‘Ultimate’ information content in solar and stellar spectra: Photospheric line asymmetries and wavelength shifts

Context. Spectral-line asymmetries (displayed as bisectors) and wavelength shifts are signatures of the hydrodynamics in solar and stellar atmospheres. Theory may precisely predict idealized lines, but accuracies in real observed spectra are limited by blends, few suitable lines, imprecise laboratory wavelengths, and instrumental imperfections. Aims. We extract bisectors and shifts until the ‘ultimate’ accuracy limits in highest-quality solar and stellar spectra, so as to understand the various limits set by (i) stellar physics (number of relevant spectral lines, effects of blends, rotational line broadening), by (ii) observational techniques (spectral resolution, photometric noise), and by (iii) limitations in laboratory data. Methods. Several spectral atlases of the Sun and bright solar-type stars were examined for those thousands of ‘unblended’ lines with the most accurate laboratory wavelengths, yielding bisectors and shifts as averages over groups of similar lines. Representative data were obtained as averages over groups of similar lines, thus minimizing the effects of photometric noise and of random blends. Results. For the solar-disk center and integrated sunlight, the 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 and very accurate laboratory wavelengths. In Procyon and other Ftype stars, a sharp blueward bend in the bisector near the spectral continuum is confirmed, revealing line saturation and damping wings in upward-moving photospheric granules. Accuracy limits are discussed: ‘astrophysical’ noise due to few measurable lines, finite instrumental resolution, superposed telluric absorption, inaccurate laboratory wavelengths, and calibration noise in spectrometers, together limiting absolute lineshift studies to ≈ 50-100 m s−1. Conclusions. Spectroscopy with resolutions λ/∆λ ≈ 300,000 and accurate wavelength calibration 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, utilizing optical interferometers and extremely large telescopes of the