Observations of the B0.2 V star \tau Scorpii have revealed unusual stellar wind characteristics : red-shifted absorption in the far-ultraviolet O VI resonance doublet up to \sim + 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 clumps 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 these clumps will fall back towards 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 ) \times 10 ^ { 7 } K in bow shocks at their leading edge .
The infalling material explains the peculiar red-shifted absorption wings seen in the O VI doublet .
Of order 10 ^ { 3 } clumps with individual masses m _ { c } \sim 10 ^ { 19 } -10 ^ { 20 } g are needed to explain the observed X-ray luminosity and also to explain the strength of the O VI absorption lines .
These values correspond to a mass loss rate in clumps of \mbox { \ . { M } } _ { c } \sim 10 ^ { -9 } to 10 ^ { -8 } M _ { \odot } yr ^ { -1 } , a small fraction of the total mass loss rate ( \mbox { \ . { M } } \sim 3 \times 10 ^ { -8 } M _ { \odot } yr ^ { -1 } ) .
We discuss the position of \tau Sco in the HR 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 .