Carbon-rich Proto-planetary Nebulae Research Articles

Molecules in the Carbon-rich Protoplanetary Nebula CRL 2688

Abstract We present observations of the carbon-rich protoplanetary nebula CRL 2688 made with the Institut de Radioastronomie Millimétrique 30 m telescope in the 3 and 2 mm bands. In total, 196 transition lines belonging to 38 molecular species and isotopologues are detected, among which, to the best of our knowledge, 153 transition lines and 13 species are the first reported for this object. Additionally, in order to contribute to future research, we have collected observational data on the molecular lines of CRL 2688 from the literature and compiled them into a single unified catalog. We find that the molecular abundance of CRL 2688 cannot be explained by the standard model of a circumstellar envelope. The implications of metal-bearing molecules on circumstellar chemistry are discussed.

Variability in Proto-planetary Nebulae. VI. Multitelescope Light Curve Studies of Several Medium-bright (V = 13–15), Carbon-rich Objects

Abstract We present 10 years of new photometric monitoring of the light variability of five evolved stars with strong mid-infrared emission from surrounding dust. Three are known carbon-rich proto-planetary nebulae (PPNe) with F−G spectral types; the nature of the other two was previously unknown. For the three PPNe, we determine or refine the pulsation periods of IRAS 04296+3429 (71 days), 06530−0213 (80 days), and 23304+6147 (84 days). A secondary period was found for each, with a period ratio P 2 /P 1 of 0.9. The light variations are small, 0.1–0.2 mag. These are similar to values found in other PPNe. The other two are found to be giant stars. IRAS 09296+1159 pulsates with a period of only 47 days but reaches pulsational light variations of 0.5 mag. Supplemental spectroscopy reveals the spectrum of a CH carbon star. IRAS 08359−1644 is a G1 III star that does not display pulsational variability; rather, it shows nonperiodic decreases of brightness of up to 0.5 mag over this 10 year interval. These drops in brightness are reminiscent of the light curves of R Corona Borealis variables, but with much smaller decreases in brightness and are likely due to transient dust obscuration. Its spectral energy distribution is very similar to that of the unusual oxygen-rich giant star HDE 233517, which possesses mid-infrared hydrocarbon emission features. These two non-PPNe turn out to be members of the rare group of giant stars with large mid-infrared excesses due to dust, objects which presumably have interesting evolutionary histories.

VARIABILITY IN PROTO-PLANETARY NEBULAE. III. LIGHT CURVE STUDIES OF MAGELLANIC CLOUD CARBON-RICH OBJECTS

We have investigated the light variability in a sample of 22 carbon-rich post-AGB stars in the Large Magellanic Cloud (LMC) and Small Magellanic Cloud (SMC), based primarily on photometric data from the OGLE survey. All are found to vary. Dominant periods are found in eight of them; these periods range from 49 to 157 days, and most of these stars have F spectral types. These eight are found to be similar to the Milky Way Galaxy (MWG) carbon-rich proto-planetary nebulae (PPNs) in several ways: (a) they are in the same period range of ~38 to ~160 days, (b) they have similar spectral types, (c) they are (all but one) redder when fainter, (d) they have multiple periods, closely spaced in time, with a average ratio of secondary to primary period of ~1.0, and as an ensemble, (e) they show a trend of decreasing period with increasing temperature, and (f) they show a trend of decreasing amplitude with decreasing period. However, they possibly differ in that the decreasing trend of period with temperature may be slightly offset from that of the MWG. These eight are classified as PPNs. The other 14 all show evidence of variability on shorter timescales. They are likely hotter PPNs or young planetary nebulae. However, in the MWG the numbers of PPNs peak in the F-G spectral types, while it appears that in the LMC they peak at a hotter B spectral type. One of the periodic ones shows a small, R Coronae Borealis-type light curve drop.

