# Nuclear Dominated Accretion Flows in Two Dimensions. II. Ejecta dynamics and nucleosynthesis for CO and ONe white dwarfs

## TL;DR

This study uses detailed 2D hydrodynamic simulations to analyze mass ejection, nucleosynthesis, and transient phenomena resulting from white dwarf mergers with neutron stars or black holes, highlighting outflow properties and potential observational signatures.

## Contribution

It provides the first comprehensive 2D simulation results on ejecta dynamics and nucleosynthesis in white dwarf merger accretion disks, including detailed outflow composition and velocity distributions.

## Key findings

- Robust outflows with velocities 10^2-10^4 km/s.
- Ejected material contains 10-30% burning products, including up to 0.01 M_sun of ^56Ni.
- Potential to produce rapidly-rising transients and high-energy events.

## Abstract

We study mass ejection from accretion disks formed in the merger of a white dwarf with a neutron star or black hole. These disks are mostly radiatively-inefficient and support nuclear fusion reactions, with ensuing outflows and electromagnetic transients. Here we perform time-dependent, axisymmetric hydrodynamic simulations of these disks including a physical equation of state, viscous angular momentum transport, a coupled $19$-isotope nuclear network, and self-gravity. We find no detonations in any of the configurations studied. Our global models extend from the central object to radii much larger than the disk. We evolve these global models for several orbits, as well as alternate versions with an excised inner boundary to much longer times. We obtain robust outflows, with a broad velocity distribution in the range $10^2-10^4$ km s$^{-1}$. The outflow composition is mostly that of the initial white dwarf, with burning products mixed in at the $\lesssim 10-30\%$ level by mass, including up to $\sim 10^{-2}M_\odot$ of ${}^{56}$Ni. These heavier elements (plus ${}^{4}$He) are ejected within $\lesssim 40^\circ$ of the rotation axis, and should have higher average velocities than the lighter elements that make up the white dwarf. These results are in broad agreement with previous one- and two-dimensional studies, and point to these systems as progenitors of rapidly-rising ($\sim $ few day) transients. If accretion onto the central BH/NS powers a relativistic jet, these events could be accompanied by high energy transients with peak luminosities $\sim 10^{47}-10^{50}$ erg s$^{-1}$ and peak durations of up to several minutes, possibly accounting for events like CDF-S XT2.