2011A&A...535A..32N

Mass-loss, occurring through radiation driven supersonic winds, is a key issue throughout the evolution of massive stars. Two outstanding problems are currently challenging the theory of radiation-driven winds: wind clumping and the weak-wind problem. We seek to obtain accurate mass-loss rates of OB stars at different evolutionary stages to constrain the impact of both problems in our current understanding of massive star winds. We perform a multi-wavelength quantitative analysis of a sample of ten Galactic OB-stars by means of the atmospheric code cmfgen, with special emphasis on the L-band window. A detailed investigation is carried out on the potential of Br_α and Pf_γ as mass-loss and clumping diagnostics. For objects with dense winds, Br_α samples the intermediate wind while Pf_γ maps the inner one. In combination with other indicators (UV, H_α, Br_γ) these lines enable us to constrain the wind clumping structure and to obtain ``true'' mass-loss rates. For objects with weak winds, Br_α emerges as a reliable diagnostic tool to constrain {dot}(M). The emission component at the line Doppler-core superimposed on the rather shallow Stark absorption wings reacts very sensitively to mass loss already at very low {dot}(M) values. On the other hand, the line wings display similar sensitivity to mass loss as H_α, the classical optical mass loss diagnostics. Our investigation reveals the great diagnostic potential of L-band spectroscopy to derive clumping properties and mass-loss rates of hot star winds. We are confident that Br_α will become the primary diagnostic tool to measure very low mass-loss rates with unprecedented accuracy.