Characterizing planetary systems with SPIRou: questions about the magnetic cycle of 55 Cnc A and two new planets around B
C. Moutou, P. Petit, P. Charpentier, P. Cristofari, C. Baruteau, P. Thébault, L. Arnold, E. Artigau, A. Carmona, N. J. Cook, F. Debras, X. Delfosse, J. -F. Donati, L. Malo, M. Ould-Elhkim
TL;DR
This paper presents CFHT/SPIRou spectropolarimetric monitoring of the 55 Cnc system, analyzing magnetic activity and radial velocities for both components. It confirms the four inner planets around 55 Cnc A with improved orbital parameters, revises the outer giant planet’s period to about $P_d \approx 4799$ d, and identifies a long-period signal potentially linked to a magnetic cycle or an additional planet near $P \approx 3827$ d. Around 55 Cnc B, SPIRou reveals two low-mass planets with periods 6.799 d and 33.747 d, adding a rare case of planetary systems around both stars in a binary with a low mass ratio. The study also shows a very weak magnetic field for 55 Cnc B and a cycle-driven, but predominantly quiet, magnetic topology for 55 Cnc A, demonstrating SPIRou’s power to disentangle stellar activity from planetary signals in the infrared.
Abstract
One of the first exoplanet hosts discovered thirty years ago, the star 55 Cnc has been constantly observed ever since. It is now known to host at least five planets with orbital periods ranging from 17 hours to 15 years. It is also one of the most extreme metal rich stars in the neighbourhood and it has a low-mass secondary star. In this article, we present data obtained at the Canada-France-Hawai'i Telescope with the SPIRou spectropolarimeter on both components of the 55 Cnc stellar system. We revisit the long-period radial-velocity signals of 55 Cnc A, with a focus on the role of the magnetic cycle, and propose the existence of a sixth planet candidate, whose period falls close to that of the magnetic cycle, or half of it. The other massive outer planet has a revised period of 13.15 years and a minimum mass of 3.8 MJup. Although some uncertainty remains on these outer planets, the characterization of the four inner planets is very robust through the combination of many different data sets, and all signals are consistent in the nIR and optical domains. In addition, the magnetic topology of the solar-type primary component of the system is observed by SPIRou at the minimum of its activity cycle, characterized by an amplitude ten times smaller than observed during its maximum in 2017. For the low-mass component 55 Cnc B, we report the discovery of two exoplanets in the system, with a period of 6.799+-0.0014 and 33.75+-0.04 days and a minimum mass of 3.5+-0.8 and 5.3+-1.4 MEarth, respectively. The secondary magnetic field is very weak and the current data set does not allow its precise characterization, setting an upper limit of 10 G. The system 55 Cnc stands out as the sixth binary system with planetary systems around both components, and the first one with non equal-mass stellar components.