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| Paper: |
Radial-Velocity Observations of Non-Radially Pulsating Stars |
| Volume: |
185, Precise Stellar Radial Velocities, IAU Colloquium 170 |
| Page: |
166 |
| Authors: |
Hatzes, A. P.; Kanaan, A.; Mkrtichian, D. |
| Abstract: |
Precise radial-velocity (PRV) measurements have recently received considerable publicity over their success at finding the first extra-solar planets. These measurements, however, can also be used for discovering new classes of low-amplitude variable stars as well as shed new light on known pulsating objects. Here we present RV results from two classes of rather different pulsating stars: cool, evolved K giant stars and hot, rapidly oscillating Ap stars. PRV measurements were the first to establish that K giants are a new class of pulsating stars. The typical RV behavior of a K giant star shows periods on two timescales: several days and hundreds of days. Although the nature of the long period variations is still unknown (surface features, pulsations, or planetary companions), the short period variability is almost certainly due to nonradial pulsations. One K giant, Arcturus, has shown at least 10 modes with periods in the range 2 to 10 days. The frequencies of these modes are approximately evenly spaced indicating that these are p-mode oscillations. Multiple short-period variations have also been seen in γ Dra and β Oph. Although we cannot exclude either planetary companions or surface features as the cause of the long-period variations, the front-running hypothesis seems to be some form of nonradial pulsations as well. The rapidly oscillating Ap stars (roAp) are a class of nonradial pulsating stars oscillating in low-degree modes with periods of 6-15 minutes (high radial order). These stars also possess a large dipole field whose axis is inclined with respect to the rotation axis. The pulsations are aligned with the magnetic rather than the rotational axis (oblique pulsator). We have recently started a program of PRV measurements of several bright roAp stars that are accessible from the northern hemisphere. The typical mean RV amplitude for an roAp star is 50-400 m/s, but this amplitude depends on the spectral region used for the measurement of the RV amplitude. A detailed line-by-line analysis reveals that the pulsational amplitude depends not only on atomic species, but on the line strength as well. In the case of γ Equ the mean pulsational amplitude for chromium lines is higher than for iron features. For a given atomic species weak spectral lines exhibit a pulsational amplitude 10-100 times higher than for strong lines. The elemental effect can be understood in the context of the inhomogeneous distribution of elements known to occur on these stars. For instance, if chromium is concentrated near the magnetic pole then it may have a higher RV amplitude than iron which may be distributed about the magnetic equator. The line strength effect is interpreted as arising from vertical structure to the pulsations since weaker lines are formed on average, deeper in the atmosphere than stronger lines. These types of measurements may prove to be a powerful tool for probing both the vertical and horizontal structure of the pulsations in roAp stars. |
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