[Paper Review] VLT/NACO astrometry of the HR8799 planetary system. L'-band observations of the three outer planets

C. Bergfors, W. Brandner+3 more|arXiv (Cornell University)|Feb 25, 2011

Stellar, planetary, and galactic studiesPhysics and Astronomy27 references16 citations

TL;DR

This study presents L'-band astrometry of HR 8799 d using VLT/NACO, analyzing its position over a 2-year baseline to assess orbital configuration. The data reject purely circular, face-on orbits, indicating HR 8799 d is likely on an inclined, possibly eccentric or non-coplanar orbit, inconsistent with coplanar, face-on models and challenging initial assumptions about the system's architecture.

ABSTRACT

HR8799 is so far the only directly imaged multiple exoplanet system. The orbital configuration would, if better known, provide valuable insight into the formation and dynamical evolution of wide-orbit planetary systems. We present L'-band observations of the HR8799 system obtained with NACO at VLT, adding to the astrometric monitoring of the planets HR8799b, c and d. We investigate how well the two simple cases of (i) a circular orbit and (ii) a face-on orbit fit the astrometric data for HR8799d over a total time baseline of ~2 years. The results indicate that the orbit of HR8799d is inclined with respect to our line of sight, and suggest that the orbit is slightly eccentric or non-coplanar with the outer planets and debris disk.

Motivation & Objective

• To refine the orbital configuration of HR 8799 d using new astrometric data from VLT/NACO.
• To test whether purely circular or face-on orbits for HR 8799 d are consistent with the observed astrometry.
• To assess the dynamical stability of the HR 8799 system under different orbital assumptions.
• To reconcile astrometric data with constraints from stellar rotation and debris disk inclination.
• To determine whether the planets' orbits are coplanar or non-coplanar, and whether eccentricity is required.

Proposed method

• Acquired L'-band imaging data of HR 8799 with NACO at the VLT on October 5 and 6, 2009, using dithered cubes to improve sky subtraction.
• Performed data reduction using IRAF and IDL, constructing sky frames by averaging and rejecting high-flux pixels to suppress stellar contamination.
• Calibrated plate scale and true north using archival astrometric binary HD 211742, yielding 27.1 mas/px and field rotation of -0.6° ± 0.2°.
• Measured the position of HR 8799 d relative to the star using photometric centroiding, with uncertainties propagated from image noise and calibration errors.
• Fitted orbital models with fixed semimajor axis and varying inclination and eccentricity, computing χ² to assess goodness of fit.
• Compared the fit of two models: (i) circular orbit (e=0) and (ii) face-on orbit (i=90°), using χ² minimization and confidence limits.

Experimental results

Research questions

• RQ1 Is the orbit of HR 8799 d consistent with a purely circular, face-on configuration given the new astrometric data?
• RQ2 What is the best-fit orbital inclination and eccentricity for HR 8799 d when constrained by the 2-year astrometric baseline?
• RQ3 How do the derived orbital parameters of HR 8799 d compare with constraints from stellar rotation and the debris disk inclination?
• RQ4 Is the observed astrometry consistent with a coplanar system of planets, or does it suggest non-coplanar orbits?
• RQ5 Can the system remain dynamically stable under the best-fit orbital configurations?

Key findings

• The orbit of HR 8799 d is inconsistent with a purely face-on configuration, with eccentricity constrained to e ≥ 0.4 at 99.7% confidence if inclination is fixed at 90°.
• For a circular orbit (e=0), the best-fit inclination is i = 63° with a 99.73% confidence lower limit of i > 43°, indicating significant inclination.
• The derived inclination of i > 43° for a circular orbit is consistent with asteroseismic constraints on stellar rotation (i ≈ 65°), but inconsistent with the 3σ upper limit of i < 40° for the debris disk.
• The high inclination required for a circular orbit implies that either the orbit is eccentric or the planets are non-coplanar, as a coplanar, face-on system is ruled out.
• The astrometric data do not support a coplanar, circular, face-on configuration for HR 8799 d, challenging initial assumptions from the 2008 discovery.
• The system likely has non-coplanar orbits or non-zero eccentricity, and continued astrometric monitoring is required to break remaining degeneracies.