2002ApJ...564L..45Z

A new metal-containing species, AlNC, 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, AlF and AlCl, exist exclusively near the stellar photosphere. Modeling of the detected transitions, assuming a spherical-shell distribution, indicates a column density of N_tot∼9x10¹¹ cm^–2 and a fractional abundance relative to H₂ of ∼3x10^–10 for AlNC. A rotational temperature of T_rot∼60 K was also derived for this molecule, suggesting that shock waves may be synthesizing AlNC 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.