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

Abstract

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 where narrow absorption components (NACs) appear in UV lines the covering factor is approximately unity. SEI line-synthesis models are used to determine the radial optical depths of blue and red components of the SiIV 1400 resonance line doublet in a sample of 12 B0 to B4 supergiants. We focus on stars with well developed NACs and relatively low terminal velocity so that the SiIV doublet components can be treated as radiatively decoupled and formed independently. 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 Si^3+ ion fraction 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 of the terminal velocity. 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 "PV mass loss discordance" in early O supergiants are discussed.

Accepted for publication in Astronomy and Astrophysics; 7 pages; 3 figures

References in corpus (5)

• Bright OB stars in the Galaxy - III. Constraints on the radial stratification of the clumping factor in hot star winds from a combined Halpha, IR and radio analysis
• Neglecting the porosity of hot-star winds can lead to underestimating mass-loss rates
• Quantitative studies of the optical and UV spectra of Galactic early B supergiants I. Fundamental parameters
• Modeling Ultraviolet Wind Line Variability in Massive Hot Stars
• Wind signatures in the X-ray emission line profiles of the late O supergiant $\zeta$ Orionis

Cited by in corpus (12)

• Towards a unified view of inhomogeneous stellar winds in isolated supergiant stars and supergiant high mass X-ray binaries
• Mass loss from inhomogeneous hot star winds III. An effective-opacity formalism for line radiative transfer in accelerating, clumped two-component media, and first results on theory and diagnostics
• New mass-loss rates of B supergiants from global wind models
• An ALMA 3mm continuum census of Westerlund 1
• On the Origin of Wind Line Variability in O Stars
• X-Shooting ULLYSES: Massive Stars at low metallicity IX: Empirical constraints on mass-loss rates and clumping parameters for OB supergiants in the Large Magellanic Cloud
• The IACOB project XI. No increase of mass-loss rates over the bistability region
• A Radio Census of the Massive Stellar Cluster Westerlund 1
• Wind properties of variable B supergiants. Evidence of pulsations connected with mass-loss episodes
• Optically-thick Structure in Early B Type Supergiant Stellar Winds at Low Metallicities
• Runaway BN supergiant star HD 93840: Progenitor of an imminent core-collapse supernova above the Galactic plane
• Structure formation in O-type stars and Wolf-Rayet stars