🔭 astrophysics

SN 2023rve: A Type II Supernova with No Nebular Oxygen

This paper presents a comprehensive analysis of the nearby Type II supernova SN 2023rve, revealing an intermediate-mass progenitor with low explosion energy and dense circumstellar material, while highlighting its unprecedented nebular spectra that lack oxygen emission, a feature potentially explained by partial fallback onto the compact remnant or alternative mechanisms like dust formation.

Original authors: Melissa Rosowsky, Stefano Valenti, Aravind P. Ravi, Yize Dong, Nicolas Meza Retamal, K. Azalee Bostroem, David J. Sand, Griffin Hosseinzadeh, Jeniveve Pearson, Anders Jerkstrand, Darshana Mehta, Jenni

Published 2026-07-01

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CC BY 4.0

Original authors: Melissa Rosowsky, Stefano Valenti, Aravind P. Ravi, Yize Dong, Nicolas Meza Retamal, K. Azalee Bostroem, David J. Sand, Griffin Hosseinzadeh, Jeniveve Pearson, Anders Jerkstrand, Darshana Mehta, Jennifer E. Andrews, Moira Andrews, Joseph Farah, Emily Hoang, D. Andrew Howell, Daryl Janzen, Saurabh W. Jha, Michael Lundquist, Curtis McCully, Megan Newsome, Estefania Padilla Gonzalez, Manisha Shrestha, Giacomo Terreran

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 a star as a massive, fiery engine that has been running for millions of years. Eventually, it runs out of fuel, collapses under its own weight, and explodes. This is a Type II supernova. Usually, when astronomers look at the "aftermath" of these explosions (the nebular phase, hundreds of days later), they see a specific set of glowing gases, much like seeing the smoke and debris after a fire. One of the most common and expected "smoke signals" is oxygen.

SN 2023rve is a supernova that broke the rules.

Here is the story of what happened, explained simply:

1. The Explosion and the "Ghost" in the Machine

Astronomers spotted SN 2023rve in a nearby galaxy called NGC 1097. At first glance, it looked like a standard stellar explosion. It got bright, stayed bright for a long time (a "plateau"), and then slowly faded away.

However, when the team looked at the explosion's "ghost" (the light emitted hundreds of days later when the debris cloud became transparent), they found something shocking: There was no oxygen.

Think of it like baking a cake. You expect to see flour, sugar, and eggs. If you cut the cake open and there is absolutely no flour, you know something strange happened. In this case, the explosion should have been rich in oxygen (since the star was massive), but the oxygen was missing from the light we could see.

2. The "Heavy" Star that Faded Quietly

Usually, a star massive enough to create a supernova (about 14 to 18 times the mass of our Sun) explodes with a lot of energy and throws out a huge amount of heavy elements like oxygen and nickel.

SN 2023rve was different:

Low Energy: The explosion was surprisingly weak, like a firecracker compared to a cannon.
Little Nickel: It produced very little radioactive nickel (the "battery" that powers the light of the explosion).
Slow Motion: The debris was moving much slower than usual.

3. Why Was the Oxygen Missing?

The paper proposes a few theories to explain why the oxygen vanished from view, using some creative scenarios:

The "Black Hole Vacuum" (Fallback): Imagine the explosion happened, but the force wasn't strong enough to throw everything out. Some of the heavy, oxygen-rich material from the center of the star might have fallen back down, like water draining into a sink, and got swallowed by the new black hole or neutron star left behind. This "fallback" would hide the oxygen from our telescopes.
The "Dust Blanket": Maybe the oxygen is still there, but it got covered up. As the star cooled, it might have formed thick clouds of cosmic dust. This dust acts like a heavy blanket, blocking the oxygen's light from escaping while letting other colors (like hydrogen) shine through.
The "Mixing Bowl": Perhaps the oxygen got mixed so thoroughly with calcium (another element) inside the star that the calcium took over the job of cooling the gas, effectively silencing the oxygen's "voice."

4. The "Fog" Around the Star

The paper also found that the star was surrounded by a thick shell of gas and dust before it exploded. Think of this like a star that was coughing up a lot of material in the years leading up to its death. When the explosion happened, the shockwave hit this dense "fog" (circumstellar material), which helped shape the light curve and made the explosion look a bit different than a standard one.

5. Why This Matters

SN 2023rve is a rare find. The team looked at 26 other similar supernovae, and only two others (which were very different in other ways) also lacked oxygen.

This discovery suggests that not all massive stars explode the same way. Some might fail to fully eject their heavy elements, or they might hide them behind dust. It adds a new piece to the puzzle of how massive stars live, die, and recycle their materials back into the universe.

In short: SN 2023rve was a massive star that exploded quietly, produced very little fuel (nickel), and hid its oxygen—either by swallowing it back up, covering it with dust, or mixing it away. It's a cosmic mystery that challenges our standard models of how stars end their lives.

