ζ Oph and the weak-wind problem

Abstract

Abstract Mass-loss rate, Ṁ is one of the key parameters affecting evolution and observational manifestations of massive stars and their impact on the ambient medium. Despite its importance, there is a factor of ∼100 discrepancy between empirical and theoretical Ṁ of late-type O dwarfs, the so-called weak-wind problem. In this Letter, we propose a simple novel method to constrain Ṁ of runaway massive stars through observation of their bow shocks and Strömgren spheres, which might be of decisive importance for resolving the weak-wind problem. Using this method, we found that Ṁ of the well-known runaway O9.5 V star ζ Oph is more than an order of magnitude higher than that derived from ultraviolet (UV) line fitting and is by a factor of 6–7 lower than those based on the theoretical recipe by Vink et al. and the Hα line. The discrepancy between Ṁ derived by our method and that based on UV lines would be even more severe if the stellar wind is clumpy. At the same time, our estimate of Ṁ agrees with that predicted by the moving reversing layer theory by Lucy.