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Since 1999, we have been conducting a radial velocity survey of 179 K giantsusing the CAT at UCO/Lick observatory. At present ~20-100 measurements havebeen collected per star with a precision of 5 to 8 m/s. Of the stars monitored,145 (80%) show radial velocity (RV) variations at a level >20 m/s, of which 43exhibit significant periodicities. Our aim is to investigate possiblemechanism(s) that cause these observed RV variations. We intend to test whetherthese variations are intrinsic in nature, or possibly induced by companions, orboth. In addition, we aim to characterise the parameters of these companions. Arelation between log g and the amplitude of the RV variations is investigatedfor all stars in the sample. Furthermore, the hypothesis that all periodic RVvariations are caused by companions is investigated by comparing their inferredorbital statistics with the statistics of companions around main sequencestars. A strong relation is found between the amplitude of the RV variationsand log g in K giant stars, as suggested earlier by Hatzes & Cochran (1998).However, most of the stars exhibiting periodic variations are located abovethis relation. These RV variations can be split in a periodic component whichis not correlated with log g and a random residual part which does correlatewith log g. Compared to main-sequence stars, K giants frequently exhibitperiodic RV variations. Interpreting these RV variations as being caused bycompanions, the orbital param eters are different from the companions orbitingdwarfs. Intrinsic mechanisms play an important role in producing RV variationsin K giants stars, as suggested by their dependence on log g. However, itappears that periodic RV variations are additional to these intrinsicvariations, consistent with them being caused by companions.

astronomy and astrophysics

Precise radial velocities of giant stars. IV. A correlation between  surface gravity and radial velocity variation and a statistical investigation  of companion properties