Migration of Low-Mass Planets

2021 ◽  
Author(s):  
Frédéric S. Masset
Keyword(s):  
Aerodynamic Drag ◽  
Semimajor Axis ◽  
Planetary Systems ◽  
High Mass ◽  
Reverse Migration ◽  
Low Viscosity ◽  
Kepler Mission ◽  
Sensitive Dependence ◽  
Low Mass ◽  
Planetary Migration

Planet migration is the variation over time of a planet’s semimajor axis, leading to either a contraction or an expansion of the orbit. It results from the exchange of energy and angular momentum between the planet and the disk in which it is embedded during its formation and can cause the semimajor axis to change by as much as two orders of magnitude over the disk’s lifetime. The migration of forming protoplanets is an unavoidable process, and it is thought to be a key ingredient for understanding the variety of extrasolar planetary systems. Although migration occurs for protoplanets of all masses, its properties for low-mass planets (those having up to a few Earth masses) differ significantly from those for high-mass planets. The torque that is exerted by the disk on the planet is composed of different contributions. While migration was first thought to be invariably inward, physical processes that are able to halt or even reverse migration were later uncovered, leading to the realization that the migration path of a forming planet has a very sensitive dependence on the underlying disk parameters. There are other processes that go beyond the case of a single planet experiencing smooth migration under the disk’s tide. This is the case of planetary migration in low-viscosity disks, a fashionable research avenue because protoplanetary disks are thought to have very low viscosity, if any, over most of their planet-forming regions. Such a process is generally significantly chaotic and has to be tackled through high-resolution numerical simulations. The migration of several low-mass planets is also is a very fashionable topic, owing to the discovery by the Kepler mission of many multiple extrasolar planetary systems. The orbital properties of these systems suggest that at least some of them have experienced substantial migration. Although there have been many studies to account for the orbital properties of these systems, there is as yet no clear picture of the different processes that shaped them. Finally, some recently unveiled processes could be important for the migration of low-mass planets. One process is aero-resonant migration, in which a swarm of planetesimals subjected to aerodynamic drag push a planet inward when they reach a mean-motion resonance with the planet, while another process is based on so-called thermal torques, which arise when thermal diffusion in the disk is taken into account, or when the planet, heated by accretion, releases heat into the ambient gas.

scholarly journals Dynamics of multiple protoplanets embedded in gas and pebble discs and its dependence on Σ and ν parameters

2018 ◽  
Vol 620 ◽  
pp. A157 ◽  
Author(s):  
M. Brož ◽  
O. Chrenko ◽  
D. Nesvorný ◽  
M. Lambrechts
Keyword(s):  
Surface Density ◽  
High Surface ◽  
Reverse Migration ◽  
Low Viscosity ◽  
Orbital Configuration ◽  
Low Mass ◽  
Convergence Zones ◽  
Hydrodynamical Simulations ◽  
Planetary Migration ◽  
Three Body

Protoplanets of super-Earth size may get trapped in convergence zones for planetary migration and form gas giants there. These growing planets undergo accretion heating, which triggers a hot-trail effect that can reverse migration directions, increase planetary eccentricities, and prevent resonant captures of migrating planets. In this work, we study populations of embryos that are accreting pebbles under different conditions, by changing the surface density, viscosity, pebble flux, mass, and the number of protoplanets. For modelling, we used the FARGO-THORIN two-dimensional (2D) hydrocode, which incorporates a pebble disc as a second pressure-less fluid, the coupling between the gas and pebbles, and the flux-limited diffusion approximation for radiative transfer. We find that massive embryos embedded in a disc with high surface density (Σ = 990 g cm−2 at 5.2 au) undergo numerous “unsuccessful” two-body encounters that do not lead to a merger. Only when a third protoplanet arrives in the convergence zone do three-body encounters lead to mergers. For a low-viscosity disc (ν = 5 × 1013 cm2 s−1), a massive co-orbital is a possible outcome, for which a pebble isolation develops and the co-orbital is further stabilised. For more massive protoplanets (5 M⊕), the convergence radius is located further out, in the ice-giant zone. After a series of encounters, there is an evolution driven by a dynamical torque of a tadpole region, which is systematically repeated several times until the co-orbital configuration is disrupted and planets merge. This may be a way to solve the problem that co-orbitals often form in simulations but they are not observed in nature. In contrast, the joint evolution of 120 low-mass protoplanets (0.1 M⊕) reveals completely different dynamics. The evolution is no longer smooth, but rather a random walk. This is because the spiral arms, developed in the gas disc due to Lindblad resonances, overlap with each other and affect not only a single protoplanet but several in the surrounding area. Our hydrodynamical simulations may have important implications for N-body simulations of planetary migration that use simplified torque prescriptions and are thus unable to capture protoplanet dynamics in its full glory.

