Planets around evolved intermediate-mass stars

III. Planet candidates and long-term activity signals in six open clusters^★,★★

¹ Instituto de Astrofísica e Ciências do Espaço, Universidade do Porto, CAUP, Rua das Estrelas, 4150-762 Porto, Portugal
e-mail: elisa.delgado@astro.up.pt
² Departamento de Física e Astronomia, Faculdade de Ciências, Universidade do Porto, Rua do Campo Alegre, 4169-007 Porto, Portugal
³ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
⁴ School of Physics & Astronomy, University of Birmingham, Edgbaston, Birmingham, B15 2TT, UK
⁵ Observatoire de Genève, Université de Genève, 51 ch. des Maillettes, 1290 Sauverny, Switzerland

Received: 12 May 2023
Accepted: 1 August 2023

Abstract

Context. We carried out a long-term campaign spanning 17 year to obtain high-precision radial velocities (RVs) with the HARPS spectrograph for a large sample of evolved stars in open clusters.

Aims. The aim of this work is to search for planets around evolved stars, with a special focus on stars more massive than 2 M_⊙ in light of previous findings that show a drop in planet occurrence around stars above this mass.

Methods. We used kima – a package for Bayesian modelling of RV and activity data with Gaussian process capability and Nested sampling for model comparison – to find the Keplerian orbits most capable of explaining the periodic signals observed in RV data, which have semiamplitudes of between 75 and 500 m s⁻¹. We also studied the variation of stellar activity indicators and photometry in order to discard stellar signals mimicking the presence of planets.

Results. We present a planet candidate in the open cluster NGC 3680 that orbits the 1.64 M_⊙ star No. 41. The planet has a minimum mass of 5.13 M_J and a period of 1155 days. We also present periodic and large-amplitude RV signals of probable stellar origin in two more massive stars (5.84 and 3.05 M_⊙ in the clusters NGC 2345 and NGC 3532). Finally, using new data, we revise the RV signals of the three stars analysed in our previous paper. We confirm the stellar origin of the signals observed in NGC 2423 No. 3 and NGC 4349 No. 127. On the other hand, the new data collected for IC4651 No. 9122 (1.79 M_⊙) seem to support the presence of a bona fide planet of 6.22 M_J at a period of 744 days, although more data will be needed to discard a possible correlation with the CCF-FWHM.

Conclusions. The targets presented in this work showcase the difficulties in interpreting RV data for evolved massive stars. The use of several activity indicators (CCF-FWHM, CCF-BIS, Hα), photometry, and long-term observations (covering several orbital and stellar rotational periods) is required to discern the true nature of the signals. However, in some cases, all this information is insufficient, and the inclusion of additional data – such as the determination of magnetic field variability or RV points in the near-infrared – will be necessary to identify the nature of the discovered signals.