the most vexing members, with substantial indirect evidence for a hot white dwarf (WD) companion. Here we report on the results of a radial velocity (RV) monitoring campaign of its central star with the HERMES échelle spectrograph of the Flemish 1.2 m Mercator telescope. We discover a single-lined spectroscopic binary with an orbital period of 1.902208 ± 0.000013 d and an RV semi-amplitude of 9.96 ± 0.13 km s−1. The high degree of nebula ionisation requires a WD companion (M ≳ 0.6M⊙), which the mass-function supports at orbital inclinations ≲ 7°, in agreement with the nebula orientation of 9°. The hard component of the X-ray spectrum may be explained by the companion accreting mass from the wind of the Roche lobe filling primary, while the softer component may be due to colliding winds. A companion with a stronger wind than the primary could produce the latter and would be consistent with models of the observed diffuse X-ray emission detected in the nebula. The diffuse X-rays may also be powered by the jets of up to 180 km s−1, and active accretion would imply that they may be the first active jets of a post-common-envelope PN, potentially making NGC 2392 an invaluable laboratory to study jet formation physics. The 1.9 d orbital period rules out a double-degenerate merger leading to a Type Ia supernova, and the weak wind of the primary likely also precludes a single-degenerate scenario. We suggest that a hard X-ray spectrum, in the absence of a late-type companion, could be a powerful tool to identify accreting WD companions.

The Temperature and Density from Permitted O ii Lines in the Planetary Nebula NGC 7009* †

Abstract We present spatial- and velocity-resolved spectroscopy of NGC 7009 acquired with the UVES spectrograph at the VLT UT2/Kueyen. We use these data to determine the structure of the electron temperature and electron density based upon O ii permitted (recombination) lines. We find a strong gradient in the O ii-based electron temperature. It agrees with the electron temperature determined by forbidden (collisionally excited) lines in part of the nebular volume, but also differs by more than 6000 K in other parts of the nebular volume. This result supports the hypothesis that NGC 7009 contains two plasma components, one of which emits both forbidden and permitted lines and the other that emits only permitted lines. For the component that emits only permitted lines, we find a lower limit to the electron density of 104 cm−3 from the O ii permitted lines, which is higher than derived from forbidden lines. We are unable to determine whether the two plasma components are in pressure equilibrium from our data, but there exist temperature and density combinations that allow this equilibrium for temperatures between 600 and 6000 K. For most of the temperature and density conditions allowed for the component that emits only permitted lines, its mass of O2+ is less than that of the plasma component that emits forbidden lines.

On the post-common-envelope central star of the planetary nebula NGC 2346

The common-envelope phase is one of the most poorly understood phases of (binary) stellar evolution, in spite of its importance in the formation of a wide-range of astrophysical phenomena ranging from cataclysmic variables to cosmologically important supernova type Ia, and even recently discovered gravitational wave producing black hole mergers. The central star of the planetary nebula NGC~2346 has long been held as one of the longest period post-common-envelope systems known with a published period of approximately 16 days, however the data presented were also consistent with much shorter periods of around 1 day (a more typical period among the known sample of post-common-envelope binary central stars). Here, using the modern high-stability, high-resolution spectrograph HERMES, we conclusively show the period to, indeed, be 16 days while also revising the surface gravity to a value typical of a sub-giant (rather than main-sequence) resulting in an intrinsic luminosity consistent with the recently published GAIA parallax distance. Intriguingly, the implied mass for the secondary ($\gtrsim$3.5 M$_\odot$) makes it, to our knowledge, the most massive post-common-envelope secondary known, whilst also indicating that the primary may be a post-RGB star.

Comprehensive Panchromatic Data Analyses and Photoionization Modeling of NGC 6781

AbstractWe characterized the dusty circumstellar nebula and central star of the C-rich bipolar planetary nebula (PN) NGC 6781 using our own Herschel data augmented with the archival data from UV to radio and constructed one of the most comprehensive photoionization PN models ever produced consisting of the ionized, atomic and molecular gas components as well as the dust component. We reproduced the observed spectral energy distribution (SED), constrained by 136 observational data points. The total nebula mass was estimated to be 0.41 M⊙, with a significant fraction (about 70 %) of it existing in the photo-dissociation region (PDR) surrounding the ionized nebula. This finding demonstrates the critical importance of the PDR in PNe, which are typically recognized as the hallmark of ionized/H+ region. It is therefore essential to characterize the PDR of the circumstellar nebula to understand material recycling in the Milky Way and other galaxies.

