Superlinear Type II Superluminous Supernovae 2017fck and 2019cmv: A Possible Origin from Interacting Thermonuclear Supernovae

Imagine the universe as a grand stage where stars perform their final, explosive acts. Usually, when a star dies, it flashes brightly and then fades away, powered by the radioactive decay of its own ashes. But sometimes, a star puts on a show so blindingly bright that it outshines entire galaxies. These are the "Superluminous Supernovae" (SLSNe), the divas of the cosmic stage.

For a long time, astronomers were puzzled by a specific type of these divas: the hydrogen-rich ones (SLSNe-II). They are so bright and so long-lasting that the standard "radioactive ash" explanation just doesn't cut it. It's like trying to power a stadium light show with a single AA battery. The usual suspect for this extra power is a thick fog of gas and dust surrounding the star (called Circumstellar Material, or CSM). When the explosion hits this fog, it creates a massive shockwave that supercharges the light.

But here's the mystery: that fog is so thick it hides the star's true identity. Is it a massive star collapsing in on itself? Or is it something else entirely?

Enter two new stars in the show: SN 2017fck and SN 2019cmv.

The "Superlinear" Mystery

The authors of this paper, led by Daichi Hiramatsu, looked closely at the light curves (the graphs showing how bright these stars get over time) of these two explosions. They noticed something weird. After reaching their peak brightness, instead of fading away quickly like a normal supernova, these two stayed incredibly bright for a very long time, fading in a straight, steady line. The authors call this a "superlinear" decline.

It's as if you turned on a flashlight, and instead of the battery dying out, it stayed at full brightness for months, slowly dimming in a perfectly straight line.

The "Ia-CSM" Connection

The team realized these two weird stars looked a lot like a different group of supernovae called SNe Ia-CSM. These are Type Ia supernovae (which usually come from white dwarfs, the dense, burnt-out cores of stars) that happen to crash into a thick shell of gas.

To test if this was the real story, the researchers built a giant digital sandbox. They used a supercomputer to simulate thousands of explosions where a white dwarf (the Type Ia) crashes into clouds of gas with different shapes and densities.

The Simulation Results:
The simulations showed that if you take a white dwarf explosion and smash it into a massive shell of gas (between 2 and 11 solar masses of material), you get exactly the "superlinear" light curve seen in 2017fck and 2019cmv.

• For SN 2017fck, the simulations suggest it crashed into a cloud weighing about 11 solar masses.
• For SN 2019cmv, the cloud was slightly lighter, around 6 solar masses.

The paper suggests that the best fit for the shape of the gas cloud was one that gets less dense as you move away from the center, following a specific mathematical rule (proportional to 1/r).

What This Means for the "Cast"

If these simulations are right, it changes the story of how these stars died.

• The Old Theory: These were massive stars (like 100 times the mass of our Sun) collapsing and exploding.
• The New Suggestion: These were likely white dwarfs (the size of Earth but as heavy as the Sun) that were part of a binary system. They were orbiting a companion star (either a massive star or an intermediate one).

The paper proposes a dramatic scenario called "common envelope evolution." Imagine the white dwarf and its companion getting so close that the companion's outer layers swallow the white dwarf whole. The white dwarf spirals inside this giant gas bubble, eventually merging with the companion's core and triggering a thermonuclear explosion. The explosion then blasts through the giant gas bubble that was just created, creating that massive, bright, long-lasting show.

What the Paper Rules Out (and What It Doesn't)

The authors are careful not to say they have solved the whole universe.

• They do not rule out that these could be massive stars collapsing, but they argue that the specific "superlinear" light curve and the spectral similarities to Type Ia-CSM make the white dwarf origin a very strong candidate.
• They explicitly suggest that the underlying explosion is thermonuclear (like a Type Ia), not the core collapse of a massive star, based on the iron signatures they see in the light and the way the light curves behave.
• They do not claim this is proven fact for every single superluminous supernova. They specifically note that one famous star, SN 2006gy, might also fit this white dwarf model, but it requires even more gas (around 13 to 19 solar masses), which is a bit harder to explain with current models.
• They admit that for SN 2019cmv, the evidence is a bit fuzzier because its light faded more slowly and looked more like a massive star explosion (IIn) than a Type Ia. However, the overall pattern still points toward the white dwarf scenario.

The Verdict

The paper suggests that SN 2017fck and SN 2019cmv are likely the result of a white dwarf exploding inside a massive, dense cloud of gas, creating a "superlinear" light show that lasts for hundreds of days.

The authors estimate the gas clouds were huge: ~11 solar masses for 2017fck and ~6 solar masses for 2019cmv. They believe this gas was likely created when the two stars in the system got tangled up in a "common envelope" before the explosion.

While the simulations match the observations beautifully, the paper concludes that this is a possible origin. It's a compelling new chapter in the story of how stars die, suggesting that some of the brightest explosions in the universe aren't from massive stars collapsing, but from white dwarfs crashing into their neighbors and blowing up in a cloud of their own making. To be absolutely sure, astronomers will need to wait for the "nebular phase" (when the gas clears out completely) to see the chemical fingerprints of the explosion, which might take years to observe.