scholarly journals Pebble accretion in self-gravitating protostellar discs

2019 ◽  
Vol 485 (4) ◽  
pp. 4465-4473
Author(s):  
D H Forgan
Keyword(s):  
Time Scales ◽  
Planet Formation ◽  
Density Ratio ◽  
Streaming Instability ◽  
Planetary Cores ◽  
Low Mass ◽  
Spiral Structures ◽  
Protostellar Discs ◽  
Core Accretion ◽  
Solid Bodies

Abstract Pebble accretion has become a popular component to core accretion models of planet formation, and is especially relevant to the formation of compact, resonant terrestrial planetary systems. Pebbles initially form in the inner protoplanetary disc, sweeping outwards in a radially expanding front, potentially forming planetesimals and planetary cores via migration and the streaming instability. This pebble front appears at early times, in what is typically assumed to be a low-mass disc. We argue this picture is in conflict with the reality of young circumstellar discs, which are massive and self-gravitating. We apply standard pebble accretion and streaming instability formulae to self-gravitating protostellar disc models. Fragments will open a gap in the pebble disc, but they will likely fail to open a gap in the gas, and continue rapid inward migration. If this does not strongly perturb the pebble disc, our results show that disc fragments will accrete pebbles efficiently. We find that in general the pebble-to-gas-density ratio fails to exceed 0.01, suggesting that the streaming instability will struggle to operate. It may be possible to activate the instability if 10 cm grains are available, and spiral structures can effectively concentrate them in regions of low gravito-turbulence. If this occurs, lunar mass cores might be assembled on time-scales of a few thousand years, but this is likely to be rare, and is far from proven. In any case, this work highlights the need for study of how self-gravitating protostellar discs define the distribution and properties of solid bodies, for future planet formation by core accretion.

scholarly journals Planet formation around M dwarfs via disc instability

2020 ◽  
Vol 633 ◽  
pp. A116
Author(s):  
Anthony Mercer ◽  
Dimitris Stamatellos
Keyword(s):  
Planet Formation ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Dwarf Stars ◽  
Low Mass ◽  
Protostellar Discs ◽  
Stellar Masses ◽  
M Dwarf Stars ◽  
Core Accretion ◽  
M Dwarf

Context. Around 30 per cent of the observed exoplanets that orbit M dwarf stars are gas giants that are more massive than Jupiter. These planets are prime candidates for formation by disc instability. Aims. We want to determine the conditions for disc fragmentation around M dwarfs and the properties of the planets that are formed by disc instability. Methods. We performed hydrodynamic simulations of M dwarf protostellar discs in order to determine the minimum disc mass required for gravitational fragmentation to occur. Different stellar masses, disc radii, and metallicities were considered. The mass of each protostellar disc was steadily increased until the disc fragmented and a protoplanet was formed. Results. We find that a disc-to-star mass ratio between ~0.3 and ~0.6 is required for fragmentation to happen. The minimum mass at which a disc fragment increases with the stellar mass and the disc size. Metallicity does not significantly affect the minimum disc fragmentation mass but high metallicity may suppress fragmentation. Protoplanets form quickly (within a few thousand years) at distances around ~50 AU from the host star, and they are initially very hot; their centres have temperatures similar to the ones expected at the accretion shocks around planets formed by core accretion (up to 12 000 K). The final properties of these planets (e.g. mass and orbital radius) are determined through long-term disc-planet or planet–planet interactions. Conclusions. Disc instability is a plausible way to form gas giant planets around M dwarfs provided that discs have at least 30% the mass of their host stars during the initial stages of their formation. Future observations of massive M dwarf discs or planets around very young M dwarfs are required to establish the importance of disc instability for planet formation around low-mass stars.

scholarly journals New Low-mass Accretors in the Scorpius-Centaurus OB Association

2015 ◽  
Vol 10 (S314) ◽  
pp. 58-62
Author(s):  
Simon J. Murphy ◽  
Warrick A. Lawson ◽  
Joao Bento
Keyword(s):  
Time Scales ◽  
Planet Formation ◽  
Formation Time ◽  
Lower Mass ◽  
Circumstellar Material ◽  
Hα Emission ◽  
Low Mass

AbstractWe describe the serendipitous discovery of two new lithium-rich M5 members of the Scorpius-Centaurus OB Association (Sco-Cen). Both stars exhibit large 12 and 22 μm excesses and strong, variable Hα emission which we attribute to accretion from circumstellar discs. Such stars are thought to be incredibly rare at the ~16 Myr median age of much of Sco-Cen. The serendipitous discovery of two accreting stars hosting large quantities of circumstellar material may be indicative of a sizeable age spread in Sco-Cen, or further evidence that disc dispersal and planet formation time-scales are longer around lower-mass stars.

