Theoretical wind clumping predictions from 2D LDI models of O-star winds at different metallicities

Context. 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. Aims. We theoretically investigated the degree of wind clumping due to the line-deshadowing instability (LDI) as a function of Z. Methods. 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 (Q) over bar describing the total ensemble of driving lines. Results. 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) equivalent to /(2) on metallicity yields f(cl) proportional to Z(0.15 +/- 0.01). This implies that empirically derived power-law dependencies of mass-loss rate (M)over dot on metallicity - which were previously inferred from spectral diagnostics effectively depending on (M)over dot root 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.

Automated summary

In hot, massive (OB) stars, the amount of wind driving and mass loss is dependent on the stellar metallicity. In addition, the line-driven winds are inhomogeneous and clumpy, and the degree of wind clumping is found to decrease with metallicity. However, the decrease is rather weak, suggesting that the power-law dependencies of mass-loss rate on metallicity should be only modestly altered by clumping.