Detection of the linear SiC${}_3$ and SiC${}_5$ radicals in IRC\,+10216

We detected the linear ${}^3{\mathrm{\Sigma }}^{-}$ radicals SiC${}_3$ and SiC${}_5$ toward IRC+10216 using an ultrasensitive line survey gathered with the Yebes 40,m radio telescope. The derived column densities of $l$-SiC${}_3$ and $l$-SiC${}_5$ are (3.6$\pm$0.4)$\times$10${}^{12}$ cm${}^{-2}$ and (1.8$\pm$0.2)$\times$10${}^{12}$ cm${}^{-2}$, respectively. The linear SiC${}_3$ radical is $\sim$2 times less abundant that its singlet rhomboidal prolate isomer, for which we provide a new analysis based on recent sensitive observations in the Q band (7,mm), and at 3 and 2,mm with the IRAM 30m telescope. The emission detected from these species arises from the cool external layers of the circumstellar envelope. We speculate whether ion-neutral routes involving SiC${}_n$H${}_m$${}^{+}$ cations or neutral-neutral reactions involving Si and SiC${}_2$ could efficiently synthesize these species.