The Center of High angular resolution astronomy (CHARA table) at Georgia State University detailed images generated of the early stages of two nova explosions detected in 2021. Using near-infrared interferometry, a process that combines light from multiple telescopes, the CHARA Array was able to capture in high resolution the rapidly changing conditions of their first post-explosion phase.
A nova is an astronomical phenomenon that occurs in a binary system when a white dwarf strips its companion star of hydrogen-rich gas, causing a runaway thermonuclear reaction on the white dwarf’s surface. The name comes from the sudden brightening that makes it appear as if a new star has appeared in the night sky. However, the ejecta immediately following the explosion are small and difficult to observe, and until now astronomers could only infer the early stages by indirect methods.
“The images give us a close-up view of how matter is ejected from the star during the explosion,” explain Gail Schaefer, director of the CHARA Array. “Detecting these transient events requires flexibility to adapt our nightly schedule as new targets of opportunity are discovered.”
Explosive results
Schaeffer and his team observed V1674 Herculis, a nova in the constellation Hercules, and V1405 Cassiopeiae, a nova in Cassiopeia. V1674 was one of the fastest novas ever recorded, reaching its peak brightness less than 16 hours after its discovery and fading rapidly in just a few days. In contrast, the V1405 took 53 days to reach peak brightness and remained bright for around 200 days.
Images taken 2.2 days (left) and 3.2 days (center) after the explosion caused by the nova V1674 Herculis. As the arrows indicate, two ejecta streams formed. On the right is an illustration of the explosion image.
The image of V1674, taken just days after its discovery, shows an explosion that is clearly not spherical; there are two ejecta streams, one to the northwest and the other to the southeast with an elliptical structure radiating almost perpendicular to them. This is direct evidence that the explosion involved multiple ejecta interacting with each other.
Spectroscopic observations also detected different velocity components in the Balmer series of hydrogen atoms. While the absorption line before the peak was about 3,800 km/s, the component that appeared after the peak reached about 5,500 km/s.
Timing is important. The new ejecta stream appeared in the image at the same time as the detection of high-energy gamma rays by NASA’s Fermi Gamma-ray Space Telescope. The collision of the different speed streams formed a powerful shock wave emitting gamma rays.
The V1405 results were even more surprising. The first two observations during the peak period showed only a bright central light source and some surrounding ejections. The diameter of the central region was approximately 0.99 milliarcseconds, which, when converted to distance, corresponds to a radius of approximately 0.85 au (au represents the astronomical unit, the distance between the Earth and the sun).
If the accumulated outer layer of hydrogen-rich gas had been ejected at the start of the explosion, it would have expanded in 53 days and had a radius of 23 to 46 au. This huge gap means that most of the outer layer has not been completely ejected after more than 50 days. In other words, it is believed that the outer layer of V1405 was in the common envelope phase that enveloped the entire binary system up to the visible light peak.
By the third observation, the structure had changed dramatically. The central light source accounted for only about half of the total radiation, while the rest was emitted from the broader region. At this time, a large emission component of approximately 2,100 km/s appeared. Subsequently, the release of material generated new shock waves and high-energy emissions were observed.
Novae as Space Labs
This discovery demonstrates that novae are much more complex than a simple explosion. Observations made over the past 15 years by the Fermi telescope have detected gamma rays on the order of gigaelectronvolts coming from more than 20 novae. Novae can now be considered as a laboratory for the study of shock waves and particle acceleration.
Observations of V1405 suggest the possibility that the orbital motion of its binary system acts as a force pushing back the outer layers that expanded due to the explosion. In slowly evolving novae, the state in which the expanded outer layers envelop the entire binary system continues for several weeks. Such phenomena provide valuable opportunities to directly observe what happens when two stars come so closely together, a process that probably occurs in more than 10 percent of the stars in the universe. However, the detailed mechanisms remain unexplained.
Novae, once thought to be simple explosions, are turning out to be much richer and more fascinating than when they were first observed. Thanks to the new window of direct imaging by near-infrared interferometry, the true nature of some of the most dramatic phenomena in the universe is just beginning to be revealed.
This story was originally published on WIRED Japan and was translated from Japanese.