Discovery of MgS and NaS in the Interstellar Medium and Tentative Detection of CaO

We report the first detection of the metal-bearing molecules sodium sulfide and magnesium sulfide and the tentative detection of calcium monoxide in the interstellar medium toward the Galactic center molecular cloud G+0.693-0.027. The derived column densities are (5.0 ± 1.1) × 10¹⁰ cm⁻², (6.0 ± 0.6) × 10¹⁰ cm⁻², and (2.0 ± 0.5) × 10¹⁰ cm⁻², respectively. This translates into fractional abundances with respect to H₂ of (3.7 ± 1.0) × 10⁻¹³, (4.4 ± 0.8) × 10⁻¹³, and (1.5 ± 0.4) × 10⁻¹³, respectively. We have also searched for other Na-, Mg-, and Ca-bearing species toward this source but none of them have been detected and thus we provide upper limits for their abundances. We discuss the possible chemical routes involved in the formation of these molecules containing metals under interstellar conditions. Finally, we compare the ratio between sulfur-bearing and oxygen-bearing molecules with and without metals, finding that metal-bearing sulfur molecules are much more abundant than metal-bearing oxygen ones, in contrast with the general trend found in the ratios between other nonmetal-oxygen- and sulfur-bearing molecules. This further strengthens the idea that sulfur may be a little depleted in G+0.693-0.027 as a result of the low-velocity shocks present in this source sputtering large amounts of material from dust grains.