HST/WFPC2 imaging analysis and cloudy modelling of the multiple-shell planetary nebulae NGC 3242, NGC 6826, and NGC 7662

We performed a detailed photometric analysis and photoionisation modelling on three high excitation multiple shell planetary nebulae: NGC 3242, NGC 6826 and NGC 7662. Archival HST/WFPC2 narrow band filter images were used to investigate shocked regions by two independent methods: using low excitation ions (via H alpha/[N II] vs H alpha/[S II] extended diagnostic diagrams) and by means of high excitation species looking for regions of enhanced [O III]/H alpha line ratios. Shocked region analysis via low excitation ions shows that major deviations from the general inside to outside ionisation trend correspond only to regions where FLIERs or LISs are located. The reduction on the SNR at the outskirts of the [O III]/H alpha ratio maps, made us unable to unambiguously identified for an enhancement on the [O III]/H alpha line ratio as an indicator for shocks. For non-shocked regions we performed a photoionisation modelling using Cloudy. Fittings to the [O III] and H alpha observed radial profiles lead us to constrain on the free parameters of the density laws and filling factors together to temperatures and luminosities for the CSPNe. Best fit models show a very well representation of the [O III] and H alpha emission. Discrepancies in the model fittings to the [N II] and [S II] profiles at NGC 6826 and NGC 3242, can be attributed in the former case, due to a contamination by the light of the CSPN and, in the latter case, either due to gas inhomogeneities within the clumps or to a leaking of UV radiation.

Dissecting the X-Ray Emission in the Young Planetary Nebula NGC 7027

Abstract 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.

Mapping Excitation in the Inner Regions of the Planetary Nebula NGC 5189 Using HST WFC3 Imaging

Abstract The planetary nebula (PN) NGC 5189 around a Wolf–Rayet [WO] central star demonstrates one of the most remarkable complex morphologies among PNe with many multiscale structures, showing evidence of multiple outbursts from an asymptotic giant branch (AGB) progenitor. In this study, we use multiwavelength Hubble Space Telescope Wide Field Camera 3 observations to study the morphology of the inner 0.3 pc × 0.2 pc region surrounding the central binary that appears to be a relic of a more recent outburst of the progenitor AGB star. We applied diagnostic diagrams based on emission-line ratios of Hα λ6563, [O iii] λ5007, and [S ii] 6716,6731 images to identify the location and morphology of low-ionization structures within the inner nebula. We distinguished two inner, low-ionization envelopes from the ionized gas, within a radius of 55 arcsec (∼0.15 pc) extending from the central star: a large envelope expanding toward the northeast, and its smaller counterpart envelope in the opposite direction toward the southwest of the nebula. These low-ionization envelopes are surrounded by a highly ionized gaseous environment. We believe that these low-ionization expanding envelopes are a result of a powerful outburst from the post-AGB star that created shocked wind regions as they propagate through the previously expelled material along a symmetric axis. Our diagnostic mapping using high-angular resolution line-emission imaging can provide a novel approach to detection of low-ionization regions in other PNe, especially those showing a complex multiscale morphology.

The interaction of the halo around the butterfly planetary nebula NGC 650-1 with the interstellar medium

With its bright and wide equatorial waist seen almost edge-on (the butterfly body) and the faint and broad bipolar extensions (the butterfly wings), NGC650-1 is the archetypical example of bipolar planetary nebula (PN) with butterfly morphology. We present here deep high-resolution broad- and narrow-band optical images that expose the rich and intricate fine structure of this bipolar PN, with small-scale bubble-like features and collimated outflows. A SHAPE spatio-kinematical model indicates that NGC650-1 has a broad central torus with an inclination angle of 75 degrees with respect to the line of sight, whereas that of the bipolar lobes, which are clearly seen in the position-velocity maps, is 85 degrees. Large field of view deep images show, for first time, an arc-like diffuse envelope in low- and high-excitation emission lines located up to 180 arcsec towards the East-Southeast of the central star, well outside the main nebula. This morphological component is confirmed by Spitzer MIPS and WISE infrared imaging, as well as by long-slit low- and high-dispersion optical spectroscopic observations. HST images of NGC650-1 obtained at two different epochs (14 yrs apart), reveal the proper motion of the central star along this direction. We propose that this motion of the star through the interstellar medium compresses the remnant material of a slow Asymptotic Giant Branch wind, producing this bow-shock-like feature.

Erratum: SALT HRS discovery of a long-period double-degenerate binary in the planetary nebula NGC 1360

Erratum: SALT HRS discovery of a long-period double-degenerate binary in the planetary nebula NGC 1360