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تسجيل مستخدم جديدThe formation of planets by disc fragmentation
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تأليف
Dimitris Stamatellos
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I discuss the role that disc fragmentation plays in the formation of gas giant and terrestrial planets, and how this relates to the formation of brown dwarfs and low-mass stars, and ultimately to the process of star formation. Protostellar discs may fragment, if they are massive enough and can cool fast enough, but most of the objects that form by fragmentation are brown dwarfs. It may be possible that planets also form, if the mass growth of a proto-fragment is stopped (e.g. if this fragment is ejected from the disc), or suppressed and even reversed (e.g by tidal stripping). I will discuss if it is possible to distinguish whether a planet has formed by disc fragmentation or core accretion, and mention of a few examples of observed exoplanets that are suggestive of formation by disc fragmentation .
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We investigate the formation and fragmentation of discs using a suite of three-dimensional smoothed particle radiative magnetohydrodynamics simulations. Our models are initialised as 1M$_odot$ rotating Bonnor-Ebert spheres that are threaded with a un
iform magnetic field. We examine the effect of including ideal and non-ideal magnetic fields, the orientation and strength of the magnetic field, and the initial rotational rate. We follow the gravitational collapse and early evolution of each system until the final classification of the protostellar disc can be determined. Of our 105 models, 41 fragment, 21 form a spiral structure but do not fragment, and another 12 form smooth discs. Fragmentation is more likely to occur for faster initial rotation rates and weaker magnetic fields. For stronger magnetic field strengths, the inclusion of non-ideal MHD promotes disc formation, and several of these models fragment, whereas their ideal MHD counterparts do not. For the models that fragment, there is no correlation between our parameters and where or when the fragmentation occurs. Bipolar outflows are launched in only 17 models, and these models have strong magnetic fields that are initially parallel to the rotation axis. Counter-rotating envelopes form in four slowly-rotating, strong-field models -- including one ideal MHD model -- indicating they form only in a small fraction of the parameter space investigated.
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ever, close binaries/multiples from disk fragmentation can also evolve to wide binaries over a few initial crossing times of the stellar cluster through tidal evolution. Finding an isolated low mass wide binary system in the earliest stage of formation, before tidal evolution could occur, would prove that turbulent fragmentation is a viable mechanism for (very) low mass wide binaries. Here we report high resolution ALMA observations of a known wide-separation protostellar binary, showing that each component has a circumstellar disk. The system is too young to have evolved from a close binary and the disk axes are misaligned, providing strong support for the turbulent fragmentation model. Masses of both stars are derived from the Keplerian rotation of the disks; both are very low mass stars.
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However, whether or not such a process actually operates, or can play a significant role in the formation of planetary-mass objects, is still unclear. We do have a few examples of directly imaged objects that may have formed in this way, but we have yet to constrain how often disc fragmentation may actually form such objects. What we want to consider here is whether or not we can constrain the likely population of planetary-mass objects formed via disc fragmentation by considering how a population of objects at large radii ($a > 50$) au - if they do exist - would evolve under perturbations from more distant stellar companions. We find that there is a specific region of parameter space to which such objects would be scattered and show that the known exoplanets in that region have properties more consistent with that of the bulk exoplanet population, than with having been formed via disc fragmentation at large radii. Along with the scarcity of directly-imaged objects at large radii, our results provide a similar, but independent, constraint on the frequency of objects formed via disc fragmentation.
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