The first detection of SiC₂ in the interstellar medium

¹ Centro de Astrobiología (CAB), INTA-CSIC, Carretera de Ajalvir km 4, Torrejón de Ardoz, 28850 Madrid, Spain
e-mail: smassalkhi@cab.inta-csic.es
² Star and Planet Formation Laboratory, Cluster for Pioneering Research, Riken, 2-1 Hirosawa, Wako, Saitama 351-0198, Japan
³ European Southern Observatory, Alonso de Córdova, 3107, Vitacura, Santiago 763-0355, Chile
⁴ Joint ALMA Observatory, Alonso de Córdova 3107, Vitacura 763 0355, Santiago, Chile
⁵ Observatorio de Yebes (IGN), Cerro de la Palera s/n, 19141 Guadalajara, Spain
⁶ Department of Physics, Astronomy and Geosciences, Towson University, 8000 York Road, Towson, MD 21252, USA

Received: 5 May 2023
Accepted: 17 July 2023

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.