Planetary Companions to HD 12661, HD 92788, and HD 38529 and Variations in Keplerian Residuals of Extrasolar Planets

Abstract

Precision Doppler observations at the Lick and Keck observatories have revealed Keplerian velocity variations in the stars HD 12661, HD 92788, and HD 38529. HD 12661 (G6 V) has an orbital period of 252.7 ± 2.7 days, velocity semiamplitude K = 88.4 ± 2.0 m s-1, and orbital eccentricity e = 0.23 ± 0.024. Adopting a stellar mass of 1.07 M☉, we infer a companion mass of M sin i = 2.79 MJ and a semimajor axis of a = 0.79 AU. HD 92788 (G5 V) has an orbital period of 326.7 ± 3.2 days, velocity semiamplitude K = 99.9 ± 2.4, and orbital eccentricity e = 0.30 ± 0.06. The adopted stellar mass of 1.06 M☉ yields a companion mass of M sin i = 3.34 MJ and a semimajor axis of a = 0.95 AU. HD 38529 (G4 IV) has an orbital period of 14.3 ± 0.8 days, velocity semiamplitude K = 53.8 ± 2.0 m s-1, and eccentricity e = 0.27 ± 0.03. The stellar mass of 1.4 M☉ sets M sin i = 0.77 MJ, with a semimajor axis of a = 0.13 AU for this companion. In addition to the 14.3 day periodicity, the velocity residuals for HD 38529 show curvature over the three years of observations. Based on a measurement of Ca II H and K emission, all three stars are chromospherically inactive. Based on both spectral synthesis modeling and narrowband photometry, HD 12661, HD 92788, and HD 38529 all appear to be metal-rich stars, reinforcing the correlation of high metallicity in the host stars of gas giant extrasolar planets. We examine the velocity residuals to the Keplerian fits for a subsample of 12 planet-bearing stars that have been observed longer than two years at the Lick Observatory. Five of the 12 (Ups Andromedae, τ Boo, 55 Cnc, HD 217107, and HD 38529) exhibit coherent variations in the residual velocities that are consistent with additional companions. Except for Upsilon Andromedae, the source of the velocity variation remains speculative pending completion of one full orbit. GJ 876 exhibits residual velocities with high rms scatter (24 m s-1), lacking identifiable coherence. The residual velocities for six of the 12 stars (51 Peg, 70 Vir, 16 Cyg B, ρ CrB, 47 UMa, and HD 195019) exhibit rms velocity scatter of ~7 m s-1, consistent with errors. The residual velocity trends suggest that known planet-bearing stars appear to harbor a distant (>3 AU) detectable companion more often than other stars in our planet survey.