A SEARCH FOR NEAR INFRARED BANDS OF THE FULLERENE CATION C$_{60}^+$ IN THE PROTOPLANETARY NEBULA IRAS 01005+7910

IRAS 01005+7910 is a carbon-rich protoplanetary nebula with a recently reported detection of mid-IR vibrational transitions of the fullerene C{sub 60} by Zhang and Kwok. We present new high spectral resolution (R ∼ 57, 000) observations of this object obtained at the 3.6 m Telescopio Nazionale Galileo, showing the presence of two absorption bands at 9577 and 9632 A which are consistent with laboratory measurements of the C{sub 60}{sup +} cation. If these two bands were produced by C{sub 60}{sup +} in the material surrounding the central post-asymptotic giant branch star, we estimate that ∼1% of carbon could be trapped in this ionized form of fullerenes which would be more abundant than the neutral species in this protoplanetary nebulae. The central star with an effective temperature of T ≥ 20, 000 K can provide the ionizing photons required. These observations bring further evidence for the presence of fullerenes in protoplanetary nebulae and suggest that a significant production takes place in this late stage of stellar evolution. Mid-IR bands of C{sub 60}{sup +} could be present in the 7-20 μm spectrum of IRAS 01005+7910 and are also likely to be detected in the spectra of planetary nebulae. High-resolution spectroscopy will bemore » required for a reliable determination of the excitation temperatures and the relative abundance of neutral and ionized fullerenes in these objects.« less

STUDIES OF VARIABILITY IN PROTO-PLANETARY NEBULAE. II. LIGHT AND VELOCITY CURVE ANALYSES OF IRAS 22272+5435 AND 22223+4327

We have carried out a detailed observational study of the light, color, and velocity variations of two bright, carbon-rich proto-planetary nebulae, IRAS 22223+4327 and 22272+5435. The light curves are based upon our observations from 1994 to 2011, together with published data by Arkhipova and collaborators. They each display four significant periods, with primary periods for IRAS 22223+4327 and 22272+5435 being 90 and 132 days, respectively. For each of them, the ratio of secondary to primary period is 0.95, a value much different from that found in Cepheids, but which may be characteristic of post-AGB stars. Fewer significant periods are found in the smaller radial velocity data sets, but they agree with those of the light curves. The color curves generally mimic the light curves, with the objects reddest when faintest. A comparison in seasons when there exist contemporaneous light, color, and velocity curves reveals that the light and color curves are in phase, while the radial velocity curves are 0.25 out of phase with the light curves. Thus they differ from what is seen in Cepheids, in which the radial velocity curve is 0.50 P out of phase with the light curve. Comparison of the observed periods and amplitudes with those of post-AGB pulsation models shows poor agreement, especially for the periods, which are much longer than predicted. These observational data, particularly the contemporaneous light, color, and velocity curves, provide an excellent benchmark for new pulsation models of cool stars in the post-AGB, proto-planetary nebula phase.

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� .

Infrared spectral evolution of carbon stars

We collected almost all Highly Processed Data Products (HPDP) of ISO SWS01 spectra for the Galactic visual carbon stars, infrared carbon stars, extreme carbon stars and carbon-rich proto-planetary nebulae (PPNs). Those infrared spectra are primarily analyzed and discussed. It is shown that either spectral shapes/peaks, or main molecular/dust features are evidenced to change in the sequence of visual carbon stars, infrared carbon stars, extreme carbon stars and carbon-rich PPNs. Statistically, in this sequence, continua are gradually changed from blue to red and locations of spectral peaks of continua are also gradually changed from short wavelengths to long wavelengths. In addition, in this sequence, intensities of main molecular/dust features are also gradually changed from prominent in the short wavelengths to prominent in the long wavelengths. Furthermore, from 2MASS and IRAS photometric data, the sequence is also proved. Results in this paper strongly support the previous suggestion for the evolution sequence of carbon-rich objects in our Galaxy, that is the sequence of visual carbon stars → infrared carbon stars → extreme carbon stars → carbon-rich PPNs.

