The luminous blue variables: Astrophysical geysers

Abstract

Some of the most luminous stars have sporadic, violent mass-loss eventswhose causes are not understood. These evolved hot stars are calledluminous blue variables (LBVs), and their instability may shape theappearance of the upper Hertzsprung-Russell (HR) diagram. LBV eruptionsare interestingly reminiscent of geysers or even volcanos. They havereceived considerable observational attention since 1980, buttheoretical work to explain the instability has been scarce. In atypical LBV eruption, the star's photosphere expands and the apparenttemperature decreases to near 8000 K. During these normal eruptions thebolometric luminosity remains constant, as typified by S Doradus, AGCarinae, and R 127. A few LBV's, specifically Eta Carinae, P Cygni, V12in NGC 2403, and SN 1961V, have giant eruptions in which the totalluminosity actually increases by more than one or two magnitudes. Thestar may expel as much as a solar mass or more with a total luminousoutput rivaling a supernova. The classical LBVs have luminositiesgreater than M_{Bol} approximately equal to -9.6 mag, suggestinginitial mass greater than 50 solar mass. These stars have very likelynot been red supergiants as there are no evolved cool stars ofcomparable luminosity. Their instability may prevent their evolution tothe red supergiant region. There is also a group of less luminous LBVs(M_{Bol} approximately equal to -8 to -9 mag) with lowtemperatures, smaller amplitudes, and lower mass-loss rates. These starshave probably been red supergiants and have shed a lot of mass prior totheir current unstable state. Although the physical cause of the LBVinstability is not yet understood, the most likely mechanisms involveradiation pressure (the opacity-modified Eddington limit) or dynamicalinstabilities in the outer layers as the star evolves off the mainsequence. In this review, we summarize the physical characteristics andbehavior of LBVs and discuss their brief but critical role in massivestar evolution, and possible mechanisms for their remarkableinstability.