Peering into the young planetary system AB Pic

Abstract

Aims. We aim to revisit the formation pathway of AB Pic b, one of the first companions ever imaged straddling the exoplanet/browndwarf boundary. We based this study on a rich set of observations which allows us to investigate its orbital and atmospheric properties. Methods. We composed a spectrum of AB Pic b merging archival medium-resolution (R ∼ 4000) VLT/SINFONI K-band (1.96 -2.45 µm) data, with published spectra at J and H-bands from SINFONI, at Lp-band from Magellan-AO/CLIO2, and photometric measurements from HST (visible) and Spitzer (mid-infrared). We modeled the spectrum implementing ForMoSA, following a forwardmodeling approach based on two atmospheric models: ExoREM and BT-SETTL13. In parallel, we determined the orbital properties of AB Pic b fitting orbital solutions to astrometric measurements from NaCo (2003) and SPHERE (2015. Results. The orbital solutions favor a semi-major axis of 190 +200 −50 au on a highly-inclined orbit (edge-on), but with a poorly constrained eccentricity. From the atmospheric modeling with Exo-REM, we derive an effective temperature of 1700 ± 50 K and surface gravity of 4.5 ± 0.3 dex, consistent with previous works, and we report for the first time a C / O ratio of 0.58 ± 0.08, consistent with the Sun's value. The posteriors are sensitive to the wavelength interval and the family of models used. Given the published rotation period of 2.1 hr and our derived vsin(i) of 73 +11 −27 km/s, we estimate for the first time the true obliquity of AB Pic b to be between 45 and 135 deg, indicating a rather significant misalignment between the planet's spin and orbit orientations. Finally, the existence of a proper motion anomaly between the Hipparcos and Gaia Early Data Release 3 compared to our SPHERE detection limits and adapted radial velocity limits indicate the potential existence of a ∼6 M Jup inner planet orbiting from 2 to 10 au (40 to 200 mas). Conclusions. The possible existence of an inner companion, together with the likely miss-alignment of the spin axis orientation, strongly favor a formation path by gravitational instability or core accretion within a disk closer inside followed by a dynamical interaction which scattered AB Pic b to its current location. Confirmation and characterization of this unseen inner exoplanet and access to a broader wavelength coverage for the characterization of AB Pic b will be essential to probe the uncertainties associated with the atmospheric and orbital parameters.

Rotation Periods, Inclinations, and Obliquities of Cool Stars Hosting Directly Imaged Substellar Companions: Spin–Orbit Misalignments Are Common

“…However, this sample may be beneficial to explore the statistical distributions of spin-spin orientations between the host stars (i * ) and companions (i p ). A handful of obliquities have now been measured for individual brown dwarf companions and giant planets by combining rotation periods, radii, and projected rotational velocities (e.g., Bowler et al 2020b;Bryan et al 2020aBryan et al , 2021Zhou et al 2020;Palma-Bifani et al 2023), but larger samples are needed to assess the prevalence of stellar-companion spin-spin alignment in order to interpret this in the context of formation scenarios (e.g., Jennings & Chiang 2021). Companions to host stars with existing inclination constraints from this study may be promising targets for follow-up photometric monitoring and high-resolution spectroscopy in the future.…”

The JWST Early Release Science Program for Direct Observations of Exoplanetary Systems. V. Do Self-consistent Atmospheric Models Represent JWST Spectra? A Showcase with VHS 1256–1257 b

“…However, constraining the formation history of VHS 1256 b using its atmospheric properties estimated from the forward-modeling approach remains challenging. A supersolar metallicity is usually used to quantify the solid material that is accreted during the formation, and the C/O is considered as a tracer of the birth location of the planet (see Nowak et al 2020;Petrus et al 2021;Hoch et al 2022;Palma-Bifani et al 2023), but with this study, we have shown that the estimates of these commonly used tracers of formation were strongly dependent on the model and the spectral information used for the fit. Moreover, Mollière et al (2022) have demonstrated that the relationship between chemical abundances and the formation mechanisms described by the models (e.g., Öberg et al 2011) is more complex than initially anticipated, and it is dependent on various intricate parameters, particularly in the case of formation within a disk.…”

“…The imposition of these assumptions leads to a limited number of free parameters (<6), and the quality of the fit heavily relies on the quality of the models. The impacts of these systematic errors on the estimation of atmospheric properties have already been recognized for L and M spectral types (Cushing et al 2008;Stephens et al 2009;Bonnefoy et al 2014;Manjavacas et al 2014;Lachapelle et al 2015;Bayo et al 2017;Petrus et al 2020Petrus et al , 2023Suárez et al 2021;Lueber et al 2023;Palma-Bifani et al 2023). These studies have interpreted these discrepancies as indications of a deficiency in the modeling of dust and haze within the synthetic atmosphere models.…”

Atmospheric properties of AF Lep b with forward modeling

“…We report the predictions of these models in terms of iso-mass (red-yellow lines) and iso-temperature (gray lines) curves as a function of R and log(g) for different ages (see Fig. 3), as done previously in Palma-Bifani et al (2023). A way to interpret the curves in Fig.…”

“…ForMoSA is a Bayesian forward-modeling code based on a nested sampling algorithm performing parameter exploration given a likelihood function (Skilling 2006). A full description of the code and applications on planetary-mass companions can be found in Petrus et al (2021) for HIP 65426 b, in Petrus et al (2023) for VHS 1256 AB b, and in Palma-Bifani et al (2023) for AB Pic b. Here we exploit its capabilities for studying the atmosphere of a companion at low spectral resolution (R λ ∼ 30).…”