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

This paper presents the first detection of carbon monoxide (CO) and dust in a Type Ib supernova, SN 2024rbc, based on optical and near-infrared observations that reveal the formation of approximately $5.2 \times 10^{-4} M_{\odot}$ of CO and $1.3 \times 10^{-3} M_{\odot}$ of dust just 62 days post-explosion, offering new insights into early dust formation in the universe.

Original authors: Ryan Hwangbo, Jeonghee Rho, Aravind P. Ravi, Seong Hyun Park, Harim Jin, Sung-Chul Yoon, T. R. Geballe, Ryan Foley, Kirsty Taggart, Kyle W. Davis, Kishore C. Patra, S. Tinyanont, Jesper Sollerman, Ste

Published 2026-03-26

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CC BY 4.0

Original authors: Ryan Hwangbo, Jeonghee Rho, Aravind P. Ravi, Seong Hyun Park, Harim Jin, Sung-Chul Yoon, T. R. Geballe, Ryan Foley, Kirsty Taggart, Kyle W. Davis, Kishore C. Patra, S. Tinyanont, Jesper Sollerman, Steve Schulze, Natalie LeBaron, Chang Liu, Charles D. Kilpatrick

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine the universe as a giant, dusty construction site. For a long time, astronomers believed that the "dust" needed to build new stars and planets mostly came from aging, dying stars (like Red Giants) that gently puff out their outer layers over billions of years. But there's a problem: the very first galaxies in the universe were already full of dust when they were just babies, less than a billion years old. Aging stars take too long to make that much dust. So, where did it come from?

The leading suspect is the violent death of massive stars: Supernovae. These are the cosmic "explosions" that happen when a giant star runs out of fuel and collapses.

This paper is about a specific supernova, named SN 2024rbc, which exploded in August 2024. It's a "Type Ib" supernova, which means the star that died had already lost its outer hydrogen coat before it blew up. The team of astronomers, led by Ryan Hwangbo, did something special: they caught this explosion very early and found two smoking guns that prove these explosions are indeed dust factories.

Here is the story of what they found, explained simply:

1. The "Ghost" in the Machine (Carbon Monoxide)

When a star explodes, it throws out a cloud of hot gas. Usually, this gas is too hot for molecules (groups of atoms stuck together) to survive. But as the cloud cools down, things start to stick together.

The most important molecule to look for is Carbon Monoxide (CO). Think of CO as the "glue" or the "first brick" in the dust-making process. If you have CO, you are on your way to making dust.

The Discovery: In the past, astronomers had seen CO in other types of supernovae, but never in a Type Ib. It was like looking for a specific type of fingerprint and only finding it on one hand, never the other.
The Breakthrough: At just 62 days after the explosion, the team pointed their telescopes at SN 2024rbc and saw a huge, broad glow in the infrared part of the spectrum. This glow was the unmistakable signature of CO molecules forming.
Why it matters: This is the first time anyone has ever seen CO in a Type Ib supernova. It proves that even these "stripped" stars (which lost their hydrogen) are still capable of making the chemical building blocks for dust very quickly.

2. The Warm Blanket (Dust)

Once you have the CO "glue," the next step is turning it into actual solid dust grains.

The Discovery: The team didn't just see the gas; they saw a "warm blanket" of dust forming around the explosion. By looking at the light, they calculated that this dust was about 910 degrees Kelvin (roughly 1,200°F or 640°C). That's hot enough to glow, but cool enough to be solid.
The Amount: They estimated that about 0.0013 times the mass of our Sun had already turned into dust in just two months.
The Analogy: Imagine a campfire. At first, it's just hot smoke and gas. But if you wait a little while, you see the embers glowing red and the ash starting to settle. SN 2024rbc is that campfire, and the astronomers caught the moment the ash started to form.

3. The "Heavier" Star Mystery

The paper also tried to figure out what kind of star exploded.

The Clues: By looking at how bright the explosion got and how fast the light faded, they used computer models to guess the star's size.
The Result: The star was likely a Helium star (a star that lost its hydrogen skin) about 3 times the mass of our Sun before it died. It exploded with a force of about 100 billion billion billion billion joules (a "1 Bethe" explosion).
The Twist: The star didn't have a thick shell of gas around it (circumstellar material). If it did, the explosion would have looked different, like a car crashing into a wall of water. Instead, it crashed into empty space, which explains why the light curve (the brightness over time) was so clean and sharp.

