The GJ 504 system revisited

Abstract

Context. The G-type star GJ504A is known to host a 3–35 MJup companion whose temperature, mass, and projected separation all contribute to making it a test case for planet formation theories and atmospheric models of giant planets and light brown dwarfs. Aims. We aim at revisiting the system age, architecture, and companion physical and chemical properties using new complementary interferometric, radial-velocity, and high-contrast imaging data. Methods. We used the CHARA interferometer to measure GJ504A’s angular diameter and obtained an estimation of its radius in combinationwith the HIPPARCOS parallax. The radius was compared to evolutionary tracks to infer a new independent age range for the system. We collected dual imaging data with IRDIS on VLT/SPHERE to sample the near-infrared (1.02–2.25 μm) spectral energy distribution (SED) of the companion. The SED was compared to five independent grids of atmospheric models (petitCODE,Exo-REM, BT-SETTL, Morley et al., and ATMO) to infer the atmospheric parameters of GJ 504b and evaluate model-to-model systematic errors. In addition, we used a specific model grid exploring the effect of different C/O ratios. Contrast limits from 2011 to 2017 were combined with radial velocity data of the host star through the MESS2 tool to define upper limits on the mass of additional companions in the system from 0.01 to 100 au. We used an MCMC fitting tool to constrain the companion’sorbital parameters based on the measured astrometry, and dedicated formation models to investigate its origin. Results. We report a radius of 1.35 ± 0.04 R⊙ for GJ504A. The radius yields isochronal ages of 21 ± 2 Myr or 4.0 ± 1.8 Gyr for the system and line-of-sight stellar rotation axis inclination of 162.4−4.3+3.8 degrees or 186.6−3.8+4.3 degrees. We re-detect the companion in the Y2, Y3, J3, H2, and K1 dual-band images. The complete 1–4 μm SED shape of GJ504b is best reproduced by T8-T9.5 objects with intermediate ages (≤ 1.5Gyr), and/or unusual dusty atmospheres and/or super-solar metallicities. All atmospheric models yield Teff = 550 ± 50 K for GJ504b and point toward a low surface gravity (3.5–4.0 dex). The accuracy on the metallicity value is limited by model-to-model systematics; it is not degenerate with the C/O ratio. We derive log L∕L⊙ = −6.15 ± 0.15 dex for the companion from the empirical analysis and spectral synthesis. The luminosity and Teff yield masses of M = 1.3−0.3+0.6 MJup and M = 23−9+10 MJup for the young and old age ranges, respectively. The semi-major axis (sma) is above 27.8 au and the eccentricity is lower than 0.55. The posterior on GJ 504b’s orbital inclination suggests a misalignment with the rotation axis of GJ 504A. We exclude additional objects (90% prob.) more massive than 2.5 and 30 MJup with semi-major axes in the range 0.01–80 au for the young and old isochronal ages, respectively. Conclusions. The mass and semi-major axis of GJ 504b are marginally compatible with a formation by disk-instability if the system is 4 Gyr old. The companion is in the envelope of the population of planets synthesized with our core-accretion model. Additional deep imaging and spectroscopic data with SPHERE and JWST should help to confirm the possible spin-orbit misalignment and refine the estimates on the companion temperature, luminosity, and atmospheric composition.

Atmospheric Characterization and Further Orbital Modeling of κ Andromeda b

“…We reconfirmed that κ And b is likely to have a larger eccentricity and semi-major axis than GJ 504 b (Bonnefoy et al 2018) and HR 8799 b,c,d,e (Wang et al 2018). κ And b may have experienced a strong excitation of the eccentricity by gravitational interactions between neighboring planets such as planet-planet scattering.…”

Towards the analysis of JWST exoplanet spectra: the effective temperature in the context of direct imaging

“…6.2.1). However, this last approach leads to a difference in the T eff compared to self-consistent models; for example, we observed a difference of 200-300 K higher, by applying for GJ 504 b (test done to prepare Bonnefoy et al 2018) the profile defined in Garland (2018).…”

“…The data are compared to a set of spectra: the subset of spectra that best reproduce the data give the T eff of the observed target (e.g. Macintosh et al 2015;Bonnefoy et al 2018). In this approach the uncertainty coming from the completeness of the wavelength range is not obvious to manage.…”

“…The range of T eff will either be defined by the "best" result for each model (e.g. Bonnefoy et al 2018) or by combining the range of models reproducing the data at a given level of uncertainty (e.g. Baudino et al 2015).…”

GTC/CanariCam deep mid-infrared imaging survey of northern stars within 5 pc

“…Modern imaging searches extend the accessible range of parameter space of cool companions to wider sepa-rations and longer orbital periods (a > 5 au, P > 10 yr) than those presently explored by radial velocities (RVs) or transits methods (e.g., Lafrenière et al 2007;Biller et al 2013;Chauvin et al 2015a;Meshkat et al 2015;Galicher et al 2016;Vigan et al 2017;Stone et al 2018;Baron et al 2019;Vigan et al 2021;Launhardt et al 2020). Moreover, directly detected brown dwarfs and planets provide a unique opportunity for spectroscopic characterisation and thereby for detailed study of their fundamental physical properties, in particular their atmospheres (e.g., Gauza et al 2015a;Skemer et al 2016;Bonnefoy et al 2016Bonnefoy et al , 2018Chinchilla et al 2021).…”