Asymptotic Giant Branch Star IRC Research Articles

A λ 3 mm Line Survey toward the Circumstellar Envelope of the Carbon-rich AGB Star IRC+10216 (CW Leo)

We present an unbiased λ 3 mm spectral line survey (between 84.5 and 115.8 GHz), conducted by the Purple Mountain Observatory 13.7 m radio telescope, together with updated modeling results, toward the carbon-rich asymptotic giant branch star IRC+10216 (CW Leo). A total of 75 spectral lines (96 transitions) are detected, and identified to arise from 19 molecules: C2H, l-C3H, C4H, CN, C3N, HC3N, HC5N, HCN, HNC, CH3CN, MgNC, CO, c-C3H2, SiC2, SiO, SiS, CS, C2S, C3S, and their isotopologues. Among them, one molecular emission line (H13CCCN J = 13–12) is discovered in IRC+10216 for the first time. The excitation temperature, column density, and fractional abundance of the detected species are deduced by assuming they are in local thermodynamic equilibrium. In addition, the isotopic ratios of [12C]/[13C], [32S]/[34S], [28Si]/[29Si], and [12C34S]/[13C32S] are obtained and found to be consistent with previous studies. Finally, we summarize all of the 106 species detected in IRC+10216 to date with their observed and modeled column densities for the convenience of future studies.

Spectroscopic Study of [Mg, H, N, C, O] Species: Implications for the Astrochemical Magnesium Chemistry.

Magnesium is an abundant metal element in space, and magnesium chemistry has vital importance in the evolution of interstellar medium (ISM) and circumstellar regions, such as the asymptotic giant branch star IRC+10216 where a variety of Mg compounds bearing H, C, N, and O have been detected and proposed as the important components in the gas-phase molecular clouds and solid-state dust grains. Herein, we report the formation and infrared spectroscopic characterization of the Mg-bearing molecules HMg, [Mg, N, C], [Mg, H, N, C], [Mg, N, C, O], and [Mg, H, N, C, O] from the reactions of Mg/Mg+ and the prebiotic isocyanic acid (HNCO) in the solid neon matrix. Based on their thermal diffusion and photochemical behavior, a complex reactivity landscape involving association, decomposition, and isomerization reactions of these Mg-bearing molecules is developed, which can not only help understand the chemical processes of the magnesium (iso)cyanides in astrochemistry but also provide implications on the presence of magnesium (iso)cyanates in the ISM and the chemical model for the dust grain surface reactions. It also provides a new paradigm of the key intermediate nature of the cationic complexes in the formation of neutral interstellar species.

Elusive Iron: Detection of the FeC Radical (X 3Δ i ) in the Envelope of IRC+10216

A new interstellar molecule, FeC (X 3Δ i ), has been identified in the circumstellar envelope of the carbon-rich asymptotic giant branch star IRC+10216. FeC is the second iron-bearing species conclusively observed in the interstellar medium, in addition to FeCN, also found in IRC+10216. The J = 4 → 3, 5 → 4, and 6 → 5 rotational transitions of this free radical near 160, 201, and 241 GHz, respectively, were detected in the lowest spin–orbit ladder, Ω = 3, using the Submillimeter Telescope of the Arizona Radio Observatory (ARO) for the 1 mm lines and the ARO 12 m at 2 mm. Because the ground state of FeC is inverted, these transitions are the lowest energy lines. The detected features exhibit slight U shapes with LSR velocities near V LSR ≈ −26 km s−1 and linewidths of ΔV 1/2 ≈ 30 km s−1, line parameters characteristic of IRC+10216. Radiative transfer modeling of FeC suggests that the molecule has a shell distribution with peak radius near 300 R * (∼6″) extending out to ∼500 R * (∼10″) and a fractional abundance, relative to H2, of f ∼ 6 × 10−11. The previous FeCN spectra were also modeled, yielding an abundance of f ∼ 8 × 10−11 in a larger shell situated near 800 R *. These distributions suggest that FeC may be the precursor species for FeCN. Unlike cyanides and carbon-chain molecules, diatomic carbides with a metallic element are rare in IRC+10216, with FeC being the first such detection.

