Multiple outflows and delayed ejections revealed by early imaging of novae

Abstract

Novae are thermonuclear eruptions on accreting white dwarfs in interacting binaries. Although most of the accreted envelope is expelled, the mechanism-impulsive ejection, multiple outflows or prolonged winds, or a common-envelope interaction-remains uncertain. Gigaelectronvolt γ-ray detections from >20 Galactic novae establish these eruptions as nearby laboratories for shock physics and particle acceleration, underscoring the need to determine how novae eject their envelopes. Here we report on near-infrared interferometry, supported by multiwavelength observations, of two γ-ray-detected novae. The images of the very fast 2021 nova V1674 Her, taken just 2-3 days after discovery, reveal the presence of two perpendicular outflows. The interaction between these outflows probably drives the observed γ-ray emission. Conversely, the images of the very slow 2021 nova V1405 Cas suggest that the bulk of the accreted envelope was ejected more than 50 days after the eruption began, as the nova slowly rose to its visible peak, during which the envelope engulfed the system in a common-envelope phase. These images offer direct observational evidence that the mechanisms driving mass ejection from the surfaces of accreting white dwarfs are not as simple as previously thought, revealing multiple outflows and delayed ejections.