Envelope Ejection and the Transition to Homologous Expansion in Common-Envelope Events

TL;DR

This study presents a detailed 3-D simulation of a common-envelope event, demonstrating complete envelope ejection driven solely by orbital energy and identifying a phase of homologous expansion that could aid observational modeling.

Contribution

It provides the first long-timescale 3-D simulation showing envelope ejection driven by orbital energy alone and identifies a homologous expansion phase post-ejection.

Findings

Envelope fully ejected in about 1500 days

80% of envelope ejected in 400 days

Envelope enters homologous expansion phase around 550 days

Abstract

We conduct a long-timescale ($5000\,$d) 3-D simulation of a common-envelope event with a $2\,M_{\odot}$ red giant and a $1\,M_{\odot}$ main sequence companion, using the moving-mesh hydrodynamic solver MANGA. Starting with an orbital radius of $52\,R_{\odot}$, our binary shrinks to an orbital radius of $5\,R_{\odot}$ in $200\,$d. We show that over a timescale of about $1500\,$d, the envelope is completely ejected while $80$ per cent is ejected in about $400\,$d. The complete ejection of the envelope is solely powered by the orbital energy of the binary, without the need for late-time reheating from recombination or jets. Motivated by recent theoretical and observational results, we also find that the envelope enters a phase of homologous expansion about $550\,\rm d$ after the start of our simulation. We also run a simplified 1-D model to show that heating from the central binary in the envelope at late times does not influence the ejection. This homologous expansion of the envelope would likely simplify calculations of the observational implications such as light curves.