OB stars and Luminous Blue Variables

The most direct method of deriving empirical mass-loss rates for hot stars is through analysis of the UV resonance transitions from dominant metal ions. A major advance is anticipated through the recent HST ULLYSES programme, involving high resolution STIS and COS spectroscopy of OB stars in the Magellanic Clouds, which has been followed up with VLT visible Xshooter spectroscopy: XshootU (Xshooting ULLYSES)

Binary evolution is now recognised to play a dominate role for OB stars, from previous Galactic and LMC studies, the latter using VLT/FLAMES spectroscopy of OB stars in 30 Doradus (VFTS). A recent initiative focused on multiple epoch spectroscopy of SMC OB stars VLT/FLAMES is currently underway: BLOeM with initial results already available.

General predictions of close binary evolution involve mass transfer from the original primary (mass donor)  to the original secondary (mass gainer), leading to the primary being stripped of its H-rich envelope, leading to a stripped (Helium) star. Such stars have recently been detected in the Magellanic Clouds.

Regarding, LBVs, I have monitored a Luminous Blue Variable star in the Magellanic galaxy NGC 2366 10,000,000 light years away. This star has a B supergiant spectral type, except that it is currently undergoing a giant eruption, such that one solar earth mass of material is being ejected every day!

Such high and constant mass-loss is exceptional even amongst massive stars, although all are irregular and structured at a basic level. It is believed that such intense episodes of mass-loss are responsible for the removal of hydrogen envelopes in massive stars, causing evolution to the Wolf-Rayet phase.