Atmospheric molecular blobs shape up circumstellar envelopes of AGB stars.

During their thermally pulsing phase, asymptotic giant branch (AGB) stars eject material that forms extended dusty envelopes1. Visible polarimetric imaging found clumpy dust clouds within two stellar radii of several oxygen-rich stars2-6. Inhomogeneous molecular gas has also been observed in multiple emission lines within several stellar radii of different oxygen-rich stars, including W Hya and Mira7-10. At the stellar surface level, infrared images have shown intricate structures around the carbon semiregular variable R Scl and in the S-type star π1 Gru11,12. Infrared images have also shown clumpy dust structures within a few stellar radii of the prototypical carbon AGB star IRC+10°216 (refs. 13,14), and studies of molecular gas distribution beyond the dust formation zone have also shown complex circumstellar structures15. Because of the lack of sufficient spatial resolution, however, the distribution of molecular gas in the stellar atmosphere and the dust formation zone of AGB carbon stars is not known, nor is how it is subsequently expelled. Here we report observations with a resolution of one stellar radius of the recently formed dust and molecular gas in the atmosphere of IRC+10°216. Lines of HCN, SiS and SiC2 appear at different radii and in different clumps, which we interpret as large convective cells in the photosphere, as seen in Betelgeuse16. The convective cells coalesce with pulsation,causing anisotropies that, together with companions17,18, shape its circumstellar envelope.

Grain Alignment in the Circumstellar Shell of IRC+10° 216

Dust-induced polarization in the interstellar medium (ISM) is due to asymmetric grains aligned with an external reference direction, usually the magnetic field. For both the leading alignment theories, the alignment of the grain’s angular momentum with one of its principal axes and the coupling with the magnetic field requires the grain to be paramagnetic. Of the two main components of interstellar dust, silicates are paramagnetic, while carbon dust is diamagnetic. Hence, carbon grains are not expected to align in the ISM. To probe the physics of carbon grain alignment, we have acquired Stratospheric Observatory for Infrared Astronomy/Higch-resolution Airborne Wideband Camera-plus far-infrared photometry and polarimetry of the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10° 216. The dust in such CSEs are fully carbonaceous and thus provide unique laboratories for probing carbon grain alignment. We find a centrosymmetric, radial, polarization pattern, where the polarization fraction is well correlated with the dust temperature. Together with estimates of a low fractional polarization from optical polarization of background stars, we interpret these results to be due to a second-order, direct radiative external alignment of grains without internal alignment. Our results indicate that (pure) carbon dust does not contribute significantly to the observed ISM polarization, consistent with the nondetection of polarization in the 3.4 μm feature due to aliphatic CH bonds on the grain surface.

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.

Discovery of multiple dust shells beyond 1 arcmin in the circumstellar envelope of IRC +10216 usingHerschel/PACS

We present new Herschel/PACS images at 70, 100, and 160 micron of the well-known, nearby, carbon-rich asymptotic giant branch star IRC+10216 revealing multiple dust shells in its circumstellar envelope. For the first time, dust shells (or arcs) are detected until 320 arcsec. The almost spherical shells are non-concentric and have an angular extent between 40 deg and 200 deg. The shells have a typical width of 5 arcsec - 8 arcsec, and the shell separation varies in the range of 10 arcsec - 35 arcsec, corresponding to 500-1700 yr. Local density variations within one arc are visible. The shell/intershell density contrast is typically 4, and the arcs contain some 50% more dust mass than the smooth envelope. The observed (nested) arcs record the mass-loss history over the past 16 000 yr, but Rayleigh-Taylor and Kelvin-Helmholtz instabilities in the turbulent astropause and astrosheath will erase any signature of the mass-loss history for at least the first 200 000 yr of mass loss. Accounting for the bowshock structure, the envelope mass around IRC+10216 contains >2Msun of gas and dust mass. It is argued that the origin of the shells is related to non-isotropic mass-loss events and clumpy dust formation.

