Universe Today
Supernova explosions occur when massive stars reach the end of their lives. The outward force of their fusion can no longer support their mass against their gravity, and they collapse and explode. These cataclysmic explosions can light up the sky for months.
But some supernovae illuminate the sky for much longer than a few months. Some last for years, as their expanding energy and debris slam into clouds of dense gas that surround the star. These are called interacting supernovae, and their behaviour is heavily influenced by the nature of the cloud of gas, called circumstellar material (CSM). However, the origin of the CSM has puzzled astrophysicists for years.
A new research letter in The Astrophysical Journal Letters may have solved this puzzle. It's titled "Interacting Binary Stars as Progenitors for Interacting Supernovae," and the lead author is Sung-Han Tsai. Tsai is from the Institute of Astronomy and Astrophysics in Academia Sinica, Taiwan.
Most stars are in binary relationships, not solitary like the Sun. So it stands to reason that many stars that explode as supernovae have binary partners. This is at the heart of this research.
"Dense, compact circumstellar media (CSM) are required to power strongly interacting supernovae (SNe), yet their physical origin remains uncertain," the paper states. In this work, the researchers completed a systematic study of different models of binary star evolution to see how the dense and compact CSM originates. This created a grid of the different models of binary stellar evolution.
They found that mass transfer between the stars is responsible. Specifically, a type of transfer called Case C mass transfer.
Before a massive star detonates as a supernova, it first swells up to an enormous size. When this happens, material from its outer layers overflows its Roche lobe and spills onto its companion star. But not all of the gas sticks around. Some escapes and forms the cocoon of CSM that surrounds both stars. "Case C mass transfer—initiated after core helium ignition—naturally produces the dense, nearby CSM inferred in interacting events," the researchers write.
Thousands of years after the material is ejected and creates the CSM, the massive star explodes. The shockwave from the explosion travels outward at thousands of kilometers per second. It slams into the cocoon of CSM, transforming kinetic energy into light. This is what's behind interacting supernova, some of the brightest supernovae possible.
*This simulation snapshot illustrates some of the complexity in a supernova explosion. High-velocity ejecta from the explosion slams into the CSM, where a lot of the kinetic energy is transformed into light. Powerful hydrodynamic instabilities and turbulence develop, which creates complex structures that look like ocean waves. The orange-red protrusions seen in the image are Rayleigh–Taylor instability (RTI) fingers, formed by the growth of the Rayleigh–Taylor instability. RTI happens when a lighter material slams into a denser material. These striking upward-rising bubble like features, along with falling spikes reminiscent of mushrooms, reveal the intense interaction between the expanding supernova remnant and the surrounding gas. Image credit: ASIAA/Ke-Jung Chen*
Timing is key. Mass transfer can occur much earlier in a massive star's life as it approaches its end, and it can overflow into a cocoon that surrounds both stars, just like Case C mass transfer. But if the star doesn't go supernova soon enough after the transfer, within a few thousand years, the CSM has travelled too far outward, and the same extended brightness doesn't occur.
"We found that binary stars can prepare the stage for interacting supernovae with remarkable timing," said Tsai. "The companion star helps create a dense cocoon around the dying star just before the explosion, providing the fuel that powers these cosmic fireworks."
"Across a grid of binary models, we find that donors of 10–20 M⊙ in binaries with separations of ∼1000–2700 R⊙ undergo late-stage Roche-lobe overflow within ∼103 yr prior to core collapse, ejecting ∼0.01–0.2 M⊙ and forming CSM extending to ∼1016–1018 cm," the authors write.
The authors say that these interacting, Case C mass transfer supernovae are not particularly rare. They could account for 13% of core-collapse supernovae (CCSNe).
They also say that a subset of the Case C transfers agress with observations of known interacting supernovae like SN 2014C. Supernovae explosions forge elements that decay in the aftermath, creating luminosity that can persist. Previous research into SN 2014C suggested that it had produced 56Ni that decayed and created its extended luminosity, but other research concluded that it would had to have produced an impossibly enormous amount of 56Ni to account for it. The Case C mass transfer is a better fit.
"In contrast to earlier binary interactions or single-star mass loss, Case C transfer operates at the right time and scale to shape the immediate pre-SN environment without requiring ad hoc eruptive mechanisms," the researchers explain.
There are still some uncertainties. The specific geometry and dynamics of the Roche Lobe overflow, along with radiative cooling, affects how dense the CSM is and how it's spatially distributed. "Reproducing the most compact CSM configurations inferred in some events likely requires mass transfer occurring even closer to core collapse or a more efficient confinement of the outflow," the authors write.
But even without filling all of the blanks, this work reaches a clear conclusion about interactive supernovae.
"Our results identify late-stage binary interaction as a robust and physically motivated channel for producing the dense CSM that powers interacting SNe," the authors conclude.
