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

Abstract

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 $2.3-2.4$ $μ$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.

Figures (13)

• Figure 1: Light curves of SN 2024rbc in the BgcVroi bands (ordered by color). ZTF data are in triangles, ATLAS in circles, and Nickel in squares. The c and o bands correspond to the cyan and orange ATLAS filters. The apparent magnitudes are in AB, and the absolute magnitudes are shown on the right. The vertical dashed lines indicate the peaks for the $g$ and $r$ bands.
• Figure 2: The optical spectra of SN 2024rbc. The lines of important metals are marked at rest wavelength (in STP air) in magenta. The neutral helium and oxygen features are marked at the approximate absorption minima in cyan and yellow, respectively. The ionized calcium line at $\sim$$8567$ Å refers to the mean of the NIR Ca II triplet (8498, 8542, and 8662 Å).
• Figure 3: The NIR spectra of SN 2024rbc. Key spectral lines are indicated at the top; lines that overlap on the scale of this figure are marked together. The lines of important metals are marked at rest wavelength (in vacuum) in magenta. The neutral helium and oxygen features are marked at the approximate absorption minima in cyan and yellow, respectively. Smoothed $+40$d and $+62$d spectra (black) are overlaid on the original spectra to improve feature visibility. At $+62$d, an estimate of the dust continuum is indicated by a red dotted line, and the velocity-broadened first CO overtone is marked above. The gray regions indicate unreliability due to low atmospheric transmission.
• Figure 4: The bolometric light curve of SN 2024rbc as derived by SuperBol.
• Figure 5: Top: Light curves of a $3.1$$M_{\odot}$ He star progenitor Type Ib SN model superimposed on the photometry of SN 2024rbc. ZTF data are in triangles, ATLAS in circles, and Nickel in squares; stars mark the quasi-bolometric magnitudes. Semi-transparent data points were not used when fitting the model. The M$_{\text{Bol}}$ light curve is the same as \ref{['Figure:BolometricLuminosity']}. The early peak in M$_{\text{Bol}}$ of the model is due to neglecting shock cooling jin23. Bottom: Color evolution in the $gro$ bands of SN 2024rbc (dotted) and the best-fit model (solid). Shaded regions about the data represent the uncertainty in color magnitude. The units of the y-axes are magnitudes.