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| Paper: |
Importance of Precise Radial Velocities for Cepheid Binaries |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
211 |
| Authors: |
Szabados, L. |
| Abstract: |
Classical Cepheids are fundamental objects in determining the cosmic distance scale via the well known (but not precisely enough) period-luminosity-color relation. Cepheids belonging to binary (and multiple) systems are among the best calibrators of the zero-point, provided a wealth of spectroscopic (including radial velocity) information is available. The recently established frequency of occurrence of binaries among Cepheids is above 50 per cent (Szabados 1995). Although there is a strong selection effect, 30 new SB-systems were discovered among the Cepheids of mean V-brightness 7.2-12.4 mag. (Szabados 1996, Pont and Szabados 1998) based on the radial velocity data whose first epoch dataset was the classical but by no means precise enough series obtained by Joy (1937). This drawback set an uncomfortable lower limit of about 4 km/s for the gamma-velocity change below which no orbital motion could be established. When summarizing the state of the art of the study of binary Cepheids, this contribution calls for precise radial velocity observations. The promising outcome of such studies are: - Determination of exact orbital elements and the distance of such binaries; - Revealing many more Cepheids belonging to binaries, unrecognized so far because the amplitude of the orbital effect is lower than the limit of detection constrained by classical equipment; - Combination of radial velocity data with astrometric measurements (Hipparcos and the next generation GAIA and DIVA microarcsec data, plus ground-based interferometric observations) for derivation of the absolute orbit. The strange behavior of Hipparcos-parallaxes of binary Cepheids and its implication is also mentioned. |
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