[Paper Review] The HARPS search for southern extra-solar planets XXXVI: Five new long-period giant planets and a system update

C. Moutou, G. Lo Curto+11 more|arXiv (Cornell University)|Dec 20, 2014

Stellar, planetary, and galactic studies1 references17 citations

TL;DR

This study presents the discovery of five new long-period giant exoplanets and a refined analysis of two previously known systems using radial velocity measurements from the HARPS spectrograph on the ESO 3.6 m telescope. The key contribution is the detection of planets with orbital periods from 4.56 to 1684 days, including a hot-Jupiter around the active star HD 103720, and improved orbital parameters for planets around HD 113538 and BD-114672, highlighting challenges from stellar activity in low-noise environments.

ABSTRACT

We describe radial-velocity time series obtained by HARPS on the 3.60 m telescope in La Silla (ESO, Chile) over ten years and report the discovery of five new giant exoplanets in distant orbits; these new planets orbit the stars HD 564, HD 30669, HD 108341, and BD-114672. Their periods range from 492 to 1684 days, semi-major axes range from 1.2 to 2.69 AU, and eccentricities range from 0 to 0.85. Their minimum mass ranges from 0.33 to 3.5 Mjup. We also refine the parameters of two planets announced previously around HD 113538, based on a longer series of measurements. The planets have a period of 663+-8 and 1818+-25 days, orbital eccentricities of 0.14+-0.08 and 0.20+-0.04, and minimum masses of 0.36+-0.04 and 0.93+-0.06 Mjup. Finally, we report the discovery of a new hot-Jupiter planet around an active star, HD 103720; the planet has a period of 4.5557+-0.0001 days and a minimum mass of 0.62+-0.025 Mjup. We discuss the fundamental parameters of these systems and limitations due to stellar activity in quiet stars with typical 2m/s radial velocity precision.

Motivation & Objective

• To detect long-period giant exoplanets in the southern sky using high-precision radial velocity measurements.
• To improve orbital parameter estimates for previously announced planets in the HD 113538 and BD-114672 systems through extended observation baselines.
• To investigate the impact of stellar activity on radial velocity precision, particularly in quiet stars with low intrinsic jitter.
• To characterize the host stars' physical parameters and assess their influence on planet detection limits.
• To contribute to a more complete picture of planetary system architectures by bridging radial velocity and direct imaging detection spaces.

Proposed method

• Radial velocity time series were collected over ten years using the HARPS spectrograph on the ESO 3.6 m telescope at La Silla, Chile.
• Stellar parameters including effective temperature, surface gravity, metallicity, and mass were derived using spectroscopic analysis of combined HARPS spectra and ARES code.
• Orbital solutions were fitted to radial velocity data using a Keplerian model, with uncertainties estimated via Markov Chain Monte Carlo (MCMC) methods.
• Stellar activity was monitored using auxiliary diagnostics: bisector span, FWHM of the cross-correlation function, and CaII H and K index.
• Bayesian analysis was applied to derive stellar masses, radii, and ages by comparing observed parameters to theoretical isochrones.
• Residuals after orbital fitting were analyzed to assess the significance of planetary signals and to detect potential activity-induced noise.

Experimental results

Research questions

• RQ1 What are the orbital and physical parameters of five newly discovered long-period giant exoplanets in the HARPS southern survey?
• RQ2 How do extended observation baselines improve the precision of orbital parameters for previously detected planets in the HD 113538 and BD-114672 systems?
• RQ3 To what extent does stellar activity limit the detection of low-amplitude radial velocity signals from long-period planets?
• RQ4 What is the impact of stellar magnetic cycles on radial velocity measurements, particularly in systems like BD-114672?
• RQ5 How do the properties of the host stars, such as metallicity and age, correlate with the detected planetary systems?

Key findings

• Five new long-period giant exoplanets were discovered: HD 103720 b (P = 4.5557 ± 0.0001 days, m sin i = 0.620 ± 0.025 M Jup ), HD 564 b (P = 492.3 ± 2.3 days, m sin i = 0.33 ± 0.03 M Jup ), HD 108341 b (P = 1684 ± 61 days, m sin i = 0.47 ± 0.06 M Jup ), BD-114672 b (P = 1667 ± 33 days, m sin i = 0.53 ± 0.05 M Jup ), and a second planet around BD-114672 with P = 1129 ± 8 days.
• The orbital parameters of two planets around HD 113538 were refined: HD 113538 b has P = 663.2 ± 8.4 days, e = 0.14 ± 0.08, and m sin i = 0.36 ± 0.04 M Jup ; HD 113538 c has P = 1818 ± 25 days, e = 0.20 ± 0.04, and m sin i = 0.93 ± 0.06 M Jup .
• The planet around HD 103720 is a hot-Jupiter with a period of 4.5557 ± 0.0001 days and a minimum mass of 0.620 ± 0.025 M Jup , detected despite the host star's high activity level.
• The residual scatter (σ(O-C)) after orbital fitting ranged from 1.1 to 13.1 m s⁻¹, with the lowest values (1.1–2.9 m s⁻¹) indicating high-fidelity solutions for the most stable systems.
• The system BD-114672, previously attributed to a magnetic cycle, now shows a planetary signal with a period of 1129 ± 8 days and eccentricity e = 0.05 ± 0.06, suggesting a planetary origin after long-term monitoring.
• Stellar activity, particularly in the form of magnetic cycles and surface inhomogeneities, was found to be a major source of radial velocity noise, with activity indicators like log R′HK and bisector span used to correct for these effects.