Observational Connections Between LBV’S and Other Stars, With Emphasis On Wolf-Rayet Stars

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Abstract

We suggest that the LBV mechanism is an essential step to “force” massive stars (M(ZAMS) ≳ 40M ⊙) to finally enter the Wolf-Rayet (W-R) domain in the Hertzsprung-Russel diagram (HRD). Just as massive supergiants show increasing variability as they approach the Humphreys-Davidson (H-D) instability limit (horizontally in the HRD diagram), so the W-R stars show decreasing variability as they recede from the H-D limit (at first horizontally into the WNL domain, then, with their high mass loss rates, plunging irreversably downwards as ever hotter, smaller and fainter, strong-line W-R stars). Among the W-R stars, the luminous WNL subtypes (especially WN8) are the most variable, probably as a consequence of blob ejection in the wind. The underlying mechanism which triggers this ejection is possibly related to wind instabilities and may thus be quite different from the source of variability in luminous supergiants or LBV’s in quiescence, where photospheric effects dominate.