scholarly journals Understanding exoplanet formation, structure and evolution in 2010

2010 ◽  
Vol 6 (S276) ◽  
pp. 171-180
Author(s):  
Gilles Chabrier ◽  
Jérémy Leconte ◽  
Isabelle Baraffe
Keyword(s):  
Planet Formation ◽  
Large Fraction ◽  
Brown Dwarfs ◽  
Short Review ◽  
Transiting Planets ◽  
The Core ◽  
Theoretical Mass ◽  
Low Mass ◽  
Core Accretion ◽  
Present Understanding

AbstractIn this short review, we summarize our present understanding (and non-understanding) of exoplanet formation, structure and evolution, in the light of the most recent discoveries. Recent observations of transiting massive brown dwarfs seem to remarkably confirm the predicted theoretical mass-radius relationship in this domain. This mass-radius relationship provides, in some cases, a powerful diagnostic to distinguish planets from brown dwarfs of same mass, as for instance for Hat-P-20b. If confirmed, this latter observation shows that planet formation takes place up to at least 8 Jupiter masses. Conversely, observations of brown dwarfs down to a few Jupiter masses in young, low-extinction clusters strongly suggests an overlapping mass domain between (massive) planets and (low-mass) brown dwarfs, i.e. no mass edge between these two distinct (in terms of formation mechanism) populations. At last, the large fraction of heavy material inferred for many of the transiting planets confirms the core-accretion scenario as been the dominant one for planet formation.

scholarly journals A giant exoplanet orbiting a very-low-mass star challenges planet formation models

Science ◽  
2019 ◽  
Vol 365 (6460) ◽  
pp. 1441-1445 ◽  
Author(s):  
J. C. Morales ◽  
A. J. Mustill ◽  
I. Ribas ◽  
M. B. Davies ◽  
A. Reiners ◽  
...  
Keyword(s):  
Near Infrared ◽  
Planet Formation ◽  
Giant Planet ◽  
Mass Star ◽  
Low Mass Stars ◽  
Migration Rates ◽  
Low Mass ◽  
And Migration ◽  
Core Accretion ◽  
Planet Accretion

Surveys have shown that super-Earth and Neptune-mass exoplanets are more frequent than gas giants around low-mass stars, as predicted by the core accretion theory of planet formation. We report the discovery of a giant planet around the very-low-mass star GJ 3512, as determined by optical and near-infrared radial-velocity observations. The planet has a minimum mass of 0.46 Jupiter masses, very high for such a small host star, and an eccentric 204-day orbit. Dynamical models show that the high eccentricity is most likely due to planet-planet interactions. We use simulations to demonstrate that the GJ 3512 planetary system challenges generally accepted formation theories, and that it puts constraints on the planet accretion and migration rates. Disk instabilities may be more efficient in forming planets than previously thought.

scholarly journals Connecting substellar and stellar formation: the role of the host star’s metallicity

2019 ◽  
Vol 624 ◽  
pp. A94 ◽  
Author(s):  
J. Maldonado ◽  
E. Villaver ◽  
C. Eiroa ◽  
G. Micela
Keyword(s):  
Formation Mechanism ◽  
Formation Process ◽  
Planet Formation ◽  
Maximum Efficiency ◽  
Stellar Formation ◽  
Wide Range ◽  
Solar Type ◽  
Low Metallicity ◽  
Low Mass ◽  
Core Accretion

Context. Most of our current understanding of the planet formation mechanism is based on the planet metallicity correlation derived mostly from solar-type stars harbouring gas-giant planets. Aims. To achieve a more extensive grasp on the substellar formation process, we aim to analyse in terms of their metallicity a diverse sample of stars (in terms of mass and spectral type) covering the whole range of possible outcomes of the planet formation process (from planetesimals to brown dwarfs and low-mass binaries). Methods. Our methodology is based on the use of high-precision stellar parameters derived by our own group in previous works from high-resolution spectra by using the iron ionisation and equilibrium conditions. All values were derived in an homogeneous way, except for the M dwarfs where a methodology based on the use of pseudo equivalent widths of spectral features was used. Results. Our results show that as the mass of the substellar companion increases the metallicity of the host star tends to lower values. The same trend is maintained when analysing stars with low-mass stellar companions and a tendency towards a wide range of host star’s metallicity is found for systems with low-mass planets. We also confirm that more massive planets tend to orbit around more massive stars. Conclusions. The core-accretion formation mechanism for planet formation achieves its maximum efficiency for planets with masses in the range 0.2–2 MJup. Substellar objects with higher masses have higher probabilities of being formed as stars. Low-mass planets and planetesimals might be formed by core-accretion even around low-metallicity stars.

