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Abstract Calcium dicarbide, CaC₂, has been characterized at high resolution in the laboratory, and its main isotopologue,⁴⁰CaC₂, has been assigned to 14 rotational emission lines between 14 and 115 GHz, including 12 previously unassigned lines, in the expanding molecular envelope of the evolved carbon star IRC+10216. Aided by high-level quantum calculations and measurements of multiple isotopologues, CaC₂is determined to be a T-shaped molecule with a highly ionic bond linking the metal atom to the C₂unit, very similar in structure to isovalent magnesium dicarbide (MgC₂). The excitation of CaC₂is characterized by a very low rotational temperature of 5.8 ± 0.6 K and a kinetic temperature of 36 ± 16 K, similar to values derived for MgC₂. On the assumption that the emission originates from a 30″ shell in IRC+10216, the column density of CaC₂is (5.6 ± 1.7) × 10¹¹cm⁻². CaC₂is only the second Ca-bearing molecule besides CaNC and only the second metal dicarbide besides MgC₂identified in space. Owing to the similarity between the predicted ion–molecule chemistry of Ca and Mg, a comparison of the CaC₂abundance with that of MgC₂and related species permits empirical inferences about the radiative association–dissociative recombination processes postulated to yield metal-bearing molecules in IRC+10216 and similar objects. 

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.