The first detection of SiC${}_2$ in the interstellar medium

Abstract

We report the first detection of SiC${}_2$ in the interstellar medium. The molecule was identified through six rotational transitions toward G\,+0.693$-$0.027, a molecular cloud located in the Galactic center. The detection is based on a line survey carried out with the GBT, the Yebes 40m, and the IRAM 30m telescopes covering a range of frequencies from 12 to 276 GHz. We fit the observed spectra assuming local thermodynamic equilibrium and derive a column density of ($1.02\pm 0.04)\times 10^{13}$ cm${}^{-2}$, which gives a fractional abundance of $7.5\times 10^{-11}$ with respect to H${}_2$, and an excitation temperature of $5.9\pm 0.2$ K. We conclude that SiC${}_2$ can be formed in the shocked gas by a reaction between the sputtered atomic silicon and C${}_2$H${}_2$, or it can be released directly from the dust grains due to disruption. We also search for other Si-bearing molecules and detect eight rotational transitions of SiS and four transitions of Si${}^{18}$O. The derived fractional abundances are $3.9\times 10^{-10}$ and $2.1\times 10^{-11}$, respectively. All Si-bearing species toward G\,+0.693$-$0.027 show fractional abundances well below what is typically found in late-type evolved stars.

8 pages, 5 figures