Near-Infrared and Optical Observations of SN 2024rbc: The First Early Detection of CO and Dust in a Type Ib Supernova

We present optical and near-infrared (NIR) observations of the Type Ib supernova (SN) 2024rbc. Emission from the first CO overtone, resting on a dust continuum at $2.3-2.4$ $\mu$m, was observed at 62 days post-explosion. The CO band heads are not seen; the emission is broad and devoid of sharp spectral structure. This is the first observation of CO in the ejecta of a Type Ib SN reported in literature. Fitting a LTE model to the CO overtone derives a mass of $(5.2\pm 1.2)$ $\times$ 10${}^{-4}$ $M_{\odot }$, a temperature of $4040\pm 435$ K, and a velocity of $5905\pm 1960$ km s${}^{-1}$. We also fitted a modified blackbody model to the dust continuum, deriving a dust temperature of $910\pm 10$ K and a mass of $(1.3\pm 0.1)$ $\times$ ${10}^{-3}$ $M_{\odot }$. Furthermore, the spectra of SN 2024rbc exhibit strong He I lines and numerous neutral and ionized metal lines. Comparing the spectral evolution of SN 2024rbc to other Type Ib, Ic, and IIb SNe indicates it is a Type Ib SN. Additionally, fitting SN light curve models of helium star progenitors computed with the STELLA code to photometric observations indicates a ${}^{56}$Ni mass of $0.07$ $M_{\odot }$ and an ejecta mass of $1.7$ $M_{\odot }$. We also compare the velocities of key optical lines to examine the evolution of the ejecta. Lastly, we discuss the observed CO and dust emission and its implications for early-Universe dust formation.