Building protoplanetary disks from the molecular cloud: redefining the disk timeline

K Baillié,J Marques,L Piau

doi:10.1051/0004-6361/201730677

Abstract

Context. Planetary formation models are necessary to understand the characteristics of the planets that are the most likely to survive. Their dynamics, their composition and even the probability of their survival depend on the environment in which they form. We therefore investigate the most favorable locations for planetary embryos to accumulate in the protoplanetary disk: the planet traps. Aims. We study the formation of the protoplanetary disk by the collapse of a primordial molecular cloud, and how its evolution leads to the selection of specific types of planets. Methods. We use a hydrodynamical code that accounts for the dynamics, thermodynamics, geometry and composition of the disk to numerically model its evolution as it is fed by the infalling cloud material. As the mass accretion rate of the disk onto the star determines its growth, we can calculate the stellar characteristics by interpolating its radius, luminosity and temperature over the stellar mass from pre-calculated stellar evolution models. The density and midplane temperature of the disk then allow us to model the interactions between the disk and potential planets and determine their migration. Results. At the end of the collapse phase, when the disk reaches its maximum mass, it pursues its viscous spreading, similarly to the evolution from a minimum mass solar nebula (MMSN). In addition, we establish a timeline equivalence between the MMSN and a “collapse-formed disk” that would be older by about 2 Myr. Conclusions. We can save various types of planets from a fatal type-I inward migration: in particular, planetary embryos can avoid falling on the star by becoming trapped at the heat transition barriers and at most sublimation lines (except the silicates one). One of the novelties concerns the possible trapping of putative giant planets around a few astronomical units from the star around the end of the infall. Moreover, trapped planets may still follow the traps outward during the collapse phase and inward after it. Finally, this protoplanetary disk formation model shows the early possibilities of trapping planetary embryos at disk stages that are anterior by a few million years to the initial state of the MMSN approximation.

Highlights

The huge diversity of observed exoplanets is a challenge for planetary formation scenarios, which must explain the trends in the distribution of the exoplanet orbital periods and masses and retrieve the observational constraints of the solar system
We show that the disk tends to warm up during the collapse phase due to the stellar luminosity and that planet traps are carried away from the star, in particular at the sublimation lines and the heat transition barriers where the dominant heating process changes between viscous heating and stellar irradiation heating
While previous works relied on an initial disk density profile following the controversial minimum mass solar nebula (MMSN) model, we here address that debate by bringing the initial condition back to the parameters of the molecular cloud at the origin of the star and disk

Summary

Introduction

The huge diversity of observed exoplanets is a challenge for planetary formation scenarios, which must explain the trends in the distribution of the exoplanet orbital periods and masses and retrieve the observational constraints of the solar system. This environment is the key element that determines which planets will fall onto their host star by spiraling inward by type-I migration in a few hundred thousand years (Goldreich & Tremaine 1979; Artymowicz 1993; Ward 1997) and which ones will avoid that by becoming trapped at the density gradient discontinuities (Masset et al 2006; Paardekooper & Papaloizou 2009a), or at the opacity transitions (Menou & Goodman 2004) These latter transitions potentially result from the sublimation lines of the disk dust species (Baillié et al 2015, 2016, hereafter referred to as BCP15 and BCP16). Timescales are critical here since forming a planet by gas accretion on a solid core requires a few million years (Pollack et al 1996), while the gas of the disk dissipates on a similar timescale (Font et al 2004; Alexander & Armitage 2007, 2009; Owen et al 2010), as confirmed by disk observations by Beckwith & Sargent (1996) and Hartmann et al (1998)

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Discussion

Conclusion