SIMBAD references

2000ApJ...534..348H - Astrophys. J., 534, 348-358 (2000/May-1)

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

HOWK J.C., CASSINELLI J.P., BJORKMAN J.E. and LAMERS H.J.G.L.M.

Abstract (from CDS):

Observations of the B0.2 V star τ Scorpii have revealed unusual stellar wind characteristics: redshifted absorption in the far-ultraviolet O VI resonance doublet up to ~+250 km.s–1 and extremely hard X-ray emission implying gas at temperatures in excess of 107 K. We describe a phenomenological model to explain these properties. We assume the wind of τ 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 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)x107 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 103 clumps with individual masses mc∼1019-1020 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 M{dot}c∼10–9 to 10–8 M.yr–1, a small fraction of the total mass-loss rate (M{dot}∼3x10–8 M.yr–1). We discuss the position of τ Sco in the H-R diagram, concluding that τ Sco is in a crucial position on the main sequence. Hotter stars near the spectral type of τ 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.

Abstract Copyright:

Journal keyword(s): Stars: Early-Type - stars: individual (τ Scorpii) - Stars: Winds, Outflows - ultraviolet: stars - X-rays: stars

Status at CDS:  

Simbad objects: 3

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2020.04.03-13:44:12

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