scholarly journals Which planets trigger longer-lived vortices: low-mass or high-mass?

2021 ◽  
Author(s):  
Michael Hammer ◽  
Min-Kai Lin ◽  
Kaitlin M Kratter ◽  
Paola Pinilla
Keyword(s):  
High Mass ◽  
Hydrodynamic Simulations ◽  
Low Viscosity ◽  
Aspect Ratios ◽  
Planetary Cores ◽  
Gap Opening ◽  
Low Mass ◽  
Low Levels ◽  
Outward Migration ◽  
Protoplanetary Discs

Abstract Recent ALMA observations have found many protoplanetary discs with rings that can be explained by gap-opening planets less massive than Jupiter. Meanwhile, recent studies have suggested that protoplanetary discs should have low levels of turbulence. Past computational work on low-viscosity discs has hinted that these two developments might not be self-consistent because even low-mass planets can be accompanied by vortices instead of conventional double rings. We investigate this potential discrepancy by conducting hydrodynamic simulations of growing planetary cores in discs with various aspect ratios (H/r = 0.04, 0.06, 0.08) and viscosities (1.5 × 10−5 ≲ α ≲ 3 × 10−4), having these cores accrete their gas mass directly from the disc. With α < 10−4, we find that sub-Saturn-mass planets in discs with H/r ≤ 0.06 are more likely to be accompanied by dust asymmetries compared to Jupiter-mass planets because they can trigger several generations of vortices in succession. We also find that vortices with H/r = 0.08 survive >6000 planet orbits regardless of the planet mass or disc mass because they are less affected by the planet’s spiral waves. We connect our results to observations and find that the outward migration of vortices with H/r ≥ 0.08 may be able to explain the cavity in Oph IRS 48 or the two clumps in MWC 758. Lastly, we show that the lack of observed asymmetries in the disc population in Taurus is unexpected given the long asymmetry lifetimes in our low viscosity simulations (α ∼ 2 × 10−5), a discrepancy we suggest is due to these discs having higher viscosities.

scholarly journals Characterizing K2 Candidate Planetary Systems Orbiting Low-mass Stars. III. A High Mass and Low Envelope Fraction for the Warm Neptune K2-55b

2018 ◽  
Vol 156 (2) ◽  
pp. 70 ◽  
Author(s):  
Courtney D. Dressing ◽  
Evan Sinukoff ◽  
Benjamin J. Fulton ◽  
Eric D. Lopez ◽  
Charles A. Beichman ◽  
...  
Keyword(s):  
Planetary Systems ◽  
High Mass ◽  
Low Mass Stars ◽  
Low Mass

scholarly journals C/O vs. Mg/Si ratios in solar type stars: The HARPS sample

2018 ◽  
Vol 614 ◽  
pp. A84 ◽  
Author(s):  
L. Suárez-Andrés ◽  
G. Israelian ◽  
J. I. González Hernández ◽  
V. Zh. Adibekyan ◽  
E. Delgado Mena ◽  
...  
Keyword(s):  
Solar System ◽  
Planetary Systems ◽  
Elemental Abundance ◽  
The Other ◽  
High Mass ◽  
The Sun ◽  
Solar Type ◽  
Low Mass

