# An outburst powered by the merging of two stars inside the envelope of a giant

## TL;DR

This study uses 3D hydrodynamical simulations to explore how the merger of two stars inside a giant's envelope causes asymmetric outflows, potentially explaining certain irregular planetary nebulae and transient events.

## Contribution

It introduces a simulation framework for energy deposition during stellar mergers inside giant envelopes, linking mergers to asymmetric nebulae and transient phenomena.

## Key findings

- Mergers produce highly non-spherical outflows.
- Energy deposition can lead to transient luminous events.
- Results suggest a connection between stellar mergers and messy planetary nebulae.

## Abstract

We conduct three-dimensional hydrodynamical simulations of energy deposition into the envelope of a red giant star as a result of the merger of two close main sequence stars or brown dwarfs, and show that the outcome is a highly non-spherical outflow. Such a violent interaction of a triple stellar system can explain the formation of `messy', i.e., lacking any kind of symmetry, planetary nebulae (PNe) and similar nebulae around evolved stars. We do not simulate the merging process, but simply assume that after the tight binary system enters the envelope of the giant star the interaction with the envelope causes the two components, stars or brown dwarfs, to merge and liberate gravitational energy. We deposit the energy over a time period of about nine hours, which is about one per cent of the orbital period of the merger product around the centre of the giant star. The ejection of the fast hot gas and its collision with previously ejected mass are very likely to lead to a transient event, i.e., an intermediate luminosity optical transient (ILOT).