The HARPS search for southern extra-solar planets
Authors: Frensch Y. G. C. , Lo Curto G. , Bouchy F. , Mayor M. , Hébrard G. , Lovis C. , Moutou C. , Pepe F. A. , Queloz D. , Santos N. , Segransan D. , Udry S. , Unger N. 
Published: June 2023

Context. The long-term ongoing HARPS radial velocity survey of extra-solar planets initiated in 2003 provides a unique data set with a 19-yr baseline that allows the detection of long-period exoplanets, brown dwarfs, and low-mass binaries.

Aims. Our aim is to detect and characterise long-period companions around main sequence stars (spectral types late F to early M). Only 6% of the planets discovered so far have periods longer than 3 yr; we are probing this still largely unknown population. Methods. We use the radial velocity method to search for exoplanets around stars. The radial velocity variations are measured with HARPS at the ESO 3.6m telescope. Difficulties in characterising long-period exoplanets arise from the entanglement of the radial velocity with the stellar magnetic cycle. We thoroughly examined the stellar activity indicators to rule out magnetic cycles as the source of the observed variation. The true mass and inclination of our heavier companions are provided by astrometry, for which we use proper motions from HIPPARCOS and Gaia.

Results. Five Jupiter-mass exoplanets are reported to orbit HIP54597, BD-210397 (×2), HD 74698, and HD 94771 with 8.9 yr, 5.2 yr, 17.4 yr, 9.4 yr, and 5.9 yr orbits, and to have minimum masses of 2.01 ± 0.03, 0.7 ± 0.1, 2.4−0.2+1.5, 0.40 ± 0.06, and 0.53 ± 0.03 MJ respectively. HD 74698 also hosts a highly irradiated Neptune in a 15 day orbit with a minimum mass of 0.07 ± 0.01 MJ. The mass and inclination of the exoplanets cannot yet be well constrained by astrometric measurements. Only HIP54597 b, HD 74698 c, and BD-210397 c have weak constraints. The mass of HIP54597 b can maximally increase by 10%–30%, the minimum mass of HD 74698 c is likely equal to its true mass, and BD-210397 c has a mass of 2.66−0.32+0.63 MJ. HD 62364 hosts a brown dwarf with a true mass of 18.77−0.63+0.66 MJ in an orbit of 14 yr. The mass of HD62364 b is around the limit of the masses of brown dwarfs, but its orbit is highly eccentric (e = 0.607 ± 0.005), which is more common among brown dwarfs than exoplanets. HD56380B, HD221638B, and HD 33473C have minimum masses within the brown dwarf limits, in orbits of 8.9 yr, 16.6 yr, and 50 yr respectively; however, astro-metric measurements reveal them to be stellar binaries, with masses of 375.3−8.4+8.6, 110.0−3.7+3.9, and 271.0−3.8+3.9 MJ. The orbits of the stellar binaries HD 11938 and HD 61383 are incomplete. The preliminary result for HD 61383 is a 0.190 M⊙ binary in a 39 yr orbit. The secondary of the binary system HD 11938 has a mass of 0.33 M⊙, which is confirmed by a secondary peak in the cross-correlation function, and a preliminary period of 35 yr. The origin of the 3.0 yr radial velocity signal of HD 3964 is uncertain as it shows entanglement with the magnetic cycle of the star. We finally report one more star, HD 11608, with a magnetic cycle that mimics a planetary signal.

Conclusions. We present the discovery of six exoplanets, one uncertain exoplanet candidate, one brown dwarf, and five stellar binaries around main sequence stars. We also improve the orbital solution of the stellar binary HD 33473C thanks to long-term monitoring.