Five New Multicomponent Planetary Systems

Abstract

We report Dopplermeasurements for six nearby G- andK-type main-sequence stars that show multiple low-mass companions, at least one of which has planetarymass.One system has three planets, the fourth triple-planet system known around a normal star, and another has an extremely low minimum mass of 18M?. HD 128311 (K0 V) has two planets (one previously known) with minimum masses (M sin i)of2.18MJ and 3.21MJ and orbital periods of 1.26 and 2.54 yr, suggesting a possible 2:1 resonance. For HD 108874 (G5 V), the velocities reveal two planets (one previously known) having minimummasses and periods of (M sin ib ¼ 1:36MJ, Pb ¼ 1:08 yr) and (M sin ic ¼ 1:02MJ, Pc ¼ 4:4 yr). HD 50499 (G1 V) has a planet with P ¼ 6:8yr and M sin i ¼ 1:7MJ, and the velocity residuals exhibit a trend of ?4.8 m s?1 yr?1, indicating a more distant companion with P >10 yr and minimum mass of 2MJ. HD 37124 (G4 IV–V) has three planets, one having M sin i ¼ 0:61MJ and P ¼ 154:5 days, as previously known. We find two plausible triple-planet models that fit the data, both having a second planet near P ¼ 840 days, with the more likely model having its third planet in a 6 yr orbit and the other one in a 29 day orbit. For HD 190360, we confirm the planet having P ¼ 7:9yr and M sin i ¼ 1:5MJ as found by the Geneva team, but we find a distinctly noncircular orbit with e ¼ 0:36 ? 0:03, rendering this not an analog of Jupiter as had been reported. Our velocities also reveal a second planet with P ¼ 17:1 days andM sin i ¼ 18:1M?.HD217107 (G8 IV) has a previously known ‘‘hot Jupiter’’ with M sin i ¼ 1:4MJ and P ¼ 7:13 days, and we confirm its high eccen- tricity, e ¼ 0:13. The velocity residuals reveal an outer companion in an eccentric orbit, having minimum mass of M sin i >2MJ, eccentricity e ? 0:5, and a period P >8 yr, implying a semimajor axis a > 4 AU and providing an opportunity for direct detection.We have obtained high-precision photometry of five of the six planetary host stars with three of the automated telescopes at Fairborn Observatory.We can rule out significant brightness variations in phase with the radial velocities in most cases, thus supporting planetary reflex motion as the cause of the velocity variations. Transits are ruled out to very shallow limits for HD 217107 and are also shown to be unlikely for the prospective inner planets of theHD37124 andHD108874 systems.HD128311 is photometrically variablewith an amplitude of 0.03 mag and a period of 11.53 days,which ismuch shorter than the orbital periods of its two planetary companions. This rotation period explains the origin of periodic velocity residuals to the two-planet model of this star. All of the planetary systems here would be further constrained with astrometry by the Space Interferometry Mission.