Center for High angular resolution astronomy (CHARA board) at Georgia State University has detailed images generated showing the early stages of two nova explosions detected in 2021. Using near-infrared interferometry, a process that combines delicate from multiple telescopes, CHARA was able to capture the rapidly changing conditions of their early post-explosion phase in high resolution.
A nova is an astronomical phenomenon that occurs in a binary system when a white dwarf star 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 appears as if a up-to-date star appeared in the night sky. However, the ejecta immediately following the explosion is petite and challenging to observe, and until now astronomers could only infer the early stages using indirect methods.
“The images allow us to see up close how material is ejected from the star during the explosion.” explains Gail Schaefer, director of CHARA Array. “Capturing these transient events requires the flexibility to adjust our nightly schedule as new opportunities are discovered.”
Explosive results
Schaefer and her team observed V1674 Herculis, a nova in the constellation Hercules, and V1405 Cassiopeiae, a nova in Cassiopeia. V1674 was one of the fastest novae ever recorded, reaching peak brightness in less than 16 hours after discovery and rapidly fading in just a few days. In contrast, V1405 took 53 days to reach maximum brightness and remained dazzling for about 200 days.
An image of V1674, taken just a few days after its discovery, shows an explosion that is clearly not spherical; there are two ejecta jets, 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 interactions of ejecta with each other.
The term is significant. The up-to-date ejecta stream appeared in the image simultaneously with the detection of high-energy gamma rays by NASA’s Fermi Gamma Space Telescope. The collision of jets of different speeds created a powerful shock wave emitting gamma radiation.
V1405’s results were even more surprising. The first two observations during the peak period showed only a dazzling central delicate source and a few ejecta surrounding it. The diameter of the central region was about 0.99 milliarcseconds, which when converted to distance corresponds to a radius of about 0.85 au (au stands for astronomical unit, the distance between the Earth and the Sun).