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

Abstract

Calcium dicarbide, CaC _2 , has been characterized at high resolution in the laboratory, and its main isotopologue, ^40 CaC _2 , 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, CaC _2 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 (MgC _2 ). The excitation of CaC _2 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 MgC _2 . On the assumption that the emission originates from a 30″ shell in IRC+10216, the column density of CaC _2 is (5.6 ± 1.7) × 10 ^11 cm ^−2 . CaC _2 is only the second Ca-bearing molecule besides CaNC and only the second metal dicarbide besides MgC _2 identified in space. Owing to the similarity between the predicted ion–molecule chemistry of Ca and Mg, a comparison of the CaC _2 abundance with that of MgC _2 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.

Keywords

• Circumstellar envelopes
• Carbon stars
• Metal-containing molecules
• Laboratory astrophysics