Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate

TL;DR

This review synthesizes recent theoretical advances in understanding the formation and early evolution of giant planets, brown dwarfs, and low-mass stars, highlighting the impact of accretion, irradiation, and initial conditions.

Contribution

It provides a comprehensive comparison of formation models, emphasizes the importance of accretion effects, and proposes a new observational criterion to distinguish planets from brown dwarfs.

Findings

Accretion influences the structure and luminosity of young brown dwarfs.

Spherical collapse models overestimate core densities, affecting initial conditions.

Young stars and brown dwarfs appear over an extended HR diagram region, not on a single birth line.

Abstract

We review recent theoretical progress aimed at understanding the formation and the early stages of evolution of giant planets, low-mass stars and brown dwarfs. Calculations coupling giant planet formation, within a modern version of the core accretion model, and subsequent evolution yield consistent determinations of the planet structure and evolution. Because of the uncertainties in the initial conditions, however, it is not possible to say whether young planets are faint or bright compared with low-mass young brown dwarfs. We review the effects of irradiation and evaporation on the evolution of short period planets and argue that substantial mass loss may have occurred for these objects. Concerning star formation, geometrical effects in protostar core collapse are examined by comparing 1D and 3D calculations. Spherical collapse is shown to overestimate the core inner density and temperature and thus to yield incorrect initial conditions for PMS or young brown dwarf evolution. Accretion is also shown to occur over a very limited fraction of the protostar surface. Accretion affects the evolution of young brown dwarfs and yields more compact structures for a given mass and age, thus fainter luminosities. This can lead to severe misinterpretations of the mass and/or age of young accreting objects from their location in the HR diagram. We argue that newborn stars and brown dwarfs should appear rapidly over an extended area in the HR diagram, depending on their accretion history, rather than on a well defined birth line. Finally, we suggest that the distinction between planets and brown dwarfs be based on an observational diagnostic, reflecting the different formation mechanisms between these two distinct populations, rather than on an arbitrary, confusing definition.