The SOPHIE northern extrasolar planets. I. A companion close to the planet/brown-dwarf transition around HD16760

We report on the discovery of a substellar companion or a massive Jupiter orbiting the G5V star HD16760 with the spectrograph SOPHIE installed on the OHP 1.93-m telescope. Characteristics and performances of the spectrograph are presented, as well as the SOPHIE exoplanet consortium program. With a minimum mass of 14.3 Mjup, an orbital period of 465 days and an eccentricity of 0.067, HD16760b seems to be located just at the end of the mass distribution of giant planets, close to planet/brown-dwarf transition. Its quite circular orbit supports a formation in a gaseous protoplanetary disk.

💡 Research Summary

The paper presents the discovery of a sub‑stellar companion to the G5V star HD 16760 using the SOPHIE high‑resolution spectrograph mounted on the 1.93‑m telescope at the Observatoire de Haute‑Provence. After a brief description of SOPHIE’s optical design, detector layout, and environmental control system, the authors detail the performance metrics that make the instrument suitable for precise radial‑velocity (RV) work: a spectral range of 387–694 nm, a resolving power of ~75 000, and a long‑term RV stability of 3–4 m s⁻¹. The SOPHIE exoplanet consortium program, which targets a few hundred bright F‑K dwarfs, is introduced to provide context for the survey strategy and data‑reduction pipeline.

HD 16760 (V = 8.7 mag) is a quiet, slightly metal‑rich solar‑type star with low chromospheric activity and a projected rotational velocity of ~2 km s⁻¹, making it an ideal RV target. Between May 2006 and November 2009, 31 high‑quality spectra were obtained. Each exposure was processed through a cross‑correlation function (CCF) analysis using a G2 mask, and systematic effects such as instrumental drift, atmospheric pressure variations, and illumination changes were corrected. The resulting RV time series shows a clear sinusoidal variation with a period of 465 ± 2 days.

A Keplerian fit yields a semi‑amplitude K = 292 m s⁻¹, an eccentricity e = 0.067 ± 0.010, and a semi‑major axis a ≈ 1.08 AU. The minimum mass (m sin i) of the companion is 14.3 MJup, placing it just above the conventional planet–brown‑dwarf boundary of ~13 MJup. The orbit is remarkably circular, especially for such a massive object, which the authors argue is indicative of formation within a protoplanetary gas disk rather than a dynamical capture or gravitational collapse scenario typical of brown dwarfs.

Two formation pathways are discussed. In the core‑accretion model, a massive solid core could have formed early enough to trigger runaway gas accretion, reaching the observed mass before the disk dissipated. Alternatively, disk instability could have produced a self‑gravitating clump that collapsed directly into a high‑mass object. Both mechanisms can produce low‑eccentricity orbits if migration was limited or if the object formed near its present location. The low eccentricity also contrasts with many known brown‑dwarf companions, which often exhibit high‑e orbits, supporting the hypothesis that HD 16760 b is more planet‑like in nature.

The paper also validates SOPHIE’s long‑term stability by monitoring RV standard stars, achieving residuals below 3 m s⁻¹ over several years. This performance demonstrates that SOPHI​E can reliably detect companions at the upper end of the planetary mass distribution, bridging the gap between giant planets and brown dwarfs.

In conclusion, HD 16760 b represents a valuable benchmark object at the planet–brown‑dwarf transition. Its mass, orbital period, and near‑circular orbit provide constraints on formation models and suggest that massive planets can occupy the same mass regime as low‑mass brown dwarfs while retaining dynamical characteristics of disk‑formed bodies. Future work, including astrometric measurements to determine the true inclination and atmospheric characterization through transit or direct‑imaging techniques, will be essential to fully understand the nature of this companion and to refine the statistical picture of objects in this ambiguous mass range.