[Paper Review] Gaseous Planets, Protostars And Young Brown Dwarfs : Birth And Fate

G. Chabrier, I. Baraffe+4 more|arXiv (Cornell University)|Feb 13, 2006

Astrophysics and Star Formation Studies3 citations

TL;DR

This paper proposes a unified theoretical framework linking core accretion with planet migration and disk evolution to resolve the timescale problem in giant planet formation, while demonstrating that non-spherical accretion in protostars leads to inaccurate initial conditions in spherical models. Key results show that young planets' initial luminosities are highly uncertain due to variable initial conditions, and accreting brown dwarfs appear fainter and scattered across the HR diagram, challenging the concept of a well-defined 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.

Motivation & Objective

• To resolve the long-standing timescale problem in core accretion model for giant planet formation.
• To assess the impact of non-spherical accretion on protostellar core collapse and initial conditions for pre-main sequence evolution.
• To evaluate how accretion affects the luminosity and evolutionary tracks of young brown dwarfs and planets.
• To challenge the conventional deuterium-burning distinction between planets and brown dwarfs and propose a mechanism-based alternative.
• To provide observational diagnostics differentiating planets from brown dwarfs based on formation mechanism and composition.

Proposed method

• Adaptation of the core accretion model with inclusion of planet migration and disk evolution to simulate planet formation in protostellar nebulae.
• Comparison of 1D spherical and 3D multidimensional hydrodynamical simulations of protostellar core collapse to assess geometric effects.
• Use of time-dependent, multi-zone models to compute thermal and structural evolution of young planets and brown dwarfs post-disk dissipation.
• Incorporation of irradiation and evaporation effects on short-period planets to model atmospheric mass loss.
• Analysis of accretion history effects on radius, luminosity, and HR diagram placement for young objects.
• Proposed observational diagnostics based on mechanical (mass-radius) and spectroscopic signatures reflecting formation mechanism differences.

Experimental results

Research questions

• RQ1 Can the inclusion of planet migration and disk evolution resolve the timescale problem in core accretion for giant planet formation?
• RQ2 How do non-spherical accretion and 3D collapse dynamics affect the initial density, temperature, and structure of prestellar cores?
• RQ3 To what extent does accretion alter the luminosity and evolutionary track of young brown dwarfs compared to non-accreting models?
• RQ4 Why is the conventional deuterium-burning boundary between planets and brown dwarfs no longer physically meaningful?
• RQ5 What observational diagnostics can reliably distinguish planets from brown dwarfs based on formation mechanism rather than mass alone?

Key findings

• The inclusion of migration and disk evolution in core accretion models shortens the gas accretion phase, resolving the long formation timescale problem for giant planets.
• Uncertainties in initial conditions—especially initial radius—lead to a factor of ~100 uncertainty in the initial luminosity of young 1 M_Jup planets, making it impossible to determine if they are bright or faint.
• 3D simulations show that spherical collapse overestimates core inner density and temperature, yielding incorrect initial conditions for pre-main sequence evolution.
• Non-spherical accretion covers only a limited fraction of the protostar surface, allowing most of the surface to radiate freely and leading to cooler, more extended cores than in spherical models.
• Accreting young brown dwarfs are more compact and fainter than non-accreting counterparts of the same mass and age, which can lead to significant errors in age and mass estimation from HR diagram placement.
• Due to variable accretion histories, young stars and brown dwarfs do not follow a well-defined birth line in the HR diagram but instead appear scattered over an extended region, challenging the validity of standard mass-age calibrations.