HERSCHEL /HIFI OBSERVATIONS OF IRC+10216: WATER VAPOR IN THE INNER ENVELOPE OF A CARBON-RICH ASYMPTOTIC GIANT BRANCH STAR

We report the results of observations of ten rotational transitions of water vapor toward the carbon-rich AGB (asymptotic giant branch) star IRC+10216 (CW Leonis), carried out with Herschel's HIFI instrument. Each transition was securely detected by means of observations using the dual beam switch mode of HIFI. The measured line ratios imply that water vapor is present in the inner outflow at small distances (few x 1.E+14 cm) from the star, confirming recent results reported by Decin et al. from observations with Herschel's PACS and SPIRE instruments. This finding definitively rules out the hypothesis that the observed water results from the vaporization of small icy objects in circular orbits. The origin of water within the dense C-rich envelope of IRC+10216 remains poorly understood. We derive upper limits on the H2-17O/H2-16O and H2-18O/H2-16O isotopic abundance ratios of ~ 5.E-3 (3 sigma), providing additional constraints on models for the origin of the water vapor in IRC+10216.

THE ARIZONA RADIO OBSERVATORY 1 mm SPECTRAL SURVEY OF IRC +10216 AND VY CANIS MAJORIS (215–285 GHz)

A low noise (1{sigma} rms {approx} 3 mK) 1 mm spectral survey (214.5-285.5 GHz) of the oxygen-rich supergiant VY Canis Majoris and the carbon-rich asymptotic giant branch star IRC +10216 has been conducted using the Arizona Radio Observatory's 10 m Submillimeter Telescope. Here the complete data set is presented. This study, carried out with a new ALMA-type receiver, marks the first continuous band scan of an O-rich circumstellar envelope, and the most sensitive survey to date of IRC +10216. In VY CMa, 130 distinct molecular lines were detected, 14 of which cannot be identified; in IRC +10216, 717 lines were observed, with 126 features remaining unidentified. In the 1 mm bands of VY CMa and IRC +10216, emission is present from 18 and 32 different chemical compounds, respectively, with 10 species common to both sources. Many narrow emission lines were observed in both circumstellar shells, arising from vibrationally excited molecules and from refractory-containing species. Line profiles in VY CMa also exhibit a variety of different shapes, caused by the complex, asymmetric outflow of this object. The survey highlights the fact that C-rich and O-rich circumstellar envelopes are chemically interesting, and both are sources of new interstellar molecules. The high numbermore » of unidentified lines and the unreliable rest frequencies for known species such as NaCN indicate the need for additional laboratory spectroscopy studies.« less

DENSITY-ENHANCED GAS AND DUST SHELLS IN A NEW CHEMICAL MODEL FOR IRC+10216

A new chemical model is presented for the carbon-rich circumstellar envelope (CSE) of the asymptotic giant branch star IRC+10216. The model includes shells of matter with densities that are enhanced relative to the surrounding circumstellar medium. The chemical model uses an updated reaction network including reactions from the RATE06 database and a more detailed anion chemistry. In particular, new mechanisms are considered for the formation of CN{sup -}, C{sub 3}N{sup -}, and C{sub 2}H{sup -}, and for the reactions of hydrocarbon anions with atomic nitrogen and with the most abundant cations in the CSE. New reactions involving H{sup -} are included which result in the production of significant amounts of C{sub 2}H{sup -} and CN{sup -} in the inner envelope. The calculated radial molecular abundance profiles for the hydrocarbons C{sub 2}H, C{sub 4}H, and C{sub 6}H and the cyanopolyynes HC{sub 3}N and HC{sub 5}N show narrow peaks which are in better agreement with observations than previous models. Thus, the narrow rings observed in molecular microwave emission surrounding IRC+10216 are interpreted as arising in regions of the envelope where the gas and dust densities are greater than the surrounding circumstellar medium. Our models show that CN{sup -} and C{sub 2}H{sup -}more » may be detectable in IRC+10216 despite the very low theorized radiative electron attachment rates of their parent neutral species. We also show that magnesium isocyanide (MgNC) can be formed in the outer envelope through radiative association involving Mg{sup +} and the cyanopolyyne species.« less

