9 research outputs found
Formation of TRAPPIST-1 and other compact systems
TRAPPIST-1 is a nearby 0.08 M M-star, which was recently found to harbor a
planetary system of at least seven Earth-mass planets, all within 0.1 au. The
configuration confounds theorists as the planets are not easily explained by
either in situ or migration models. In this Paper we present a scenario for the
formation and orbital architecture of the TRAPPIST-1 system. In our model,
planet formation starts at the H2O iceline, where pebble-size particles --
whose origin is the outer disk -- concentrate to trigger streaming
instabilities. After their formation, planetary embryos quickly mature by
pebble accretion. Planet growth stalls at Earth masses, where the planet's
gravitational feedback on the disk keeps pebbles at bay. Planets are
transported by Type I migration to the inner disk, where they stall at the
magnetospheric cavity and end up in mean motion resonances. During disk
dispersal, the cavity radius expands and the inner-most planets escape
resonance. We argue that the model outlined here can also be applied to other
compact systems and that the many close-in super-Earth systems are a scaled-up
version of TRAPPIST-1. We also hypothesize that few close-in compact systems
harbor giant planets at large distances, since they would have stopped the
pebble flux from the outer disk.Comment: 8 pages, accepted for publication in A&
A Lagrangian Model for Dust Evolution in Protoplanetary Disks: Formation of Wet and Dry Planetesimals at Different Stellar Masses
We introduce a new Lagrangian smooth-particle method to model the growth and
drift of pebbles in protoplanetary disks. The Lagrangian nature of the model
makes it especially suited to follow characteristics of individual (groups of)
particles, such as their composition. In this work we focus on the water
content of solid particles. Planetesimal formation via streaming instability is
taken into account, partly based on previous results on streaming instability
outside the water snowline that were presented in Schoonenberg & Ormel (2017).
We validate our model by reproducing earlier results from the literature and
apply our model to steady-state viscous gas disks (with constant gas accretion
rate) around stars with different masses. We also present various other models
where we explore the effects of pebble accretion, the fragmentation velocity
threshold, the global metallicity of the disk, and a time-dependent gas
accretion rate. We find that planetesimals preferentially form in a local
annulus outside the water snowline, at early times in the lifetime of the disk
(), when the pebble mass fluxes are high enough to
trigger the streaming instability. During this first phase in the planet
formation process, the snowline location hardly changes due to slow viscous
evolution, and we conclude that assuming a constant gas accretion rate is
justified in this first stage. The efficiency of converting the solids
reservoir of the disk to planetesimals depends on the location of the water
snowline. Cooler disks with a closer-in water snowline are more efficient at
producing planetesimals than hotter disks where the water snowline is located
further away from the star. Therefore, low-mass stars tend to form
planetesimals more efficiently, but any correlation may be overshadowed by the
spread in disk properties.Comment: 18 pages, 15 figures, accepted for publication in A&
What pebbles are made of: Interpretation of the V883 Ori disk
Recently, an Atacama Large Millimeter/submillimeter Array (ALMA) observation
of the water snow line in the protoplanetary disk around the FU Orionis star
V883 Ori was reported. The radial variation of the spectral index at
mm-wavelengths around the snow line was interpreted as being due to a pileup of
particles interior to the snow line. However, radial transport of solids in the
outer disk operates on timescales much longer than the typical timescale of an
FU Ori outburst (-- yr). Consequently, a steady-state pileup is
unlikely. We argue that it is only necessary to consider water evaporation and
re-coagulation of silicates to explain the recent ALMA observation of V883 Ori
because these processes are short enough to have had their impact since the
outburst. Our model requires the inner disk to have already been optically
thick before the outburst, and our results suggest that the carbon content of
pebbles is low.Comment: Accepted to A&A Letter
Dark matter subhalos and unidentified sources in the Fermi 3FGL source catalog
If dark matter consists of weakly interacting massive particles (WIMPs), dark
matter subhalos in the Milky Way could be detectable as gamma-ray point sources
due to WIMP annihilation. In this work, we perform an updated study of the
detectability of dark matter subhalos as gamma-ray sources with the Fermi Large
Area Telescope (Fermi LAT). We use the results of the Via Lactea II simulation,
scaled to the Planck 2015 cosmological parameters, to predict the local dark
matter subhalo distribution. Under optimistic assumptions for the WIMP
parameters --- a 40 GeV particle annihilating to with a thermal
cross-section, as required to explain the Galactic center GeV excess --- we
predict that at most subhalos might be present in the third Fermi LAT
source catalog (3FGL). This is a smaller number than has been predicted by
prior studies, and we discuss the origin of this difference. We also compare
our predictions for the detectability of subhalos with the number of subhalo
candidate sources in 3FGL, and derive upper limits on the WIMP annihilation
cross-section as a function of the particle mass. If a dark matter
interpretation could be excluded for all 3FGL sources, our constraints would be
competitive with those found by indirect searches using other targets, such as
known Milky Way satellite galaxies.Comment: 17 pages, 8 figure
Planetesimal formation near the snowline: in or out?
