Unveiling the dust nucleation zone of IRC+10216 with ALMA

Abstract

We report the detection in IRC+10216 of lines of HNC $J$=3-2 pertaining to 9 excited vibrational states with energies up to $\sim$5300 K. The spectrum, observed with ALMA, also shows a surprising large number of narrow, unidentified lines that arise in the vicinity of the star. The HNC data are interpreted through a 1D--spherical non--local radiative transfer model, coupled to a chemical model that includes chemistry at thermochemical equilibrium for the innermost regions and reaction kinetics for the external envelope. Although unresolved by the present early ALMA data, the radius inferred for the emitting region is $\sim$0.06'' (i.e., $\simeq$ 3 stellar radii), similar to the size of the dusty clumps reported by IR studies of the innermost region ($r<$ 0.3''). The derived abundance of HNC relative to H${}_2$ is $10^{-8}<$ $\chi$(HNC) $<10^{-6}$, and drops quickly where the gas density decreases and the gas chemistry is dominated by reaction kinetics. Merging HNC data with that of molecular species present throughout the inner envelope, such as vibrationally excited HCN, SiS, CS, or SiO, should allow us to characterize the physical and chemical conditions in the dust formation zone.

Accepted for publication in ApJL

References in corpus (9)

• Oxygen Chemistry in the Circumstellar Envelope of the Carbon-Rich Star IRC+10216
• Molecular abundances in the inner layers of IRC +10216
• Detection of circumstellar CH2CHCN, CH2CN, CH3CCH and H2CS
• A Detailed Analysis of the Dust Formation Zone of IRC+10216 Derived from Mid-IR Bands of C2H2 and HCN
• Warm water vapour in the sooty outflow from a luminous carbon star
• The circumstellar envelope of IRC+10216 from milli-arcsecond to arcmin scales
• The size, luminosity and motion of the extreme carbon star IRC+10216 (CW Leonis)
• An independent distance estimate to CW Leo
• Herschel/HIFI observation of highly excited rotational lines of HNC toward IRC +10 216

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