Laboratory and Astronomical Discovery of Magnesium Dicarbide, MgC<sub>2</sub>
P. Bryan Changala, H. M. Sen Gupta, J. Cernicharo, J. R. Pardo, M. Agúndez, C. Cabezas, B. Tercero, M. Guélin, Michael McCarthy
Abstract
Abstract We report the detection of magnesium dicarbide, MgC 2 , in the laboratory at centimeter wavelengths and assign 24 MgC 2 , 25 MgC 2 , and 26 MgC 2 to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC 2 is found to be T-shaped with a highly ionic bond between the metal atom and the C 2 unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC 2 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) × 10 12 cm −2 . The abundance of MgC 2 relative to the magnesium–carbon chains MgCCH, MgC 4 H, and MgC 6 H 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.