Theoretical wind clumping predictions from 2D LDI models of O-star winds
  at different metallicities
  Hot, massive (OB) stars experience strong line-driven stellar winds and mass
loss. As the majority of efficient driving lines are metallic, the amount of
wind driving and mass loss is dependent on the stellar metallicity Z. In
addition, line-driven winds are intrinsically inhomogeneous and clumpy.
However, to date, neither theoretical nor empirical studies of line-driven
winds have investigated how such wind clumping may also depend on Z. We
theoretically investigated the degree of wind clumping due to the
line-deshadowing instability (LDI) as a function of Z. We performed
two-dimensional hydrodynamic simulations of the LDI with an assumed
one-dimensional radiation line force for a grid of O-star wind models with
fixed luminosity, but with different metal contents by varying the accumulative
line strength Qbar describing the total ensemble of driving lines. We find
that, for this fixed luminosity, the amount of wind clumping decreases with
metallicity. The decrease is clearly seen in the statistical properties of our
simulations, but is nonetheless rather weak; a simple power-law fit for the
dependence of the clumping factor f_cl = <rho^2>/<rho>^2 on metallicity yields
f_cl ~ Z^(0.15 +/- 0.01). This implies that empirically derived power-law
dependencies of mass-loss rate Mdot on metallicity -- which were previously
inferred from spectral diagnostics effectively depending on Mdot*sqrt(f_cl) but
without having any constraints on f_cl(Z) -- should be only modestly altered by
clumping. We expect that this prediction can be directly tested using new data
from the Hubble Space Telescope Ultraviolet Legacy Library of Young Stars as
Essential Standards (ULLYSES) project.