4. Why This Changes the Story

The big picture here is about the Early Universe.

The Problem: We see dust in galaxies that are billions of light-years away (meaning we are seeing them as they were billions of years ago). There wasn't enough time for slow, aging stars to make all that dust.
The Solution: Massive stars die quickly (in just a few million years) and explode. If they can make dust as fast as SN 2024rbc did (in just 62 days), they are the perfect candidates to explain why the early universe was so dusty.

The Takeaway

Think of SN 2024rbc as a cosmic "time-lapse video." The astronomers hit "record" just two months after the star died and saw the universe's dust factory kick into high gear. They found the CO glue and the dust bricks forming almost immediately.

This discovery is a huge piece of the puzzle. It tells us that when massive stars die, they don't just destroy things; they are incredibly efficient at recycling their own guts into the dust that will eventually form new stars, planets, and even us.

In short: We found a new type of supernova making dust faster than we thought possible, proving that these violent explosions are the secret ingredient that allowed the early universe to build the stars we see today.

1. Problem Statement

The origin of dust in the early Universe ( $z \ge 6$ ) remains a significant astrophysical puzzle. While Asymptotic Giant Branch (AGB) stars are the primary dust producers in the local Universe, their long maturation times ( $10^8$ – $10^{10}$ years) and low individual dust yields ( $\sim 10^{-3} M_\odot$ ) cannot account for the massive dust reservoirs observed in high-redshift galaxies. Core-Collapse Supernovae (CCSNe) are hypothesized to be a more efficient early dust source, but direct observational evidence of rapid dust formation in stripped-envelope supernovae (SESNe), specifically Type Ib and Ic, has been limited.

A critical precursor to dust formation is the formation of Carbon Monoxide (CO) molecules, which act as coolants allowing gas to condense into dust grains. While CO has been detected in Type Ic SNe as early as 63 days post-explosion, no confirmed detection of CO had previously been reported in a Type Ib SN. This gap in knowledge hinders the understanding of whether Type Ib progenitors (which retain some helium envelopes) can facilitate the rapid chemical conditions necessary for early dust formation compared to Type Ic progenitors.

2. Methodology

The authors conducted a multi-wavelength observational campaign of SN 2024rbc, a Type Ib supernova discovered by the Zwicky Transient Facility (ZTF) in the host galaxy NGC 39 ( $z=0.016$ , $D \approx 66.8$ Mpc).

Observations:
- Optical Photometry: Spanning <1 to ~160 days post-explosion using ZTF, ATLAS, and the Lick Observatory (Nickel telescope).
- Optical Spectroscopy: Seven epochs from +6 to +61 days using Keck I (LRIS), Palomar 60" (SEDM), NOT (ALFOSC), and Lick (Kast).
- Near-Infrared (NIR) Spectroscopy: Three epochs (+40, +47, +62 days) using Keck II (NIRES), IRTF (SpeX), and Gemini-North (GNIRS).
Data Reduction: NIR data were reduced using three independent pipelines (xdgnirs, Figaro, and pypeit) to ensure robustness. Optical spectra were reduced using pypeit and the UCSC Spectral Pipeline.
Modeling & Analysis:
- Explosion Date: Constrained using non-detections from ZTF, ATLAS, and LAST to determine $t_0 = \text{MJD } 60525.15$ .
- Progenitor Fitting: Bolometric light curves were generated using SuperBol and fitted against 1D radiative-hydrodynamical models (STELLA code) for Helium star progenitors to derive ejecta mass, $^{56}\text{Ni}$ mass, and explosion energy.
- NIR Component Modeling: The +62d NIR spectrum was decomposed into three components:
  1. Hot photospheric blackbody emission.
  2. Warm dust continuum (modeled via a modified blackbody with wavelength-dependent absorption).
  3. CO first overtone emission (modeled using Local Thermodynamic Equilibrium (LTE) assumptions and line data from Goorvitch 1994).

