More metal cyanide species: Detection of A1NC (X¹ ∑ ⁺) Toward IRC + 10216

A new metal-containing species, A1NC, has been detected toward the circumstellar envelope of the late-type carbon star IRC + 10216, using the IRAM 30 m telescope. The J = 11 → 10, 12 → 11, and 13 → 12 rotational transitions at 2 mm and the J = 18 → 17 and J = 21 → 20 transitions at 1.2 mm of this linear, closed-shell molecule were observed in this object. The line profiles appear U-shaped, indicating a source ≃20″, and the horn-to-center ratios suggest a shell-like distribution. In contrast, the other two Al-bearing molecules previously detected, A1F and A1C1, exist exclusively near the stellar photosphere. Modeling of the detected transitions, assuming a spherical-shell distribution, indicates a column density of N_tot ∼9 × 10¹¹ cm^-2 and a fractional abundance relative to H₂ of ∼ 3 × 10^-10 for A1NC. A rotational temperature of T_rot ∼ 60 K was also derived for this molecule, suggesting that shock waves may be synthesizing A1NC in the outer envelope. This species is the fourth metal cyanide/isocyanide compound discovered in this object, along with MgNC, MgCN, and NaCN. These data suggest that cyanide/isocyanide species are the major molecular carriers of metals in circumstellar gas.