IRC + 10°216 mass loss properties through the study ofλ3 mm emission

19 pags., 24 figs., 3 tabs.
Low-mass evolved stars are major contributors to interstellar medium enrichment as a consequence of the intense mass-loss process these stars experience at the end of their lives. The study of the gas in the envelopes surrounding asymptotic giant branch (AGB) stars through observations in the millimetre wavelength range provides information about the history and nature of these molecular factories. Here we present ALMA observations at subarsecond resolution, complemented with IRAM-30 m data, of several lines of SiO, SiS, and CS towards the best-studied AGB circumstellar envelope, IRC + 10°216. We aim to characterise their spatial distribution and determine their fractional abundances mainly through radiative transfer and chemical modelling. The three species display extended emission with several enhanced emission shells. CS displays the most extended distribution reaching distances up to approximately 20''. SiS and SiO emission have similar sizes of approximately 11'', but SiS emission is slightly more compact. We have estimated fractional abundances relative to H2, which on average are equal to f(SiO) 10-7, f(SiS) 10-6, and f(CS) 10-6 up to the photo-dissociation region. The observations and analysis presented here show evidence that the circumstellar material displays clear deviations from an homogeneous spherical wind, with clumps and low density shells that may allow UV photons from the interstellar medium (ISM) to penetrate deep into the envelope, shifting the photo-dissociation radius inwards. Our chemical model predicts photo-dissociation radii compatible with those derived from the observations, although it is unable to predict abundance variations from the starting radius of the calculations ( 10 R*), which may reflect the simplicity of the model. We conclude that the spatial distribution of the gas proves the episodic and variable nature of the mass loss mechanism of IRC + 10°216, on timescales of hundreds of years.
Our team acknowledges the support given by ERC through the grant ERC-2013-Syg-610256 “NANOCOSMOS”, the Spanish MINECO through the grants AYA2012-32032 and AYA2016-75066-C2-1-P and the CONSOLIDERIngenio program “ASTROMOL” CSD 2009-00038. LVP acknowledges support from the Swedish Research Council and the ERC consolidator grant 614264. This work is based on observations carried out under projects numbers 014-13 and 055-15 with the IRAM-30 m telescope. IRAM is supported by INSU/CNRS (France), MPG (Germany) and IGN (Spain). This paper makes use of the following ALMA data: ADS/JAO.ALMA#2013.1.00432.S. ALMA is a partnership of ESO (representing its member states), NSF (USA) and NINS (Japan), together with NRC (Canada) and NSC and ASIAA (Taiwan), in cooperation with the Republic of Chile. The Joint ALMA Observatory is operated by ESO, AUI/NRAO and NAOJ. This research has made use of NASA’s Astrophysics Data System. This work has made use of GILDAS2 and CASA3 softwares to reduce and analyse data.