Inferring hot-star-wind acceleration from Line Profile Variability

Abstract

The migration of profile sub-peaks identified in time-monitored optical emission lines of Wolf-Rayet star spectra provides a direct diagnostic of the dynamics of their stellar winds via a measured line-of-sight velocity change per unit time. Inferring the associated wind acceleration scale from such an apparent acceleration then relies on the adopted intrinsic velocity of the wind material at the origin of this variable pattern. We develop radiative transfer tools to characterise the Line Emission Region (LER) of the program lines, including turbulence and line-optical depth effects. We find that monitored-lines can be fitted well with a pure optically thin formation mechanism, line-broadening resulting from the finite velocity extent of the LER rather than turbulence.Our new estimates of LER velocity centroids are systematically shifted outwards closer to terminal velocity compared to previous determinations, now suggesting WR-wind acceleration length scales of ca. 10-20Rsun, a factor of a few smaller than previously inferred. Based on radiation-hydrodynamics simulations of the line-driven-instability mechanism, we compute synthetic line profile variability for CIII5696A, for WR111 The results match well the measured observed migration of 20-30 m/s2, However, our model stellar radius of 19 Rsun, typical of an O-type supergiant, is a factor 2--10 larger than generally expected for WR core radii.Such small radii leave inferred acceleration scales to be more extended than expected from dynamical models of line driving, but the severity of the discrepancy is substantially reduced compared to previous analyses. We conclude with a discussion of how using lines formed deeper in the wind would provide a stronger constraint on the key wind dynamics as well as information on the onset of wind clumping.