| |
 |
| Paper: |
Steps toward Absolute Accuracy in Radial Velocities |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
58 |
| Authors: |
Gullberg, D. |
| Abstract: |
We are developing methods to reach high absolute accuracy in spectroscopic radial velocities for stars of different spectral types. The echelle spectrograph Elodie (R=40,000) at the Observatoire de Haute-Provence has been used to observe, in particular, Hyades stars and stars in the open cluster of Ursa Major. Several radial-velocity standard stars and others were also observed as was the Moon, for calibration purposes. To obtain low-noise spectra without contamination, long integration times with ThAr calibrations in between object exposures were used, as opposed to the standard mode of operation which is to do short exposures with the ThAr superimposed in the interorder space, giving simultaneous calibration. We get a precision of 15 ms-1 or better for our Moon observations. To compute the spectroscopic radial velocity of an observed star, its spectrum is cross-correlated with an FTS solar spectrum of very high SNR and resolution (Kurucz et.al 1984). Both datasets are normalized, flipped to yield an emission-like spectrum, the average subtracted, and resampled to give equidistant samples with the same sampling frequency. The CCF maximum is determined by assuming that the three topmost points outline a parabolic function, defining the radial velocity at its maximum. The uncertainty is computed from the curvature of the parabolic function, the local SNR for the spectrum and the local slope of the template. The uncertainty of the CCF maximum depends also on template mismatch, which may cause systematic effects as well. Elodie software was used to compute radial velocities from the two standard templates available, F0 and K0 masks. Together with our own calculations we have data for F0, G2, and K0 masks and this is used to deduce some properties of the template mismatch. The Moon observations are used to bring the measured velocities to an absolute scale by comparing with the JPL ephemeris. |
|
|
|
|
|
