A conceptually novel model - one in which stimulated (i.e. induced) resonance Rayleigh scattering causes ions of select species to become accelerated to very high terminal velocities in the stellar winds of OB-type stars - is proposed to explain the general appearance of so-called P Cygni profiles that often dominate the vacuum ultraviolet (VUV) spectra of such stars. In the unit step of the proposed nonlinear scattering mechanism, an ion moving in the stellar wind with radial velocity v becomes accelerated by the nonlinear process when three quantum-mechanical exchanges of energy simultaneously occur. (1) The ion, which is assumed to possess a strongly-allowed optical transition at frequency 
, absorbs a photon at 
from the illuminating star’s continuum. (2) At the same time, the ion also emits a photon at frequency 
that propagates radially inwards (i.e. towards the star). (3) As a result of (1) and (2), the radial velocity v of the ion becomes increased by an amount 
, thereby allowing overall conservation of both energy and momentum to occur in the unit nonlinear scattering step. A weak monochromatic wave at 
initially forms at some distance from the star, and becomes enormously amplified in stimulating the nonlinear scattering process as it propagates radially inwards towards the star, all the while retaining a high degree of monochromaticity. Nonlinear absorption of continuum light, induced by the presence of the intense 
wave, both pumps the stimulated scattering process and produces the spectrally-wide blueshifted region of continuum absorption that characterizes a P Cygni profile. Close to the photosphere of a hot star, the presence of the high intensity wave at 
can potentially make the stimulated scattering process somewhat more probable than linear scattering. It is here suggested that the high rate of stimulated scattering may enable ions displaying P Cygni profiles to become accelerated rapidly enough to avoid being slowed down via Coulomb coupling with stellar wind protons, thereby allowing such ions to attain terminal velocities as high as a few thousand km/sec. Since stimulated scattering processes processes are characterized by pump power thresholds, the model readily explains why only select species are accelerated to very high velocities in OB-type stellar winds, and also why a dramatic P Cygni profile for a given ion species can often discontinuously be present or absent when spectra of stars varying only slightly in spectral type are compared.
By: P. P. Sorokin; J. H. Glownia
Published in: RC23550 in 2005
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