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SN 2020bij and a Possible Slow-Rise High-Velocity Subclass of Type IIP Supernovae

This paper proposes a new subclass of Type IIP supernovae, exemplified by SN 2020bij and four similar events, characterized by slow-rising light curves and high expansion velocities that are best explained by weak or absent circumstellar material interaction, thereby offering new insights into the late-stage mass-loss diversity of red supergiant progenitors.

Original authors: Sondos Mohsen-Tanev, Iair Arcavi, Shahar Bracha, K. Azalee Bostroem, Griffin Hosseinzadeh, Jesper Sollerman, Claudia P. Gutiérrez, Priscila J. Pessi, Joseph Anderson, Mariusz Gromadzki, Avishay Gal-Ya

Published 2026-07-03

📖 5 min read🧠 Deep dive

CC BY 4.0

Original authors: Sondos Mohsen-Tanev, Iair Arcavi, Shahar Bracha, K. Azalee Bostroem, Griffin Hosseinzadeh, Jesper Sollerman, Claudia P. Gutiérrez, Priscila J. Pessi, Joseph Anderson, Mariusz Gromadzki, Avishay Gal-Yam, Daichi Hiramatsu, Jamison Burke, Koichi Itagaki, Ting-Wan Chen, D. Andrew Howell, Curtis McCully, Megan Newsome, Estefania Padilla Gonzalez, Craig Pellegrino

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

The Big Picture: A New Kind of Stellar Explosion

Imagine a massive star as a giant, aging balloon. Usually, just before it pops (explodes as a supernova), it leaks air (mass) rapidly, creating a thick cloud of dust and gas right around it. When the balloon finally bursts, the shockwave hits this cloud immediately, causing a bright, fast flash.

For a long time, astronomers thought all Type IIP supernovae (the most common kind of exploding star) worked this way: a quick flash, a fast rise to maximum brightness, and then a long, steady plateau.

This paper introduces a new character: SN 2020bij. It's a supernova that broke the rules. Instead of a quick flash, it took its time to get bright. It's like a car that doesn't speed up immediately when you hit the gas; it revs slowly for a while before taking off. The authors found four other stars that behave exactly the same way. They propose that these aren't just "weird" outliers, but a distinct new subclass of exploding stars.

The Mystery: Why the Slow Rise?

In the world of supernovae, a "slow rise" is a big clue.

The Old Theory: If a star explodes and hits a thick cloud of gas (circumstellar material) right away, the light curve (the graph of brightness over time) shoots up fast. It's like a car hitting a wall of water; the splash is immediate and huge.
The New Discovery: SN 2020bij and its cousins rose to their peak brightness over about 14 days. Most other supernovae do this in less than 10 days.

The authors used computer models to figure out why. They found that these slow-rising stars didn't need a thick cloud of gas to explain their behavior. In fact, the models worked best when they assumed there was almost no gas around the star when it exploded.

The Analogy: Imagine two fireworks.

Firework A is launched into a thick fog. The explosion hits the fog instantly, creating a massive, immediate burst of light.
Firework B is launched into clear, empty air. It takes a moment for the explosion to expand and become fully visible, resulting in a slower, more gradual brightening.

SN 2020bij is Firework B. It suggests that the star didn't have a "fog" (dense gas cloud) around it right before it died.

The "High-Velocity" Twist

There's another twist to this story. Usually, if a star has a thick cloud of gas around it, the explosion slows down as it pushes through that cloud. But these slow-rising stars are actually moving very fast.

It's like a race car that takes a long time to get up to speed (slow rise) but then is traveling at 200 mph (high velocity) once it's moving. This combination—slow rise + high speed—is the unique fingerprint of this new group.

The Investigation: How They Figured It Out

The team didn't just guess; they did some serious detective work:

The Discovery: They found SN 2020bij in a galaxy called NGC 3463 using a telescope in Japan. They caught it just as it was starting to glow.
The Comparison: They looked at the "library" of known supernovae and found four others (ASASSN-14kg, SN 2018fif, SN 2021yja, and SN 2023axu) that had the same slow, lazy rise to brightness.
The Models: They ran two types of computer simulations:
- Analytical Models: Simple math equations to see if the light could be explained by the star just cooling down after the shockwave (shock cooling).
- Numerical Models: Complex, 3D simulations of the star's explosion, testing different amounts of surrounding gas.

The Result: The models showed that for these five stars, the "no gas" (or very little gas) scenario fit the data perfectly. You didn't need a thick cloud to explain the slow rise. In fact, adding a thick cloud made the models worse.

What This Means for the Stars

This discovery changes how we think about the final days of massive stars (Red Supergiants).

The Old View: We thought these stars were constantly shedding mass, creating a dense "cocoon" of gas around them right before they died.
The New View: SN 2020bij and its friends suggest that some stars might be much more "contained." They might not shed as much mass, or the gas they do shed might stay very close to the star, not forming a large, dense cloud that the explosion has to punch through.

The Conclusion

The authors aren't saying all Type IIP supernovae are like this. Most still seem to have that "fog" and rise quickly. But they are proposing that there is a rare, special club of supernovae that:

Rise slowly to their peak brightness.
Have very high expansion speeds.
Likely exploded with very little surrounding gas.

By studying these rare events, astronomers hope to understand the different ways massive stars lose their weight in their final moments. It's like realizing that while most people sweat a lot before a marathon, some people run dry, and understanding why helps us understand the biology of the runner.

