The efficient star-forming regions of stripped-envelope supernovae

Stars are the engines that drive the chemical evolution of galaxies. When massive stars, those at least eight times heavier than our Sun, reach the end of their lives, they explode as supernovae. These violent events are not just spectacular displays; they are fundamental to how galaxies work. The explosions scatter heavy elements into space, create dust, and inject energy that can either trigger or suppress the birth of new stars. To understand how galaxies change over time, astronomers must understand the stars that explode. Specifically, they need to know what kind of stars are dying and what the environment looks like at the exact moment of the explosion.

There are two main families of these massive star explosions. One family, called hydrogen-rich supernovae, still holds onto its outer layer of hydrogen gas when it explodes. The other family, known as stripped-envelope supernovae, has lost its hydrogen layer before the explosion, leaving behind a bare core. For decades, scientists have debated why these two types of stars explode in different ways. One theory suggests that stripped-envelope stars are simply much heavier and die younger, while another proposes that they are part of binary systems where a companion star strips away the hydrogen. The key to solving this mystery lies in measuring how efficiently the gas around the explosion is turning into new stars.

A team of astronomers has now taken a significant step toward answering this question by looking directly at the birthplaces of these exploding stars. Using powerful telescopes, they measured the amount of cold gas available to make stars and the rate at which that gas was actually being converted into new stars at the precise locations where supernovae had recently occurred. They focused on a specific group of nearby galaxies, observing them with enough detail to see individual clouds of gas and star-forming regions, rather than just the blurry glow of the entire galaxy. Their goal was to calculate the star-formation efficiency, a measure of how hard a region is working to turn its gas supply into stars.

The researchers combined data from two of the world's most advanced observatories. They used the Very Large Telescope to detect light from hot, ionized gas, which acts as a beacon for recent star formation. Simultaneously, they used the Atacama Large Millimeter/submillimeter Array to map the cold molecular gas, which is the raw fuel for making stars. By comparing the brightness of the hot gas to the mass of the cold gas, they could determine how efficiently each region was producing stars. This method allowed them to see the immediate surroundings of forty-two different supernovae, including twenty-one hydrogen-rich explosions and nineteen stripped-envelope explosions.

The results revealed a striking difference between the two types of explosions. The stripped-envelope supernovae were found in regions that were forming stars eight times more efficiently than the regions where the hydrogen-rich supernovae occurred. In other words, the environments where stripped-envelope stars exploded were converting their gas into new stars at a much faster rate. However, the total amount of gas available in these regions was roughly the same for both types of supernovae. This finding is crucial because it suggests that the difference is not simply about having more fuel, but about how intensely that fuel is being used.

This observation challenges the idea that stripped-envelope stars are exclusively the most massive stars in the universe. If they were simply the heaviest stars, they would be expected to explode in the densest pockets of gas, where the fuel supply is largest. Instead, the data shows they explode in regions where the gas is being processed with extreme efficiency. The authors suggest two possible explanations for this phenomenon. One possibility is that the initial mass of the stars in these regions is skewed toward the very top end, meaning the most massive stars are more common there than in typical star-forming regions. This would boost the star formation rate without necessarily increasing the total gas mass.

The second, and perhaps more compelling, explanation involves binary star systems. In this scenario, the stripped-envelope stars are not necessarily the most massive single stars, but rather stars that have interacted with a companion. The intense turbulence and high efficiency of star formation in these regions might encourage the formation of binary pairs. When two stars form close together, they can strip each other of their outer layers, leading to a stripped-envelope explosion. The high efficiency of star formation in these regions would also explain the bright glow of ionized gas observed, as binary systems are known to produce more ionizing radiation than single stars of the same mass.

The study confirms that stripped-envelope supernovae prefer to explode in environments characterized by intense, efficient star formation, rather than just environments with a large amount of gas. While the researchers cannot yet say definitively which of the two scenarios is correct, their data strongly suggests that the standard picture of these stars as merely "very massive single stars" is incomplete. The findings point toward a complex interplay where the efficiency of the star-forming environment, and the likelihood of stars forming in pairs, plays a decisive role in determining how a massive star ends its life. This work provides a new way to look at the death of stars, showing that the efficiency of their birth environment leaves a lasting mark on the nature of their final explosion.