Detection of Formaldehyde toward the Extreme Carbon Star IRC +10216

We report the detection of H2CO (formaldehyde) around the carbon-rich asymptotic giant branch star IRC +10216. We find a fractional abundance with respect to molecular hydrogen of x(H2CO) = 1.3 × 10-8. This corresponds to a formaldehyde abundance with respect to water vapor of x(H2CO)/x(H2O) = (1.1 ± 0.2) × 10-2, in line with the formaldehyde abundances found in solar system comets, and indicates that the putative extrasolar cometary system around IRC +10216 may have a similar chemical composition to solar system comets. However, we also failed to detect CH3OH (methanol) around IRC +10216, and our upper limit of x(CH3OH)/x(H2O) < 7.7 × 10-4 (3 σ) indicates that methanol is substantially underabundant in IRC +10216 compared to solar system comets. We also conclude, on the basis of offset observations, that formaldehyde has an extended source in the envelope of IRC +10216 and may be produced by the photodissociation of a parent molecule, similar to the production mechanism for formaldehyde in solar system comet comae. Preliminary mapping observations also indicate a possible asymmetry in the spatial distribution of formaldehyde around IRC +10216, but higher signal-to-noise ratio observations are required to confirm this finding. By serendipity, our observations have led to the detection of the J = 17-16 transition of Al37Cl at 241.855 GHz. Our analysis of the measured line flux, along with those of previously observed lower frequency transitions, yields a total AlCl (aluminum monochloride) abundance in the range (2-8) × 10-8 relative to H2; this range, which is a factor of 10 smaller than an abundance estimate that has appeared previously in the literature, amounts to ~4%-16% of the solar elemental abundance of chlorine, a fraction that is in accord with the predictions of thermochemical equilibrium models for cool stellar photospheres.

A Chemical Route to the Formation of Water in the Circumstellar Envelopes around Carbon-rich Asymptotic Giant Branch Stars: Fischer-Tropsch Catalysis

Fischer-Tropsch catalysis has been suggested as a means of driving hydrocarbon chemistry in oxygen rich regions such as the protosolar nebula. In addition to producing hydrocarbons, Fischer-Tropsch catalysis also produces water, and it is therefore possible that such processes could account for the recent observations of water in the circumstellar envelope of asymptotic giant branch star IRC +10216.

Methylpolyynes and Small Hydrocarbons in CRL 618

We report on the detection with the Infrared Space Observatory of strong infrared absorption from NH3 and C2H4 in CRL 618. The observed NH3 and C2H4 bands arise from a region with kinetic temperatures 200 K, i.e., the dense gas in the photodissociation region associated to the dense torus surrounding the central star, as was the case for the polyynes and cyanopolyynes (see the companion Letter). Several absorption bands, probably arising from small gas-phase hydrocarbons, are observed between 5.5 and 11 μm. Two of these species have been identified with the 30 m IRAM telescope as the methylpolyynes CH3C2H and CH3C4H. However, the absorption around 6.2 μm is particularly broad and could arise from the combination of these small hydrocarbons and from the aromatic C=C stretching of polycyclic aromatic hydrocarbons of moderate size. These bands and those associated to the polyynes, cyanopolyynes, methylpolyynes, and benzene are not present in the infrared spectrum of the asymptotic giant branch star IRC +10216.