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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. 

Grasses are among the most resilient plants, and some can survive prolonged desiccation in semiarid regions with seasonal rainfall. However, the genetic elements that distinguish grasses that are sensitive versus tolerant to extreme drying are largely unknown. Here, we leveraged comparative genomic approaches with the desiccation-tolerant grass Eragrostis nindensis and the related desiccation-sensitive cereal Eragrostis tef to identify changes underlying desiccation tolerance. These analyses were extended across C4 grasses and cereals to identify broader evolutionary conservation and divergence. Across diverse genomic datasets, we identified changes in chromatin architecture, methylation, gene duplications, and expression dynamics related to desiccation in E. nindensis. It was previously hypothesized that transcriptional rewiring of seed desiccation pathways confers vegetative desiccation tolerance. Here, we demonstrate that the majority of seed-dehydration–related genes showed similar expression patterns in leaves of both desiccation-tolerant and -sensitive species. However, we identified a small set of seed-related orthologs with expression specific to desiccation-tolerant species. This supports a broad role for seed-related genes, where many are involved in typical drought responses, with only a small subset of crucial genes specifically induced in desiccation-tolerant plants.