Mass-loss rates for O and early B stars powering bowshock nebulae:
  evidence for bi-stability behavior
  Second only to initial mass, the rate of wind-driven mass loss determines the
final mass of a massive star and the nature of its remnant. Motivated by the
need to reconcile observational values and theory, we use a recently vetted
technique to analyze the mass-loss rates in a sample of OB stars that generate
bowshock nebulae. We measure peculiar velocities from new Gaia parallax and
proper motion data and their spectral types from new optical and infrared
spectroscopy. For our sample of 67 central stars in morphologically selected
bowshocks nebulae, 67 are OB stars. The median peculiar velocity is 11 km/s,
significantly smaller than classical `runaway star' velocities. Mass-loss rates
for these O and early B stars agree with recently lowered theoretical
predictions, ranging from ~10^-7 Msun/yr for mid-O dwarfs to 10^-9 Msun/yr for
late-O dwarfs---a factor of about 2.7 lower than the often-used Vink et al.
(2001) formulation. Our results provide the first observational mass-loss rates
for B0--B3 dwarfs and giants---10^-9 to 10^-8 Msun/yr. We find evidence for an
increase in the mass-loss rates below a critical effective temperature,
consistent with predictions of the bi-stability phenomenon in the range
Teff=19,000--27,000 K. The sample exhibits a correlation between modified wind
momentum and luminosity, consistent in slope but lower by 0.43 dex in magnitude
compared to canonical wind-luminosity relations. We identify a small subset of
objects deviating most significantly from theoretical expectations as probable
radiation-driven bow wave nebulae by virtue of their low stellar-to-nebular
luminosity ratios. For these, the inferred mass-loss rates must be regarded as
upper limits.