Context. Aims. We aim to present a detailed study of the magnesium-to-silicon and carbon-to-oxygen ratios (Mg/Si and C/O) and their importance in determining the mineralogy of planetary companions. Methods. Using 499 solar-like stars from the HARPS sample, we determined C/O and Mg/Si elemental abundance ratios to study the nature of the possible planets formed. We separated the planetary population in low-mass planets (<30 M⊙) and high-mass planets (>30 M⊙) to test for a possible relation with the mass. Results. We find a diversity of mineralogical ratios that reveal the different kinds of planetary systems that can be formed, most of them dissimilar to our solar system. The different values of the Mg/Si and C/O can determine different composition of planets formed. We found that 100% of our planetary sample present C/O < 0.8. 86% of stars with high-mass companions present 0.8 > C/O > 0.4, while 14% present C/O values lower than 0.4. Regarding Mg/Si, all stars with low-mass planetary companion showed values between one and two, while 85% of the high-mass companion sample does. The other 15% showed Mg/Si values below one. No stars with planets were found with Mg/Si > 2. Planet hosts with low-mass companions present C/O and Mg/Si similar to those found in the Sun, whereas stars with high-mass companions have lower C/O.

scholarly journals The role of disc torques in forming resonant planetary systems

2020 ◽  
Vol 635 ◽  
pp. A204 ◽  
Author(s):  
S. Ataiee ◽  
W. Kley
Keyword(s):  
Planetary System ◽  
Planetary Systems ◽  
Accurate Method ◽  
Parameter Study ◽  
Resonant Systems ◽  
Low Mass ◽  
Hydrodynamical Simulations ◽  
Planetary Migration ◽  
Self Gravity ◽  
The Impact

Context. The most accurate method for modelling planetary migration and hence the formation of resonant systems is using hydrodynamical simulations. Usually, the force (torque) acting on a planet is calculated using the forces from the gas disc and the star, while the gas accelerations are computed using the pressure gradient, the star, and the planet’s gravity, ignoring its own gravity. For a non-migrating planet the neglect of the disc gravity results in a consistent torque calculation while for a migrating case it is inconsistent. Aims. We aim to study how much this inconsistent torque calculation can affect the final configuration of a two-planet system. We focus on low-mass planets because most of the multi-planetary systems, discovered by the Kepler survey, have masses around ten Earth masses. Methods. Performing hydrodynamical simulations of planet–disc interaction, we measured the torques on non-migrating and migrating planets for various disc masses as well as density and temperature slopes with and without considering the self-gravity of the disc. Using this data, we found a relation that quantifies the inconsistency, used this relation in an N-body code, and performed an extended parameter study modelling the migration of a planetary system with different planet mass ratios and disc surface densities, to investigate the impact of the torque inconsistency on the architecture of the planetary system. Results. Not considering disc self-gravity produces an artificially larger torque on the migrating planet that can result in tighter planetary systems. The deviation of this torque from the correct value is larger in discs with steeper surface density profiles. Conclusions. In hydrodynamical modelling of multi-planetary systems, it is crucial to account for the torque correction, otherwise the results favour more packed systems. We examine two simple correction methods existing in the literature and show that they properly correct this problem.

scholarly journals Formation of compact systems of super-Earths via dynamical instabilities and giant impacts

2019 ◽  
Vol 491 (4) ◽  
pp. 5595-5620 ◽  
Author(s):  
Sanson T S Poon ◽  
Richard P Nelson ◽  
Seth A Jacobson ◽  
Alessandro Morbidelli
Keyword(s):  
Initial Conditions ◽  
Post Processing ◽  
Significant Modification ◽  
Kepler Mission ◽  
Giant Impacts ◽  
Low Mass ◽  
In Situ Formation ◽  
Dynamical Instabilities ◽  
Gaseous Envelopes

