The wings of Ca II H and K as photospheric diagnostics and the reliability of one-dimensional photosphere modeling

TL;DR

This study evaluates the effectiveness of using the wings of Ca II H and K lines as diagnostics for photospheric temperature stratification in the Sun and cool stars, highlighting their reliability and limitations in one-dimensional modeling.

Contribution

It calibrates the use of Ca II H and K wings in one-dimensional spectral interpretation using multi-dimensional simulations, assessing accuracy and identifying sources of error.

Findings

Spatially-averaged models match simulations well except in deep layers.

Large thermal inhomogeneities cause significant errors in temperature estimates.

One-dimensional fits are satisfactory for small-scale structures despite some inaccuracies.

Abstract

The extended wings of the Ca II H and K lines provide excellent diagnostics of the temperature stratification of the photosphere of the Sun and of other cool stars, thanks to their LTE opacities and source functions and their large span in formation height. The aim of this study is to calibrate the usage of the H and K wings in one-dimensional interpretation of spatially averaged spectra and in deriving per-pixel stratifications from resolved spectra. I use multi-dimensional simulations of solar convection to synthesize the H and K wings, derive one-dimensional models from these wings as if they were observed, and compare the resulting models to the actual simulation input. I find that spatially-averaged models constructed from the synthesized wings generally match the simulation averages well, except for the deepest layers of the photosphere where large thermal inhomogeneities and Planck-function nonlinearity gives large errors. The larger the inhomogeneity, the larger the latter. The presence of strong network fields increases such inhomogeneity. For quiet photospheric conditions the temperature excesses reach about 200 K. One-dimensional stratification fits of discrete structures such as granulation and small-scale magnetic concentrations give satisfactory results with errors that are primarily due to steep temperature gradients and abrupt changes of temperature with depth. I conclude that stratification modeling using the H and K wings is a useful technique for the interpretation of solar high-resolution observations.