[Paper Review] First direct detection of an exoplanet by optical interferometry; Astrometry and K-band spectroscopy of HR8799 e

S. Lacour, M. Nowak+86 more|Kölner Universitäts PublikationsServer (Universität zu Köln)|Mar 28, 2019

Stellar, planetary, and galactic studies4 references44 citations

TL;DR

The study demonstrates direct exoplanet detection with optical interferometry using GRAVITY, providing high-precision astrometry and a K-band spectrum for HR8799 e, and constraining its orbit and atmospheric properties.

ABSTRACT

To date, infrared interferometry at best achieved contrast ratios of a few times $10^{-4}$ on bright targets. GRAVITY, with its dual-field mode, is now capable of high contrast observations, enabling the direct observation of exoplanets. We demonstrate the technique on HR8799, a young planetary system composed of four known giant exoplanets. We used the GRAVITY fringe tracker to lock the fringes on the central star, and integrated off-axis on the HR8799e planet situated at 390 mas from the star. Data reduction included post-processing to remove the flux leaking from the central star and to extract the coherent flux of the planet. The inferred K band spectrum of the planet has a spectral resolution of 500. We also derive the astrometric position of the planet relative to the star with a precision on the order of 100$\,μ$as. The GRAVITY astrometric measurement disfavors perfectly coplanar stable orbital solutions. A small adjustment of a few degrees to the orbital inclination of HR 8799 e can resolve the tension, implying that the orbits are close to, but not strictly coplanar. The spectrum, with a signal-to-noise ratio of $\approx 5$ per spectral channel, is compatible with a late-type L brown dwarf. Using Exo-REM synthetic spectra, we derive a temperature of $1150\pm50$\,K and a surface gravity of $10^{4.3\pm0.3}\,$cm/s$^{2}$. This corresponds to a radius of $1.17^{+0.13}_{-0.11}\,R_{ m Jup}$ and a mass of $10^{+7}_{-4}\,M_{ m Jup}$, which is an independent confirmation of mass estimates from evolutionary models. Our results demonstrate the power of interferometry for the direct detection and spectroscopic study of exoplanets at close angular separations from their stars.

Motivation & Objective

• Demonstrate direct detection of an exoplanet at close angular separation using optical interferometry.
• Obtain high-precision astrometry of HR8799 e relative to its host star.
• Acquire a K-band spectrum of HR8799 e to constrain atmospheric properties and physical parameters.
• Assess the orbital architecture for coplanarity among the HR8799 system.
• Compare interferometric results with existing imaging-based measurements and models.

Proposed method

• Use GRAVITY fringe tracker to lock onto the central star and integrate off-axis on HR8799 e at ~390 mas.
• Extract coherent flux (VISDATA) for planet and star, then derive planet position by fitting flux models.
• Remove stellar coherent flux by linear decomposition using planet position and stellar models.
• Normalize phase and amplitude by the stellar coherent flux to obtain the planet’s complex visibility V_planet.
• Compute planet spectrum from V_planet and assumed planet diameter via F_planet = |V_planet| F_star / [2J1(pi theta_planet u)/(pi theta_planet u)].
• Derive astrometry by fitting wavelength-dependent phase to obtain optimal OPDs and combine baselines to determine planet position with uncertainties.

Experimental results

Research questions

• RQ1 Can optical interferometry determine the position of a directly imaged exoplanet at sub-mas precision?
• RQ2 What are the astrometric constraints on HR8799 e’s orbit when combined with previous measurements?
• RQ3 What does the GRAVITY K-band spectrum reveal about HR8799 e’s atmospheric properties and temperature?
• RQ4 Is HR8799 e consistent with coplanar stable orbital configurations with the other planets?
• RQ5 How do interferometric measurements inform mass and radius estimates via atmospheric models?

Key findings

• GRAVITY achieves astrometric precision at the ~100 microarcsecond level for HR8799 e, disfavoring perfectly coplanar stable orbital solutions for the system.
• Single GRAVITY astrometry point yields a semimajor axis of 16.4+2.1-1.1 AU, eccentricity 0.15±0.08, and inclination 25°±8°.
• K-band spectrum (R~500) shows features consistent with a late-type L brown dwarf; CO band head at 2.29 μm is prominent and CH4 is not clearly detected.
• Exo-REM fitting gives Teff = 1150±50 K and log g = 4.3±0.3, corresponding to a radius ~1.17+0.13-0.11 RJup and a mass ~10+7-4 MJup.
• Spectral type inferred to be near L7 BD; observations support a low-gravity, young-planet interpretation compatible with a self-luminous giant planet.
• Results showcase interferometry’s ability to directly detect and characterize exoplanets at sub-arcsecond separations and with high spectral resolution.