VARIABILITY IN PROTO-PLANETARY NEBULAE. I. LIGHT CURVE STUDIES OF 12 CARBON-RICH OBJECTS

We have carried out long-term (14 years) V and R photometric monitoring of 12 carbon-rich proto-planetary nebulae. The light and color curves display variability in all of them. The light curves are complex and suggest multiple periods, changing periods, and/or changing amplitudes, which are attributed to pulsation. A dominant period has been determined for each and found to be in the range of ~150 d for the coolest (G8) to 35-40 d for the warmest (F3). A clear, linear inverse relationship has been found in the sample between the pulsation period and the effective temperature and also an inverse linear relationship between the amplitude of light variation and the effective temperature. These are consistent with the expectation for a pulsating post-AGB star evolving toward higher temperature at constant luminosity. The published spectral energy distributions and mid-infrared images show these objects to have cool (200 K), detached dust shells and published models imply that intensive mass loss ended a few thousand years ago. The detection of periods as long as 150 d in these requires a revision in the published post-AGB evolution models that couple the pulsation period to the mass loss rate and that assume that intensive mass loss ended when the pulsation period had decreased to 100 d. This revision will have the effect of extending the time scale for the early phases of post-AGB evolution. It appears that real time evolution in the pulsation periods of individual objects may be detectable on the time scale of two decades.

ASPITZER/IRS SPECTRUM OF THE 2008 LUMINOUS TRANSIENT IN NGC 300: CONNECTION TO PROTO-PLANETARY NEBULAE

We present a Spitzer/IRS low-resolution mid-infrared (mid-IR) spectrum (5–14 μm) of the luminous transient discovered in the nearby galaxy NGC 300 in 2008 May. This transient had peak luminosity MV ≃ −13, showed an optical spectrum dominated by relatively narrow Balmer and Ca ii lines in emission, and its progenitor was identified in pre-explosion images as a dust-enshrouded ∼10 M☉ star, characteristics that make it a twin of SN 2008S. The Spitzer spectrum, obtained three months after discovery, shows that the transient is very luminous in the mid-IR. Furthermore, the spectrum shows strong, broad emission features at 8 μm and 12 μm that are observed in Galactic carbon-rich proto-planetary nebulae. Combining these data with published optical and near-IR photometry obtained at the same epoch, we find that the mid-IR excess traced by the Spitzer spectrum accounts for ∼20% of the total energy output. This component can be well explained by emission from ∼3 × 10−4 M☉ of pre-existing progenitor dust at temperature T ∼ 400 K. The spectral energy distribution of the transient also shows a near-IR excess that can be explained by emission from newly formed dust in the ejecta. Alternatively, both the near-IR and mid-IR excesses can together be explained by a single pre-existing geometrically thick dust shell. In light of the new observations obtained with Spitzer, we revisit the analysis of the optical spectra and kinematics, which were compared to the massive yellow-hypergiant IRC+10420 in previous studies. We show that proto-planetary nebulae share many properties with the NGC 300 transient and SN 2008S. We conclude that even though the explosion of a massive star (M ≳ 10 M☉) cannot be ruled out, an explosive event on a massive (M ∼ 6–10 M☉) carbon-rich AGB/super-AGB or post-AGB star is consistent with all observations of the transients and their progenitors presented thus far.

A Search for Phosphine in Circumstellar Envelopes: PH 3 in IRC +10216 and CRL 2688?

