An Updated Formalism For Line-Driven Radiative Acceleration and Implications for Stellar Mass Loss

TL;DR

This paper updates the formalism for line-driven radiative acceleration by calculating new force multipliers from extensive spectral line data, improving models of stellar winds and mass loss, especially for stars cooler than 15,000 K.

Contribution

It introduces a new fitting function for line force multipliers that accounts for saturation effects, enhancing the accuracy of stellar wind models across different temperatures.

Findings

New force multipliers for 4.5 million spectral lines.

Better fit with saturation model than power-law.

Predicted wind quenching below 15,000 K.

Abstract

Radiation contributes to the acceleration of large-scale flows in various astrophysical environments because of the strong opacity in spectral lines. Quantification of the associated force is crucial to understanding these line-driven flows, and a large number of lines (due to the full set of elements and ionization stages) must be taken into account. Here we provide new calculations of the dimensionless line strengths and associated opacity-dependent force multipliers for an updated list of approximately 4.5 million spectral lines compiled from the NIST, CHIANTI, CMFGEN, and TOPbase databases. To maintain generality of application to different environments, we assume local thermodynamic equilibrium, illumination by a Planck function, and the Sobolev approximation. We compute the line forces in a two-dimensional grid of temperatures (i.e., values between 5,200 and 70,000 K) and densities (varying over 11 orders of magnitude). Historically, the force multiplier function has been described by a power-law function of optical depth. We revisit this assumption by fitting alternative functions that include a saturation to a constant value (Gayley's $\bar{Q}$ parameter) in the optically-thin limit. This alternate form is a better fit than the power-law form, and we use it to calculate example mass-loss rates for massive main-sequence stars. Because the power-law force multiplier does not continue to arbitrarily small optical depths, we find a sharp decrease, or quenching, of line-driven winds for stars with effective temperatures less than about 15,000 K.