[Paper Review] The HARPS search for southern extrasolar planets. XXV. Four new planets in orbit around the moderatly active dwarfs HD 63765, HD 104067, HD 125595, and HIP 70849

D. Ségransan, M. Mayor+11 more|arXiv (Cornell University)|Jul 1, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy32 references43 citations

TL;DR

This paper reports the discovery of four new extrasolar planets using radial velocity measurements from the HARPS spectrograph on the ESO 3.6-m telescope at La Silla. The planets include two Neptune-mass worlds around moderately active K-dwarfs (HD 104067 b and HD 125595 b), a Saturn-mass planet with eccentric orbit around a G9 dwarf (HD 63765 b), and a massive gas giant (3–15 M_Jup) in a long-period orbit around a low-mass K7 dwarf (HIP 70849 b), challenging current planet formation models for low-mass stars.

ABSTRACT

We report the detection of four new extrasolar planets in orbit around the moderately active stars HD 63765, HD 104067, HIP 70849, and HD 125595 with the HARPS Echelle spectrograph mounted on the ESO 3.6-m telescope at La Silla. The first planet, HD 63765 b, has a minimum mass of 0.64MJup, a period of 358 days, and an eccentricity of 0.24. It orbits a G9 dwarf at 0.94 AU. The second planet, HD 104067 b, is a 3.6 Neptune-mass-planet with a 55.8-day-period. It orbits its parent K2 dwarf, in a circular orbit with a semi-major axis of a=0.26 AU. Radial velocity measurements present a \approx 500-day-oscillation that reveals significant magnetic cycles. The third planet is a 0.77 Neptune-mass-planet in circular orbit around the K4 dwarf, HD 12595, with a 9.67-day-period. Finally, HIP 7849 b is a long-period (5< P <75 years) and massive planet of m. sin i \approx 3.5-15 MJup that orbits a late K7 dwarf.

Motivation & Objective

• To detect low-mass and intermediate-mass planets around moderately active stars using high-precision radial velocity measurements.
• To improve understanding of planetary system architectures and host star properties in a statistically robust sample of nearby stars.
• To investigate the impact of stellar activity and magnetic cycles on radial velocity detection and orbital parameter determination.
• To test current planet formation models, particularly for low-mass stars and intermediate-mass planets in the 0.1–0.5 AU range.
• To characterize the frequency and properties of gas giants and Neptune-mass planets in systems with active stars.

Proposed method

• Radial velocity measurements were obtained using the HARPS echelle spectrograph on the ESO 3.6-m telescope at La Silla Observatory.
• Stellar parameters, including effective temperature, metallicity, and age, were derived via spectroscopic analysis and theoretical isochrones with Bayesian estimation.
• Chromospheric activity was quantified using the log R′HK index, and its variability was analyzed to assess stellar magnetic cycles and rotational periods.
• Orbital solutions were derived using Keplerian modeling of radial velocity data, with uncertainties estimated via Monte Carlo simulations.
• The presence of activity-induced signals was assessed by correlating radial velocity residuals with the log R′HK index, revealing significant correlations (e.g., r = 0.59 for HD 104067).
• A genetic algorithm was employed to optimize orbital solution fitting, particularly for systems with complex signals or long-period companions.

Experimental results

Research questions

• RQ1 How do stellar activity cycles and chromospheric variability affect the detection and characterization of low-mass exoplanets via radial velocity methods?
• RQ2 What is the frequency and distribution of Neptune-mass and gas giant planets around moderately active K- and G-type dwarfs?
• RQ3 Can the presence of massive planets (≥3 M_Jup) around low-mass stars (K7 V) be reconciled with current core accretion and disk migration models?
• RQ4 Why are intermediate-mass planets (1–10 M_Nept) in the 0.1–0.5 AU range more common than predicted by current formation theories?
• RQ5 To what extent do radial velocity signals from long-period companions (e.g., HIP 70849 b) mimic or mask planetary signals, and how can they be disentangled from stellar activity?

Key findings

• HD 63765 b is a 0.64 M_Jup planet with a 358-day period, eccentricity e = 0.24, and semi-major axis a = 0.94 AU, orbiting a G9 dwarf.
• HD 104067 b is a 3.44 M_Nept mass planet with a 55.8-day period, in a circular orbit at 0.2643 AU, and exhibits a 500-day activity cycle in radial velocity residuals.
• HD 125595 b is a 0.766 M_Nept mass planet with a 9.67-day period and circular orbit, located at 0.0809 AU from its K4 dwarf host.
• HIP 70849 b is a massive planet with m sin i = 3–15 M_Jup, orbiting a K7 dwarf with a period between 5 and 75 years, suggesting a highly eccentric or long-period orbit.
• The radial velocity residuals for HD 104067 show a significant correlation (Pearson r = 0.59) with the chromospheric activity index, indicating activity-induced signals.
• The detection of four planets, including three with masses between 0.77 and 3.44 M_Nept, challenges current planet formation models, especially for low-mass stars and intermediate-mass planets in the 0.1–0.5 AU range.