Ionisation fractions and mass-loss in O stars

Extract

The most sensitive indicators of mass-loss for stars in the upper left part of the HR diagram are the UV P Cygni profiles observed in the resonance lines of common ions such as N V, Si IV, and C IV. We present here some results from a study of these lines in the high resolution IUE spectra of 197 Ï stars. Profile fits were carried out in the manner described by Prinja & Howarth (1986) for all unsaturated P Cygni resonance doublets. The parameterisations adopted enable the product of mass-loss rate (Ṁ) and ion fraction (qi) to be determined at a given velocity, such that Ṁ qi°C Ni R_* v_∞ , where N_i is the column density of the observed ion i, v_∞ is the terminal velocity, and R_⋆ is the stellar radius. The accompanying figures illustrate the behaviour of Ṁ q_i (evaluated at 0.5 v_∞ ) for N V and C IV.