The first detection of SiC2 in the interstellar medium

Abstract

We report the first detection of SiC2 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 ± 0.04) 1013 cm-2, which gives a fractional abundance of 7.5 - 10-11 with respect to H2, and an excitation temperature of 5.9 ± 0.2 K. We conclude that SiC2 can be formed in the shocked gas by a reaction between the sputtered atomic silicon and C2H2, 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 Si18O. The derived fractional abundances are 3.9 - 10-10 and 2.1 - 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.