ABSTRACT The NASA’s Kepler mission discovered ∼700 planets in multiplanet systems containing three or more transiting bodies, many of which are super-Earths and mini-Neptunes in compact configurations. Using N-body simulations, we examine the in situ, final stage assembly of multiplanet systems via the collisional accretion of protoplanets. Our initial conditions are constructed using a subset of the Kepler five-planet systems as templates. Two different prescriptions for treating planetary collisions are adopted. The simulations address numerous questions: Do the results depend on the accretion prescription?; do the resulting systems resemble the Kepler systems, and do they reproduce the observed distribution of planetary multiplicities when synthetically observed?; do collisions lead to significant modification of protoplanet compositions, or to stripping of gaseous envelopes?; do the eccentricity distributions agree with those inferred for the Kepler planets? We find that the accretion prescription is unimportant in determining the outcomes. The final planetary systems look broadly similar to the Kepler templates adopted, but the observed distributions of planetary multiplicities or eccentricities are not reproduced, because scattering does not excite the systems sufficiently. In addition, we find that ∼1 per cent of our final systems contain a co-orbital planet pair in horseshoe or tadpole orbits. Post-processing the collision outcomes suggests that they would not significantly change the ice fractions of initially ice-rich protoplanets, but significant stripping of gaseous envelopes appears likely. Hence, it may be difficult to reconcile the observation that many low-mass Kepler planets have H/He envelopes with an in situ formation scenario that involves giant impacts after dispersal of the gas disc.

scholarly journals On the emergent system mass function: the contest between accretion and fragmentation

2020 ◽  
Vol 500 (2) ◽  
pp. 1697-1707
Author(s):  
Paul C Clark ◽  
Anthony P Whitworth
Keyword(s):  
Star Formation ◽  
Standard Deviation ◽  
Mass Distribution ◽  
Power Law ◽  
Accretion Rate ◽  
Mass Function ◽  
High Mass ◽  
New Model ◽  
System Formation ◽  
Low Mass

ABSTRACT We propose a new model for the evolution of a star cluster’s system mass function (SMF). The model involves both turbulent fragmentation and competitive accretion. Turbulent fragmentation creates low-mass seed proto-systems (i.e. single and multiple protostars). Some of these low-mass seed proto-systems then grow by competitive accretion to produce the high-mass power-law tail of the SMF. Turbulent fragmentation is relatively inefficient, in the sense that the creation of low-mass seed proto-systems only consumes a fraction, ${\sim }23{{\ \rm per\ cent}}$ (at most ${\sim }50{{\ \rm per\ cent}}$), of the mass available for star formation. The remaining mass is consumed by competitive accretion. Provided the accretion rate on to a proto-system is approximately proportional to its mass (dm/dt ∝ m), the SMF develops a power-law tail at high masses with the Salpeter slope (∼−2.3). If the rate of supply of mass accelerates, the rate of proto-system formation also accelerates, as appears to be observed in many clusters. However, even if the rate of supply of mass decreases, or ceases and then resumes, the SMF evolves homologously, retaining the same overall shape, and the high-mass power-law tail simply extends to ever higher masses until the supply of gas runs out completely. The Chabrier SMF can be reproduced very accurately if the seed proto-systems have an approximately lognormal mass distribution with median mass ${\sim } 0.11 \, {\rm M}_{\odot }$ and logarithmic standard deviation $\sigma _{\log _{10}({M/M}_\odot)}\sim 0.47$).

scholarly journals The formation of high-mass black holes in low-mass X-ray binaries

New Astronomy ◽  
1999 ◽  
Vol 4 (4) ◽  
pp. 313-323 ◽  
Author(s):  
G.E. Brown ◽  
C.-H. Lee ◽  
Hans A. Bethe
Keyword(s):  
Black Holes ◽  
High Mass ◽  
X Ray ◽  
Low Mass ◽  
X Ray Binaries

scholarly journals Planetary populations according to the orbital angular momentum

