Proto-planetary Nebula CRL Research Articles

Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

Exploring the Ionized Core of the Proto-Planetary Nebula CRL 618 and Its Vicinity with ALMA

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.

The spectra of evolved stars at 20–25 GHz: Tracing circumstellar chemistry during the asymptotic giant branch to planetary nebula transition

Abstract We report an unbiased radio line survey towards the circumstellar envelopes of evolved stars at the frequency range from 20 to 25 GHz, aiming to obtain a more complete unbiased picture of the chemical evolution in the final stages of stellar evolution. The observation sample includes the asymptotic giant branch (AGB) star IRC +10216, the proto-planetary nebulae (PPNs) CRL 2688 and CRL 618, and the young planetary nebula (PN) NGC 7027, representing an evolutionary sequence spanning about 10000 years. Rotational transitions from cyanopolyyne chains and inversion lines from ammonia are detected in the AGB star and PPNs, while the PN displays several recombination lines. The different spectral behaviors of these evolved stars clearly reflect the evolution of circumstellar chemistry during the AGB–PPN–PN transitions.

Submillimetre polarization and magnetic field properties in the envelopes of protoplanetary nebulae CRL 618 and OH 231.8+4.2

We have carried out continuum and line polarisation observations of two Proto-planetary nebulae (PPNe), CRL 618 and OH 231.8+4.2, using the Submillimeter Array (SMA) in its compact configuration. The frequency range of observations, 330-345 GHz, includes the CO(J=3-2) line emission. CRL 618 and OH 231.8+4.2 show quadrupolar and bipolar optical lobes, respectively, surrounded by a dusty envelope reminiscent of their AGB phase. We report a detection of dust continuum polarised emission in both PPNe above 4 sigma but no molecular line polarisation detection above a 3 sigma limit. OH 231.8+4.2 is slightly more polarised on average than CRL 618 with a mean fractional polarisation of 4.3 and 0.3 per cent, respectively. This agrees with the previous finding that silicate dust shows higher polarisation than carbonaceous dust. In both objects, an anti-correlation between the fractional polarisation and the intensity is observed. Neither PPNe show a well defined toroidal equatorial field, rather the field is generally well aligned and organised along the polar direction. This is clearly seen in CRL 618 while in the case of OH 231.8+4.2, the geometry indicates an X-shaped structure coinciding overall with a dipole/polar configuration. However in the later case, the presence of a fragmented and weak toroidal field should not be discarded. Finally, in both PPNe, we observed that the well organised magnetic field is parallel with the major axis of the 12CO outflow. This alignment could indicate the presence of a magnetic outflow launching mechanism. Based on our new high resolution data we propose two scenarios to explain the evolution of the magnetic field in evolved stars.

The molecular envelope of CRL 618: A new model based onHerschel/HIFI observations

We study the physical properties and molecular excitation of the different warm gas components found in the protoplanetary nebula CRL 618. We revise our previous Herschel/HIFI observations, which consist of several 12CO and 13CO lines in the far-infrared/sub-mm band. These data have been re-analyzed in detail by improving calibration, the signal-to-noise-ratio, and baseline substraction. We identify the contributions of the different nebular components to the line profiles. We have used a spatio-kinematical model to better constrain the temperature, density, and kinematics of the molecular components probed by the improved CO observations. The 12CO and 13CO J=16-15, J=10-9, and J=6-5 transitions are detected in this source. The line profiles present a composite structure showing spectacular wings in some cases, which become dominant as the energy level increases. Our analysis of the high-energy CO emission with the already known low-energy J=2-1 and J=1-0 lines confirms that the high-velocity component, or fast bipolar outflow, is hotter than previously estimated with a typical temperature of ~300 K. This component may then be an example of a very recent acceleration of the gas by shocks that has not yet cooled down. We also find that the dense central core is characterized by a very low expansion velocity, ~5 km/s, and a strong velocity gradient. We conclude that this component is very likely to be the unaltered circumstellar layers that are lost in the last AGB phase, where the ejection velocity is particularly low. The physical properties of the other two nebular components, the diffuse halo and the double empty shell, more or less agrees with the estimations derived in previous models.

