Carbon-rich Planetary Nebulae Research Articles

FAST Detection of OH Emission in the Carbon-rich Planetary Nebula NGC 7027

We present the first detection of the ground-state OH emission line at 1612 MHz toward the prototypical carbon-rich planetary nebula (PN) NGC 7027, utilizing the newly installed ultrawideband (UWB) receiver of the Five-hundred-meter Aperture Spherical radio Telescope (FAST). This emission is likely to originate from the interface of the neutral shell and the ionized region. The other three ground-state OH lines at 1665, 1667, and 1721 MHz are observed in absorption and have velocities well matched with that of HCO+ absorption. We infer that the OH absorption is from the outer shell of NGC 7027, although the possibility that they are associated with a foreground cloud cannot be completely ruled out. All the OH lines exhibit a single blueshifted component with respect to the central star. The formation of OH in carbon-rich environments might be via photodissociation-induced chemical processes. Our observations offer significant constraints for chemical simulations, and they underscore the potent capability of the UWB receiver of FAST to search for nascent PNe.

Seimei KOOLS-IFU mapping of the gas and dust distributions in Galactic planetary nebulae: the case of IC 2165

ABSTRACT We investigated the physical and chemical properties of the gas and dust components in a carbon-rich planetary nebula (PN) IC 2165 using two-dimensional emission-line maps with superior resolution. The extinction map is generated in a self-consistent and assumption-free manner. The circumstellar gas-to-dust mass ratio (GDR) map ranges radially from 1210 in the central nebula filled with hot gas plasma to 120 near the ionization front. The determined GDR is comparable to ∼400, which is commonly adopted for carbon-rich asymptotic giant branch (AGB) stars, and ∼100 for interstellar medium. Except for the inner regions, the GDR in IC 2165 is nearly the same as in such AGB stars, indicating that most dust grains withstand the harsh radiation field without being destroyed. The gas and dust mass distributions concentrated in the equatorial plane may be related to the non-isotropic mass loss during the AGB phase and nebula shaping. The spatial distributions of electron densities/temperatures and ionic/elemental abundances were investigated herein. We determined 13 elemental abundances using point-spread-function-matched spatially integrated multiwavelength spectra extracted from the same aperture. Their values are consistent with values predicted by a theoretical model for stars of initially 1.75 M⊙ and Z = 0.003. Finally, we constructed the photoionization model using our distance measurement to be consistent with all derived quantities, including the GDR, gas and dust masses, and post-AGB evolution. Thus, we demonstrate the capability of the Kyoto Okayama Optical Low-dispersion Spectrograph with optical-fibre Integral Field Unit (KOOLS-IFU) and how the spatial variation of the gas and dust components in PNe derived from IFU observations can help understand the evolution of the circumstellar/interstellar medium.

Magnetism and Astronomical Infrared Spectrum of Fullerene C<sub>60</sub> and Void Induced Graphene Molecules

Magnetism of fullerene C60 was studied by three methods of the density functional theory (DFT) calculation, laboratory experiment and astronomical observation. DFT revealed that the most stable spin state was non-magnetic one of Sz=0/2. This is contrary to our recent study on void induced graphene molecules of C23 and C53 to be magnetic one of Sz=2/2. Two graphene molecules combined model suggested that two up-spin at every carbon pentagon ring may cancel each other to bring Sz=0/2. Similar cancelation may occur on C60. Molecular vibrational infrared spectrum of C60 show four major bands, which coincide with gas-phase laboratory experiment, also with astronomically observed one of carbon rich planetary nebula Tc1 and Lin49. However, there remain many unidentified bands on astronomical one. We supposed multiple voids on graphene sheet, which may create both C60 and complex graphene molecules. It was revealed that spectrum of two voids induced graphene molecule coincident well with major astronomical bands. Simple sum of C60 and graphene molecules could successfully reproduce astronomical bands in detail.

