O-star mass-loss rates at low metallicity

Astrophysics Group, Blackett Laboratory, Imperial College London, Prince Consort Road, London SW7 2AZ, UK
e-mail: l.lucy@imperial.ac.uk

Received: 18 April 2012
Accepted: 11 May 2012

Abstract

Mass fluxes J are computed for the extragalactic O stars investigated by Tramper et al. (2011, ApJ, 741, 8 – TSKK). For one early-type O star, computed and observed rates agree within errors. However, for two late-type O stars, theoretical mass-loss rates underpredict observed rates by  ~1.6 dex, far exceeding observational errors. A likely cause of the discrepancy is overestimated observed rates due to the neglect of wind-clumping. A less likely but intriguing possibility is that, in observing O stars with Z/Z_⊙ ~ 1/7, TSKK have serendipitously discovered an additional mass-loss mechanism not evident in the spectra of Galactic O stars because of their powerful radiation-driven winds. Constraints on this unknown mechanism are discussed. In establishing that the discrepancies, if real, are inescapable for purely radiation-driven winds, failed searches for high-J solutions are reported and the importance stressed of a numerical technique that cannot spuriously create or destroy momentum. The Z-dependences of the computed rates for Z/Z_⊙ ∈ (1/30,2) show significant departures from a single power law, and these are attributed to curve-of-growth effects in the differentially expanding reversing layers. The best-fitting power-law exponents range from 0.68−0.97.