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| Paper: |
Non-turbulent Pulsation: Metallicism in Pulsating Stars |
| Volume: |
135, A Half Century of Stellar Pulsation Interpretations: a Tribute to Arthur N. Cox |
| Page: |
420 |
| Authors: |
Kurtz, D. |
| Abstract: |
In the late 1970s after I showed that evolved Am stars can also pulsate near the red edge of the instability strip, Art Cox, King and Hodson calculated that the shift in mass fraction of the He II ionisation zone should be enough to start pulsation in Am star which has already undergone diffusion. No one has had anything to say theoretically about this problem since then, although I have continued to find more extreme objects which both pulsate and have strong abundance anomalies. The latest is HD 40765 which shows that pulsation with surface radial velocity amplitudes of 10 km/s can coexist with evolved-Am (rho Pup) abundance anomalies. If diffusion is the correct model for the abundance anomalies in these stars, then their pulsation is extremely laminar with 10 km/s pulsation velocities generating no turbulent velocites great than a fraction of a cm/s. Otherwise, diffusion is an incorrect, or incomplete theory for the peculiarities in A-star atmopheres. Mode selection and the driving mechanism are also important, since the high-overtone roAp stars show both pulsation and the most extreme abundance anomalies known. It looks in their case like H-driving near the surface may be excitation mechanism, and may also be the mode selection mechanism. Why similar H driving does not excite high overtones in non- peculiar delta Scuti stars is an interesting question. Li & Stix suggest that H- -driving does matter in delta Scuti stars, and may be able to fix the blue edge problem. |
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