We present the results of a search for the JK = 10→ 00 transition of PH3 (phosphine) at 267 GHz toward several circumstellar envelopes using the Arizona Radio Observatory 10 m Submillimeter Telescope (SMT). In the carbon-rich shells of IRC +10216 and CRL 2688, we have detected emission lines exactly at the PH3 frequency. Toward the oxygen-rich supergiant VY Canis Majoris, only an upper limit was obtained, while in the evolved carbon-rich proto-planetary nebula CRL 618, the transition is contaminated by vibrationally excited HC3N (ν7 = 4). The line shape in IRC +10216 appears to consist of two distinct components: a flat-topped profile with a width of ~28 km s−1, as is typical for this source, and a narrower feature approximately 4 km s−1 wide. The narrow component likely arises from the inner envelope (r < 8R*) where the gas has not reached the terminal expansion velocity, or it is nonthermal emission. Based on the broader component, the abundance of PH3 with respect to H2 is estimated to be 5 × 10−8 in a region with a radius of r < 150R*. If the narrower component is thermal, it implies a phosphine abundance of ~5 × 10−7 close to the stellar photosphere (r < 8R*). In CRL 2688, the PH3 abundance is less constrained, with plausible values ranging from 3 × 10−8 to 4 × 10−7, assuming a spherical distribution. Phosphine appears to be present in large concentrations in the inner envelope of C-rich AGB stars, and thus may function as a parent molecule for other phosphorus species.

Aromatic, aliphatic, and the unidentified 21 micron emission features in proto-planetary nebulae

AbstractAromatic features at 3.3, 6.2, 7.7, 8.6, 11.3 μm are observed in proto-planetary nebulae (PPNe) as well as in PNe and H ii regions. Aliphatic features at 3.4 and 6.9 μm are also observed; however, these features are often stronger in PPNe than in PNe. These observations suggest an evolution in the features from simple molecules (C2H2) in AGB stars to aliphatics in PPNe to aromatics in PNe. In the same carbon-rich PPNe, a strong, broad, unidentified 21 μm emission feature has been found. We will present recent observations of the aromatic, aliphatic, and 21 μm emission features, along with C2H2 (13.7 μm) and a new feature at 15.8 μm, and discuss correlations among them and other properties of these PPNe.

A Study of the 3.3 and 3.4 μm Emission Features in Proto–Planetary Nebulae

Medium-resolution spectra have been obtained of seven carbon-rich proto-planetary nebulae (PPNs) and one young planetary nebula from 3.2 to 3.8 μm, an interval containing the prominent hydrocarbon C–H stretches at 3.3 and 3.4 μm due to aromatic and aliphatic structures, respectively. The 3.3 μm feature is newly identified in IRAS 23304+6147, 22223+4327, and 06530-0213 and is confirmed in Z02229+6208. Three of the PPNs emit in the 3.4 μm feature, two of these being new identifications, IRAS 20000+3239 and 01005+7910, with two others showing possible detections. The 3.3 and 3.4 μm emission features in IRAS 22272+5435 are seen in the nebula offset from the star but not at the position of the central star, consistent with the 2003 results of Goto et al. A similar distribution is seen for the 3.3 μm feature in IRAS 22223+4327. All of the PPNs except IRAS 22272+5435 show Class A 3 μm emission features. These observations, when combined with those of the approximately equal number of other carbon-rich PPNs previously observed, demonstrate that there are large differences in the 3 μm emission bands, even for PPNs with central stars of similar spectral type, and thus that the behavior of the bands does not depend solely on spectral type. We also investigated other possible correlations to help explain these differences. These differences do not depend on the C/O value, since the Class B sources fall within the C/O range found for Class A. All of these 3.3 μm sources also show C2 absorption and 21 μm emission features, except IRAS 01005+7910, which is the hottest source at B0.

Diffraction‐Limited 3 μm Spectroscopy of IRAS 04296+3429 and IRAS 05341+0852: Spatial Extent of Hydrocarbon Dust Emission and Dust Evolutionary Sequence

We present 3 um spectroscopy of the carbon-rich proto-planetary nebulae IRAS 04296+3429 and IRAS 05341+0852 conducted with the adaptive optics system at the Subaru Telescope. We utilize the nearly diffraction-limited spectroscopy to probe the spatial extent of the hydrocarbon dust emitting zone. We find a hydrocarbon emission core extending up to 100--160 mas from the center of IRAS 04296+3429, corresponding to a physical diameter of 400--640 AU, assuming a distance of 4 kpc. On the other hand, we find that IRAS 05341+0852 is not spatially resolved with this instrumentation. The physical extent of these proto-planetary nebulae, along with the reanalyzed data of IRAS 22272+5435 published previously, suggests a correlation between the physical extent of the hydrocarbon dust emission and the spectral evolution of the aliphatic to aromatic features in these post-AGB stars. These measurements represent the first direct test of the proposed chemical synthesis route of carbonaceous dust in the circumstellar environment of evolved stars.

