Multiple outflows and delayed ejections revealed by early imaging of novae

TL;DR

This study uses early imaging and multiwavelength observations of two gamma-ray detected novae to reveal complex ejection mechanisms, including multiple outflows and delayed envelope ejections, challenging previous simplified models.

Contribution

It provides the first direct observational evidence of multiple outflows and delayed ejections in novae, highlighting complex mass ejection processes.

Findings

Fast nova V1674 Her shows two perpendicular outflows.

Slow nova V1405 Cas exhibits delayed ejection over 50 days.

Interaction of outflows likely causes gamma-ray emission.

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. GeV $\gamma$-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 with multiwavelength observations, of two $\gamma$-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 likely drives the observed $\gamma$-ray emission. Conversely, the images of the very slow 2021 nova V1405~Cas suggest a delay in the ejection of the bulk of the accreted envelope of more than 50 days after the start of eruption, as the nova slowly rises to visible peak and during which the envelope engulfed the system in a common envelope phase. These unprecedented 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.