A Survey of 3–5.4 μm Emission from Planetary Nebulae Using SOFIA/FLITECAM

Abstract Here we present the results of an airborne 3–5.4 μm spectroscopic study of three young, carbon-rich planetary nebulae (PNs) IC 5117, PNG 093.9-00.1, and BD +30 3639. These observations were made using the grism spectroscopy mode of the First Light Infrared TEst CAMera (FLITECAM) instrument during airborne science operations on board NASA’s Stratospheric Observatory for Infrared Astronomy (SOFIA). The goal of this study is to characterize the 3.3 and 5.25 μm polycyclic aromatic hydrocarbon (PAH) dust emission in PNs and study the evolution of PAH features within evolved stars before their incorporation into new stellar systems in star-forming regions. Targets were selected from Infrared Astronomical Satellite, Kuiper Airborne Observatory and Infrared Space Observatory source lists, and were previously observed with FLITECAM on the 3 m Shane telescope at Lick Observatory to allow direct comparison between the ground and airborne observations. We measure PAH emission equivalent width and central wavelength, classify the shape of the PAH emission, and determine the PAH/aliphatic ratio for each target. The 3.3 μm PAH emission feature is observed in all three objects. PNG 093.9-00.1 exhibits NGC 7027-like aliphatic emission in the 3.4–3.6 μm region while IC 5117 and BD +30 3639 exhibit less aliphatic structure. All three PNs additionally exhibit PAH emission at 5.25 μm.

Witnessing the emergence of a carbon star

Abstract During the late stages of their evolution, Sun-like stars bring the products of nuclear burning to the surface. Most of the carbon in the Universe is believed to originate from stars with masses up to a few solar masses. Although there is a chemical dichotomy between oxygen-rich and carbon-rich evolved stars, the dredge-up itself has never been directly observed. In the last three decades, however, a few stars have been shown to display both carbon- and oxygen-rich material in their circumstellar envelopes. Two models have been proposed to explain this dual chemistry: one postulates that a recent dredge-up of carbon produced by nucleosynthesis inside the star during the Asymptotic Giant Branch changed the surface chemistry of the star. The other model postulates that oxygen-rich material exists in stable keplerian rotation around the central star. The two models make contradictory, testable, predictions on the location of the oxygen-rich material, either located further from the star than the carbon-rich gas, or very close to the star in a stable disc. Using the Faint Object InfraRed CAmera (FORCAST) instrument on board the Stratospheric Observatory for Infrared Astronomy (SOFIA) Telescope, we obtained images of the carbon-rich planetary nebula BD +30° 3639 which trace both carbon-rich polycyclic aromatic hydrocarbons and oxygen-rich silicate dust. With the superior spectral coverage of SOFIA, and using a 3D photoionization and dust radiative transfer model we prove that the O-rich material is distributed in a shell in the outer parts of the nebula, while the C-rich material is located in the inner parts of the nebula. These observations combined with the model, suggest a recent change in stellar surface composition for the double chemistry in this object. This is evidence for dredge-up occurring ∼103 yr ago.

Oxygen enrichment in carbon-rich planetary nebulae

We study the relation between the chemical composition and the type of dust present in a group of 20 Galactic planetary nebulae (PNe) that have high quality optical and infrared spectra. The optical spectra are used, together with the best available ionization correction factors, to calculate the abundances of Ar, C, Cl, He, N, Ne, and O relative to H. The infrared spectra are used to classify the PNe in two groups depending on whether the observed dust features are representative of oxygen-rich or carbon-rich environments. The sample contains one object from the halo, eight from the bulge, and eleven from the local disc. We compare their chemical abundances with nucleosynthesis model predictions and with the ones obtained in seven Galactic H II regions of the solar neighbourhood. We find evidence of O enrichment (by $\sim$ 0.3 dex) in all but one of the PNe with carbon-rich dust (CRD). Our analysis shows that Ar, and especially Cl, are the best metallicity indicators of the progenitors of PNe. There is a tight correlation between the abundances of Ar and Cl in all the objects, in agreement with a lockstep evolution of both elements. The range of metallicities implied by the Cl abundances covers one order of magnitude and we find significant differences in the initial masses and metallicities of the PNe with CRD and oxygen-rich dust (ORD). The PNe with CRD tend to have intermediate masses and low metallicities, whereas most of the PNe with ORD show higher enrichments in N and He, suggesting that they had high-mass progenitors.

