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Warm water vapour in the sooty outflow from a luminous carbon star

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Abstract

The detection1 of circumstellar water vapour around the ageing carbon star IRC +10216 challenged the current understanding of chemistry in old stars, because water was predicted2 to be almost absent in carbon-rich stars. Several explanations for the water were postulated, including the vaporization of icy bodies (comets or dwarf planets) in orbit around the star1, grain surface reactions3, and photochemistry in the outer circumstellar envelope4. With a single water line detected so far from this one carbon-rich evolved star, it is difficult to discriminate between the different mechanisms proposed. Here we report the detection of dozens of water vapour lines in the far-infrared and sub-millimetre spectrum of IRC +10216 using the Herschel satellite5. This includes some high-excitation lines with energies corresponding to 1,000 K, which can be explained only if water is present in the warm inner sooty region of the envelope. A plausible explanation for the warm water appears to be the penetration of ultraviolet photons deep into a clumpy circumstellar envelope. This mechanism also triggers the formation of other molecules, such as ammonia, whose observed abundances6 are much higher than hitherto predicted7.

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Figure 1: Unblended ortho - and para -water lines detected with Herschel in IRC +10216.
Figure 2: Schematic overview of the envelope around a carbon-rich AGB star.
Figure 3: Fractional abundances in the clumpy circumstellar environment of IRC +10216.
Figure 4: HC 3 N as observed in the envelope of IRC +10216.

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Acknowledgements

Herschel is an ESA space observatory with science instruments provided by European-led Principal Investigator (PI) consortia and with important participation from NASA. PACS has been developed by a consortium of institutes led by MPE (Germany) and including UVIE (Austria); KUL, CSL, IMEC (Belgium); CEA, OAMP (France); MPIA (Germany); IFSI, OAP/AOT, OAA/CAISMI, LENS, SISSA (Italy); and IAC (Spain). This development has been supported by the funding agencies BMVIT (Austria), ESA-PRODEX (Belgium), CEA/CNES (France), DLR (Germany), ASI (Italy) and CICT/MCT (Spain). SPIRE has been developed by a consortium of institutes led by Cardiff University (UK) and including the University of Lethbridge (Canada); NAOC (China); CEA, LAM (France); IFSI, the University of Padua (Italy); IAC (Spain); Stockholm Observatory (Sweden); Imperial College London, RAL, UCL-MSSL, UKATC, University of Sussex (UK); and Caltech, JPL, NHSC, the University of Colorado (USA). This development has been supported by national funding agencies: CSA (Canada); NAOC (China); CEA, CNES, CNRS (France); ASI (Italy); MCINN (Spain); SNSB (Sweden); STFC (UK); and NASA (USA). IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain).

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Contributions

L.D. identified the molecular lines, analysed and interpreted the PACS and SPIRE data and performed the non-local-thermodynamic equilibrium (non-LTE) radiative transfer computations; M.A. and J.C. were responsible for the chemical modelling; M.J.B. was mainly responsible for the Herschel MESS Key program observations performed with the SPIRE instrument; M.A.T.G. is PI of the Herschel MESS Key program, F.D. checked the non-LTE radiative transfer calculations, R.L. modelled the spectral energy distribution; E.D.B. identified molecular lines; P.R. and B.V. were responsible for the calibration of the PACS observations; R.W. and E.T.P. were responsible for the calibration of the SPIRE observations; the rest of the authors belong to the Herschel MESS consortium, the framework in which the Herschel PACS and SPIRE spectroscopic observations were performed.

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Correspondence to L. Decin.

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This file contains Supplementary Notes 1-2, Supplementary Figures 1-6 with legends, Supplementary Table 1 and additional references. (PDF 371 kb)

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Decin, L., Agúndez, M., Barlow, M. et al. Warm water vapour in the sooty outflow from a luminous carbon star. Nature 467, 64–67 (2010). https://doi.org/10.1038/nature09344

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