Mass-loss predictions for O and B stars as a function of metallicity

We have calculated a grid of massive star wind models and mass-loss rates f or a wide range of metal abundances between 1/100 less than or equal to Z/Z (circle dot) less than or equal to 10. The calculation of this grid complet es the Vink et al, (2000) mass-loss recipe with an additional parameter Z. We have found that the exponent of the power law dependence of mass loss vs . metallicity is constant in the range between 1/30 less than or equal to Z /Z(circle dot) less than or equal to 3. The mass-loss rate scales as M prop ortional to Z(0.85) v(infinity)(p) with p = -1.23 for stars with T-eff grea ter than or similar to 25 000 K, and p = -1.60 for the B supergiants with T -eff less than or similar to 25 000 K Taking also into account the metallic ity dependence of v(infinity) using the power law dependence v(infinity) pr oportional to Z(0.13) from Leitherer et al. (1992) the overall result of ma ss loss as a function of metallicity can be represented by M proportional t o Z(0.69) for stars with T-eff greater than or similar to 25 000 K, and M p roportional to Z(0.64) for B supergiants with T-eff less than or similar to 25 000 K although it is derived that the exponent of the mass loss vs. met allicity dependence is constant over a large range ill Z, one should be awa re of the presence of bi-stability jumps at specific temperatures. Here the character of the line driving changes drastically due to recombinations of dominant metal species resulting in jumps in the mass loss. We have invest igated the physical origins of these jumps and have derived formulae that c ombing mass loss recipes for both sides of such jumps. As observations of d ifferent galaxies show that the ratio Fe/O varies with metallicity: we make a distinction between the metal abundance Z derived an the basis of iron o r oxygen lines. Our mass-loss predictions are successful in explaining the observed mass-loss rates for Galactic and Small Magellanic CIoud O-type sta rs, as well as in predicting the observed Galactic bi-stability jump. Hence , we believe that our predictions are reliable and suggest that our mass-lo ss recipe be used in future evolutionary calculations of massive stars at d ifferent metal abundance. A computer routine to calculate mass loss is publ icly available.