Two New Long-Period Giant Planets from the McDonald Observatory Planet Search and Two Stars with Long-Period Radial Velocity Signals Related to Stellar Activity Cycles

TL;DR

This study reports the discovery of two long-period Jupiter-like planets around stars HD 95872 and psi1 Draconis B, using over 15 years of radial velocity data, and discusses stellar activity effects that can mimic planetary signals.

Contribution

First detection of two new long-period giant planets with detailed radial velocity analysis, and identification of stellar activity signals that can be mistaken for planets.

Findings

HD 95872b has a minimum mass of 4.6 M_Jup at 5.2 AU.

psi1 Dra Bb has a minimum mass of 1.5 M_Jup at 4.4 AU.

Stellar activity cycles can mimic planetary signals in radial velocity data.

Abstract

We report the detection of two new long-period giant planets orbiting the stars HD 95872 and HD 162004 (psi1 Draconis B) by the McDonald Observatory planet search. The planet HD 95872b has a minimum mass of 4.6 M_Jup and an orbital semi-major axis of 5.2 AU. The giant planet psi1 Dra Bb has a minimum mass of 1.5 M_Jup and an orbital semi-major axis of 4.4 AU. Both of these planets qualify as Jupiter analogs. These results are based on over one and a half decades of precise radial velocity measurements collected by our program using the McDonald Observatory Tull Coude spectrograph at the 2.7 m Harlan J. Smith telescope. In the case of psi1 Draconis B we also detect a long-term non-linear trend in our data that indicates the presence of an additional giant planet, similar to the Jupiter-Saturn pair. The primary of the binary star system, psi1 Dra A, exhibits a very large amplitude radial velocity variation due to another stellar companion. We detect this additional member using speckle imaging. We also report two cases - HD 10086 and HD 102870 (beta Virginis) - of significant radial velocity variation consistent with the presence of a planet, but that are probably caused by stellar activity, rather than reflexive Keplerian motion. These two cases stress the importance of monitoring the magnetic activity level of a target star, as long-term activity cycles can mimic the presence of a Jupiter-analog planet.