'Ultimate' Information Content in Solar and Stellar Spectra: Photospheric line asymmetries and wavelength shifts

TL;DR

This paper investigates the ultimate accuracy limits in measuring spectral line asymmetries and shifts in solar and stellar spectra, revealing insights into stellar atmospheres and the constraints of current observational techniques.

Contribution

It introduces a method to extract bisectors and shifts from high-quality spectra using accurate laboratory wavelengths, advancing the understanding of stellar atmospheric dynamics.

Findings

Bisectors and shifts for new species like Fe II, Ti I, Ti II, Cr II, Ca I, C I were measured.

Line saturation and damping wings in F-type stars were observed near the spectral continuum.

Absolute lineshift studies are limited to 50-100 m/s by various noise sources.

Abstract

CONTEXT: Spectral-line asymmetries and wavelength shifts are signatures of hydrodynamics in solar and stellar atmospheres. Theory may precisely predict idealized lines, but observed spectra are limited by blends, too few suitable lines, imprecise laboratory wavelengths, and by instrumental imperfections. AIMS: Bisectors and shifts are extracted until the 'ultimate' accuracy limits in highest-quality solar and stellar spectra, to understand limits set by stellar physics, observational techniques, and limitations in laboratory data. METHODS: Spectral atlases of the Sun and bright solar-type stars were examined for thousands of 'unblended' lines with the most accurate laboratory wavelengths, yielding bisectors and shifts as averages over groups of similar lines, thus minimizing effects of photometric noise and of random blends. RESULTS: For solar spectra, bisector shapes and shifts were extracted for previously little-studied species (Fe II, Ti I, Ti II, Cr II, Ca I, C I), using recently determined very accurate laboratory wavelengths. In Procyon and other F-type stars, a blueward bend in the bisector near the spectral continuum reveals line saturation and damping wings in upward-moving photospheric granules. Accuracy limits set by 'astrophysical' noise, finite instrumental resolution, superposed telluric absorption, and inaccurate wavelengths, together limit absolute lineshift studies to approximately 50-100 m/s. CONCLUSIONS: Spectroscopy with resolutions R = 300,000 will enable bisector studies for many stars. Circumventing remaining limits of astrophysical noise in line-blends and rotationally smeared profiles may ultimately require spectroscopy across spatially resolved stellar disks.