Laboratory and Astronomical Discovery of Magnesium Dicarbide, MgC 2

We report the detection of magnesium dicarbide, MgC2, in the laboratory at centimeter wavelengths and assign 24MgC2, 25MgC2, and 26MgC2 to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC2 is found to be T-shaped with a highly ionic bond between the metal atom and the C2 unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC2 emission lines observed in IRC+10216 yields a very low rotational temperature of 6 ± 1 K, a kinetic temperature of 22 ± 13 K, and a column density of (1.0 ± 0.3) × 1012 cm−2. The abundance of MgC2 relative to the magnesium–carbon chains MgCCH, MgC4H, and MgC6H is 1:2:22:20 and provides a new constraint on the sequential radiative association–dissociative recombination mechanisms implicated in the production of metal-bearing molecules in circumstellar environments.