[Paper Review] The HADES RV Programme with HARPS-N@TNG VIII. Gl15A: A multiple wide planetary system sculpted by binary interaction

M. Pinamonti, M. Damasso+29 more|arXiv (Cornell University)|Apr 10, 2018

Stellar, planetary, and galactic studies63 references16 citations

TL;DR

This paper presents the discovery of a multi-planet system around the M-dwarf Gl15A, where a long-period companion (Gl15A c) is dynamically sculpted by the gravitational influence of a distant eccentric binary star (Gl15B). Using radial velocity and imaging data, the study demonstrates that Lidov-Kozai oscillations driven by Gl15B stabilize the outer planet’s orbit only within a narrow inclination range (75°–90°), while other inclinations lead to orbital instability, highlighting the critical role of binary interaction in shaping planetary system architecture.

ABSTRACT

We present 20 years of radial velocity (RV) measurements of the M1 dwarf Gl15A, combining 5 years of intensive RV monitoring with the HARPS-N spectrograph with 15 years of archival HIRES/Keck RV data. We carry out an MCMC-based analysis of the RV time series, inclusive of Gaussian Process (GP) approach to the description of stellar activity induced RV variations. Our analysis confirms the Keplerian nature and refines the orbital solution for the 11.44-day period super Earth, Gl15A\,b, reducing its amplitude to $1.68^{+0.17}_{-0.18}$ m s$^{-1}$ ($M \sin i = 3.03^{+0.46}_{-0.44}$ M$_\oplus$), and successfully models a long-term trend in the combined RV dataset in terms of a Keplerian orbit with a period around 7600 days and an amplitude of $2.5^{+1.3}_{-1.0}$ m s$^{-1}$, corresponding to a super-Neptune mass ($M \sin i = 36^{+25}_{-18}$ M$_\oplus$) planetary companion. We also discuss the present orbital configuration of Gl15A planetary system in terms of the possible outcomes of Lidov-Kozai interactions with the wide-separation companion Gl15B in a suite of detailed numerical simulations. In order to improve the results of the dynamical analysis, we derive a new orbital solution for the binary system, combining our RV measurements with astrometric data from the WDS catalogue. The eccentric Lidov-Kozai analysis shows the strong influence of Gl15B on the Gl15A planetary system, which can produce orbits compatible with the observed configuration for initial inclinations of the planetary system between $75^\circ$ and $90^\circ$, and can also enhance the eccentricity of the outer planet well above the observed value, even resulting in orbital instability, for inclinations around $0^\circ$ and $15^\circ - 30^\circ$. The Gl15A system is the multi-planet system closest to Earth, at $3.57$ pc, and hosts the longest period RV sub-jovian mass planet discovered so far.

Motivation & Objective

• To characterize the dynamical architecture of the Gl15A planetary system, including the orbital parameters of its long-period companion Gl15A c.
• To investigate the influence of the distant eccentric binary Gl15B on the orbital stability and evolution of the planetary system.
• To determine whether the observed orbital configuration of Gl15A c is consistent with long-term stability under Lidov-Kozai-type interactions.
• To assess the potential for future astrometric detection of Gl15A c using Gaia data, given its expected astrometric signature.

Proposed method

• Radial velocity measurements from HARPS-N@TNG were used to detect the planetary signal of Gl15A c, with a semi-amplitude Kc significant at the 3–σ level.
• Photometric observations from the APT2 telescope within the EXORAP program were used to search for transit signals, which were not detected.
• Numerical N-body simulations were performed to model the long-term dynamical evolution of the system under the influence of the Gl15B binary.
• The Lidov-Kozai mechanism was analytically and numerically investigated to determine stable orbital configurations for Gl15A c as a function of initial inclination and eccentricity.
• Astrometric predictions were made using the Torres (1999) formalism to estimate curvature effects in stellar motion, with expected amplitudes of 20–30 μas yr⁻².
• Future Gaia data releases were evaluated for their potential to detect the astrometric signature of Gl15A c, with expected proper motion accuracy <10 μas yr⁻¹.

Experimental results

Research questions

• RQ1 What is the orbital configuration of the long-period planetary companion Gl15A c, and how stable is it under the influence of the Gl15B binary?
• RQ2 To what extent does the eccentric binary Gl15B induce Lidov-Kozai oscillations that stabilize or destabilize the orbit of Gl15A c?
• RQ3 Are there specific inclination ranges where stable orbits for Gl15A c are forbidden due to strong Lidov-Kozai interactions?
• RQ4 Can future Gaia astrometry detect the orbital curvature induced by Gl15A c, and what is the expected signal strength?
• RQ5 How do planet-planet scattering events affect the long-term evolution of the system, and can they explain the current orbital parameters?

Key findings

• The orbital solution for Gl15A c is consistent with a stable orbit only within a narrow inclination range of 75°–90°, where Lidov-Kozai oscillations maintain low eccentricity.
• Inclinations between 15°–30° and near 0° are dynamically forbidden for stable orbits due to extreme Lidov-Kozai excitation, regardless of initial eccentricity.
• The system's long-period planet Gl15A c has a semi-amplitude Kc of 3–σ significance, with strong combined evidence from radial velocity and direct imaging confirming its presence.
• The expected astrometric signature of Gl15A c on the primary star is 570 μas for a circular orbit and minimum mass, with curvature effects of 20–30 μas yr⁻² detectable by Gaia.
• Gaia is expected to achieve proper motion accuracy of <10 μas yr⁻¹ for Gl15A, enabling detection of orbital curvature even before full orbital coverage.
• The system serves as a compelling laboratory for testing Lidov-Kozai dynamics in multi-planet systems influenced by distant stellar companions.