Sulphur-bearing carbon chains in IRC+10216

We have updated the chemical model of IRC+10216 developed by Millar, Herbst & Bettens to include recent routes to the formation of sulphuretted hydrocarbons. The routes are based on a quantum chemical study of the S+C₂H system. In addition, we have altered the parent species for sulphur to reflect new observational results. We find that the model calculations give excellent agreement with the observed column densities, and discuss the significance of these reactions to the formation of species as yet unobserved and to dark interstellar clouds.     

NCCN in TMC-1 and IRC+10216

We employ quantum chemical calculations using the CBS-RAD ('Complete Basis Set – Radicals') technique on the C₂N₂H potential energy surface to show that the reaction of HNC with CN is a viable and plausible route to NCCN in cold astrophysical environments. We use detailed chemical kinetic models to predict the abundance of NCCN in TMC-1 and IRC+10216. Radio-astronomical detection of NCCN is precluded by its lack of a dipole moment. We discuss other prospects for its observation in interstellar and circumstellar environments, by space-borne infrared spectroscopy, indirectly by detection of the NCCNH⁺ ion, or inferentially by detection of its higher-energy, polar isomer CNCN.     

Large molecules in the envelope surrounding IRC+10216

A new chemical model of the circumstellar envelope surrounding the carbon-rich star IRC+10216 is developed that includes carbon-containing molecules with up to 23 carbon atoms. The model consists of 3851 reactions involving 407 gas-phase species. Sizeable abundances of a variety of large molecules including carbon clusters, unsaturated hydrocarbons and cyanopolyynes have been calculated. Negative molecular ions of chemical formulae and C _n H (7 n 23) exist in considerable abundance, with peak concentrations at distances from the central star somewhat greater than their neutral counterparts. The negative ions might be detected in radio emission, or even in the optical absorption of background field stars. The calculated radial distributions of the carbon-chain C _n H radicals are looked at carefully and compared with interferometric observations.     

A Submillimeter HCN Laser in IRC +10216

We present subarcsecond thermal infrared imaging of HD 98800, a young quadruple system composed of a pair of low-mass spectroscopic binaries separated by 0&farcs;8 (38 AU), each with a K-dwarf primary. Images at wavelengths ranging from 5 to 24.5 μm show unequivocally that the optically fainter binary, HD 98800B, is the sole source of a comparatively large infrared excess on which a silicate emission feature is superposed. The excess is detected only at wavelengths of 7.9 μm and longer, peaks at 25 μm, and has a best-fit blackbody temperature of 150 K, indicating that most of the dust lies at distances greater than the orbital separation of the spectroscopic binary. We estimate the radial extent of the dust with a disk model that approximates radiation from the spectroscopic binary as a single source of equivalent luminosity. Given the data, the most likely values of disk properties in the ranges considered are Rin=5.0+/-2.5 AU, DeltaR=13+/-8 AU, lambda0=2+4-1.5 μm, gamma=0+/-2.5, and sigmatotal=16+/-3 AU2, where Rin is the inner radius, DeltaR is the radial extent of the disk, lambda0 is the effective grain size, gamma is the radial power-law exponent of the optical depth tau, and sigmatotal is the total cross section of the grains. The range of implied disk masses is 0.001-0.1 times that of the Moon. These results show that, for a wide range of possible disk properties, a circumbinary disk is far more likely than a narrow ring.     

1.3 mm Plateau de Bure Observations of IRC+10216

The Plateau de Bure millimeter-wave interferometer has been used at 1.3 mm wavelength to observe the prototype carbon-star IRC+10216 in the continuum and in the v = 1, J = 13 − 11 transition of SiS at 234.8 GHz. The 3 mm continuum and the v = (0, 2⁰, 2), J = 1 − 0 HCN transition were simultaneously observed. The source of vibrationally excited SiS appears resolved, and a source size of ∼ 0.4″ or 7 10¹⁴cm (10 stellar radii) is derived, in agreement with a crude model of line formation. This revised version was published online in September 2006 with corrections to the Cover Date.     

Gaseous refractory-element molecules in IRC10216

The list of detected refractory-element (RE) species in IRC10216 is now large enough to try to assess their chemistry, and the fraction of each that escapes in gas phase to the ISM. The former may tell us how grains are formed, the latter whether mass-loss from evolved stars is important in determining interstellar elemental depletions as distinct from accretion processes in the ISM. We expect that much of the Si chemistry is now understood and about 25% of Si escapes as a gas. Other REs are less well understood but most should be more volatile than Si. For many of the REs, O-rich CSEs should behave similarly to C-rich ones.Operated by Associated Universities, Inc., under cooperative agreement with the National Science Foundation     

The birth of a planetary nebula around the carbon star IRC+10216

Our current understanding of the evolution of solar-type stars suggests that after a period as a red giant star, during which mass loss occurs continuously in the form of a stellar wind, a period of intense mass loss known as a superwind occurs, during which a significant fraction of the envelope of the star is ejected into space, forming the material from which a planetary nebula (PN) will be constructed. It has been suggested that this superwind ejects material from the star in a toroidal or disc-like fashion, rather than isotropically. Here we present Hubble Space Telescope optical images of a toroidal superwind caught in the act: our images of the carbon star IRC+10216, which is believed to be in the final stages of red giant evolution, show that most of its optical emission is a bipolar reflection nebula. We show that the full spectral energy distribution and these images can be modelled as an equatorially enhanced dusty superwind, providing the first direct observational support for the toroidal superwind model, and supporting the 'interacting winds' model of PN formation.     