scholarly journals SPH simulations of star/planet formation triggered by cloud-cloud collisions

2007 ◽  
Vol 3 (S249) ◽  
pp. 271-278 ◽  
Author(s):  
Spyridon Kitsionas ◽  
Anthony P. Whitworth ◽  
Ralf S. Klessen
Keyword(s):  
Star Formation ◽  
Planet Formation ◽  
Mass Range ◽  
Stellar Mass ◽  
Hydrodynamic Simulations ◽  
Early Stages ◽  
Gravitational Instabilities ◽  
Planetary Mass ◽  
Low Mass ◽  
Protostellar Discs

AbstractWe present results of hydrodynamic simulations of star formation triggered by cloud-cloud collisions. During the early stages of star formation, low-mass objects form by gravitational instabilities in protostellar discs. A number of these low-mass objects are in the sub-stellar mass range, including a few objects of planetary mass. The disc instabilities that lead to the formation of low-mass objects in our simulations are the product of disc-disc interactions and/or interactions between the discs and their surrounding gas.

scholarly journals The Story of Planets: Anchoring Numerics in Reality

2013 ◽  
Vol 8 (S299) ◽  
pp. 123-130
Author(s):  
Zoë M. Leinhardt
Keyword(s):  
Numerical Simulations ◽  
Extrasolar Planets ◽  
Planet Formation ◽  
Planetary System ◽  
Numerical Models ◽  
Building Blocks ◽  
Solar Mass ◽  
Hot Jupiters ◽  
Planetary Cores ◽  
Low Mass

AbstractBuilding a complete coherent model of planet formation has proven difficult. There are gaps in the observational record, difficult physical processes that we have yet to fully understand, such as planetesimal formation, and an extensive list of observationally determined constraints that the model must fulfil. For example, the diversity of extrasolar planets detected to date is staggering – from single hot-Jupiters to multiple planet systems with several tightly packed super-Earths. In addition, the characteristics of the host stars are broad from single solar-mass stars to tight binaries and low mass, low metalicity stars. Even more surprising, perhaps, is the frequency of detection and thus, the implied efficiency of the planet formation process. Any theoretical model must not just be able to explain how planets form but must also explain the frequency and diversity of planetary systems. So why is planet formation so prolific? What parameters determine the type of planetary system that will result? How important are the initial parameters of the protoplanetary disk, such as composition, versus stochastic effects, such as gravitational scattering events, that occur during the evolution of the planetary system?Current observations of extrasolar planets provide snapshots in time of the earliest and latest stages of planet formation but do not show the evolution between the two. It is at this point that we must rely on numerical models to evolve proto-planetary disks into planets. But how can we validate the results of our numerical simulations if the middle stages of planet formation are effectively invisible? Collisions are a core component of planet formation. Planetesimals, the building blocks of planets, collide with one another as they grow and evolve into planets or planetary cores and are viscously stirred by larger protoplanets and fully-formed planets. The range of impact parameters encountered during growth from planetesimals to planets span multiple collision outcome regimes: cratering, merging, disruption, and hit-and-run events. Most of these collisions produce significant debris and dust. If we have a good understanding of the production of collisional debris we can use it as an indirect tracer of on-going planetary evolution even if the planets themselves are not directly detectable.In this paper I will show how numerical simulations of planet formation including realistic collision modelling can be used to predict, and be constrained by, observations.

scholarly journals Planet formation and disc mass dependence in a pebble-driven scenario for low-mass stars

2020 ◽  
Vol 499 (3) ◽  
pp. 3510-3521
Author(s):  
Spandan Dash ◽  
Yamila Miguel
Keyword(s):  
Planet Formation ◽  
Type I ◽  
Mass Dependence ◽  
M Dwarfs ◽  
Low Mass Stars ◽  
Mass Distributions ◽  
Streaming Instability ◽  
Resonant Systems ◽  
Low Mass ◽  
Initial Distributions

