2024ApJ...966L..28G

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, CaC₂ is determined to be a T-shaped molecule with a highly ionic bond linking the metal atom to the C₂ unit, very similar in structure to isovalent magnesium dicarbide (MgC₂). The excitation of CaC₂ is 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 CaC₂ is (5.6 ± 1.7) × 10¹¹ cm^–2. CaC₂ is only the second Ca-bearing molecule besides CaNC and only the second metal dicarbide besides MgC₂ identified in space. Owing to the similarity between the predicted ion-molecule chemistry of Ca and Mg, a comparison of the CaC₂ abundance with that of MgC₂ and related species permits empirical inferences about the radiative association-dissociative recombination processes postulated to yield metal-bearing molecules in IRC+10216 and similar objects.