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.
We report the detection of magnesium dicarbide, MgC2, in the laboratory at centimeter wavelengths and assign24MgC2,25MgC2, and26MgC2to 14 unidentified lines in the radio spectrum of the circumstellar envelope of the evolved carbon star IRC+10216. The structure of MgC2is found to be T-shaped with a highly ionic bond between the metal atom and the C2unit, analogous to other dicarbides containing electropositive elements. A two-temperature excitation model of the MgC2emission 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) × 1012cm−2. The abundance of MgC2relative to the magnesium–carbon chains MgCCH, MgC4H, and MgC6H 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.
more » « less- Award ID(s):
- 2110489
- NSF-PAR ID:
- 10382610
- Publisher / Repository:
- DOI PREFIX: 10.3847
- Date Published:
- Journal Name:
- The Astrophysical Journal Letters
- Volume:
- 940
- Issue:
- 2
- ISSN:
- 2041-8205
- Format(s):
- Medium: X Size: Article No. L42
- Size(s):
- Article No. L42
- Sponsoring Org:
- National Science Foundation
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