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| Paper: |
Radial-Velocity Variations of the Rapidly Oscillating Ap Star 33 Lib |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
183 |
| Authors: |
Hatzes, A. P.; Kanaan, A.; Mkrtichian, D. |
| Abstract: |
We present a time series of precise stellar radial velocity measurements of the rapidly oscillating Ap (roAp) star 33 Lib. High speed observations were made approximately every minute continuously for a total of 3 hours using the 2-D coude spectrograph of the 2.7-m telescope at McDonald Observatory. The setup of this prism cross-dispersed echelle spectrograph provided a wavelength coverage of 4800-6900 Å at a resolving power of 60,000. A molecular gas iodine absorption cell placed in the light path just before the spectrograph slit provided a stable wavelength reference for measuring radial velocities to a precision of about 20 m/s per observation. A Fourier analysis of the data clearly shows the 8.2 min pulsation period found previously by photometric investigations and gives a peak-to-peak (2K) amplitude of about 80 m/s. This corresponds to a RV-to-photometric amplitude of 27 km/s/mag in B (based on &Delta B = 3.0 mmag) which is comparable to the value for classical Cepheids. However, we find, like in other roAp stars we have studied, that the RV amplitude depends on the spectral region used for measuring the RV amplitude and is as high as 57 plus or minus 4.7 m/s in the region 5411-5500 Å and as low as 7 plus or minus 3 m/s in the region 5877-5976 A. The amplitude versus wavelength (spectral order) is complicated and is not strictly a decreasing function of increasing wavelength. A detailed line-by-line analysis shows considerable scatter in the RV amplitude derived from individual lines and can be as high as 320 m/s and as low as 7 m/s. There is an overall trend of increasing RV amplitude with decreasing line strength, although there is considerable scatter about the mean curve. This result is similar to what we have found for other roAp stars. We also found that spectral lines due to iron lines have a slightly higher mean RV semi-amplitude (128 plus or minus 44 m/s) than the amplitude for chromium (61 plus or minus 44 m/s) and titanium (98 plus or minus 53 m/s), although this result is somewhat uncertain due to the fewer chromium and titanium lines used for computing the mean RV amplitude. If this result is true, then it is counter to what we have found in the roAp star &gamma Equ. Spectral lines from nickel exhibited the highest RV semi-amplitude (138 plus or minus 68 m/s). We believe that the line strength variations are an indication of vertical atmospheric structure to the pulsations and that the elemental differences are related to the inhomogeneous distribution of elements known to occur on Ap stars. Precise stellar radial velocity studies of roAp stars may be a powerful tool for studying both the spatial (surface) and vertical structure to the pulsational velocity field. |
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