3D LTE spectral line formation with scattering in red giant stars

TL;DR

This study examines how coherent isotropic continuum scattering influences spectral line formation in red giant stars, revealing significant effects on flux levels, line profiles, and abundance measurements, especially at shorter wavelengths and lower metallicities.

Contribution

It provides a detailed analysis of scattering effects in 3D and 1D LTE models of red giants, highlighting their impact on spectral line formation and abundance corrections.

Findings

Scattering increases continuum flux beneath 5000 Å, especially at short wavelengths and low metallicities.

Line depths are enhanced by scattering, affecting abundance determinations for high-excitation and ionized species.

Strong lines can have abundance corrections up to -0.5 dex at 3000 Å in 3D models.

Abstract

We investigate the effects of coherent isotropic continuum scattering on the formation of spectral lines in local thermodynamic equilibrium (LTE) using 3D hydrodynamical and 1D hydrostatic model atmospheres of red giant stars. Continuum flux levels, spectral line profiles and curves of growth for different species are compared with calculations that treat scattering as absorption. Photons may escape from deeper, hotter layers through scattering, resulting in significantly higher continuum flux levels beneath a wavelength of 5000 A. The magnitude of the effect is determined by the importance of scattering opacity with respect to absorption opacity; we observe the largest changes in continuum flux at the shortest wavelengths and lowest metallicities; intergranular lanes of 3D models are more strongly affected than granules. Continuum scattering acts to increase the profile depth of LTE lines: continua gain more brightness than line cores due to their larger thermalization depth in hotter layers. We thus observe the strongest changes in line depth for high-excitation species and ionized species, which contribute significantly to photon thermalization through their absorption opacity near the continuum optical surface. Scattering desaturates the line profiles, leading to larger abundance corrections for stronger lines, which reach -0.5 dex at 3000 A for Fe II lines in 3D with excitation potential 2 eV at [Fe/H]=-3.0. The corrections are less severe for low-excitation lines, longer wavelengths, and higher metallicity. Velocity fields increase the effects of scattering by separating emission from granules and intergranular lanes in wavelength. 1D calculations exhibit similar scattering abundance corrections for weak lines, but those for strong lines are generally smaller compared to 3D models and depend on the choice of microturbulence.