The [ITAL]ISO[/ITAL]/SWS Spectrum of IRC +10216: The Vibrational Bands of C[TINF]2[/TINF]H[TINF]2[/TINF] and HCN

Agúndez

Kahane

et al. 2011

A&A

We report the detection in IRC +10216 of 63 rotational transitions of HCN, most of them with quantum numbers J = 3-2, pertaining to 28 different vibrational states with energies up to 10 700 K (ν 1 +3ν 2 +ν 3 ). Some of the transitions were also observed for the rare isotopologue H 13 CN. The observations were carried out with the IRAM 30-m telescope. The HCN lines with level energies above 5000 K arise within 1.5 stellar radius from the photosphere. Their intensities imply a vibrational temperature of T vib 2400 K and a fractional HCN abundance relative to H 2 , x(HCN) = 5-7 × 10 −5 . These high-energy levels are mainly populated by photospheric radiation, and their vibrational temperature yields a direct measurement of the stellar photospheric temperature. The lines with energy levels between 2000 and 5000 K yield T vib 1300 K and x(HCN) 0.8-2 × 10 −5 . They are radiatively excited and arise from a shell extending between 1.5 and 5 stellar radii. Finally, the lines from the low-energy vibrational states (<2000 K) trace a larger region of the envelope (5-28 stellar radii) with T vib = 400-600 K and x(HCN) = 4-6× 10 −6 . They are strongly affected by mid and near-infrared pumping. The line widths increase from 5 km s −1 near the 2400 K photosphere, to 19 km s −1 in the warm 400 K shell. This provides a unique insight into the physical conditions of the gas acceleration and dust formation region.

Section: Observations and Resultsmentioning

confidence: 99%

Section: Introductionmentioning

confidence: 99%

Probing the dust formation region in IRC +10216 with the high vibrational states of hydrogen cyanide

Agúndez

Kahane

et al. 2011

A&A

“…ISO observed a full infrared scan from 2.4 to 197 μm, albeit with a low spatial and spectral resolution, that probed the inner layers of IRC +10216 through the lines of abundant molecules such as CO, C 2 H 2 , and HCN (Cernicharo et al 1996(Cernicharo et al , 1999. More recently, spectral surveys carried out with Herschel with a high angular resolution have detected many high energy rotational transitions of molecules such as CO, HCN, CS, SiS, SiO, and SiC 2 , at low spectral resolution using SPIRE and PACS in the 55−672 μm range (Decin et al 2010a), and at high spectral resolution in the 488−1901 GHz range covered by HIFI .…”

Section: Introductionmentioning

confidence: 99%

Molecular abundances in the inner layers of IRC +10216

Agúndez

Fonfría

et al. 2012

A&A

136

259

Context. The inner layers of circumstellar envelopes around asymptotic giant branch stars are sites where a variety of processes such as thermochemical equilibrium, shocks induced by the stellar pulsation, and condensation of dust grains determine the chemical composition of the material that is expelled into the outer envelope layers and, ultimately, into interstellar space. Aims. We aim at studying the abundances, throughout the whole circumstellar envelope of the carbon star IRC +10216, of several molecules formed in the inner layers in order to constrain the different processes at work in such regions. Methods. Observations towards IRC +10216 of CS, SiO, SiS, NaCl, KCl, AlCl, AlF, and NaCN have been carried out with the IRAM 30-m telescope in the 80−357.5 GHz frequency range. A large number of rotational transitions covering a wide range of energy levels, including highly excited vibrational states, are detected in emission and serve to trace different regions of the envelope. Radiative transfer calculations based on the LVG formalism have been performed to derive molecular abundances from the innermost out to the outer layers. The excitation calculations include infrared pumping to excited vibrational states and inelastic collisions, for which up-to-date rate coefficients for rotational and, in some cases, ro-vibrational transitions are used. Results. We find that in the inner layers CS, SiO, and SiS have abundances relative to H 2 of 4 × 10 −6 , 1.8 × 10 −7 , and 3 × 10 −6 , respectively, and that CS and SiS have significant lower abundances in the outer envelope, which implies that they actively contribute to the formation of dust. Moreover, in the inner layers, the amount of sulfur and silicon in gas phase molecules is only 27% for S and 5.6% for Si, implying that these elements have already condensed onto grains, most likely in the form of MgS and SiC. Metal-bearing molecules lock up a relatively small fraction of metals, although our results indicate that NaCl, KCl, AlCl, AlF, and NaCN, despite their refractory character, are not significantly depleted in the cold outer layers. In these regions a few percent of the metals Na, K, and Al survive in the gas phase, either in atomic or molecular form, and are therefore available to participate in the gas phase chemistry in the outer envelope.

“…The derived C 2 H 2 abundances suggest a possible condensation of this species onto the dust grains beyond the first dust formation shell, explaining the last acceleration process at 15 − 20 R * . The vibrational levels of C 2 H 2 and HCN are out of LTE, supporting the existence of a radiative pumping mechanism in the middle region related to the near-IR radiation field (Cernicharo et al 1999). The deviations from LTE depend on the molecule, being very different for C 2 H 2 and HCN because of their own radiative selection rules (e.g., C 2 H 2 ν 4 (π g ) → G.S.…”

mentioning

confidence: 76%

“…−5 M yr −1 Cernicharo et al 1999), turn the CSE into an environment friendly to high molecular abundances. By now, 60 different molecular species have been detected, with CO the most abundant species with a fractional abundance of 8 × 10 −4 , C 2 H 2 with 8 × 10 −5 and HCN with 4 × 10 −5 (Keady & Ridgway 1993;Cernicharo et al 1996).…”

Section: Introductionmentioning

confidence: 99%

Physical and Chemical Conditions in the Dust Formation Zone of IRC+10216

Fonfría

Richter

et al. 2006

IAU

Abstract.A mid-infrared high-resolution spectral survey of the source IRC+10216 (CW Leo) has been carried out between 11 and 14 µm. A large number of lines of C 2H2 and HCN and their most abundant isotopologues, have been identified. Lines involving high-energy ro-vibrational levels allow an accurate derivation of the physical and chemical conditions in the innermost envelope. We have developed a radiative transfer model capable of fitting the observed lines satisfactorily. The fit of more than 200 ro-vibrational lines allowed us to get the kinetic, vibrational and rotational temperatures and the abundances of the C 2H2 and HCN between 1 and 300 R * .