Non-LTE analysis of the Ofpe/WN9 star HDE 269227 (R84)

A spectral analysis of the Ofpe/WN9 star HD 269227 (R84) is presented. The equations of radiative transfer are solved for a spherically expanding atmosphere. A non-LTE model is used to predict the spectrum of hydrogen and helium. In almost all cases, the calculated and observed line profiles agree within observational error. The stellar parameters determined for R84 are L = 10^5.7 L_⊙, T_eff = 28,500 K, R_* = 30 R_⊙, M = 10^-4.6 M_⊙ yr^-1, V_∞ = 400 km s^-1, and Y = 0.63 by mass. Since the blueshift of the helium line P Cygni absorptions depends upon the line strength, the observed helium lines allow the velocity law to be probed from the terminal velocity of the wind down to approximately 35 km s^-1. Thus, we can empirically determine the shape of the velocity law. The above stellar parameters are used in a subsequent hydrodynamic calculation assuming that the stellar wind is driven by radiation pressure. This model reproduces both the observed mass-loss rate and the terminal wind velocity. A comparison of computed velocity law with the law determined by the spectroscopic analysis reveals good agreement, which strongly supports the argument that the stellar wind is radiation-driven. The observational opportunity to investigate the wind's velocity law makes Ofpe/WN9 stars ideally suited objects of testing predictions of radiation-driven wind theory. From its stellar parameters R84 can be identified as a star which has lost about half its initial mass. A comparison of the stellar parameters with those calculated from evolutionary models reveals that R84 is probably a post-red supergiant and that it experienced violent mass loss during its red supergiant phase. R84's close spectroscopic similarity to S Doradus variables at minimum visual brightness suggests that these stars have already experienced a phase of violent mass loss in their past during which they lost a significant fraction of their initial mass. Whether or not this enhanced mass loss occurred during the red supergiant phase cannot be determined.