Stagnation and infall of dense clumps in the stellar wind of τ Scorpii

Observations of the B0.2 V star tau Scorpii have revealed unusual stellar wind characteristics: redshifted absorption in the far-ultraviolet O VI resonance doublet up to similar to+250 km s(-1) and extremely hard X-ray emission implying gas at temperatures in excess of 10(7) K. We describe a phenomenological model to explain these properties. We assume the wind of tau Sco consists of two components: ambient gas in which denser clumps are embedded. The clumps are optically thick in the UV resonance lines primarily responsible for accelerating the ambient wind. The reduced acceleration causes the clumps to slow and even infall, all the while being confined by the ram pressure of the outflowing ambient wind. We calculate detailed trajectories of the dumps in the ambient stellar wind, accounting for a line radiation driving force and the momentum deposited by the ambient wind in the form of drag. We show that these clumps will fall back toward the star with velocities of several hundred km s(-1) for a broad range of initial conditions. The velocities of the clumps relative to the ambient stellar wind can approach 2000 km s(-1), producing X-ray-emitting plasmas with temperatures in excess of (1-6) x 10(7) K in bow shocks at their leading edge. The infalling material explains the peculiar redshifted absorption wings seen in the O VI doublet. Of order 10(3) clumps with individual masses m(c) similar to 10(19)-10(20) g are needed to explain the observed X-ray luminosity and also to explain the strength of the O vr absorption lines. These values correspond to a mass-loss rate in clumps of M-c similar to 10(-9) to 10(-8) M-. yr(-1) a small fraction of the total mass-loss rate ((M) over dot similar to 3 x 10(-8) M-. yr(-1)). We discuss the position of Sco In the H-R diagram, concluding that tau Sco is in a crucial position on the main sequence. Hotter stars near the spectral type of tau Sco have too powerful winds for clumps to fall back to the stars, and cooler stars have too low mass-loss rates to produce observable effects. The model developed here can be generally applied to line-driven outflows with clumps or density irregularities.