Spectral line asymmetries in the metal-poor red giant HD 122563: CO5BOLD predictions versus observations

TL;DR

This study compares 3D hydrodynamical model predictions with high-quality observations of Fe I spectral lines in the metal-poor red giant HD 122563, assessing the models' accuracy in reproducing convection-induced line asymmetries and shifts.

Contribution

It provides a detailed comparison between CO5BOLD 3D model predictions and observed spectral line asymmetries and shifts in a metal-poor red giant, highlighting areas of agreement and discrepancy.

Findings

Good agreement in Doppler shifts of Fe I line cores

Theoretical bisectors match observed ones reasonably well

Discrepancies in weak and strong line predictions

Abstract

We study the influence of convection on the asymmetries and Doppler shifts of Fe I spectral lines in the metal-poor red giant HD 122563. To this end, we compute theoretical Fe I line shifts and line bisectors using 3D hydrodynamical model atmosphere of HD 122563 calculated with the CO5BOLD code. We then make a detailed comparison of the theoretical line shifts and bisectors with those derived from the high quality HARPS spectrum of HD 122563 taken from the ESO Science Archive Facility (R = 115 000, average signal-to-noise ratio, S/N = 310). In general, we find a good agreement between the theoretically predicted and observed Doppler shifts of Fe I line cores, with somewhat larger discrepancies seen in the case of weaker (equivalent width W < 5 pm) and stronger lines (W > 11 pm). Both observed and theoretical coreshifts cover a range between 0 and -1 km/s, with increasingly stronger blueshifts for weaker lines and slight hints of a coreshift dependence on wavelength. Theoretical bisectors reproduce the observed ones reasonably well too, however, theoretical bisectors of the weak red (lambda > 600 nm) Fe I lines have blueshifts that are by up to 200 m/s larger than observed. The obtained results therefore suggest that the current CO5BOLD models are capable of reproducing the large-scale velocity fields in the atmosphere of HD 122563 suficiently well. Nevertheless, further efforts are needed in order to understand the physical reasons behind the discrepancies in theoretical predictions and observed properties of the weakest and strongest Fe I lines.