A Search for Exoplanets around Northern Circumpolar Stars. XI. A Planetary Companion around HD 112987 and a Planetary Companion Candidate around HD 58545

We present precise radial velocity (RV) measurements of the northern circumpolar giant stars HD 58545 and HD 112987 obtained over 8 yr with the Bohyunsan Observatory Echelle Spectrograph and supplemented during the final 3 yr by the Network of Robotic Echelle Spectrographs. RV variations in giant stars may arise from orbiting companions or intrinsic phenomena, including pulsations, chromospheric activity, and rotational modulation. To distinguish these possibilities, we analyzed line-profile variations, line bisectors, chromospheric activity indicators, and photometric data, and modeled the RV time series with Keplerian orbits using Markov Chain Monte Carlo sampling. For HD 58545, the best-fitting Keplerian model yields an orbital period of 450.44 − 4.63 + 3.35 days and a minimum companion mass of 3.65 − 0.58 + 0.61 M J . For HD 112987, we derive an orbital period of 554.96 − 4.67 + 4.14 days and a minimum companion mass of 8.48 − 0.73 + 0.96 M J . None of the activity diagnostics for HD 112987 exhibits significant variability at the RV period, consistent with a Keplerian interpretation. In contrast, several activity diagnostics of HD 58545 exhibit periodicities comparable to the RV period, which is also consistent within the uncertainties with the projected rotational timescale, P rot / sin i = 462 ± 33 days . Thus, rotationally modulated stellar variability may contribute to the RV signal. Although the signal can be described by a Keplerian model, the current data cannot distinguish unambiguously between a planetary companion and intrinsic stellar variability. We therefore conclude that HD 112987 hosts a planetary companion, whereas further monitoring is required to determine the origin of the RV signal in HD 58545.