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Shock cooling emission from late-time mass loss in low-mass He star binaries

This paper models low-mass helium star binaries undergoing intense pre-supernova mass loss to demonstrate that shock cooling from the resulting dense circumstellar material can explain the early light curves of certain Type Ib/n supernovae and fast blue optical transients, while predicting distinctive late-time radio emission as a key observational test.

Original authors: Samantha C. Wu, Anthony L. Piro

Published 2026-07-23

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CC BY 4.0

Original authors: Samantha C. Wu, Anthony L. Piro

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 grand, chaotic dance floor where stars are the dancers. Sometimes, two stars get so close they become a binary pair, holding hands and spinning around a common center. As they age, one star might run out of fuel and swell up like a giant, overinflated balloon. If it gets too big, it spills its outer layers onto its partner or into space, creating a cloud of gas and dust called circumstellar material (CSM). When the star finally runs out of fuel completely and collapses, it explodes in a supernova—a cosmic firework that outshines entire galaxies. But here's the twist: if that explosion happens inside a thick cloud of its own making, the blast wave hits the gas before it even hits the empty space. This creates a unique, bright flash of light called "shock cooling emission," which is like the steam rising instantly when a hot pan hits cold water. Astronomers care about this because these flashes act as a flashlight, illuminating the messy, hidden history of how stars die and interact with their neighbors.

This paper dives into a specific, dramatic scenario: what happens when a low-mass star that has already lost its hydrogen "coat" (a stripped star) is in a binary system and starts losing mass again right before it explodes? The authors, Samantha Wu and Anthony Piro, used powerful computer simulations to model these dying stars. They imagined these stars in different-sized orbits, from tight hugs to distant spins, and watched how they shed material in the final months of their lives. They then "exploded" these simulated stars in a virtual lab to see what the resulting light curves (the story of how bright the explosion gets and fades over time) would look like.

The team found that these explosions are like a two-act play. First, the shockwave hits the dense cloud of gas the star just dumped nearby, creating a super-bright, fast-rising flash of light. This is the "shock cooling" from the circumstellar material. But then, the light doesn't just fade away; it hits a second stage. The star's remaining helium envelope (its inner skin) is also puffed up and extended. As the shockwave cools this helium layer, it creates a long, glowing plateau—a flat, bright stretch of time that lasts for weeks. This helium plateau is a bit like a slow-burning ember that keeps the fire going after the initial burst.

The simulations suggest that these specific explosions could explain a mysterious group of real-life events astronomers have seen: fast, blue, and incredibly bright transients known as "Fast Blue Optical Transients" (FBOTs). Some of the models the authors built evolve so quickly and stay so hot that they look exactly like these elusive cosmic speedsters. However, the authors are careful to note that this is a simulation-based suggestion, not a final proof. They also predict a "ghost" signal: because these stars likely dumped a lot of material far out into space years before the explosion, the shockwave might hit that distant gas years later, creating a bright radio signal that rises slowly long after the optical light has faded.

In short, the paper proposes that the chaotic final days of a binary star system, where a stripped star dumps a massive amount of helium-rich gas just before dying, can create the bright, fast, and hot explosions we see in the sky. It's a story of a star's final, frantic dance, leaving a trail of gas that lights up the universe in a unique way, offering a new way to measure the size and mass of these dying stars. The authors hope that by looking for these specific light patterns and the delayed radio signals, we can confirm if this is indeed the secret life cycle of some of the universe's most energetic explosions.

Technical Summary: Shock Cooling Emission from Late-Time Mass Loss in Low-Mass He Star Binaries

Problem Statement
A subset of hydrogen-poor core-collapse supernovae (SNe), including Type Ib/c and Type Ibn events, exhibit signatures of interaction with dense circumstellar material (CSM). Observations suggest these events originate from progenitors undergoing extreme mass loss shortly before core collapse. While previous work has explored mass transfer in binary systems, the specific appearance of SNe from low-mass stripped stars (He cores $\lesssim 4 M_\odot$ ) that undergo significant radial expansion and late-time mass transfer remains incompletely characterized. Specifically, there is a need to model how the interplay between dense CSM formed by late-stage binary interaction and the extended helium envelopes of stripped stars shapes the early light curves, potentially explaining fast blue optical transients (FBOTs) and ultra-stripped SNe.

Methodology
The authors employ a multi-stage numerical approach to simulate these systems:

