Unveiling the Dust Nucleation Zone of IRC+10216 with ALMA
Résumé
We report the detection in IRC+10216 of lines of HNC J = 3 - 2 pertaining to nine excited vibrational states with energies up to ~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 current early ALMA data, the radius inferred for the emitting region is ~0.''06 (i.e., ~= 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 H2 is 10-8 < χ(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.