Far-infrared Detection of C3 in Sagittarius B2 and IRC +10216

We report on the detection of nine lines of the nu2 bending mode of triatomic carbon, C3, in the direction of Sagittarius B2. The R(4) and R(2) lines of C3 have been also detected in the carbon-rich star IRC +10216. The abundances of C3 in the direction of Sgr B2 and IRC +10216 are approximately 3x10-8 and approximately 10-6, respectively. In Sgr B2 we have also detected the 23-12 line of NH with an abundance of a few times 10-9. Polyatomic molecules will have a weak contribution from their pure rotational spectrum to the emission/absorption in the far-infrared. We suggest, however, that they could be, through their low-lying vibrational bending modes, the dominant carriers of emission/absorption in the spectrum of bright far-infrared sources.     

The molecular emission-line spectrum of IRC +10216 between 330 and 358 GHz

We have conducted a spectral line survey of IRC +10216 using the Caltech Submillimeter Observatory to an average sensitivity of < or approximately 95 mK. A deconvolution algorithm has been used to derive the continuous single-sideband spectrum from 330.2 to 358.1 GHz. A total of 56 spectral lines were detected of which 54 have been identified with 8 molecules and a total of 18 isotopomers. The observed lines are used to derive column densities and relative abundances for the detected species. Within this frequency range the spectral lines detected contribute the majority of the total flux emitted by IRC +10216. We use the derived column densities and excitation temperatures to simulate the molecular line emission (assuming LTE) at frequencies up to 1000 GHz. The observed and simulated flux from line emission is compared to broadband total flux measurements and to dust emission assuming a power-law variation of the dust emissivity. We conclude that significant corrections for the line flux must be made to broadband flux measurements of IRC +10216 at wavelengths longer than approximately 750 micrometers.     

Intrinsic polarization of the infrared carbon star IRC+10216 (CW Leo)

Conclusions Analysis of the polarization observations of IRC+10216 has shown that to explain the large polarization of the object it is necessary to have alignment of particles in an envelope through which the initially unpolarized radiation of the star passes on its way to the observer. Among the considered orientation mechanisms, paramagnetic relaxation near the photosphere for comparatively large fields (B35³ G) is the most obvious in view of the rate of alignment of the particles compared with the disorientation by the thermal motion of the gas. A confirmation of the presence of a magnetic field could be given by observations of circular polarization, which according to our estimates could reach 3–4% in the case of IRC+10216.The periodic variability of the degree of polarization of the object is due to a change in the diameter of the dust particles resulting from their partial evaporation at the phases of maximal light of the object.A. F. Ioffe Physicotechnical Institute; Leningrad State University. Translated from Astrofizika, Vol. 15, No. 2, pp. 297–309, April–June, 1979.     

Understanding the chemical complexity in Circumstellar Envelopes of C-Rich AGB stars: the case of IRC +10216

The circumstellar envelopes of carbon-rich AGB stars show a chemical complexity that is exemplified by the prototypical object IRC +10216, in which about 60 different molecules have been detected to date. Most of these species are carbon chains of the type C _n H, C _n H₂, C _n N, HC _n N. We present the detection of new species (CH₂CHCN, CH₂CN, H₂CS, CH₃CCH and C₃O) achieved thanks to the systematic observation of the full 3 mm window with the IRAM 30m telescope plus some ARO 12m observations. All these species, known to exist in the interstellar medium, are detected for the first time in a circumstellar envelope around an AGB star. These five molecules are most likely formed in the outer expanding envelope rather than in the stellar photosphere. A pure gas phase chemical model of the circumstellar envelope is reasonably successful in explaining the derived abundances, and additionally allows to elucidate the chemical formation routes and to predict the spatial distribution of the detected species.     

Plateau de bure observations of IRC+10216: high sensitivity maps of SiC2, SiS,and CS

We have obtained high angular resolution ( 3), and high sensitivity maps of IRC+10216. SiC₂ is found both in a spherical shell and in the very central region, indicating it is formed both in the inner envelope close to the star, and in the outer shell. The molecules SiS and CS are mostly found in the inner parts of the envelope, but are still detectable in the outer region (r 15) where the products of photochemistry are found. The maps show that IRC+10216 has a very clumpy envelope, with strong departures from spherical symmetry; an axis oriented NS-SW (P.A. 20°) can be seen in all maps. The radial brightness distribution of CS has secondary maxima, at the radius where the SiC₂ shell has its peak emission. A preliminary map shows CN in the same shell, but also in a still larger outer shell. Time variations in the mass loss rate, could be invoked to explain the multiple shell structure of this envelope.     

The circumstellar environment of the pecular F hypergiant IRC +10420

IRC +10420 is to date the only object that has been proposed to be in the transition from the Red Supergiant Phase to the Wolf-Rayet phase. In this contribution we report on new high resolution optical spectra of IRC +10420.     

Probing the IRC+10 ° 216 circumstellar envelope using spectroscopic observations of background stars

A unique and novel set of observations has been undertaken to study the circumstellar envelope (CSE) of the nearby (130 pc) carbon star IRC +10° 216 using optical absorption spectroscopy towards twobackground stars lying beyond the envelope.The primary aim of the observations is to search for diffuse band (DIB)carriers in the CSE. The circumstellar H+2H₂column density expected along the line of sight towards the targets islarge compared to that derived from the small interstellar reddening inthis region, E _B-V< 0.03 mag. We summarise our mainfindings, already reported in Kendall et al. (2002): The 6284Å DIB is detected in the VLT/UVES spectrum of one target, but clearly arises inthe foreground ISM. No other DIB is seen. Hence the DIB carriers, if present in the CSE, have a low abundance relative to H in the C-rich envelope of IRC +10°216, in comparison with this ratio in theISM. An important new result, previously unreported, concerns the detectionof circumstellar C₂ via absorption of the Phillips bandnear 8760Å.