PO and PN in the wind of the oxygen-rich AGB star IK Tauri

Phosphorus-bearing compounds have only been studied in the circumstellar environments (CSEs) of the asymptotic giant branch (AGB) star IRC +10216 and the protoplanetary nebula CRL 2688, both C-rich objects, and the O-rich red supergiant VY CMa. The current chemical models cannot reproduce the high abundances of PO and PN derived from observations of VY CMa. No observations have been reported of phosphorus in the CSEs of O-rich AGB stars. We aim to set observational constraints on the phosphorous chemistry in the CSEs of O-rich AGB stars, by focussing on the Mira-type variable star IK Tau. Using the IRAM 30m telescope and the Submillimeter Array (SMA), we observed four rotational transitions of PN (J=2-1,3-2,6-5,7-6) and four of PO (J=5/2-3/2,7/2-5/2,13/2-11/2,15/2-13/2). The IRAM 30m observations were dedicated line observations, while the SMA data come from an unbiased spectral survey in the frequency range 279-355 GHz. We present the first detections of PN and PO in an O-rich AGB star and estimate abundances X(PN/H2) of about 3x10^-7 and X(PO/H2) in the range 0.5-6.0x10^-7. This is several orders of magnitude higher than what is found for the C-rich AGB star IRC +10216. The diameter (<=0.7") of the PN and PO emission distributions measured in the interferometric data corresponds to a maximum radial extent of about 40 stellar radii. The abundances and the spatial occurrence of the molecules are in very good agreement with the results reported for VY CMa. We did not detect PS or PH3 in the survey. We suggest that PN and PO are the main carriers of phosphorus in the gas phase, with abundances possibly up to several 10^-7. The current chemical models cannot account for this, underlining the strong need for updated chemical models that include phosphorous compounds.

The chemical composition of the post-asymptotic giant branch F supergiant CRL 2688

We present an analysis of a high resolution (R~50,000) optical spectrum of the central region of the proto-planetary nebula CRL 2688. This object is thought to have recently moved off the AGB, and display abundance patterns of CNO and heavy elements that can provide us with important clues to understand the nucleosynthesis, dredge-up and mixing experienced by the envelope of the central star during its AGB stage of evolution. The analysis of the molecular features, presumably originated from the circumstellar matter provides further constraints on the chemistry and velocity of the expanding shell, expelled as a consequence of the strong mass loss experienced by the central star. We confirm that the central star shows a spectrum typical of an F-type supergiant with Teff=7250 K, log g=0.5 and [Fe/H]=-0.3 dex. We find that the abundance pattern of this object is characterized by enhancements of Carbon ([C/Fe]=0.6), Nitrogen ([N/Fe]=1.0) and Na ([Na/Fe]=0.7), similar to other previously known carbon-rich post-AGB stars. Yttrium is also enhanced while the [Ba/Y] ratio is very low (-1.0), indicating that only the light s-process elements are enhanced. The Zinc abundance is found to be normal, [Zn/Fe]=0.0, suggesting that there is no depletion of refractory elements. The Halpha, Na I and K I resonance lines show prominent emission components, whose helio-centric radial velocities are offsetted by -41 km/s relative to the photospheric metal-absorption lines. The molecular features of C_2 and CN also show emission components, whose velocities are consistent with the emission components of the Halpha, Na I, and K I lines. On the other hand, their absorption components are more highly blue shifted than the corresponding emission components, which suggests that the regions where the emission and absorption components arise are expanding at different velocities.

STIS optical spectroscopy of the lobes of CRL 618

Context. Many proto-planetary nebulae show collimated structures sometimes showing multiple outflows.Aims. We present the results of new optical spectroscopic observations (both medium (with a dispersion of 0.56 A pixel-1 ) and low (2.73 and 4.92 A pixel-1 for the G430L and G750L gratings) spectral resolution) of the lobes of the proto-planetary nebula CRL 618 obtained with the Space Telescope Imaging Spectrograph on board of the Hubble Space Telescope. Methods. We analyse the density structure and the excitation conditions of the lobes of CRL 618. The spectra obtained at medium spectral resolution (~50 km s-1 ) allow us to quote the fraction of unscattered (intrinsic) Hα emission. We have also obtained de-reddened emission line ratios of several features from the low spectral resolution spectra.Results. We find that most of the analysed line ratios are reproduced by planar shocks moving through a dense medium (with pre-shock densities ~104 cm-3 ) with shock velocities from 30 to 40 km s-1 (except the [O III]/Hβ line ratios which require shock velocities of 80 to 90 km s-1 ). We find that the [S II]-weighted ionization fraction ranges from 0.015 to 0.06. The total densities derived from the electron density and the ionization fraction are ~105 to 106 cm-3 . Conclusions. We conclude that the spectra of the lobes of CRL 618 can be margially reproduced by steady plane–parallel shock models for shock velocities which are significantly lower than the velocities at which the jet moves outwards (~200 km s-1 ). These results are consistent with the predictions of a jet with a variable ejection velocity. The mirror-symmetry, the luminosity asymmetry between both lobes and the ejection velocity variability suggest that its central source may host a binary system.

