Free access article

A&A 411, 581-586 (2003)
DOI: 10.1051/0004-6361:20031396

Intrinsic spectral blueshifts in rapidly rotating stars?

Søren Madsen, Dainis Dravins, Hans-Günter Ludwig and Lennart Lindegren

Lund Observatory, Box 43, 22100 Lund, Sweden
    e-mail: soren,dainis,hgl,lennart@astro.lu.se
(Received 23 June 2003 / Accepted 29 August 2003)

Abstract
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 ^-1 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, 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).

Key words: methods: data analysis -- techniques: astrometry -- spectroscopy -- radial velocities -- stars: rotation -- open clusters and associations: general

Offprint request: Søren Madsen, soren@astro.lu.se

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