The Center for High Angular Resolution Astronomy (CHARA Array) at Georgia State University has created detailed images in the first phases of two nova explosions detected in 2021. Through near-infrared interferometry, a process that combines light from several telescopes, the CHARA Array was able to capture in high resolution the rapidly changing conditions of their early post-explosion phase.

A nova is an astronomical event that occurs in a binary system when a white dwarf ejects its companion star of hydrogen-rich gas, causing a thermonuclear runaway reaction at the surface of the white dwarf. The name comes from the sudden brightness that appears as if a new star has appeared in the night sky. However, the ejecta after the explosion is small and a challenge to observe, and until now astronomers could only imagine the early stages through indirect methods.

“The images give us a closer look at how material is ejected from the star during the explosion,” Explains Gail Schaefer, director of the CHARA Array. “Capturing these transient events requires the flexibility to adjust our evening schedule while discovering new targets of opportunity.”

Fireworks Results

Schaeffer and his team spotted V1674 Herculis, a nova in the Hercules constellation, and V1405 Cassiopeiae, a nova in Cassiopeia. V1674 is one of the fastest novae ever recorded, reaching peak brightness less than 16 hours after its discovery and rapidly fading in just a few days. In contrast, V1405 took 53 days to reach its peak brightness and remained bright for about 200 days.

Images taken 2.2 days (left) and 3.2 days (middle) after the explosion caused by nova V1674 Herculis. As indicated by the arrows, two ejecta flows are formed. On the right is an illustration of an 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 one to the southeast with an elliptical structure radiating almost perpendicular to it. This is direct evidence that the explosion involved multiple ejecta interacting with each other.

Spectroscopic observations have also detected different velocity components in the Balmer series of hydrogen atoms. While the absorption line before the peak is about 3,800 km/s, the component that appears after the peak reaches about 5,500 km/s.

Timing is important. The new flow of ejecta appeared in the image along with the detection of high-energy gamma rays by NASA’s Fermi Gamma-ray Space Telescope. The collision of the different velocity streams forms a powerful, gamma-ray emitting shock wave.

The results of V1405 are even more surprising. The first two observations during the peak showed only a bright central light source and some surrounding ejections. The diameter of the central region is approximately 0.99 milliarcseconds, which when converted to distance corresponds to a radius of approximately 0.85 au (au stands for the astronomical unit, the distance between the Earth and the sun).