Observational identification of a sample of likely recent common-envelope events

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. The interaction that ensues leads to a great variety of astrophysical systems and associated phenomena, but happens over a very short timescale. Unfortunately, direct empirical studies of this momentous and complex phase are difficult at present because few objects experiencing, or having just experienced, common-envelope evolution are known. Here we present Atacama Large Millimeter/submillimeter Array observations of minor CO isotopologues towards a sample of sources known as water fountains, which reveal that almost all of them recently lost a substantial fraction of their initial mass over a timescale of less than a few tens to a few hundreds of years. The only known mechanism able to explain such rapid mass ejection, corresponding to a large fraction of the stellar mass, is the common-envelope evolution. A stellar population analysis shows that the number of water-fountain sources in the Milky Way is comparable to the expected number of common-envelope events that involve low-mass evolved stars. Thus, the known sample of water-fountain sources accounts for a large fraction of the systems undergoing a common-envelope phase in our Galaxy. As one of the distinguishing characteristics of water-fountain sources is their fast bipolar outflow, we conclude that outflows and jets play an important role right before, during or immediately after the common-envelope phase.