New images reveal complex nova eruptions ^{CEFR B2}

Astronomers obtained direct interferometric images of two novae within days of their eruptions, at a level of detail not seen before. The observations, published in Nature Astronomy and made with the CHARA Array in California, provide clear evidence that nova eruptions are more complex than previously thought, with multiple interacting outflows and, in one case, a long delay before material is expelled.

Interferometry combines light from several telescopes to reach very high angular resolution. Gail Schaefer, director of the CHARA Array, noted that the method lets scientists catch transient events by adapting observing schedules when new targets appear. Novae themselves arise when a white dwarf accretes material from a companion and triggers a runaway nuclear reaction; early eruption stages were until now usually inferred because the expanding gas appeared as an unresolved point of light. Direct images now show ejecta being thrown out and colliding, which explains how strong shocks can form.

NASA’s Fermi Large Area Telescope first revealed that novae can emit high-energy gamma rays. In its first 15 years, Fermi-LAT detected GeV emission from more than 20 novae, establishing them as galactic gamma-ray sources and valuable targets for multi-messenger studies. The team imaged two different 2021 novae: V1674 Herculis, one of the fastest on record, brightened and faded in just days and showed two distinct perpendicular outflows that appeared while Fermi detected gamma rays, directly linking shock-powered emission to colliding flows; and V1405 Cassiopeiae, which retained its outer layers for more than 50 days before a delayed expulsion that led to new shocks and gamma-ray emission.

Interferometric images were supported by spectra from major observatories such as Gemini; emerging spectral features matched structures in the images and provided one-to-one confirmation of how flows form and collide. Coauthors John Monnier and Laura Chomiuk emphasized that the results open a new window onto how stars explode and generate high-energy radiation. The observations were obtained through the CHARA open-access program funded by the National Science Foundation, with institutional support from Georgia State’s College of Arts & Sciences, Office of the Provost, and Office of the Vice President for Research and Economic Development.