[Previous] | [Session 64] | [Next]
A.P. Hatzes (McDonald Observatory), D.E. Mkrtichian (Odessa State University), A. Kanaan (Instituto de Fisica UFRGS)
The rapidly oscillating Ap stars (roAp) are a class of nonradial pulsating stars oscillating in low-degree modes (\ell = 1--3) with periods of 4-15 minutes. We present precise radial velocity (RV) measurements for 5 roAp stars (33 Lib, \gamma Equ, HR 1217, HD 134214, and HD 122970) taken with an iodine gas absorption cell at McDonald Observatory's 2.7-m telescope. Pulsational RV variations are detected in all five stars with amplitudes ranging from 50-400 m\thinspace s-1. For the roAp star HR 1217 we were able to detect 5 of the 6 known pulsation modes present in the star.
All the roAp stars we have studied so far exhibit a pulsational RV amplitude that depends on the spectral region that is examined. This RV amplitude as measured from different spectral regions can differ by a factor of up to 100 for the same star. A detailed line-by-line analysis of the radial velocities reveals that the pulsational amplitude depends not only on atomic species, but on the line strength as well. For a given atomic species the RV amplitude increases with decreasing line strength. This 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. The elemental effect can be understood in the context of the inhomogeneous distribution of elements known to occur on these stars and that is believed to result from the global dipole magnetic fields that are present. For instance, if an element is concentrated near the magnetic pole then it may have a higher RV amplitude than one that is distributed about the magnetic equator. This surface distribution of elements can act as a spatial filter allowing us to detect high degree modes which is not possible on stars with a more uniform distribution of elements due to cancellation effects. Precise RV measurements may prove to be a powerful tool for probing both the vertical and horizontal structure of the pulsations in roAp stars.