2009 ◽  
Vol 5 (S265) ◽  
pp. 420-421
Author(s):  
João A. S. Amarante ◽  
Helio J. Rocha-Pinto
Keyword(s):  
Angular Momentum ◽  
Solar System ◽  
Orbital Angular Momentum ◽  
Momentum Distribution ◽  
Semimajor Axis ◽  
Terrestrial Planets ◽  
Planetary Mass ◽  
Exoplanetary Systems ◽  
Planetary Migration ◽  
Two Populations

AbstractWe investigate the angular momentum distribution of known exoplanetary systems, as a function of the planetary mass, orbital semimajor axis and metallicity of the host star. We find exoplanets seems to be classified according to at least two ‘populations’, with respect to their angular momentum properties. This classification is independent on the composition of the planet and seems to be valid for both jovian and neptunian planets, and probably can be extrapolated to the terrestrial planets of the Solar System. We analyse these ‘populations’ considering the phenomenon of planetary migration.

scholarly journals ATLASGAL – Ammonia observations towards the southern Galactic plane

2018 ◽  
Vol 609 ◽  
pp. A125 ◽  
Author(s):  
M. Wienen ◽  
F. Wyrowski ◽  
K. M. Menten ◽  
J. S. Urquhart ◽  
C. M. Walmsley ◽  
...  
Keyword(s):  
Standard Deviation ◽  
Hyperfine Structure ◽  
Optical Depth ◽  
Line Width ◽  
Rotational Temperature ◽  
High Mass ◽  
Mass Star ◽  
Star Forming ◽  
Star Forming Regions ◽  
Low Mass

Context. The initial conditions of molecular clumps in which high-mass stars form are poorly understood. In particular, a more detailed study of the earliest evolutionary phases is needed. The APEX Telescope Large Area Survey of the whole inner Galactic disk at 870 μm, ATLASGAL, has therefore been conducted to discover high-mass star-forming regions at different evolutionary phases. Aims. We derive properties such as velocities, rotational temperatures, column densities, and abundances of a large sample of southern ATLASGAL clumps in the fourth quadrant. Methods. Using the Parkes telescope, we observed the NH3 (1, 1) to (3, 3) inversion transitions towards 354 dust clumps detected by ATLASGAL within a Galactic longitude range between 300° and 359° and a latitude within ± 1.5°. For a subsample of 289 sources, the N2H+ (1–0) line was measured with the Mopra telescope. Results. We measured a median NH3 (1, 1) line width of ~ 2 km s-1, rotational temperatures from 12 to 28 K with a mean of 18 K, and source-averaged NH3 abundances from 1.6 × 10-6 to 10-8. For a subsample with detected NH3 (2, 2) hyperfine components, we found that the commonly used method to compute the (2, 2) optical depth from the (1, 1) optical depth and the (2, 2) to (1, 1) main beam brightness temperature ratio leads to an underestimation of the rotational temperature and column density. A larger median virial parameter of ~ 1 is determined using the broader N2H+ line width than is estimated from the NH3 line width of ~ 0.5 with a general trend of a decreasing virial parameter with increasing gas mass. We obtain a rising NH3 (1, 1)/N2H+ line-width ratio with increasing rotational temperature. Conclusions. A comparison of NH3 line parameters of ATLASGAL clumps to cores in nearby molecular clouds reveals smaller velocity dispersions in low-mass than high-mass star-forming regions and a warmer surrounding of ATLASGAL clumps than the surrounding of low-mass cores. The NH3 (1, 1) inversion transition of 49% of the sources shows hyperfine structure anomalies. The intensity ratio of the outer hyperfine structure lines with a median of 1.27 ± 0.03 and a standard deviation of 0.45 is significantly higher than 1, while the intensity ratios of the inner satellites with a median of 0.9 ± 0.02 and standard deviation of 0.3 and the sum of the inner and outer hyperfine components with a median of 1.06 ± 0.02 and standard deviation of 0.37 are closer to 1.

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