PREPRINT

# Peering into the Young Planetary System AB Pic. Atmosphere, Orbit, Obliquity & Second Planetary Candidate

P. Palma-Bifani, G. Chauvin, M. Bonnefoy, P. M. Rojo, S. Petrus, L. Rodet, M. Langlois, F. Allard, B. Charnay, C. Desgrange, D. Homeier, A. -M. Lagrange, J. -L. Beuzit, P. Baudoz, A. Boccaletti, A. Chomez, P. Delorme, S. Desidera, M. Feldt, C. Ginski, R. Gratton, A. -L. Maire, M. Meyer, M. Samland, I. Snellen, A. Vigan, Y. Zhang

Submitted on 2 November 2022

## Abstract

We aim to revisit the system AB Pic which has a known companion at the exoplanet/ brown-dwarf boundary. We based this study on a rich set of observations to investigate the companion's orbit and atmosphere. We composed a spectrum of AB Pic b merging archival VLT/SINFONI K-band data, with published spectra at J and H-band (SINFONI) and Lp-band (Magellan-AO), and photometric measurements (HST and Spitzer). We modeled the spectrum with ForMoSA, based on two atmospheric models: ExoREM and BT-SETTL13. We determined the orbital properties of b fitting the astrometric measurements from NaCo (2003 and 2004) and SPHERE (2015). The orbital solutions favor a semi-major axis of $\sim$190au viewed edge-on. With Exo-REM, we derive a T${}_{eff}$ of 1700$±$50K and surface gravity of 4.5$±$0.3dex, consistent with previous works, and we report for the first time a C/O ratio of 0.58$±$0.08 ($\sim$solar). The posteriors are sensitive to the wavelength interval and the family of models used. Given the 2.1hr rotation period and our vsin(i) of $\sim$73km/s, we estimate for the first time the true obliquity to be $\sim$45 or $\sim$135deg, indicating a significant misalignment between the planet's spin and orbit orientations. Finally, the existence of a proper motion anomaly between the Hipparcos and Gaia eDR3 compared to our SPHERE detection limits and adapted radial velocity limits indicate the existence of a $\sim$6M${}_{Jup}$ inner planet orbiting from 2 to 10au (40-200mas). 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 dynamical interactions. Confirmation and characterization of planet c and access to a broader wavelength coverage for planet b will be essential to probe the uncertainties associated with the parameters.

## Preprint

Comment: 17 pages, 13 Figures, 6 Tables. Accepted for publication in A&A (31 of October)

Subject: Astrophysics - Earth and Planetary Astrophysics