  Spectroscopic radial velocities for several nearby open clusters suggest that
spectra of (especially earlier-type) rapidly rotating stars are systematically
blueshifted by 3 km/s or more, relative to the spectra of slowly rotating ones.
Comparisons with astrometrically determined radial motions in the Hyades
suggests this to be an absolute blueshift, relative to wavelengths naively
expected from stellar radial motion and gravitational redshift. Analogous
trends are seen also in most other clusters studied (Pleiades, Coma Berenices,
Praesepe, Alpha Persei, IC 2391, NGC 6475, IC 4665, NGC 1976 and NGC 2516).
Possible mechanisms are discussed, including photospheric convection, stellar
pulsation, meridional circulation, and shock-wave propagation, as well as
effects caused by template mismatch in determining wavelength displacements.
For early-type stars, a plausible mechanism is shock-wave propagation upward
through the photospheric line-forming regions. Such wavelength shifts thus
permit studies of certain types of stellar atmospheric dynamics and -
irrespective of their cause - may influence deduced open-cluster membership
(when selected from common velocity) and deduced cluster dynamics (some types
of stars might show fortuitous velocity patterns).
