A critical reassessment of the fundamental properties of Gas Giant Host Star Gliese 504

A critical reassessment of the fundamental properties of GJ 504: Chemical composition and age

Authors:

D'Orazi et al

Abstract:

The recent development of brand new observational techniques and theoretical models have greatly advanced the exoplanet research field. Despite significant achievements, which have allowed the detection of thousands extrasolar systems, a comprehensive understanding of planetary formation and evolution mechanisms is still desired. One relevant limitation is given by the accuracy in the measurements of planet-host star ages. The star GJ 504 has been found to host a substellar companion whose nature is strongly debated. There has been a recent difference of opinion in the literature owing to the uncertainty on the age of the system: a young age of ∼ 160 Myr would imply a giant planet as a companion, but a recent revision pointing to a solar age (∼ 4 Gyr) instead suggests a brown dwarf. With the aim of shedding light on this debated topic, we have carried out a high-resolution spectroscopic study of GJ 504 to derive stellar parameters, metallicity, and abundances of both light and heavy elements, providing a full chemical characterisation. The main objective is to infer clues on the evolutionary stage (hence the age) of this system. We performed a strictly differential (line-by-line) analysis of GJ 504 with respect to two reference stars, that is the planet-host dwarf ι Hor and the subgiant HIP 84827. The former is crucial in this context because its stellar parameters (hence the evolutionary stage) is well constrained from asteroseismic observations. Regardless of the zero point offsets, our differential approach allows us to put tight constraints on the age of GJ 504 with respect to ι Hor, thereby minimising the internal uncertainties. We found that the surface gravity of GJ 504 is 0.2 ± 0.07 dex lower than that of the main-sequence star ι Hor, suggesting a past turn-off evolution[...]

T Class Brown Dwarf Gliese 504b Formed Like a Planet, NOT a Star

The LEECH Exoplanet Imaging Survey: Characterization of the Coldest Directly Imaged Exoplanet, GJ 504 b, and Evidence for Super-Stellar Metallicity

Authors:

Skemer et al

Abstract:

As gas giant planets and brown dwarfs radiate away the residual heat from their formation, they cool through a spectral type transition from L to T, which encompasses the dissipation of cloud opacity and the appearance of strong methane absorption. While there are hundreds of known T-type brown dwarfs, the first generation of directly-imaged exoplanets were all L-type. Recently, Kuzuhara et al. (2013) announced the discovery of GJ 504 b, the first T dwarf exoplanet. GJ 504 b provides a unique opportunity to study the atmosphere of a new type of exoplanet with a ~500 K temperature that bridges the gap between the first directly imaged planets (~1000 K) and our own Solar System's Jupiter (~130 K). We observed GJ 504 b in three narrow L-band filters (3.71, 3.88, and 4.00 microns), spanning the red end of the broad methane fundamental absorption feature (3.3 microns) as part of the LEECH exoplanet imaging survey. By comparing our new photometry and literature photometry to a grid of custom model atmospheres, we were able to fit GJ 504 b's unusual spectral energy distribution for the first time. We find that GJ 504 b is well-fit by models with the following parameters: T_eff=544+/-10 K, g less than 600 m/s^2, [M/H]=0.60+/-0.12, cloud opacity parameter of f_sed=2-5, R=0.96+/-0.07 R_Jup, and log(L)=-6.13+/-0.03 L_Sun, implying a hot start mass of 3-30 M_jup for a conservative age range of 0.1-6.5 Gyr. Of particular interest, our model fits suggest that GJ 504 b has a super-stellar metallicity. Since planet formation can create objects with non-stellar metallicities, while binary star formation cannot, this result suggests that GJ 504 b formed like a planet, not like a binary companion.

How Old is the Gliese 504 System

ON THE AGE OF GLIESE 504

Authors:

Fuhrmann et al

Abstract:

Direct imaging observations of the solar-type star Gl 504 have recently uncovered a faint companion that, on the supposition that the host star has an age of $160_{-60}^{+350}$ Myr, was announced to be a $4_{-1.0}^{+4.5}$ MJ Jovian exoplanet. Here we present the observational evidence that Gl 504 A is an evolved turn-off star of about solar age and by inference its faint companion a low-mass brown dwarf. As with our previous work on Gl 504 A several years ago, we suggest the accretion of a substellar object to account for the otherwise unexplained high rotation of Gl 504 A. We also propose that with the distant Gl 504 B we may now well be facing the driving agent for the former merger.