AN EXPERIMENTAL AND THEORETICAL STUDY ON THE IONIZATION ENERGIES OF POLYYNES (H-(C≡C)n-H;n= 1-9)

We present a combined experimental and theoretical work on the ionization energies of polyacetylene -- organic molecules considered as important building blocks to form polycyclic aromatic hydrocarbons (PAHs) in the proto planetary nebulae such as of CRL 618. This set of astrophysical data can be utilized with significant confidence in future astrochemical models of photon-dominated regions and also of the proto planetary nebulae CRL 618. We recommend ionization energies of polyacetylenes from diacetylene up to heptaacetylene with an experimental accuracy of +- 0.05 eV: 10.03 eV (diacetylene), 9.45 eV (triacetylene), 9.08 eV (tetraacetylene), 8.75 eV (pentaacetylene), 8.65 eV (hexaacetylene), and 8.50 eV (heptaacetylene); further, ionization energies and with an accuracy of +- 0.1 eV: 8.32 eV (octaacetylene) and 8.24 eV (nonaacetylene) were computed. Implications of these energies to the redox chemistry involved in the multiply charged metal-ion mediated chemistry of hydrocarbon-rich atmospheres of planets and their moons such as Titan are also discussed.

Dissociation of the benzene molecule by ultraviolet and soft X-rays in circumstellar environment

Benzene molecules, present in the proto-planetary nebula CRL 618, are ionized and dissociated by ultraviolet (UV) and X-ray photons originated from the hot central star and by its fast wind. Ionic species and free radicals produced by these processes can lead to the formation of new organic molecules. The aim of this work is to study the photoionization and photodissociation processes of the benzene molecule, using synchrotron radiation and time-of-flight mass spectrometry. Mass spectra were recorded at different energies corresponding to the vacuum UV (21.21 eV) and soft X-ray (282–310 eV) spectral regions. The production of ions from the benzene dissociative photoionization is here quantified, indicating that C6H6 is more efficiently fragmented by soft X-ray than UV radiation, where 50 per cent of the ionized benzene molecules survive to UV dissociation while only about 4 per cent resist to X-rays. Partial ion yields of H+ and small hydrocarbons, such as C2H+2, C3H+3, C4H+2, are determined as a function of photon energy. Absolute photoionization and dissociative photoionization cross-sections have also been determined. From these values, half-life of benzene molecule due to UV and X-ray photon fluxes in CRL 618 was obtained.

A MOLECULAR LINE SURVEY OF THE HIGHLY EVOLVED CARBON STAR CIT 6

We present a spectral line survey of the C-rich envelope CIT 6 in the 2mm and 1.3mm bands carried out with the Arizona Radio Observatory (ARO) 12m telescope and the Heinrich Hertz Submillimeter Telescope (SMT). The observations cover the frequency ranges of 131--160 GHz, 219--244 GHz, and 252--268 GHz with typical sensitivity limit of T_R<10 mK. A total of 74 individual emission features are detected, of which 69 are identified to arise from 21 molecular species and isotopologues, with 5 faint lines remaining unidentified. Two new molecules (C4H and CH3CN) and seven new isotopologues (C17O, 29SiC2, 29SiO, 30SiO, 13CS, C33S, and CS) are detected in this object for the first time. The column densities, excitation temperatures, and fractional abundances of the detected molecules are determined using rotation diagram analysis. Comparison of the spectra of CIT 6 to that of IRC+10216 suggests that the spectral properties of CIT 6 are generally consistent with those of IRC+10216. For most of the molecular species, the intensity ratios of the lines detected in the two objects are in good agreement with each other. Nevertheless, there is evidence suggesting enhanced emission from CN and HC3N and depleted emission from HCN, SiS, and C4H in CIT 6. Based on their far-IR spectra, we find that CIT 6 probably has a lower dust-to-molecular gas ratio than IRC+10216. To investigate the chemical evolution of evolved stars, we compare the molecular abundances in the AGB envelopes CIT 6 and IRC+10216 and those in the bright proto-planetary nebula CRL 618. The implication on the circumstellar chemistry is discussed.

