Dust formation and mass loss around intermediate-mass AGB stars with initial metallicity $Z_{\rm ini} \leq 10^{-4}$ in the early Universe I: Effect of surface opacity on the stellar evolution and dust-driven wind

TL;DR

This study investigates how surface opacity affects dust formation and mass loss in low-metallicity AGB stars, revealing key conditions for efficient dust-driven winds and providing formulas for mass-loss rates relevant to early Universe dust production.

Contribution

It introduces the impact of different low-temperature opacity treatments on dust-driven wind efficiency in early Universe AGB stars, with new fitting formulas for mass-loss rates based on stellar parameters.

Findings

Opacity treatment significantly influences dust formation and mass loss.

Efficient dust-driven winds require specific temperature and carbon excess conditions.

Derived formulas enable better estimates of dust yields from early Universe AGB stars.

Abstract

Dust formation and resulting mass loss around Asymptotic Giant Branch (AGB) stars with initial metallicity in the range of $0 \leq Z_{\rm ini} \leq 10^{-4}$ and initial mass $2\leq M_{\rm ini}/M_{\odot} \leq 5$ are explored by the hydrodynamical calculations of dust-driven wind (DDW) along the AGB evolutionary tracks. We employ the MESA code to simulate the evolution of stars, assuming an empirical mass-loss rate in the post-main sequence phase, and considering the three types of low-temperature opacities (scaled-solar, CO-enhanced, and CNO-enhanced opacities) to elucidate the effect on the stellar evolution and the DDW. We find that the treatment of low-temperature opacity strongly affects the dust formation and resulting DDW; in the carbon-rich AGB phase, the maximum $\dot{M}$ of $M_{\rm ini} \geq$ 3 $M_{\odot}$ star with the CO-enhanced opacity is at least one order of magnitude smaller than that with the CNO-enhanced opacity. A wide range of stellar parameters being covered, a necessary condition for driving efficient DDW with $\dot{M} \ge 10^{-6}$ $M_{\odot}$ yr$^{-1}$ is expressed as the effective temperature $T_{\rm eff} \lesssim 3850$ K and $\log(\delta_{\rm C}L/\kappa_{\rm R} M) \gtrsim 10.43\log T_{\rm eff}-32.33 $ with the carbon excess $\delta_{\rm C}$ defined as $\epsilon_{\rm C} - \epsilon_{\rm O}$ and the Rosseland mean opacity $\kappa_{\rm R}$ in units of cm$^2$g$^{-1}$ in the surface layer, and the stellar mass (luminosity) $M$ $(L)$ in solar units. The derived fitting formulae of gas and dust mass-loss rates in terms of input stellar parameters could be useful for investigating the dust yield from AGB stars in the early Universe being consistent with the stellar evolution calculations.