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| Paper: |
Applications of Precision Doppler Velocity Measurements in Variable Star Studies |
| Volume: |
135, A Half Century of Stellar Pulsation Interpretations: a Tribute to Arthur N. Cox |
| Page: |
201 |
| Authors: |
Butler, R. P. |
| Abstract: |
Techniques developed over the last 10 years have improved the precision of Doppler velocity measurements by more than two orders of magnitude. While most of this work has centered on the discovery of extrasolar planets, a number of applications have been developed for the study of variable stars. Work carried out with the Lick Observatory Iodine absorption cell has resulted in the direct detection of velocity gradients in Cepheid photospheres, the first precision comparison of Doppler velocities in the visible and near IR, and a 6 year survey of photometrically non-variable supergiants in and about the Cepheid instability strip. Most of the photometric non-variables show Doppler velocity variations of 50 to 1,000 m/s and periods of 40 to 250 days. In addition, the Lick Planet Survey has shown that slowly rotating solar-type stars are intrinsically stable to at least 3 m/s. The Keck Iodine absorption cell has been used for search for solar-type oscillations in the bright K1 dwarf star 107 Psc. A precision of 2 m/s was obtained on a six hour data string of 200 observations. The detection threshold was 55 cm/s over the expected oscillation frequency window. We did not detect any oscillations at this level. In comparison, the solar 5 minute oscillation has an amplitude of 0.23 m/s, thus this technique is already within a factor of two of achieving the necessary detection threshold. The Lick Observatory precision Doppler technique makes use of a fast echelle spectrograph at resolution of R=62,000 and a large format CCD which acquires the entire visible and near IR spectrum in each exposure. Starlight is sent through an iodine absorption cell placed at the spectrometer entrance slit. The resulting superimposed iodine lines provide a fiducial wavelength scale against which to measure radial velocity shifts. The shapes of iodine lines convey the PSF of the spectrometer to account for changes in spectrometer optics and illumination on all time scales. |
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