A SPECTRAL LINE SURVEY OF CRL 2688 IN THE RANGE OF 85-116 GHz

A spectral line survey of the proto-planetary nebula CRL 2688 was carried out in the frequency range of 85-116 GHz with the 14 m radio telescope at the Taeduk Radio Astronomy Observatory. Eight detected molecules are all carbon bearing molecules: HCN, HNC, C2H, HC3N, CS, CCS, CN, and CO, and their isotopic species H13CN and 13CO. A CCS line was newly detected in CRL 2688. The molecular lines detected in CRL 2688 were also compared with those of the proto-typical AGB star IRC +10216. The SiS (J = 5-4, v = 0) molecular line detected in IRC +10216 was not detected in CRL 2688, indicating a lower abundance of the SiS molecule in CRL 2688 than in IRC +10216. Excitation temperatures and column densities of HC3N in both CRL 2688 and IRC +10216 were derived using standard LTE rotational diagram analysis. The HC3N column density of CRL 2688 was found to be lower than that of IRC +10216. This fact, together with the lower abundance of the SiS molecule in CRL 2688, may be related to the chemical evolution of the circumstellar envelope of CRL 2688.

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.

Proto–Planetary Nebulae as Explosions: Bullets versus Jets and Nebular Shaping

Many proto-planetary nebulae (PPN) appear as narrow collimated structures sometimes showing multiple, roughly aligned lobes. In addition, many PPN flows have been shown to have short acceleration times. In this paper we explore whether jet or ``bullet'' (a massive clump) models fit the observations of individual collimated lobes adequately by comparing simulations of both radiatively cooled (stable) jets and bullets. We find that the clump model is somewhat favored over jets because (1) it leads to greater collimation of outflows (2) it accounts better and more naturally for ring-like structures observed in the PPN CRL 618, and (3) it is more successful in reproducing the Hubble-flow character of observed kinematics in some PPN. In addition, bullets naturally account for observed multipolar flows, since the likely MHD launch mechanisms required to drive outflows make multiple non-aligned jets unlikely. Thus we argue that PPN outflows may be driven by explosive MHD launch mechanisms such as those discussed in the context of supernovae (SNe) and gamma-ray bursts(GRB).

A Search for ortho -benzyne ( o -C 6 H 4 ) in CRL 618

Polycyclic aromatic hydrocarbons (PAHs) have been proposed as potential carriers of the unidentified infrared bands (UIRs) and the diffuse interstellar bands (DIBs). PAHs are not likely to form by gas-phase or solid-state interstellar chemistry, but rather might be produced in the outflows of carbon-rich evolved stars. PAHs could form from acetylene addition to the phenyl radical (C6H5), which is closely chemically related to benzene (C6H6) and ortho-benzyne (o-C6H4). To date, circumstellar chemical models have been limited to only a partial treatment of benzene-related chemistry, and so the expected abundances of these species are unclear. A detection of benzene has been reported in the envelope of the proto-planetary nebula (PPN) CRL 618, but no other benzene-related species has been detected in this or any other source. The spectrum of o-C6H4 is significantly simpler and stronger than that of C6H5, and so we conducted deep Ku-, K- and Q-band searches for o-C6H4 with the Green Bank Telescope. No transitions were detected, but an upper limit on the column density of 8.4x10^13 cm^-2 has been determined. This limit can be used to constrain chemical models of PPNe, and this study illustrates the need for complete revision of these models to include the full set of benzene-related chemistry.

Submillimeter Array Observation of the Proto-Planetary Nebula CRL 618 in the CO J = 6-5 Line

We report on the results of a Submillimeter Array interferometric observation of the proto-planetary nebula CRL 618 in the 12CO J=6-5 line. With the new capability of SMA enabling us to use two receivers at a time, we also observed simultaneously in the 12CO J=2-1 and 13CO J=2-1 lines. The 12CO J=6-5 and 13CO J=2-1 lines were first interferometrically observed toward CRL 618. The flux of the high velocity component of the 12CO J=6-5 line is almost fully recovered, while roughly 80% of the flux of the low velocity component is resolved out. The low recovery rate suggests that the emission region of the low velocity component of the 12CO J=6-5 line is largely extended. Continuum emission is detected both at 230 and 690 GHz. The flux of the 690 GHz continuum emission seems to be partially resolved out, suggesting dust emission partly contaminates the 690 GHz continuum flux. The cavity structure, which has been confirmed in a previous observation in the 12CO J=2-1 line, is not clearly detected in the 12CO J=6-5 line, and only the south wall of the cavity is detected. This result suggests that the physical condition of the molecular envelope of CRL 618 is not exactly axial symmetric.

