The transition mass-loss rate: Calibrating the role of line-driven winds
  in massive star evolution
  A debate has arisen regarding the importance of stationary versus eruptive
mass loss for massive star evolution. The reason is that stellar winds have
been found to be clumped, which results in the reduction of unclumped empirical
mass-loss rates. Most stellar evolution models employ theoretical mass-loss
rates which are already reduced by a moderate factor of ~2-3 compared to
non-corrected empirical rates. A key question is whether these reduced rates
are of the correct order of magnitude, or if they should be reduced even
further, which would mean that the alternative of eruptive mass loss becomes
necessary. Here we introduce the transition mass-loss rate (dM/dt)_trans
between O and Wolf-Rayet (WR) stars. Its novelty is that it is model
independent. All that is required is postulating the spectroscopic transition
point in a given data-set, and determining the stellar luminosity, which is far
less model dependent than the mass-loss rate. The transition mass-loss rate is
subsequently used to calibrate stellar wind strength by its application to the
Of/WNh stars in the Arches cluster. Good agreement is found with two
alternative modelling/theoretical results, suggesting that the rates provided
by current theoretical models are of the right order of magnitude in the
~50Msun mass range. Our results do not confirm the specific need for eruptive
mass loss as Luminous Blue Variables, and current stellar evolution modelling
for Galactic massive stars seems sound. Mass loss through alternative
mechanisms might still become necessary at lower masses, and/or metallicities,
and the quantification of alternative mass loss is desirable.
