The Galactic WC and WO stars: The impact of revised distances from Gaia
  DR2 and their role as massive black hole progenitors
  Wolf-Rayet stars of the carbon sequence (WC stars) are an important
cornerstone in the late evolution of massive stars before their core collapse.
As core-helium burning, hydrogen-free objects with huge mass-loss, they are
likely the last observable stage before collapse and thus promising progenitor
candidates for type Ib/c supernovae. Their strong mass-loss furthermore
provides challenges and constraints to the theory of radiatively driven winds.
Thus, the determination of the WC star parameters is of major importance for
several astrophysical fields. With Gaia DR2, for the first time parallaxes for
a large sample of Galactic WC stars are available, removing major uncertainties
inherent to earlier studies. In this work, we re-examine the sample from Sander
et al. (2012) to derive key properties of the Galactic WC population. All
quantities depending on the distance are updated, while the underlying spectral
analyses remain untouched. Contrasting earlier assumptions, our study yields
that WC stars of the same subtype can significantly vary in absolute magnitude.
With Gaia DR2, the picture of the Galactic WC population becomes more complex:
We obtain luminosities ranging from log L = 4.9 to 6.0 with one outlier having
log L = 4.7. This indicates that the WC stars are likely formed from a broader
initial mass range than previously assumed. We obtain mass-loss rates ranging
between log Mdot = -5.1 and -4.1, with Mdot propto L^0.68 and a linear scaling
of the modified wind momentum with luminosity. We discuss the implications for
stellar evolution, including unsolved issues regarding the need of envelope
inflation to address the WR radius problem, and the open questions in regard to
the connection of WR stars with Gamma-ray bursts. WC and WO stars are
progenitors of massive black holes, collapsing either silently or in a
supernova that most-likely has to be preceded by a WO stage.
