More metal cyanide species: Detection of AlNC (X (1)Sigma(+)) toward IRC+10216

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 greater than or similar to20, 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 similar to 9x10(11) cm(-2) and a fractional abundance relative to H-2 of similar to3x10(-10) for AlNC. A rotational temperature of T-rot similar to60K 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.