Ultraviolet diagnostic of porosity-free mass-loss estimates in B stars

Abstract

Aims. We seek to establish evidence in UV P Cygni line profiles that the signs of wind clumping and porosity vary with velocity. We aim to demonstrate empirically that while at most wind velocities optically thick clumps cover only a fraction of the stellar surface, close to the terminal velocity (v ∞ ) where narrow absorption components (NACs) appear in UV lines the covering factor is approximately unity. Methods. SEI line-synthesis models are used to determine the radial optical depths (τ rad (w)) of blue and red components of the Siiv λλ1400 resonance line doublet in a sample of 12 B0 to B4 supergiants. We focus on stars with well developed NACs and relatively low v ∞ so that the Siiv doublet components can be treated as radiatively decoupled and formed independently. Results. For all 12 stars the mean optical depth ratio of the blue to red components is closer to ∼2 (i.e. the ratio of oscillator strengths) in the NACs than at intermediate and lower velocities. The product of mass-loss rate and ion fraction (Ṁ q(Si 3+ )) calculated from the NAC optical depths is a factor of ∼2 to 9 higher compared to mass-loss values sampled at ∼0.4 to 0.6 v ∞ . Conclusions. Since the wind effectively becomes "smooth" at the high NAC velocities and the column density is uniformly distributed over the stellar disk, the optical depths of the NACs are not seriously affected by porosity and this feature thus provides the most reliable measurement of mass-loss rate in the UV lines. Applications of this result to the weak-wind problem of late O-dwarf stars and the "P v mass loss discordance" in early O supergiants are discussed.

Mass loss from inhomogeneous hot star winds

“…2.3), whereas in the case of f ic = 0.1 the strong κ 0 = 100 line recovers the absorption blackness of the smooth model. Another important result of vorosity evident from the figure, is that the relative strength between the blue and red components can differ significantly from the expected factor of 2 in optical depth (stemming from the oscillator strength ratio, see also Prinja & Massa 2010;Sundqvist et al 2011;Prinja & Massa 2013); this last property is examined in detail in Sect. 3.3.…”

UV Spectroscopy of Massive Stars

“…Si IV doublets have optical depths a factor of two lower than the blue component. However, analysis of UV wind profiles in B stars showed that the best fits to the doublet profiles were generally obtained using a ratio less than two, indicating that the winds are porous [134,135].…”

Towards a Unified View of Inhomogeneous Stellar Winds in Isolated Supergiant Stars and Supergiant High Mass X-Ray Binaries

“…E.g., Cohen et al (2010a) proposed to measure mass-loss rates by analysing X-ray emission line profiles. Prinja and Massa (2013) suggested that the ratio of 23 Or from the black troughs, if present 24 Sometimes, two velocity laws with different β are combined, e.g. Todt et al 2010 narrow features observed at terminal velocities in the UV resonance lines of B-supergiants provides a clumping independent tool to estimate mass-loss rates in these objects.…”