Reconsidering the Origin of the 21 Micron Feature: Oxides in Carbon‐rich Protoplanetary Nebulae?

The origin of the so-called 21 μm feature that is especially prominent in the spectra of some carbon-rich protoplanetary nebulae (PPNs) is the matter of a lively debate. A large number of potential band carriers have been presented and discarded within the past decade. This paper gives an overview of the problems related to the hitherto proposed feature identifications, including the recently suggested candidate carrier silicon carbide. We also discuss the case for spectroscopically promising oxides. SiC is shown to produce a strong resonance band at 20-21 μm if coated by a layer of silicon dioxide. At low temperatures, core-mantle particles composed of SiC and amorphous SiO2 indeed have their strongest spectral signature at a position of 20.1 μm, which coincides with the position of the 21 μm emission band. The optical constants of another candidate carrier that has been relatively neglected so far—iron monoxide—are proven to permit a fairly accurate reproduction of the 21 μm feature profile as well, especially when low-temperature measurements of the infrared properties of FeO are taken into account. As candidate carrier of the 21 μm emission band, FeO has the advantage of being stable against further oxidation and reduction only in a narrow range of chemical and physical conditions, coinciding with the fact that this feature, too, is detected in a small group of objects only. However, it is unclear how FeO would form or survive, particularly in carbon-rich PPNs.

The Polymerization of Acetylene, Hydrogen Cyanide, and Carbon Chains in the Neutral Layers of Carbon-rich Proto-planetary Nebulae

I present chemical evolution models for the neutral layers of proto-planetary nebulae in order to explain the high abundance of polyynes, cyanopolyynes, methyl-polyynes, benzene, and other C-bearing species found in CRL 618 by Cernicharo et al. In the neutral regions behind the H II region, the radiation field produces a rich photochemistry on timescales shorter than the dynamical evolution of the central H II region, leading to the formation of large carbon-rich molecules CnH2, HC(2n+1)N, and Cn. Reactions between radicals H and H2 play a crucial role in maintaining a high abundance for these species in spite of the strong radiation field emerging from the central star. The models predict that proto-planetary nebulae in the early stages of evolution will have high abundances for species such as HC(2n+1)N, CnH4, and CnH3N. The role of reactions between neutrals and radicals in the growth of aromatic molecules is discussed. The large abundance derived for small cumulenes suggests that the growth of small carbon grains with C/H ≫ 1 could be dominated by reactions between polyynes and cumulenes. The models also explain the observed abundances of H2CO, OH, H2O, CO2, and [O I] in the region where CO is photodissociated.

An Abundance Analysis of the New Carbon‐rich Proto–Planetary Nebula IRAS 06530−0213

In this paper we present a study of the proto-planetary nebula (PPN) IRAS 06530-0213 based on low- and high-resolution spectra. The low-resolution spectrum shows that the star is an F5 supergiant with molecular C2 and C3 and enhanced s-process lines. From the high-resolution spectra, the following atmospheric parameters were determined: Teff = 6900 K, log g = 1.0, and ξt = 4.5. Abundance analysis shows that IRAS 06530-0213 is metal-poor ([Fe/H] = -0.9) and overabundant in carbon ([C/Fe] = 1.3), nitrogen ([N/Fe] = 1.0), and s-process elements ([s-process/Fe] = 1.9), indicating asymptotic giant branch (AGB) nucleosynthesis and deep convective mixing. From the analysis of circumstellar C2 and CN molecular bands in the spectrum of IRAS 06530-0213, an envelope expansion velocity of Vexp = 14 ± 1 km s-1 was determined, a typical value for post-AGB stars. Also typical of PPNs is the double-peaked spectral energy distribution. The properties of both the photosphere and circumstellar envelope suggest that IRAS 06530-0213 is unambiguously a low-mass, carbon-rich PPN. For comparison purposes, new, high-resolution spectra of the well-known PPN HD 56126 (IRAS 07134+1005) were also analyzed and compared with previous results.

