OKAYAMA PLANET SEARCH PROGRAM

Abstract

We have carried out a precise Doppler survey of G-type giants aiming to unveil the properties of planetary systems in intermediate-mass stars (<TEX>$1.5-5M_{\bigodot}$</TEX>). G-type giants are promising targets for Doppler planet searches around massive stars, because they are slow-rotators and have many sharp absorption lines in their spectra and their surface activities are relatively low in contrast to their younger counterparts on the main-sequence (B-A stars). We are now monitoring radial velocities of about 300 late G-type (including early K-type) giants using HIgh Dispersion Echelle Spectrograph (HIDES) at Okayama Astrophysical Observatory. We have achieved a Doppler precision of about 6-7 m/s over a time span of 3 years using an iodine absorption cell. We found that most of the targets have radial velocity scatters of <TEX>${\sigma}{\~} 10-20 m\;s^{-1}$</TEX> over 1-3 years, with the most stable reaching levels of 6-8 m <TEX>$s^{-1}$</TEX>. Up to now, we have succeeded in discovering the first extrasolar planet around a G-type giant star HD 104985, and also found several candidates showing significant radial velocity variations, suggesting the existence of stellar and substellar companions. Observations have continued to establish their variability.