L-band spectroscopy of Galactic OB-stars^⋆,⋆⋆

¹ Centro de Astrobiología, (CSIC-INTA), Ctra. Torrejón Ajalvir km 4, 28850 Torrejón de Ardoz, Spain
e-mail: najarro@cab.inta-csic.es
² Department of Physics, University of Cincinnati, PO Box 21001, Cincinnati, Ohio, 45221-0011, USA
e-mail: hansonmm@ucmail.uc.edu
³ Universitätssternwarte München, Scheinerstr. 1, 81679 München, Germany
e-mail: uh101aw@usm.uni-muenchen.de

Received: 26 October 2010
Accepted: 18 August 2011

Abstract

Context. 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.

Aims. 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.

Methods. 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.

Results. 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 Ṁ. 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 Ṁ values. On the other hand, the line wings display similar sensitivity to mass loss as H_α, the classical optical mass loss diagnostics.

Conclusions. 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.