ABSTRACT Measured disc masses seem to be too low to form the observed population of planetary systems. In this context, we develop a population synthesis code in the pebble accretion scenario, to analyse the disc mass dependence on planet formation around low-mass stars. We base our model on the analytical sequential model presented by Ormel, Liu, and Schoonenberg and analyse the populations resulting from varying initial disc mass distributions. Starting out with seeds the mass of Ceres formed by streaming instability inside the ice-line, we grow the planets using the pebble accretion process and migrate them inwards using type I migration. The next planets are formed sequentially after the previous planet crosses the ice line. We explore different initial distributions of disc masses to show the dependence of this parameter with the final planetary population. Our results show that compact close-in resonant systems can be pretty common around M dwarfs between 0.09 and 0.2 M⊙ only when the discs considered are more massive than what is being observed by sub-mm disc surveys. The minimum disc mass to form a Mars-like planet is found to be about 2 × 10−3 M⊙. Small variations in the disc mass distribution also manifest in the simulated planet distribution. The paradox of disc masses might be caused by an underestimation of the disc masses in observations, by a rapid depletion of mass in discs by planets growing within 1 million years, or by deficiencies in our current planet formation picture.

scholarly journals Pebble-driven planet formation around very low-mass stars and brown dwarfs

2020 ◽  
Vol 638 ◽  
pp. A88 ◽  
Author(s):  
Beibei Liu ◽  
Michiel Lambrechts ◽  
Anders Johansen ◽  
Ilaria Pascucci ◽  
Thomas Henning
Keyword(s):  
Mass Fraction ◽  
Planet Formation ◽  
Water Mass ◽  
Giant Planet ◽  
Brown Dwarfs ◽  
Mass Range ◽  
Protoplanetary Disk ◽  
Low Mass Stars ◽  
Streaming Instability ◽  
Low Mass

We conduct a pebble-driven planet population synthesis study to investigate the formation of planets around very low-mass stars and brown dwarfs in the (sub)stellar mass range between 0.01 M⊙ and 0.1 M⊙. Based on the extrapolation of numerical simulations of planetesimal formation by the streaming instability, we obtain the characteristic mass of the planetesimals and the initial mass of the protoplanet (largest body from the planetesimal populations), in either the early self-gravitating phase or the later non-self-gravitating phase of the protoplanetary disk evolution. We find that the initial protoplanets form with masses that increase with host mass and orbital distance, and decrease with age. Around late M-dwarfs of 0.1 M⊙, these protoplanets can grow up to Earth-mass planets by pebble accretion. However, around brown dwarfs of 0.01 M⊙, planets do not grow to the masses that are greater than Mars when the initial protoplanets are born early in self-gravitating disks, and their growth stalls at around 0.01 Earth-mass when they are born late in non-self-gravitating disks. Around these low-mass stars and brown dwarfs we find no channel for gas giant planet formation because the solid cores remain too small. When the initial protoplanets form only at the water-ice line, the final planets typically have ≳15% water mass fraction. Alternatively, when the initial protoplanets form log-uniformly distributed over the entire protoplanetary disk, the final planets are either very water rich (water mass fraction ≳15%) or entirely rocky (water mass fraction ≲5%).

scholarly journals Polydisperse streaming instability – III. Dust evolution encourages fast instability

2021 ◽  
Vol 502 (1) ◽  
pp. 1469-1486
Author(s):  
Colin P McNally ◽  
Francesco Lovascio ◽  
Sijme-Jan Paardekooper
Keyword(s):  
Power Law ◽  
Planet Formation ◽  
Full Range ◽  
Density Ratio ◽  
Stopping Times ◽  
Physical Parameters ◽  
Streaming Instability ◽  
Dust Distribution ◽  
Dust Evolution ◽  
Single Size

ABSTRACT Planet formation via core accretion requires the production of kilometre-sized planetesimals from cosmic dust. This process must overcome barriers to simple collisional growth, for which the streaming instability (SI) is often invoked. Dust evolution is still required to create particles large enough to undergo vigorous instability. The SI has been studied primarily with single-size dust, and the role of the full evolved dust distribution is largely unexplored. We survey the polydisperse streaming instability (PSI) with physical parameters corresponding to plausible conditions in protoplanetary discs. We consider a full range of particle stopping times, generalized dust size distributions, and the effect of turbulence. We find that while the PSI grows in many cases more slowly with an interstellar power-law dust distribution than with a single size, reasonable collisional dust evolution, producing an enhancement of the largest dust sizes, produces instability behaviour similar to the monodisperse case. Considering turbulent diffusion, the trend is similar. We conclude that if fast linear growth of PSI is required for planet formation, then dust evolution producing a distribution with peak stopping times on the order of 0.1 orbits and an enhancement of the largest dust significantly above the single power-law distribution produced by a fragmentation cascade is sufficient, along with local enhancement of the dust to gas volume mass density ratio to order unity.

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