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.