In short: SN 2020bij is the "slow starter" of the supernova world, and its behavior tells us that not all dying stars are surrounded by a thick cloud of debris. Some are clean, fast, and unique.

Technical Summary: SN 2020bij and a Possible Slow-Rise High-Velocity Subclass of Type IIP Supernovae

Problem Statement
Type IIP supernovae (SNe IIP), resulting from the core-collapse of single red supergiant (RSG) stars, have recently been found to often require interaction with circumstellar material (CSM) to reproduce the rapid rise to the plateau phase observed in their light curves. Previous studies utilizing radiation-hydrodynamical modeling (e.g., Morozova et al. 2017, 2018) and early-time spectroscopy suggest that significant CSM mass is common in the final stages of RSG evolution. However, the diversity of mass-loss episodes in RSGs remains poorly understood. This paper investigates SN 2020bij, a Type IIP event characterized by an unusually slow rise to its plateau (~14 days) and high expansion velocities, to determine if it represents a distinct physical subclass that challenges the prevailing view of ubiquitous CSM interaction.

Methodology
The authors present multi-wavelength observations of SN 2020bij, discovered in the spiral galaxy NGC 3463 (z = 0.013276) on MJD 58877.68.

Observations: Photometric data were obtained in $UBV gri$ bands using the Las Cumbres Observatory (LCO) global network, unfiltered imaging from the Itagaki Astronomical Observatory, and data from ATLAS and Gaia. Spectroscopic follow-up was conducted using ESO-NTT (EFOSC2) and LCO FLOYDS spectrographs, providing a temporal baseline from ~4 to ~51 days post-explosion.
Sample Selection: To contextualize SN 2020bij, the authors identified four other SNe IIP from the literature (ASASSN-14kg, SN 2018fif, SN 2021yja, and SN 2023axu) exhibiting similarly slow rises ( $\gtrsim$ 10 days) to the plateau.
Analysis Techniques:
1. Photometric Analysis: Light curve parameters (plateau duration $t_{PT}$ and decline rate $s_{50V}$ ) were calculated. Color evolution and bolometric luminosities were derived via blackbody fitting of the spectral energy distribution (SED).
2. Analytical Modeling: The early light curves were fitted using the analytic shock cooling models of Sapir & Waxman (2017) and Morag et al. (2023). These models assume emission powered solely by shock energy deposited in the stellar envelope, without CSM interaction.
3. Numerical Modeling: The authors utilized SNEmcee, a new open-source MCMC fitting tool, to compare the data against a grid of numerical progenitor models (KEPLER) exploded with the Supernova Explosion Code (SNEC). Fits were performed both with and without a parametrized CSM component (defined by density scaling $K$ and thickness $\Delta R_{CSM}$ ).

Key Results

Light Curve Characteristics: SN 2020bij exhibits a plateau duration of $92.07 \pm 2.35$ days and a decline rate typical of SNe IIP. However, its rise time of ~14 days is significantly longer than the typical $\lesssim$ 10 days seen in most SNe IIP. The other four events in the sample (ASASSN-14kg, SN 2018fif, SN 2021yja, SN 2023axu) share this slow-rise characteristic.
Expansion Velocities: Spectroscopic analysis reveals that SN 2020bij and the other slow-rising events possess high expansion velocities, distinguishing them from the bulk of the SNe IIP population.
Modeling Constraints:
- Shock Cooling: Both the Sapir & Waxman (2017) and Morag et al. (2023) models successfully reproduce the early optical light curves of SN 2020bij and the other slow-rising events without invoking CSM interaction. The models yield progenitor radii consistent with RSGs ( $\sim$ 860 $R_\odot$ for SN 2020bij) and envelope masses of $3-5 M_\odot$ .
- Discrepancies: The models slightly underestimate the U-band flux, a discrepancy also noted in other events (e.g., SN 2023axu), which the authors attribute to uncertainties in UV line blanketing rather than CSM interaction.
- CSM Interaction: When SNEmcee numerical models including CSM were applied, the addition of CSM did not significantly improve the fit. Bayesian Information Criterion (BIC) analysis strongly favored the model without CSM ( $\Delta$ BIC $\approx$ 5.91), suggesting the data do not require a CSM component.
Sample Consistency: The four additional slow-rising SNe also showed evidence of weak to no CSM interaction in previous literature and in the authors' re-analysis, with shock cooling alone being sufficient to explain their early emission.

Significance and Claims
The paper proposes that SN 2020bij and the identified slow-rising, high-velocity SNe constitute a new, rare subclass of Type IIP supernovae. Unlike the majority of SNe IIP which appear to require CSM interaction to explain rapid light curve rises, this subclass is characterized by:

Slow Rise Times: $\gtrsim$ 10 days to the plateau.
High Expansion Velocities.
Weak to No CSM Interaction: Their early emission is consistent with pure shock cooling from a confined or absent CSM environment.

The authors conclude that these events likely originate from progenitors with relatively confined CSM, offering a counterexample to the trend of ubiquitous, dense CSM mass-loss in RSGs shortly before explosion. Understanding this subclass is presented as a key step toward constraining the full diversity of late-stage mass-loss mechanisms in massive stars. The paper emphasizes that future early-time photometric and spectroscopic coverage is necessary to further map this subclass and its physical properties.