Physical properties of fullerene-containing Galactic planetary nebulae

We searched the Spitzer Space Telescope data archive for Galactic planetary nebulae (PNe), which show the characteristic 17.4 and 18.9 μm features due to C60, also known as buckminsterfullerene. Out of 338 objects with Spitzer/Infrared Spectrograph data, we found eleven C60-containing PNe, six of which (Hen2-68, IC2501, K3-62, M1-6, M1-9 and SaSt2-3) are new detections, not known to contain C60 prior to this work. The strongest 17.4 and 18.9 μm C60 features are seen in Tc1 and SaSt2-3, and these two sources also prominently show the C60 resonances at 7.0 and 8.5 μm. In the other nine sources, the 7.0 and 8.5 μm features due to C60 are much weaker. We analysed the spectra, along with ancillary data, using the photoionization code cloudy to establish the atomic line fluxes, and determine the properties of the radiation field, as set by the effective temperature of the central star. In addition, we measured the infrared spectral features due to dust grains. We find that the polycyclic aromatic hydrocarbon (PAH) profile over 6–9 μm in these C60-bearing carbon-rich PNe is of the more chemically processed class A. The intensity ratio of 3.3 to 11.3 μm PAH indicates that the number of C-atoms per PAH in C60-containing PNe is small compared to that in non-C60 PNe. The Spitzer spectra also show broad dust features around 11 and 30 μm. Analysis of the 30 μm feature shows that it is strongly correlated with the continuum, and we propose that a single carbon-based carrier is responsible for both the continuum and the feature. The strength of the 11 μm feature is correlated to the temperature of the dust, suggesting that it is at least partially due to a solid-state carrier. The chemical abundances of C60-containing PNe can be explained by asymptotic giant branch nucleosynthesis models for initially 1.5–2.5 M⊙ stars with Z = 0.004. We plotted the locations of C60-containing PNe on a face-on map of the Milky Way and we found that most of these PNe are outside the solar circle, consistent with low metallicity values. Their metallicity suggests that the progenitors are an older population.

Blue Luminescence and Extended Red Emission: Possible Connections to the Diffuse Interstellar Bands

AbstractBlue luminescence (BL) and extended red emission (ERE) are observed as diffuse, optical-wavelength emissions in interstellar space, resulting from photoluminescence by ultraviolet(UV)-illuminated interstellar grains. Faintness and the challenge of separating the BL and ERE from the frequently much brighter dust-scattered continuum present major observational hurdles, which have permitted only slow progress in testing the numerous models that have been advanced to explain these two phenomena. Both the ERE, peaking near 680 nm (FWHM ~ 60 - 120 nm) and the BL, asymmetrically peaking at ~ 378 nm (FWHM ~ 45 nm), were first discovered in the Red Rectangle nebula. Subsequently, ERE and BL have been observed in other reflection nebulae, and in the case of the ERE, in carbon-rich planetary nebulae, H II regions, high-latitude cirrus clouds, the galactic diffuse ISM, and in external galaxies. BL exhibits a close spatial and intensity correlation with emission in the aromatic emission feature at 3.3 micron, most likely arising from small, neutral polycyclic aromatic hydrocarbon (PAH) molecules. The spectral characteristics of the BL also agree with those of fluorescence by PAH molecules with 13 to 19 carbon atoms. The BL phenomenon is thus most readily understood as the optical fluorescence of small, UV-excited aromatic molecules. The ERE, by contrast, though co-existent with mid-IR PAH emissions, does not correlate with emissions from either neutral or ionized PAHs. Instead, the spatial ERE morphology appears to be strictly governed by the density of far-UV (E ≥ 10.5 eV) photons, which are required for the ERE excitation. The most restrictive observational constraint for the ERE process is its exceptionally high quantum efficiency. If the ERE results from photo-excitation of a nano-particle carrier by photons with E ≥ 10.5 eV in a single-step process, the quantum efficiency exceeds 100%. Such a process, in which one to three low-energy optical photons may be emitted following a single far-UV excitation, is possible in highly isolated small clusters, e.g. small, dehydrogenated carbon clusters with about 20 to 28 carbon atoms. A possible connection between the ERE carriers and the carriers of DIBs may exist in that both are ubiquitous throughout the diffuse interstellar medium and both have an abundance of low-lying electronic levels with E ≤ 2.3 eV above the ground state.

