The empirical metallicity dependence of the mass-loss rate of O- and early B-type stars

Abstract

We present a comprehensive study of the observational dependence of the mass-loss rate in stationary stellar winds of hot massive stars on the metal content of their atmospheres. The metal content of stars in the Magellanic Clouds is discussed, and a critical assessment is given of state-of-the-art mass-loss determinations of OB stars in these two satellite systems and the Milky-Way. Assuming a powerlaw dependence of mass loss on metal content,Ṁ ∝ Z m , and adopting a theoretical relation between the terminal flow velocity and metal content, v ∞ ∝ Z 0.13 (Leitherer et al. 1992, ApJ, 401, 596), we find m = 0.83 ± 0.16 for non-clumped outflows from an analysis of the wind momentum luminosity relation (WLR) for stars more luminous than 10 5.2 L . Within the errors, this result is in agreement with the prediction m = 0.69 ± 0.10 by Vink et al. (2001, A&A, 369, 574). Absolute empirical values for the mass loss, based on Hα and ultraviolet (UV) wind lines, are found to be a factor of two higher than predictions in this high luminosity regime. If this difference is attributed to inhomogeneities in the wind, and this clumping does not impact the predictions, this would imply that luminous O and early-B stars have clumping factors in their Hα and UV line forming regions of about a factor of four. For lower luminosity stars, the winds are so weak that their strengths can generally no longer be derived from optical spectral lines (essentially Hα) and one must currently rely on the analysis of UV lines. We confirm that in this low-luminosity domain the observed Galactic WLR is found to be much steeper than expected from theory (although the specific sample is rather small), leading to a discrepancy between UV mass-loss rates and the predictions by a factor 100 at luminosities of L ∼ 10 4.75 L , the origin of which is unknown. We emphasize that even if the current mass-loss rates of hot luminous stars would turn out to be overestimated as a result of wind clumping, but the degree of clumping would be rather independent of metallicity, the scalings derived in this study are expected to remain correct.

Young Star Clusters in the Small Magellanic Cloud: Impact of Local and Global Conditions on Star Formation

“…We find an average radial velocity v rad = 165.2 km/s, with a dispersion σ = 1.9 km/s, further supporting the hypothesis that NGC 346 is the result of a hierarchical collapse. Our spectroscopic analysis also indicates that the ionized gas is quiescent, with no evidence of large-scale motions, confirming that the mechanical feedback is reduced at low metallicity (see the discussion in Mokiem et al 2007), and proving that supernovae have not yet exploded (Smith et al -in preparation).…”

Analysis of Galactic late-type O dwarfs: more constraints on the weak wind problem

“…The four most luminous early-type dwarfs analyzed in the work of Martins et al (2005a) have clumped mass-loss rates. Thus, although they fall somewhat below the theoretical line in the logṀ × logṀ Vink plot and below the fit to the data of Mokiem et al (2007), they do not present a significant discrepancy if clumping is neglected 4 .…”

“…A possible reason for the aforementioned discrepancies is that mass-loss rate determinations based on UV lines could be incorrect, as was suggested by Mokiem et al (2007). The use of only one UV diagnostic line (i.e., C iv λλ1548, 1551), the abundance of carbon, the effect of X-rays, and the wind ionization structure derived from the models, are claimed to be considerable sources of errors for the derivation ofṀ.…”

“…A reason to include ζ Oph in our sample is that it is considered to have anṀ estimate based on Hα which is in good agreement with the predictions of the radiative wind theory (Mokiem et al 2005(Mokiem et al , 2007. Thus, it is interesting to check if a fit from the UV to the optical (including Hα) can be achieved with CMFGEN using a lowṀ.…”

The population of X-ray supernova remnants in the Large Magellanic Cloud

“…By measuring mass-loss rates of early-type stars in the Galaxy, LMC, and 20 The product abundance × ionisation fraction × number of available lines for metals is comparable to that of hydrogen and helium. SMC, Mokiem et al (2007) could quantify the dependence ofṀ on the metallicity asṀ ∝ Z 0.83 . It is therefore expected that in lower metallicity environment (e.g.…”