Constraints from Infrared Space Observatory Data on the Velocity Law and Clumpiness of WR 136

Observations with the Infrared Space Observatory (ISO) SWS spectrometer are used to constrain the velocity law and wind clumping of the well-studied Wolf-Rayet (W-R) star WR 136 (HD 192163, WN6). Because the free-free continuum opacity in W-R winds increases steadily with wavelength in the IR, each point in the continuous spectrum may be regarded as forming in a pseudophotosphere of larger radius for longer wavelength. Using this idea in combination with an analysis of the Doppler-broadened widths of several He II recombination lines, we can derive information about the velocity law and clumpiness of the stellar wind of WR 136. The observed line emission emerges from the region exterior to the radius of optical depth unity in the free-free opacity, corresponding to v>~0.3v_∞ for our shortest-wavelength line. The ISO observations provide the continuum shape, flux level, and seven fairly strong He II emission profiles. Adopting a β-law distribution for the outflow velocity law, we compute the continuous energy distribution and line profiles. We find that there is a broad range of β-values consistent with the continuum data if we also allow the wind temperature to be a free parameter. Interestingly, the continuum data are found to constrain the wind to have fairly low clumping values for the IR-forming region. By using the continuum results in conjunction with line profile modeling, the observational constraints are best satisfied with a clumping factor of D_c=<ρ²>/<ρ>² of 1-3 and a β-value of 2-3, although higher β-values are not strongly ruled out for a modest wind temperature. The wavelength range of our ISO data allows us to probe only the outer wind acceleration zone, but in combination with radio observations, our finding that the wind clumping is fairly small suggests that the clumping in the wind of WR 136 decreases with increasing radius.

Based on observations with ISO, an ESA project with instruments funded by ESA member states (especially the PI countries: France, Germany, the Netherlands, and the United Kingdom) and with the participation of ISAS and NASA.