Observational identification of a sample of likely recent Common-Envelope Events

TL;DR

This study uses ALMA observations of water fountain sources to identify likely recent common-envelope events, revealing rapid mass ejection in low-mass binary systems and suggesting outflows are crucial during this phase.

Contribution

It provides the first empirical evidence linking water fountain sources to recent common-envelope evolution in low-mass binaries.

Findings

Water fountains had low initial masses (<4 solar masses).

They ejected significant mass rapidly over a few hundred years.

Water fountains may represent a large fraction of recent common-envelope systems in the Galaxy.

Abstract

One of the most poorly understood stellar evolutionary paths is that of binary systems undergoing common-envelope evolution, when the envelope of a giant star engulfs the orbit of a companion. Although this interaction leads to a great variety of astrophysical systems, direct empirical studies are difficult because few objects experiencing common-envelope evolution are known. We present ALMA observations towards sources known as water fountains that reveal they had low initial masses ($<4~{\rm M}_\odot$) and ejected a significant fraction of it over less than a few hundred years. The only mechanism able to explain such rapid mass ejection is common-envelope evolution. Our calculations show that the water-fountain sample accounts for a large fraction of the systems in our Galaxy which have just experienced the common-envelope phase. Since water-fountain sources show characteristic fast bipolar outflows, outflows and jets likely play an important role right before, during or immediately after the common-envelope phase.