Laboratory and astronomical discovery of magnesium dicarbide, MgC${}_2$

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 $6\pm 1$ K, a kinetic temperature of $22\pm 13$ K, and a column density of $(1.0\pm 0.3)\times 10^{12}$ 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.

15 pages, 5 figures