Context. The formation of planetesimals in protoplanetary disks is not well-understood. Streaming instability is a promising mechanism to directly form planetesimals from pebble-sized particles, provided a high enough solids-to-gas ratio. However, local enhancements of the solids-to-gas ratio are difficult to realize in a smooth disk, which motivates the consideration of special disk locations such as the snowline – the radial distance from the star beyond which water can condense into solid ice.
Aims. In this article we investigate the viability of planetesimal formation by streaming instability near the snowline due to water diffusion and condensation. We aim to identify under what disk conditions streaming instability can be triggered near the snowline.
Methods. To this end, we adopt a viscous disk model, and numerically solve the transport equations for vapor and solids on a cylindrical, 1D grid. We take into account radial drift of solids, gas accretion on to the central star, and turbulent diffusion. We study the importance of the back-reaction of solids on the gas and of the radial variation of the mean molecular weight of the gas. Different designs for the structure of pebbles are investigated, varying in the number and size of silicate grains. We also introduce a semi-analytical model that we employ to obtain results for different disk model parameters.
Results. We find that water diffusion and condensation can locally enhance the ice surface density by a factor 3–5 outside the snowline. Assuming that icy pebbles contain many micron-sized silicate grains that are released during evaporation, the enhancement is increased by another factor ~2. In this “many-seeds” model, the solids-to-gas ratio interior to the snowline is enhanced as well, but not as much as just outside the snowline. In the context of a viscous disk, the diffusion-condensation mechanism is most effective for high values of the turbulence parameter α (10-3–10-2). Therefore, assuming young disks are more vigorously turbulent than older disks, planetesimals near the snowline can form in an early stage of the disk. In highly turbulent disks, tens of Earth masses can be stored in an annulus outside the snowline, which can be identified with recent ALMA observations
Pebble-driven planet formation for TRAPPIST-1 and other compact systems
Recently, seven Earth-sized planets were discovered around the M-dwarf star TRAPPIST-1. Thanks to transit-timing variations, the masses and therefore the bulk densities of the planets have been constrained, suggesting that all TRAPPIST-1 planets are consistent with water mass fractions on the order of 10%. These water fractions, as well as the similar planet masses within the system, constitute strong constraints on the origins of the TRAPPIST-1 system. In a previous work, we outlined a pebble-driven formation scenario. In this paper we investigate this formation scenario in more detail. We used a Lagrangian smooth-particle method to model the growth and drift of pebbles and the conversion of pebbles to planetesimals through the streaming instability. We used the N-body code MERCURY to follow the composition of planetesimals as they grow into protoplanets by merging and accreting pebbles. This code is adapted to account for pebble accretion, type-I migration, and gas drag. In this way, we modelled the entire planet formation process (pertaining to planet masses and compositions, not dynamical configuration). We find that planetesimals form in a single, early phase of streaming instability. The initially narrow annulus of planetesimals outside the snowline quickly broadens due to scattering. Our simulation results confirm that this formation pathway indeed leads to similarly-sized planets and is highly efficient in turning pebbles into planets. Our results suggest that the innermost planets in the TRAPPIST-1 system grew mostly by planetesimal accretion at an early time, whereas the outermost planets were initially scattered outwards and grew mostly by pebble accretion. The water content of planets resulting from our simulations is on the order of 10%, and our results predict a "V-shaped" trend in the planet water fraction with orbital distance: from relatively high (innermost planets) to relatively low (intermediate planets) to relatively high (outermost planets)