Granulation signatures in the spectrum of the very metal-poor red giant HD122563

TL;DR

This study analyzes the spectral line asymmetries and shifts in a very metal-poor red giant star, HD122563, using high-resolution observations and 3D hydrodynamic models, revealing surface convection signatures and validating model predictions.

Contribution

It provides the first detailed comparison of observed line asymmetries and shifts with 3D hydrodynamic simulations for a metal-poor red giant, supporting large 3D-LTE abundance corrections.

Findings

Observed line bisectors show characteristic granulation signatures.

Good agreement between observed and simulated line shifts supports model realism.

Results validate the use of 3D hydrodynamic models for abundance analysis.

Abstract

A very high resolution (R=200,000), high signal-to-noise ratio (S/N=340) blue-green spectrum of the very metal-poor ([Fe/H]=-2.6) red giant star HD122563 has been obtained by us at McDonald Observatory. We measure the asymmetries and core wavelengths of a set of unblended FeI lines covering a wide range of line strength. Line bisectors exhibit the characteristic C-shape signature of surface convection (granulation) and they span from about 100 m/s in the strongest FeI features to 800 m/s in the weakest ones. Core wavelength shifts range from about -100 to -900 m/s, depending on line strength. In general, larger blueshifts are observed in weaker lines, but there is increasing scatter with increasing residual flux. Assuming local thermodynamic equilibrium (LTE), we synthesize the same set of spectral lines using a state-of-the-art three-dimensional hydrodynamic simulation for a stellar atmosphere of fundamental parameters similar to those of HD122563. We find good agreement between model predictions and observations. This allows us to infer an absolute zero-point for the line shifts and radial velocity. Moreover, it indicates that the structure and dynamics of the simulation are realistic, thus providing support to previous claims of large 3D-LTE corrections, based on the hydrodynamic model used here, to elemental abundances and fundamental parameters of very metal-poor red giant stars obtained with standard 1D-LTE spectroscopic analyses.