Abstract
Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves. Some of them rival supernovae in total radiative output, and the mass loss is crucial for subsequent evolution. Some are supernova impostors, including SN precursor outbursts, while others are true SN events shrouded by material that was ejected earlier. Luminous Blue Variable stars (LBV’s) are traditionally cited in relation with giant eruptions, though this connection is not well established. After four decades of research, the fundamental causes of giant eruptions and LBV events remain elusive. This review outlines the basic relevant physics, with a brief summary of essential observational facts. Reasons are described for the spectrum and emergent radiation temperature of an opaque outflow. Proposed mechanisms are noted for instabilities in the star’s photosphere, in its iron opacity peak zones, and in its central region. Various remarks and conjectures are mentioned, some of them relatively unfamiliar in the published literature.
Highlights
Very massive stars occasionally expel material in colossal eruptions, driven by continuum radiation pressure rather than blast waves
Eruptions may illustrate the butterfly effect— the time and strength of each outburst may depend on seemingly minor details, and the total mass loss may differ greatly between two stars that appear identical at birth
Since only 2 to 4 giant eruptions or supernova impostors were known before 2000, they were conflated with LBV outbursts
Summary
Very massive stars lose much—and possibly most—of their mass in sporadic events driven by continuum radiation. The relatively low-luminosity outbursts may be highly abnormal phenomena (e.g., stellar mergers) that occur in only a tiny fraction of the stars In some cases they might not be above the Eddington Limit, or might not be opaque, or might be accelerated by non-radiative forces. By contrast, are highly super-Eddington, tend to look like each other regardless of their causative instabilities, and may occur in a substantial fraction of the most massive stars. This article is a descriptive review like a textbook chapter, not a survey of publications It outlines the basic physics and theoretical results with only a minimal account of the observational data.
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