THE FORMATION OF COSMIC FULLERENES FROM AROPHATIC CLUSTERS

Fullerenes have recently been identified in space and they may play a significant role in the gas and dust budget of various astrophysical objects including planetary nebulae (PNe), reflection nebulae (RNe) and H II regions. The tenuous nature of the gas in these environments precludes the formation of fullerene materials following known vaporization or combustion synthesis routes even on astronomical timescales. We have studied the processing of hydrogenated amorphous carbon (a-C:H or HAC) nano-particles and their specific derivative structures, which we name "arophatics", in the circumstellar environments of young, carbon-rich PNe. We find that UV-irradiation of such particles can result in the formation of fullerenes, consistent with the known physical conditions in PNe and with available timescales.

DISCOVERY AND ANALYSIS OF 21 μm FEATURE SOURCES IN THE MAGELLANIC CLOUDS

Spitzer Space Telescope mid-infrared spectroscopy has been obtained for 15 carbon-rich protoplanetary nebulae (PPNe) in the Large Magellanic Cloud (LMC) and for two other such stars in the Small Magellanic Cloud (SMC). Of these 17 PPNe, the unidentified 21 μm feature is strong in 7 spectra, weak in 2 spectra, and very weak or questionable in 4 spectra. Two of the four spectra without the 21 μm feature have a very strong feature near 11 μm, similar to a feature observed in some carbon-rich planetary nebulae (PNe) in the LMC. We attribute this feature to unusual SiC dust, although the feature-to-continuum ratio is much larger than for SiC features in Galactic or Magellanic Cloud carbon star spectra. The remaining two objects show typical carbon-rich PPNe spectra with no 21 μm features. One of the LMC objects that lacks the 21 μm feature and one SMC object with a questionable 21 μm detection may have mixed dust chemistries based upon their spectral similarity to Galactic [WC] PNe. The 13 objects that either definitely or may show the 21 μm feature have distinct dust shell properties compared to the Galactic 21 μm objects—the 21 μm features are weaker, the estimated dust temperatures are significantly higher, the unidentified infrared (UIR) bands are stronger, and the UIRs show more structure. Four of the 21 μm objects appear to show normal SiC emission features in their spectra. Many of the PPNe show strong 30 μm features, although this feature carries less of the total mid-infrared emission than is normally the case for the Galactic 21 μm PPNe. The LMC objects are in the LMC halo rather than in the LMC bar. The estimated luminosities of these PPNe vary from 4700 to 12,500 L� .

FULLERENES FOUND IN SPACE

LARGE AMOUNTS of the fullerenes C 60 and C 70 have been detected in a blast of gas from a dying star—the first unambiguous finding of this form of carbon in space. The discovery suggests that fullerenes form readily in space, given the right conditions. Fullerenes, stable and inert, have long been believed to be able to exist in space, particularly near the environment of carbon-rich stars, but evidence has been limited to inconclusive spectra and traces of the molecules in meteorites that fell to Earth. Astronomer Jan Cami of the University of Western Ontario and his colleagues trained NASA’s Spitzer Space Telescope on a carbon-rich planetary nebula—a star in the late stages of life—and found all of the characteristic infrared emission spectral lines of cold, neutral C 60 and C 70 ( Science, DOI: 10.1126/science.1192035). Pascale Ehrenfreund, professor of astrobiology at the University of Amsterdam, notes that the detection is “rather unambiguous,” and that these new data are ...

Crossed‐Molecular‐Beam Study on the Formation of Phenylacetylene from Phenyl Radicals and Acetylene

A hot topic! Crossed-molecular-beam experiments showed that phenylacetylene molecules—precursors to polycyclic aromatic hydrocarbons in combustion flames and in carbon-rich planetary nebulae—can be formed through reactions of phenyl radicals with acetylene under single-collision conditions (see picture).

Modelling photoluminescence from small particles

The Extended Red Emission (ERE) has been observed in virtually all possible environments in which dust is present, ranging from H regions to carbon rich planetary nebulae, to reflection nebulae, to dense interstellar clouds, to the diffuse interstellar medium. One problem in comparing ERE with laboratory photoluminescence (PL) measurements is given by the fact that laboratory samples are usually macroscopic in size, or very thin films, while interstellar dust particles are usually of sizes comparable to, or smaller than, the wavelength of visible light. We here apply a newly developed general recipe to extrapolate the expected PL of small, homogeneous spheres starting from the available laboratory results obtained for bulk samples, and infer previously unsuspected consequences for ERE-like PL phenomena.

