The limited role of recombination energy in common envelope removal

TL;DR

This study shows that recombination energy in common envelope events is mostly radiated away rather than driving the outflow, challenging previous assumptions about its role in envelope ejection.

Contribution

It provides a detailed analysis of energy transport timescales, demonstrating that recombination energy is unlikely to significantly contribute to envelope removal.

Findings

Recombination energy mostly radiates away, not powering outflows.

Energy transport times are shorter than the envelope expansion time.

Including rapid mass loss would further reduce energy transport time.

Abstract

We calculate the outward energy transport time by convection and photon diffusion in an inflated common envelope and find this time to be shorter than the envelope expansion time. We conclude therefore that most of the hydrogen recombination energy ends in radiation rather than in kinetic energy of the outflowing envelope. We use the stellar evolution code MESA and inject energy inside the envelope of an asymptotic giant branch star to mimic energy deposition by a spiraling-in stellar companion. During 1.7 years the envelope expands by a factor of more than 2. Along the entire evolution the convection can carry the energy very efficiently outwards, to the radius where radiative transfer becomes more efficient. The total energy transport time stays within several months, shorter than the dynamical time of the envelope. Had we included rapid mass loss, as is expected in the common envelope evolution, the energy transport time would have been even shorter. It seems that calculations that assume that most of the recombination energy ends in the outflowing gas might be inaccurate.