Spatially Resolved 3 Micron Spectroscopy of IRAS 22272+5435: Formation and Evolution of Aliphatic Hydrocarbon Dust in Proto–Planetary Nebulae

We present medium-resolution 3 lm spectroscopy of the carbon-rich proto–planetary nebula IRAS 22272+5435. Spectroscopy with the Subaru Telescope adaptive optics system revealed a spatial variation of hydrocarbon molecules and dust surrounding the star. The rovibrational bands of acetylene (C2H2) and hydrogen cyanide (HCN) at 3.0 lm are evident in the central star spectra. The molecules are concentrated in the compact region near the center. The 3.3 and 3.4 lm emission of aromatic and aliphatic hydrocarbons is detected at 600–1300 AU from the central star. The separation of spatial distribution between gas and dust suggests that the small hydrocarbon molecules are indeed the source of solid material and that the gas left over from the grain formation is being observed near the central star. The intensity of aliphatic hydrocarbon emission relative to the aromatic hydrocarbon emission decreases with distance from the central star. The spectral variation is well matched to that of a laboratory analog thermally annealed with different temperatures. We suggest that either the thermal process after the formation of a grain or the variation in the temperature in the dust-forming region over time determines the chemical composition of the hydrocarbon dust around the proto–planetary nebula. Subject headings: circumstellar matter — dust, extinction — infrared: ISM — ISM: evolution — stars: AGB and post-AGB — stars: individual (IRAS 22272+5435)

Westbrook's Molecular Gun: Discovery of Near-Infrared Microstructures and Molecular Bullets in AFGL 618

We present high-sensitivity near-IR images of a carbon-rich proto-planetary nebula, AFGL 618, obtained with the Infrared Camera and Spectrograph (IRCS) mounted on the 8.2m Subaru Telescope. The deep near-IR images have revealed “bullets” and “horns” extending farther out from the edges of the previously known bipolar nebulosities that consist of dust-scattered star light component and shock-excited line emission component. That these bullets and horns represent the positions from which [Fe II] IR lines arise is strongly suggested from the spatial coincidence between these near-IR microstructures and the optical collimated outflow structure observed by the recent HST/WFPC2 imaging, together with the previous detection of shock-excited, forbidden IR lines of atomic species at those locations. At these positions of the near-IR mincrostructures, we have also discovered CO clumps moving at &gt; 200 km s-1 from our re-analysis of the existing 12CO J = 1 – 0 data obtained with the Berkeley-Illinois-Maryland Association (BIMA) interferometer array. These findings indicate that fast-moving CO clumps seem to be impinging upon the surrounding ambient circumstellar shell, thereby causing shocked emission regions which manifest themselves as the near-IR microstructures at the shock interface. To deepen our understanding of the connection between the near-IR microstructures and the CO outflow structure, we are currently conducting higher resolution observation in CO lines with the BIMA array.

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.

Westbrook’s Molecular Gun: Discovery of Near-Infrared Microstructures in AFGL 618

We present high-sensitivity near-IR images of a carbon-rich proto-planetary nebula, AFGL 618, obtained with the Subaru Telescope. These images have revealed and extending farther out from the edges of the previously known bipolar lobes. The spatial coincidence between these near-IR microstructures and the optical collimated outflow structure, together with the detection of shock-excited, forbidden IR lines of atomic species, strongly suggests that these bullets and horns represent the locations from which [Fe II] IR lines arise. We have also discovered CO clumps moving at greater than 200 km s-1 at the positions of the near-IR bullets by reanalyzing the existing 12CO J = 1-0 interferometry data. These findings indicate that the near-IR microstructures represent the positions of shocked surfaces at which fast-moving molecular clumps interface with the ambient circumstellar shell.