Astrophysics: Variable snow lines affect planet formation.

The formation of planets depends on the underlying protoplanetary disc structure, which influences both the accretion and migration rates of embedded planets. The disc itself evolves on time-scales of several Myr during which both temperature and density profiles change as matter accretes onto the central star. Here we use a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model the emergence of planetary embryos preferably occurs after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time-scales are too long to compete with the migration time-scales and the dissipation time of the protoplanetary disc. Altogether, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs.

The formation of planets depends on the underlying protoplanetary disc structure, which influences both the accretion and migration rates of embedded planets. The disc itself evolves on time-scales of several Myr during which both temperature and density profiles change as matter accretes onto the central star. Here we use a detailed model of an evolving disc to determine the growth of planets by pebble accretion and their migration through the disc. Cores that reach their pebble isolation mass accrete gas to finally form giant planets with extensive gas envelopes, while planets that do not reach pebble isolation mass are stranded as ice giants and ice planets containing only minor amounts of gas in their envelopes. Unlike earlier population synthesis models, our model works without any artificial reductions in migration speed and for protoplanetary discs with gas and dust column densities similar to those inferred from observations. We find that in our nominal disc model the emergence of planetary embryos preferably occurs after approximately 2 Myr in order to not exclusively form gas giants, but also ice giants and smaller planets. The high pebble accretion rates ensure that critical core masses for gas accretion can be reached at all orbital distances. Gas giant planets nevertheless experience significant reduction in semi-major axes by migration. Considering instead planetesimal accretion for planetary growth, we show that formation time-scales are too long to compete with the migration time-scales and the dissipation time of the protoplanetary disc. Altogether, we find that pebble accretion overcomes many of the challenges in the formation of ice and gas giants in evolving protoplanetary discs.

view Abstract Citations (25) References (27) Co-Reads Similar Papers Volume Content Graphics Metrics Export Citation NASA/ADS Evolution of Magnetized Protoplanetary Disks Reyes-Ruiz, Mauricio ; Stepinski, Tomasz F. Abstract We investigate the global evolution of a turbulent protoplanetary disk in its viscous stage, incorporating the effects of Maxwell stress due to a large-scale magnetic field permeating disk. We assume that the viscous stress is given by an alpha model. A magnetic field is produced contemporaneously by an alpha omega dynamo mechanism and the resultant Maxwell stress assists the viscous stress in providing the means for disk evolution. The aim of this work is to compare the evolution of magnetized and nonmagnetized disks driven by turbulent viscosity of the same magnitude and thus assess the effects of a self-generated magnetic field on the structure and dynamical evolution of protoplanetary disks. Two illustrative examples corresponding to two different initial conditions are considered: a high-mass case that starts with a disk of 0.245 solar mass and angular momentum of 5.6 x 1052g sq cm/s, and a low-mass that case starts with a disk of 0.11 solar mass and angular momentum of 1.8 x 1052g sq cm/s. For each of these two cases the radial development of a disk is calculated numerically assuming a fiducial value of the dimensionless viscosity parameter alphass = 0.01, as well as alphass = 2 x 10-3. In all cases the central star has a mass equal to 1 solar mass. The most striking feature of magnetized disk evolution is the presence of the surface density bulge located in the region of the disk where the dynamo mechanism cannot support a magnetic field. The bulge persists for a time of the order of 105-106 yr. The presence and persistence of the surface density bulge may have important implications for the process of planet formation and the overall characteristics of resultant planetary systems. Publication: The Astrophysical Journal Pub Date: January 1995 DOI: 10.1086/175120 Bibcode: 1995ApJ...438..750R Keywords: Accretion Disks; Magnetic Fields; Planetary Evolution; Protoplanets; Viscosity; Finite Difference Theory; Ionization; Planetary Systems; Turbulence; Astrophysics; ACCRETION; ACCRETION DISKS; MAGNETOHYDRODYNAMICS: MHD; STARS: PLANETARY SYSTEMS full text sources ADS |

In the standard scenario of planet formation, terrestrial planets, ice giants, and cores of gas giants are formed by the accumulation of planetesimals. However, there are few N-body simulation studies of planetesimal accretion that correctly take into account the merging condition of planetesimals. In order to investigate a realistic accretion process of planetesimals, it is necessary to clarify the merging criteria of planetesimals at collision. We perform numerical collision experiments using smoothed particle hydrodynamics and obtain the merging criteria as a function of planetesimal mass and impact parameters for undifferentiated rocky and icy planetesimals and differentiated icy planetesimals. We vary the total mass of colliding planetesimals, their mass ratios, and the impact angle and obtain the critical impact velocity as the merging criteria distinguishing merging from hit-and-run collision. We find that the critical impact velocity normalized by the two-body surface escape velocity decreases with increasing impact angle. The critical impact velocity does not depend on the total mass, while it has a weak positive dependence on the mass ratio. These results barely depend on the composition and internal structure of the planetesimals.