Synthesis of Organic Compounds in the Circumstellar Environment

Recent developments in infrared spectroscopy have made possible the detection of the stretching and bending modes of organic molecules in space. Infrared emission bands of the C-C and C-H stretching and bending modes of aromatic compounds have been widely observed in carbon-rich planetary nebulae, the descendents of carbon stars. Infrared spectra of the transition objects between carbon stars and planetary nebulae, called proto-planetary nebulae, have shown definite signatures of a variety of aliphatic side-groups attached to aromatic rings. This suggests that molecular synthesis is actively ongoing in the circumstellar environment and the chemical nature of these compounds evolve over time scales as short as several hundred years.

Dust Features in Carbon-rich Planetary Nebulae

Carbon-rich AGB stars, protoplanetary nebulae (PPNe), and PNe show a variety of dust features attributed to carbon-based compounds. Of particular interest are the three dust features peaking near 21, 26 and 33 μm. The 26 and 33 μm features (collectively the “30 μm feature”; see Volk et al. 2002) are always observed together. The 21 μm feature is weak in the AGB phase and becomes more prominent in the PPN phase, which implies carrier evolution. We wish to see how this evolution continues into the PN phase from detailed modelling of the infrared spectra of PNe.

The Far-Infrared Spectrum of Hydrogenated Amorphous Carbon and the 21, 27, and 33 Micron Features in Carbon-rich Proto–Planetary Nebulae

We report laboratory emission spectra of hydrogenated amorphous carbon (HAC) in the wavelength range 19 μm < λ < 120 μm (525-83 cm-1) at temperatures between 300°C and 500°C. These spectra show emission features at 21, 27, and ≈33 μm (475, 370, and 303 cm-1) that can be identified with similar bands in the far-infrared spectra of extreme carbon stars, proto-planetary nebulae, and planetary nebulae. It is suggested that HAC is present in these objects but that the chemical composition of this material depends on the evolutionary state. We also detect a number of broad features at wavelengths longer than 40 μm, attributable to chemical groups in HAC. A strong feature is observed at ≈60 μm (165 cm-1) in laboratory spectra and may be present in spectra of the carbon-rich planetary nebula CPD -56°8032. We briefly discuss the nature of the chemical groups responsible for the appearance of these features.

Silicate and ice emission bands in the ISO spectrum of the PAH-emitting carbon-rich planetary nebula CPD-56°8032

Combined ISO SWS and LWS spectroscopy is presented of the late WC-type planetary nebula nucleus CPD-56†8032 and its carbon-rich nebula. The extremely broad coverage (2.4–197 μm) enables us to recognize the clear and simultaneous presence of emission features from both oxygen- and carbon-rich circumstellar materials. Removing a smooth continuum highlights bright emission bands characteristic of polycyclic aromatic hydrocarbons (hereafter PAHs) in the 3–15 μm region, bands from crystalline silicates longwards of 18 μm, and the 43- and 62-μm bands of crystalline water ice. We discuss the probable evolutionary state and history of this unusual object in terms of (a) a recent transition from an O-rich to a C-rich outflow following a helium shell flash; or (b) a carbon-rich nebular outflow encountering an O-rich comet cloud orbiting in a Kuiper-belt-like distribution.

Axially Symmetric Superwinds of Proto–Planetary Nebulae with 21 Micron Dust Features

