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D. Dravins, L. Lindegren, H.-G. Ludwig, S. Madsen (Lund Observatory, Sweden)
Wavelengths of stellar spectral lines do not have the precise values ‘naively’ expected from laboratory wavelengths merely Doppler-shifted by stellar radial motion. Slight displacements may originate as convective shifts (correlated velocity and brightness patterns in the photosphere), as gravitational redshifts, or perhaps be induced by wave motions. Intrinsic lineshifts thus reveal stellar surface structure, while possible periodic changes (during a stellar activity cycle, say) need to be segregated from variability induced by orbiting exoplanets.
Absolute lineshifts can now be studied also in some stars other than the Sun, thanks to astrometric determinations of stellar radial motion. Comparisons between spectroscopic apparent radial velocities and astrometrically determined radial motions reveal greater spectral blueshifts in F-type stars than in the Sun (as theoretically expected from their more vigorous convection), further increasing in A-type stars (possibly due to atmospheric shockwaves).
Solar spectral atlases, and high-resolution spectra (from UVES on ESO VLT) of a dozen solar-type stars are being surveyed for ‘unblended’ photospheric lines of most atomic species with accurate laboratory wavelengths available. One aim is to understand the ultimate information content of stellar spectra, and in what detail it will be feasible to verify models of stellar atmospheric hydrodynamics. These may predict line asymmetries (bisectors) and shifts for widely different classes of lines, but there will not result any comparison with observations if such lines do not exist in real spectra.
An expected near-future development in stellar physics is spatially resolved spectroscopy across stellar disks, enabled by optical interferometry and adaptive optics on very large telescopes. Stellar surface structure manifests itself in the center-to-limb wavelength changes along a stellar diameter, and their spatially resolved time variability, diagnostics which already now can be theoretically modeled.
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Bulletin of the American Astronomical Society, 36 5
© 2004. The American Astronomical Society.