Detection of Ubiquitous Circumbinary Matter Formed from Common-Envelope Ejections

Abstract The formation of compact binary systems is canonically explained by evolution through a common envelope (CE) phase. While such binaries are ubiquitous and important, e.g., as progenitors of type Ia supernovae and black hole mergers, direct observational evidence for the CE material is elusive. The short-lived and dynamic nature of CEs poses challenges in their detection and detailed study. Numerical simulations predict that a fraction of the envelope may remain bound after CE ejections. In this article, we report for the first time the detection of such bound matter across a sample of 281 hot subdwarf binary candidates, believed to have formed in recent CE ejections. In their LAMOST spectra, we find anomalously strong calcium (Ca) II K lines arising from circumbinary matter. As expected, the radial velocities are close to the systemic velocities of the binaries, with a possible tendency for slow outflows.