Calcium Chemistry in Carbon-rich Circumstellar Environments: The Laboratory and Astronomical Discovery of Calcium Dicarbide, CaC2

Calcium dicarbide, CaC2, has been characterized at high resolution in the laboratory, and its main isotopologue, (CaC2)-Ca-40, 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 C-2 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) x 10(11) 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.