Molecular Line Survey of CRL 618 from 80 to 276 GHz and Complete Model

We present the complete data set, model and line identification of a survey of the emission from the C-rich protoplanetary nebula CRL 618 performed with the IRAM-30m telescope in the following frequency ranges: 80.25-115.75 GHz, 131.25-179.25 GHz, and 204.25-275.250 GHz. A selection of lines from different species has been used in previous works to derive the structure of the source, its physical conditions and the chemical abundances in the different gas regions. In this work, we have used this information to run a global simulation of the spectrum in order to check the consistency of the model and to ease the task of line identification. The total number of lines that have a correspondence in both data and model is ~3100, although quite often in this object many lines blend into complex features so that the model, that takes into account line blending, is a key tool at this stage of the analysis. Of all the lines that we have been able to label, ~55% of them belong to the different forms of HC3N, and ~18% to those of HC5N. The density of remaining unidentified features above the 3sigma limit is only one per ~2.1 GHz (74 features), which is unprecedented in the analysis of this type of large millimeter-wave line surveys.

Molecular Abundances in CRL 618

In previous works we have modeled the different gas regions of the proto-planetary nebula CRL 618 by studying the large number of lines from the cyanopolyynes detected in a millimeter-wave line survey of this object. In this work we retrieve the rotational temperatures (Tr) and abundance ratios R with respect to HC3N (used as reference) for all molecular species detected in the survey by running grids of models in the (R,Tr) space to find the minimum of a weighted χ2 defined for this analysis. This provides the best knowledge to date of the (polar) molecular composition of CRL 618 thanks to the uniform calibration of the whole survey and the large number of lines available from each species, allowing comparisons with predictions made by chemical models of C-rich post-AGB objects. A significantly lower value of the 12C/13C ratio has been revealed in the gas closest to central star with respect to the colder and outer envelope. It can be due to 13C-rich material, produced in a late CNO cycling occurred in the central star, being currently injected into this inner gas envelope.

A Survey of Large Molecules toward the Proto–Planetary Nebula CRL 618

We present the results of our survey toward the proto-planetary nebula CRL 618 for several large, highly saturated, oxygen-bearing organic molecules of biological importance including acetaldehyde (CH3CHO), acetic acid (CH3COOH), dimethyl ether (CH3OCH3), ethanol (CH3CH2OH), formic acid (HCOOH) and methyl formate (HCOOCH3); large carbon chain molecules including methyl cyanide (CH3CN), methylcyanoacetylene (CH3C3N), cyanoacetylene (HC3N), cyanodiacetylene (HC5N), and C6H; and, finally, smaller molecules including 34SO, SO2, OC34S, and MgNC. No biologically important organic molecules were detected. However, we report the first interferometric detections of CH3CN and vibrationally excited HC3N and HC5N toward this source. The temperature and distribution of CH3CN toward CRL 618 indicates that it is formed in the outer envelope surrounding the ultracompact H II region. Furthermore, the P Cygni line profile and corresponding channel maps of vibrationally excited HC5N support its distribution in the extended envelope expanding radially from the central star. The detection of vibrationally excited HC3N confirmed the temperature structure and column density of HC3N in the inner envelope found by Wyrowski and colleagues. Finally, our observations clearly indicate that CRL 618 is a good source of large carbon chain species but a very poor source for detecting or producing organic species of biological importance.

1″ Resolution Mapping of the Molecular Envelope of the Protoplanetary Nebula CRL 618

We present 1 00 resolution interferometric maps of the CO J ¼ 2 1a nd HC3N J ¼ 25 24 emission in the protoplanetary nebula CRL 618 obtained with the millimeter array of the Owens Valley Radio Observatory (OVRO). Our CO data trace with high accuracy the spatio-kinematic structure of different molecular components: (1) the fast, bipolar outflow, which is observed up to axial distances of � 2B 5( � 2:7 ; 10 16 cm) from the nebula center with a maximal deprojected expansion velocity of Vexp � 340 km s � 1 ; (2) the roughly round outer halo, which is slowly expanding at Vexp � 17 km s � 1 ; (3) an extended structure that is elongated in the polar direction � 6 00 (� 8:1 ; 10 16 cm),surroundingtheopticallobes,andthatexpandsatVexp � 22kms � 1 (slowaxialcomponent); and(4)a dense, inner torus-likecore (Rout � 1:6 ; 10 16 cm) expanding atVexpP12kms � 1 . Components (3) and (4) have been revealed for the first time by our high angular resolution data. The geometry, kinematics, density, and temperaturedistributionoftheindividualcomponentsofthemolecularenvelopeofCRL618havebeenconstrained by fitting a spatio-kinematic model to the data. We interpret our results in terms of nebular evolution as follows. In the last k2500 yr, there have been at least two distinct episodes of mass loss at a large scale in the form of a slow wind. The firsttookplace at a rate of ˙