We present narrowband images at selected wavelengths in the 8-13 μm window of four carbon-rich, proto-planetary nebulae that have an unusual 21 μm dust feature: IRAS 04296+3429, IRAS 22272+5435, IRAS 07134+1005, and IRAS 19500-1709. We observe axially symmetric dust emission structures in IRAS 22272+5435 and IRAS 07134+1005, and tentatively in IRAS 19500-1709, while IRAS 04296+3429 is unresolved with our ~1'' angular resolution. The well-resolved morphology of IRAS 07134+1005 shows an elliptical outer shell surrounding two aligned peaks that we interpret as limb-brightened peaks of an optically thin, elliptical shell with an equatorial density enhancement. This mid-IR morphology contrasts with that observed in the better studied carbon-rich proto-planetary nebulae, AFGL 2688, AFGL 915, and AFGL 618, which show bright, unresolved cores, probably created by optically thick inner regions, and bipolar extensions that align with their optical reflection nebulosities. Using an axially symmetric dust code and assuming that the dust is composed of 0.01 μm amorphous carbon grains, we model the dust emission images and the spectral energy distributions of these four proto-planetary nebulae and of the young, carbon-rich planetary nebula IRAS 21282+5050, which also has an axially symmetric dust shell and other similarities with the proto-planetary nebulae that have the 21 μm dust feature. Marginally resolved mid-infrared images constrain the dust shell's inner radius, while well-resolved mid-infrared images additionally constrain other geometric parameters of the model (e.g., inclination angles and pole-to-equator mass-loss rate ratios). The modeling reveals that the observed axial symmetry in the dust shells of these objects coincides with an enhanced mass-loss phase (~3 × 10-5 M☉ yr-1) during which the equatorial mass-loss rate was a factor of 18-90 higher than the polar mass-loss rate, i.e., an axially symmetric superwind. Our dynamical age estimates indicate that these stars left the asymptotic giant branch approximately 300-1400 years ago, just after the superwind phase. For each object, the size and structure of the dust shell is the same for the sampled wavelengths, with the exception of IRAS 22272+5435 for which the 11.8 μm emission is larger than either the 8.2 or the 9.7 μm emission. IRAS 22272+5435's spectrum has a larger dust feature-to-dust continuum ratio than found in the other objects, and hence its 11.8 μm image is probably dominated by the 11.8 μm feature emission that has different optical properties than the underlying continuum.

ISO LWS Observations of Evolved Stars and Nebulae

This review reports, on behalf of the LWS Consortium1, some of the spectroscopic results on evolved stars and nebulae that have been obtained at far-infrared wavelengths by the Infrared Space Observatory (ISO) since its launch in November 1995. The results reported have been obtained mainly using ISO’s Long Wavelength Spectrometer (LWS; Clegg et al. 1996), but for some objects complementary spectra have also been obtained with the Short Wavelength Spectrometer (SWS; de Graauw et al. 1996). In this review I present some of the results that have been obtained (a) for oxygen-rich cool stars and (b) for carbon stars and carbon-rich post-AGB objects and planetary nebulae. Highlights from the first area include the first detection of the far-infrared rotational lines of water vapour, the dominant coolant in their outflows, and the detailed confirmation of the proposed radiative pump mechanism for circumstellar OH masers. Highlights from the second area include the detection of a rich HCN and CO far-infrared line spectrum from the carbon star IRC+10°216, with HCN found to be the dominant coolant, the detection of very strong shock-excited CO rotational lines from the post-AGB object AFGL 2688, and the first detection of the far-infared rotational lines of CH+ (from the the PDR of the carbon-rich planetary nebula NGC 7027), as well as the first detection in emission of the far-IR rotational lines of CH (also from NGC 7027).

Four Newly Identified Sources with 21 Micron Emission

view Abstract Citations (57) References (47) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Four Newly Identified Sources with 21 Micron Emission Kwok, Sun ; Hrivnak, Bruce J. ; Geballe, T. R. Abstract Mid-infrared spectroscopy and near- and mid-infrared photometry have been obtained for four carbon-rich cool IRAS sources whose IRAS low-resolution spectra are similar to those of proto-planetary nebulae (PPNs) possessing the unusual 21 microns emission feature. The 21 microns feature is detected in all four sources, nearly doubling the number of known 21 microns emitters. The energy distributions of these objects show the characteristic "double-peaked" structure, confirming that they are PPNs. Broad features near 7.7 and 11.3 microns similar to the unidentified infrared features (UIRs) seen in carbon-rich planetary nebulae (PNs) are also detected, suggesting a chemical link between these PPNs and PNs. The presence of the 21 microns feature appears to be correlated with the presence of unusually strong 3.4-3.5 microns features. Possible scenarios for the evolution of these emission features in the PPN phase are discussed. Publication: The Astrophysical Journal Pub Date: November 1995 DOI: 10.1086/176490 Bibcode: 1995ApJ...454..394K Keywords: STARS: CIRCUMSTELLAR MATTER; INFRARED: STARS; ISM: PLANETARY NEBULAE: GENERAL; STARS: CARBON full text sources ADS | data products SIMBAD (23)