Radial-Velocity Variability of G-Type Giants: First Three Years of the Okayama Planet Search Program

Abstract

Abstract We report on the radial-velocity variability of G-type giants based on the results of precise Doppler measurements of 57 stars that were observed for the first 3 years of the Okayama Planet Search Program. This program aims to search for planets around intermediate-mass stars in their evolved stages as late-G giants. We found that most of the targets with $B-V<1.0$ have radial-velocity scatters of $\sigma \sim 10 \;\mathrm{m} \,\mathrm{s}^{-1}$, with the most stable reaching levels of $6 \hbox{--} 8 \,\mathrm{m} \,\mathrm{s}^{-1}$, while those with $B-V>1.0$ typically show $\sigma \sim 20 \,\mathrm{m} \,\mathrm{s}^{-1}$. In total, about 60% of the targets are stable in radial velocity to a level of $\sigma < 15 \,\mathrm{m} \,\mathrm{s}^{-1}$ over 1–3 years, and about 90% have $\sigma < 30 \,\mathrm{m} \,\mathrm{s}^{-1}$. We found that 4 stars (HD 30557, HD 34559, HD 68077, and HD 85444) show significant long-term radial-velocity trends, suggesting that they are probably orbited by unseen stellar or substellar companions. Three stars (HD 79181, HD 104985, and HD 141680) show periodic radial-velocity variations. HD 104985 is the first planet-harboring star discovered from our survey. The properties of the variations in these 3 stars have been, and will be, extensively discussed in separate papers. Relatively large, but not apparently periodic, radial-velocity variations of $\sigma \gt 30 \,\mathrm{m} \,\mathrm{s}^{-1} \sigma > 30 \,\mathrm{m} \,\mathrm{s}^{-1}$ are found in 4 stars (HD 41597, HD 134190, HD 161178, and HD 176598). For most of the stars without showing any significant periodicity, we can exclude companions with $K\gt50 \,\mathrm{m} \,\mathrm{s}^{-1} K>50 \,\mathrm{m} \,\mathrm{s}^{-1}$, or $m_2 \sin i \gt 1.8 \,M_\mathrm{J}(a/\mathrm{AU})^{1/2}(M_{*}/{{{M}_{\odot}}})^{1/2} m_2 \sin i > 1.8 \,M_\mathrm{J}(a/\mathrm{AU})^{1/2}(M_{*}/{{{M}_{\odot}}})^{1/2}$ for orbital radii $a \lesssim 1 \hbox{--} 2 \,\mathrm{AU}$.