Detection of MgCN in IRC +10216: A new metal-bearing free radical

A new metal-containing molecule, MgCN, has been detected toward the late-type star IRC +10216, using the NRAO 12 m and IRAM 30 m telescopes. The N = 11 → 10, 10 → 9, and 9 → 8 transitions of this species, which has a ²Σ⁺ ground state, have been observed in the outer envelope of this object at 3 mm. For the N = 11 → 10 transitions, the two spin-rotation components are clearly resolved and conclusively identify this new radical. These measurements imply a column density for MgCN of N_tot ∼ 10¹² cm^-2 in the outer shell, which corresponds to a fractional abundance of f ∼ 7 ×10^-10. This molecule, the metastable isomer of MgNC, is the third metal-bearing species thus far identified in the outer shell of IRC +10216, and its detection implies a ratio of MgNC/MgCN ∼ 22/1. MgCN may be formed through a reaction scheme involving magnesium and HNC or CN, both prominent outer shell molecules, or through synthesis on grains.