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| Paper: |
Stellar Oscillations in K Giant Stars |
| Volume: |
154, Cool Stars, Stellar Systems and the Sun: Tenth Cambridge Workshop |
| Page: |
311 |
| Authors: |
Hatzes, Artie P.; Cochran, William D. |
| Abstract: |
The advent of precise stellar radial velocity (RV) measurements has shown K giants to be a new class of pulsating stars. The RV behavior of some of these stars can be quite complex, showing several periods ranging from a few days to several hundreds of days. Although the nature of the long-period variability is still unknown, the short-period variability certainly arises from nonradial pulsations. So far, short period RV variability has been detected in at least five K giant stars. The best studied of these, Arcturus, has shown, at one time or another, at least 10 short-term periods ranging in values from 2 to 8 days. These periods are approximately equally-spaced in frequency indicative of p-mode oscillations. The star gamma Dra is another K giant showing multimode variability. Over 8 consecutive nights the radial velocity for this stars showed the presence of two periods of 3.5 and 5.7 days. The fact that these stars show multi-mode oscillations means they are prime targets for asteroseismic techniques. These may enable us, for the first time, to probe the internal structure of an evolved star. The nature of the long-period variations is still unclear with possible mechanisms including planetary companions, rotational modulation, and nonradial oscillations. It is argued that rotational modulation is most likely not the cause of this RV variability. If the long period RV variability is due to pulsations then they must be g-modes since the periods are over two orders of magnitude longer than that of the fundamental radial mode. K giants may thus be particularly interesting in that they show pulsation modes spanning more than two orders of magnitude in the period. The K giant star beta Gem shows a long period of 558 days, but no evidence of short-term variability. Interestingly, this star is to the left of the coronal dividing line (CDL). Due to the small number of K giant stars that have been studied, it is not known if the CDL also represents a boundary for the onset of $p$-mode oscillations. |
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