Mass-loss Rates of 74 Bow Shock OB Stars: No Evidence for Bi-stability Behavior

Abstract

Accurate measurements of wind-driven mass-loss rates are essential for understanding the evolution and fate of massive stars. Established techniques for measuring $\dot{M}$ suffer from large dispersions and become insensitive in the regime of weak-winded late-O to early B stars. We employ a promising new technique to measure stellar mass loss for \numstars~early type stars that produce infrared stellar bow shock nebulae. We use updated stellar parameters, improved parallactic distances and proper motions, and modern dust models to characterize the bow shock nebulae and obtain new measurements of mass-loss rates. $\mdot$ ranges from $2\times10^{-9}~\moperyear$ for weak-winded late-O and early B stars at $\log\left(\lstar/L_\odot\right)\approx4.5$ to $4\times10^{-6}~\moperyear$ for the most luminous stars in our sample at $\log\left(\lstar/L_\odot\right)\approx6.3$. Stellar space velocities relative to their local standard of rest are insufficient to accurately reflect the relative star-ISM velocities for the majority of our sample, which have speeds much less than canonical runaway thresholds, suggesting the pervasive influence of bulk interstellar flows in forming bow shock nebulae. Adopting a minimum relative velocity of $20~\kms$ is physically motivated and yields mass-loss rates in excellent agreement with the latest theoretical prescriptions while requiring no adjustable parameters. We find no enhancements in $\mdot$ for stars cooler than 25~kK, in agreement with modern mass-loss recipes that do not include bi-stability effects.