Multiple outflows and delayed ejections revealed by early imaging of novae

TL;DR

This study delivers direct, high-resolution evidence that nova ejecta can comprise multiple, non-spherical flows and can undergo delayed envelope ejection. By combining early CHARA near-infrared interferometry with multi-wavelength data (optical/IR spectroscopy, Fermi-LAT gamma-rays, and Swift X-ray/UV observations) for two 2021 γ-ray–detected novae, it reveals a fast perpendicular outflow coupled with a slower component in V1674 Her, and a markedly delayed, predominantly non-ejected envelope in V1405 Cas. The results support a complex mass-loss history in novae, likely influenced by binary dynamics and possible common-envelope-like phases, and establish novae as nearby laboratories for shock physics and binary interaction in envelope ejection. The work highlights the importance of early, high-angular-resolution imaging to resolve ejecta morphologies and constrain the mechanisms powering shocks and high-energy emission in nova eruptions.

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 $γ$-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 with 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 likely drives the observed $γ$-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.

Figures (29)

• Figure 1: Early imaging of nova V1674 Her reveals a shift from spherical symmetry with potential multiple outflows.Panel (a): multiple-ejection schematic illustration---an early slower flow followed near optical peak by a faster outflow; their collision produces GeV $\gamma$-ray–emitting shocks (purple arrows). Panel (b): CHARA images at $t=2.2$ and $3.2$ d after discovery ($t_0=$ 2021-06-12.19 UT), reconstructed with BSMEM (see Section Methods). The images show a central elongated component is surrounded by an extended structure elongated roughly perpendicular; we interpret the inner feature as the slow flow and the extended feature as the fast outflow. Panel (c): H$\beta$ spectral line profiles taken on days 0.7 and 2 since $t_0$. The orange, blue, and green dashed lines represent $v_{\mathrm{rad}}$ = 0 km s$^{-1}$, = $-3800$ km s$^{-1}$, and = $-5500$ km s$^{-1}$ relative to rest wavelength, respectively. Panel (d): the AAVSO visible (green stars for $V$-band and blue points for Visual measurements) and GeV $\gamma$-rays (black squares for detections and magenta triangles for 1$\sigma$ upper limits, noted UL in the legend) light curves of V1674 Her. The error bars represent 1-$\sigma$ uncertainties. The vertical dashed lines represent the dates of the CHARA imaging.
• Figure 1: The CHARA (u,v) coverage of interferometric baselines projected on the plane of the sky in right ascension (RA) and declination (Dec.), during the observations of nova V1674 Her. Left panel is the 2021 June 14 epoch (day 2.2) and right panel is the 2021 June 15 epoch (day 3.2).
• Figure 2: Early imaging of nova V1405 Cas reveals a delay in ejection of more than 50 days into eruption.Top-left: three CHARA images obtained on days 53, 55, and 67 since discovery ($t_0$ = 2021-03-18.42 UT), reconstructed with BSMEM (see Section Methods). Top-right: H$\alpha$ spectral line profiles taken on days 53, 55, and 65, since $t_0$. The orange, blue, and green dashed lines represent $v_{\mathrm{rad}}$ = 0 km s$^{-1}$, $-700$ km s$^{-1}$, and $-2100$ km s$^{-1}$ relative to rest wavelength, respectively. Panel (c): shows the AAVSO $V$-band (green stars) and GeV $\gamma$-ray (black squares for detections and magenta triangles for 1$\sigma$ upper limits, noted UL in the legend) light curves of V1405 Cas (see Section Methods). The error bars represent 1-$\sigma$ uncertainties. The vertical dashed lines represent the dates of the CHARA epochs.
• Figure 2: The prior (top) and BSMEM images of nova V1674 Her.Top panels: the prior images used based on an elliptical Gaussian fit to the visibility data. Middle panels: the BSMEM images using a linear scale. Bottom panel: the BSMEM images using square-root intensity, to highlight low-surface brightness emission within the field-of-view.
• Figure 3: The AAVSO optical light curve of nova V1674 Her during the first 100 days of the eruption AAVSODATA. The green stars are $V$-band measurements while the black dots are visutal estimates. The error bars represent 1-$\sigma$ uncertainties.