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

We detected the linear $^3\Sigma^-$ 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.