Binary Stellar Evolution (MESA): Using the MESA code (version r15140), the authors model the evolution of single stars with initial masses $M_{\text{ZAMS}} = 13.8\text{--}14.5 M_\odot$ at solar metallicity ( $Z=0.02$ ). The hydrogen envelopes are removed once core hydrogen burning ends, creating stripped helium stars with initial masses $M_{\text{He,i}} = 2.5\text{--}2.75 M_\odot$ . These stars are placed in circular orbits with a $1.4 M_\odot$ compact companion (point mass) across a range of initial orbital periods ( $P_{\text{orb,i}} = 30, 100, 300, 400, 500$ days). The simulations track mass loss from core helium burning through carbon, neon, and oxygen burning phases. The authors assume non-conservative mass transfer, where mass and angular momentum are lost from the system as a fast wind near the accretor.
CSM Profile Construction: The density profile of the CSM is derived from the mass loss rates ( $\dot{M}_{\text{csm}}$ ) and velocities ( $v_{\text{csm}}$ ) calculated during the binary evolution. The CSM velocity is estimated based on the binary escape velocity and mass ratio, assuming outflows from the outer Lagrange point ( $L_2$ ). The resulting CSM density follows a $\rho \propto r^{-2}$ profile.
Supernova Explosion (SNEC): The final stellar profiles from MESA, with the calculated CSM attached, are exploded using the SNEC (SuperNova Explosion Code) hydrodynamic radiative transfer code. The inner core is excised to form a neutron star. The authors vary explosion energies ( $E_{\text{SN}} = 10^{50}\text{--}3\times10^{51}$ erg) and $^{56}\text{Ni}$ masses ( $M_{\text{Ni}} = 0.01\text{--}0.1 M_\odot$ ).
Radiative Transfer Updates: The authors update SNEC to estimate color temperature ( $T_{\text{color}}$ ) rather than just photospheric temperature, accounting for the fact that photon temperatures can exceed photospheric temperatures in stripped-envelope SNe. This is done by identifying the thermalization depth where the optical depth $\tau_{\text{th}} \approx 1$ .

Key Results
The simulations produce a grid of light curves characterized by two distinct phases of shock cooling emission (SCE):

Dual SCE Components: The light curves are dominated first by SCE from the dense CSM, followed by SCE from the extended helium envelope of the stripped star. The CSM SCE is typically brighter and faster, while the He envelope SCE can produce a plateau lasting $\sim 10\text{--}30$ days.
Dependence on Orbital Period and Mass:
- High CSM Mass: Models with low initial He masses ( $M_{\text{He,i}} \lesssim 2.6 M_\odot$ ) and intermediate-to-wide orbital periods ( $P_{\text{orb,i}} \gtrsim 100$ d) undergo intense late-time mass transfer ( $\dot{M} \sim 10^{-3}\text{--}10^{-1} M_\odot \text{yr}^{-1}$ ), creating CSM masses up to $\sim 0.8 M_\odot$ extending to $\sim 10^{14}$ cm. These models exhibit bright, long-duration plateaus ( $\gtrsim 10^{44}$ erg s $^{-1}$ ) where the CSM and He envelope SCE phases merge.
- Low CSM Mass: Models with higher He masses or tighter orbits ( $P_{\text{orb,i}} \sim 30$ d) have less CSM ( $\lesssim 0.03 M_\odot$ ). Their light curves show a rapid initial peak from CSM SCE followed by a dimmer, longer-lasting plateau from the He envelope.
- No CSM: Even models without significant CSM exhibit a visible SCE plateau from the extended He envelope, which is brighter than a purely radioactive decay tail.
Helium Recombination Plateau: The He envelope SCE phase is characterized by a nearly constant color temperature of $\sim 10^4$ K, driven by helium recombination. This creates a plateau analogous to the hydrogen recombination plateau in Type IIP SNe. The authors derive analytic scalings for the plateau luminosity and duration that depend on the explosion energy, total mass (ejecta + CSM), and initial radius.
Spectral and Color Properties: The models are generally hot ( $T \gtrsim 2\times10^4$ K during early SCE) and blue, consistent with FBOTs. The color temperature is often higher than the photospheric temperature. The models lack narrow emission lines in the CSM SCE phase because the shock typically breaks out at the edge of the CSM, accelerating all material; narrow lines (Type Ibn features) would require interior breakout, which is less likely in these specific high-mass-loss scenarios.

Comparison to Observations
The authors compare their model grid to observed Type Ib/n SNe and FBOTs:

Morphology: The models successfully reproduce the light curve shapes of several observed events, including SN 2019kbj (Type Ibn), iPTF14gqr (ultra-stripped), SN 2019myn, and SN 2023utc.
FBOT Connection: A significant fraction of the models, particularly those with low CSM masses and high explosion energies, evolve rapidly and remain hot, matching the properties of FBOTs classified as Type Ib/n.
Radio Predictions: The binary evolution models predict that the late-time mass transfer phase producing the nearby CSM is preceded by a carbon-burning mass transfer phase that creates a dense wind extending to $\sim 10^{18}$ cm. The authors argue that the interaction of the SN shock with this distant CSM should generate bright radio emission rising months to years after the optical peak. This offers a testable prediction: non-detection of such late-time radio emission could rule out the specific binary mass transfer scenario proposed here.

Significance and Conclusions
The paper demonstrates that low-mass stripped stars in binary systems, undergoing late-time mass transfer near core collapse, naturally produce the dense CSM and extended envelopes required to explain the diverse light curves of interacting hydrogen-poor SNe.

The study provides a physical mechanism for the "fast and blue" nature of many Type Ib/n SNe and FBOTs, linking them directly to binary evolution parameters (initial mass and orbital period).
It highlights the He envelope SCE as a potential probe for measuring the radii and masses of stripped progenitors, a capability previously limited to Type IIP SNe.
The authors conclude that while their models match the bolometric light curves of many observed events, uncertainties in CSM velocity, opacity, and $^{56}\text{Ni}$ mixing remain. They emphasize that future high-cadence surveys (e.g., LSST) and late-time radio follow-up are critical for validating this binary progenitor scenario and distinguishing it from other interaction mechanisms.

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