Technical Summary of SN 2023rve: A Type II Supernova with No Nebular Oxygen

Problem and Context
Type II supernovae (SNe II) originate from massive stars retaining hydrogen envelopes, yet a discrepancy exists between theoretical predictions (which suggest progenitors up to ~25 $M_\odot$ should produce SNe II) and direct progenitor detections (which typically find stars in the 8–17 $M_\odot$ range), known as the "RSG problem." Furthermore, the diversity of SN II light curves and spectral evolution is not fully understood, particularly regarding the role of circumstellar material (CSM) and the physical mechanisms behind low-luminosity events or "fallback" explosions where ejecta may fall back onto a compact remnant. A critical diagnostic for progenitor mass and nucleosynthesis is the presence of nebular oxygen emission ([O I] $\lambda\lambda$ 6300, 6364) at late times; its absence in a massive progenitor system poses a significant challenge to current models.

Methodology
The authors present a comprehensive multi-band photometric and spectroscopic study of SN 2023rve, discovered in NGC 1097.

Observations: The study utilizes data from the Las Cumbres Observatory (LCO) network for high-cadence photometry over 485 days, alongside spectroscopy from the Faulkes Telescopes, VLT, Keck, and SALT.
Distance and Reddening: The distance to the host galaxy was constrained using the Tully-Fisher relation and independently verified via the Expanding Photosphere Method (EPM). Host extinction was estimated using Na I D absorption and color evolution comparisons with a sample of similar SNe II.
Light Curve Analysis: A pseudo-bolometric light curve was constructed to estimate the synthesized $^{56}$ Ni mass by comparing the radioactive tail to SN 1987A.
Hydrodynamic Modeling: The authors employed the SNEC (SuperNova Explosion Code) to model the light curve. This involved a two-step process: first, varying progenitor mass and explosion energy to fit the plateau phase; second, varying CSM density and radial extent to fit the early-time rise and plateau slope.
Spectral Analysis: Nebular spectra (days 362, 422, and 485) were analyzed for emission line features, specifically searching for [O I], [Ca II], and He I. Line widths and velocities were measured to distinguish between SN ejecta and host galaxy contamination.

Key Results

Photometric Properties: SN 2023rve reached a peak absolute $V$ -band magnitude of $-17.11 \pm 0.46$ mag. It exhibited a plateau duration of approximately 135 days with a decline rate of $0.90 \pm 0.02$ mag/50 days, placing it between typical Type IIP and Type IIL events.
Progenitor and Explosion Parameters: Hydrodynamic modeling of the light curve suggests an intermediate-mass progenitor with a Zero-Age Main Sequence (ZAMS) mass of $\sim 16 M_\odot$ . The explosion energy was inferred to be low at $0.27 \times 10^{51}$ ergs. The model also indicates a dense CSM component with a radial extent of $2900 R_\odot$ and a density parameter of $10^{18}$ g cm $^{-1}$ .
Nickel Mass: The pseudo-bolometric light curve analysis yielded a low synthesized $^{56}$ Ni mass of $0.0064 \pm 0.0026 M_\odot$ .
Spectroscopic Anomalies:
- Absence of Oxygen: The most striking result is the complete absence of forbidden [O I] $\lambda\lambda$ 6300, 6364 emission in the nebular spectra, despite the presence of strong H $\alpha$ and Ca II emission. A comparison with a sample of 26 other SNe II revealed that only two other objects (SN 2015C and SN 2007od) lack oxygen signatures at similar epochs, and both have higher expansion velocities than SN 2023rve.
- Low Velocities: The ejecta velocities, particularly in the nebular phase, are notably low, even lower than those of known low-luminosity SNe IIP like SN 2005cs.
- Helium and CSM: Narrow He I lines were detected throughout the nebular phase. Features identified as "Cachito" (initially Si II, later high-velocity H $\alpha$ ) provide evidence of interaction between the SN ejecta and a dense CSM shell.

Significance and Interpretation
The paper argues that the combination of a low explosion energy, a long plateau, a small synthesized nickel mass, and the unprecedented lack of nebular oxygen emission in a system with a $\sim 16 M_\odot$ progenitor is consistent with a fallback explosion. In this scenario, a fraction of the inner, oxygen-rich ejecta fails to reach escape velocity and falls back onto the newly formed compact remnant, suppressing the oxygen emission.

The authors also discuss alternative explanations for the missing oxygen, including:

Dust Formation: Dust condensing in the inner ejecta could absorb or scatter oxygen photons, a mechanism supported by evidence of dust in SN 2023rve and SN 2007od.
Mixing: If oxygen is well-mixed with calcium in the inner ejecta, calcium may become the dominant coolant, reducing [O I] flux.
CSM Interaction: Ongoing interaction with a dense CSM shell could alter the ionization state and optical depth of the inner ejecta.

The detection of intermediate-width He I lines remains ambiguous regarding their origin (SN ejecta vs. CSM interaction). The study concludes that SN 2023rve highlights the diversity of SN II outcomes and underscores the necessity of systematic late-time spectroscopic follow-up to understand the physical mechanisms governing core-collapse supernovae, particularly those involving fallback or significant mass loss.

1. The Explosion and the "Ghost" in the Machine

2. The "Heavy" Star that Faded Quietly

3. Why Was the Oxygen Missing?

4. The "Fog" Around the Star

5. Why This Matters

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