3. Key Contributions

First CO Detection in Type Ib: This paper reports the first confirmed detection of the CO first overtone in the ejecta of a Type Ib supernova, observed at just 62 days post-explosion.
Early Dust Formation Evidence: The detection of a warm dust continuum coincident with CO emission provides direct evidence of rapid dust formation in a Type Ib event.
Comparative Analysis: The study provides a comprehensive comparison of SN 2024rbc's light curves and spectral evolution against a sample of Type Ib, Ic, and IIb SNe, confirming its classification and highlighting similarities/differences in dust formation timelines.
Progenitor Constraints: Detailed fitting of the light curve constrains the progenitor to a specific binary evolution channel, providing parameters for future theoretical models of SESN dust production.

4. Key Results

A. Classification and Light Curve

Classification: Spectral analysis confirms SN 2024rbc is a Type Ib SN, characterized by strong He I lines (e.g., 5876 Å, 1.083 µm) and the absence of Hydrogen.
Light Curve: The supernova peaked at $12\text{--}18$ days post-explosion. The decline rate and morphology align closely with other Type Ib SNe (e.g., 2007C, 2008D) and some Type Ic SNe (e.g., 2020oi).
Progenitor Parameters: Fitting to STELLA models indicates:
- Progenitor He Star Mass: $3.1 M_\odot$ (Initial ZAMS mass $\sim 11 M_\odot$ ).
- Ejecta Mass: $1.7 M_\odot$ .
- $^{56}\text{Ni}$ Mass: $0.07 M_\odot$ .
- Explosion Energy: $1.0 \times 10^{51}$ erg.
- The lack of a strong early optical peak suggests a moderate circumstellar medium (CSM) mass ( $< 1.8 \times 10^{-2} M_\odot$ ).

B. Spectral Evolution

Optical: Showed strong He I, Ca II, O I, and Fe II features. The Ca II triplet (8567 Å) and O I (7775 Å) showed P-Cygni profiles with velocities decelerating from $\sim 13,000$ km s $^{-1}$ to $\sim 8,500$ km s $^{-1}$ within the first month.
NIR: Dominated by neutral atoms (C I, S I, Mg I, Si I, Fe II, Na I, Ca I). The He I 1.083 µm line was the strongest feature.

C. CO and Dust Detection (Day +62)

The most significant finding is the detection of emission at $2.3\text{--}2.4$ µm in the +62d spectrum.

CO Overtone: The emission is broad and lacks sharp band heads, indicating high velocity broadening.
- CO Temperature: $4040 \pm 435$ K.
- CO Velocity: $5905 \pm 1960$ km s $^{-1}$ .
- CO Mass: $(5.2 \pm 1.2) \times 10^{-4} M_\odot$ (LTE estimate; non-LTE mass is expected to be $\sim 10^{-3} M_\odot$ ).
Dust Continuum: A rising continuum beneath the CO feature indicates warm dust.
- Dust Temperature: $910 \pm 10$ K.
- Dust Mass: $(1.3 \pm 0.1) \times 10^{-3} M_\odot$ .

5. Significance

Validation of SESN Dust Production: The simultaneous detection of CO and dust in a Type Ib SN at 62 days confirms that stripped-envelope progenitors (even those retaining helium) can rapidly cool and form molecules and dust. This supports the hypothesis that CCSNe are major contributors to the dust budget of the early Universe.
Implications for Early Universe: While the current dust mass ( $\sim 10^{-3} M_\odot$ ) is insufficient to explain high-redshift dust reservoirs alone, the authors note that dust mass in SNe can grow by orders of magnitude over time (citing SN 2014C). The early detection suggests SN 2024rbc has the potential to produce $\sim 0.1 M_\odot$ of dust over the next decade.
Chemical Physics: The detection challenges the notion that helium-rich environments suppress CO formation. The authors suggest that while ionized helium (He $^+$ ) can inhibit CO, the conditions in SN 2024rbc allowed for rapid molecule formation, possibly due to specific density or cooling rates.
Future Outlook: This discovery underscores the need for continued monitoring of Type Ib SNe and follow-up observations with mid-infrared instruments (e.g., JWST/MIRI) to track the long-term evolution of dust mass and composition.