Pre-explosion spiral mass loss of a binary star merger

TL;DR

This paper explains the pre-explosion behavior of a binary star merger, showing that gradual mass loss from the outer Lagrange point causes observable photometric variations and influences the merger process.

Contribution

It reveals that pre-merger mass loss via the outer Lagrange point explains observed behaviors, challenging existing models and highlighting its ubiquity in similar transients.

Findings

Pre-merger mass loss lasts thousands of orbits.

Mass loss during this phase rivals that of the dynamical coalescence.

Photometric variations are driven by the 'death spiral' outflow.

Abstract

Binary stars commonly pass through phases of direct interaction which result in the rapid loss of mass, energy, and angular momentum. Though crucial to understanding the fates of these systems, including their potential as gravitational wave sources, this short-lived phase is poorly understood and has thus far been unambiguously observed in only a single event, V1309 Sco. Here we show that the complex and previously-unexplained photometric behavior of V1309 Sco prior to its main outburst results naturally from the runaway loss of mass and angular momentum from the outer Lagrange point, which lasts for thousands of orbits prior to the final dynamical coalescence, much longer than predicted by contemporary models. This process enshrouds the binary in a "death spiral" outflow, which affects the amplitude and phase modulation of its light curve, and contributes to driving the system together. The total amount of mass lost during this gradual phase ($\sim 0.05 M_\odot$) rivals the mass lost during the subsequent dynamical interaction phase, which has been the main focus of "common envelope" modeling so far. Analogous features in related transients suggest that this behavior is ubiquitous.