Implications of the discovery of AF Lep b. The mass-luminosity relation for planets in the β Pic Moving Group and the L–T transition for young companions and free-floating planets

Abstract

Context. Dynamical masses of young planets aged between 10 and 200 Myr detected in imaging play a crucial role in shaping models of giant planet formation. Regrettably, only a few such objects possess these characteristics. Furthermore, the evolutionary pattern of young sub-stellar companions in near-infrared colour-magnitude diagrams might diverge from free-floating objects, possibly due to differing formation processes.

Aims. The recent identification of a giant planet around AF Lep, part of the β Pic moving group (BPMG), encouraged us to re-examine these points.

Methods. We considered updated dynamical masses and luminosities for the sub-stellar objects in the BPMG. In addition, we compared the properties of sub-stellar companions and free-floating objects in the BPMG and other young associations remapping the positions of the objects in the colour-magnitude diagram into a dustiness-temperature plane.

Results. We found that cold-start evolutionary models do not reproduce the mass-luminosity relation for sub-stellar companions in the BPMG. This aligns rather closely with predictions from “hot start” scenarios and is consistent with recent planet formation models. We obtain rather good agreement with masses from photometry and the remapping approach compared to actual dynamical masses. We also found a strong suggestion that the near-infrared colour-magnitude diagram for young companions is different from that of free-floating objects belonging to the same young associations.

Conclusions. If confirmed by further data, this last result would imply that cloud settling – which likely causes the transition between L and T spectral type – occurs at a lower effective temperature in young companions than in free-floating objects. This might tentatively be explained with a different chemical composition.