Context. The study of protoplanetary disc evolution and theories of planet formation has predominantly concentrated on solar- (and low-) mass stars since they host the majority of confirmed exoplanets. Nevertheless, the confirmation of numerous planets orbiting stars more massive than the Sun (up to ~3 M⊙) has sparked considerable interest in understanding the mechanisms involved in their formation, and thus in the evolution of their hosting protoplanetary discs. Aims. We aim to improve our knowledge of the evolution of the gaseous component of protoplanetary discs around intermediate-mass stars and to set the stage for future studies of planet formation around them. Methods. We study the long-term evolution of protoplanetary discs affected by viscous accretion and photoevaporation by X-ray and far-ultraviolet (FUV) photons from the central star around stars in the range of 1–3 M⊙, considering the effects of stellar evolution and solving the vertical structure equations of the disc. We explore the effect of different values of the viscosity parameter and the initial mass of the disc. Results. We find that the evolutionary pathway of protoplanetary disc dispersal due to photoevaporation depends on the stellar mass. Our simulations reveal four distinct evolutionary pathways for the gas component not reported before that are a consequence of stellar evolution and that likely have a substantial impact on the dust evolution, and thus on planet formation. As the stellar mass increases from one solar mass to ~1.5–2 M⊙, the evolution of the disc changes from the conventional inside-out clearing, in which X-ray photoevaporation generates inner holes, to a homogeneous disc evolution scenario where both inner and outer discs formed after a gap is opened by photoevaporation vanish over a similar timescale. As the stellar mass continues to increase, reaching ~2–3 M⊙, we identify a distinct pathway that we refer to as revenant disc evolution. In this scenario, the inner and outer discs reconnect after the gap opened. For the largest masses, we observe outside-in disc dispersal, in which the outer disc dissipates first due to a stronger FUV photoevaporation rate. Revenant disc evolution stands out as it is capable of extending the disc lifespan. Otherwise, the disc dispersal timescale decreases with increasing stellar mass except for low-viscosity discs.

We have obtained 8-13 ?m spectra of 30 young (1-10?Myr) low-mass pre-main-sequence stars using COMICS on the 8.2?m Subaru Telescope to examine dust evolution in protoplanetary disks. Most spectra show silicate emission features of various strengths and shapes, indicative of dust processing during the different stages of protoplanetary disk evolution. We have analyzed the observed silicate emission features using a simple model previously applied to the more massive and luminous Herbig Ae/Be systems. We determined the feature strength and shape and derived the composition and typical size of the silicate dust grains. We confirm the previously reported dependency of the silicate feature strength and shape on the grain size of the amorphous silicate dust. We examine the relation between the derived dust properties and stellar and circumstellar disk parameters, such as systemic age, luminosity of H? (LH?), disk mass, and opacity power-law index ? at radio wavelengths. A possible relation is found between silicate feature strength (grain size indicator) and the LH?, which may be an indicator of accretion activity. It implies that the turbulence induced by accretion activity may be important for grain size evolution in the disk. No clear correlation between the crystallinity and the stellar/disk parameters is found. We find that on average 5%-20% in mass of the silicate dust grains is in crystalline form, irrespective of systemic age. This latter finding supports the idea that crystalline silicate is formed at an early evolutionary phase, probably at the protostellar phase, and is remaining during the later stages.

Context. Current models of the size- and radial evolution of dust in\nprotoplanetary disks generally oversimplify either the radial evolution of the\ndisk (by focussing at one single radius or by using steady state disk models)\nor they assume particle growth to proceed monodispersely or without\nfragmentation. Further studies of protoplanetary disks - such as observations,\ndisk chemistry and structure calculations or planet population synthesis models\n- depend on the distribution of dust as a function of grain size and radial\nposition in the disk.\n Aims. We attempt to improve upon current models to be able to investigate how\nthe initial conditions, the build-up phase, and the evolution of the\nprotoplanetary disk influence growth and transport of dust.\n Methods. We introduce a new version of the model of Brauer et al. (2008) in\nwhich we now include the time-dependent viscous evolution of the gas disk, and\nin which more advanced input physics and numerical integration methods are\nimplemented.\n Results. We show that grain properties, the gas pressure gradient, and the\namount of turbulence are much more influencing the evolution of dust than the\ninitial conditions or the build-up phase of the protoplanetary disk. We\nquantify which conditions or environments are favorable for growth beyond the\nmeter size barrier. High gas surface densities or zonal flows may help to\novercome the problem of radial drift, however already a small amount of\nturbulence poses a much stronger obstacle for grain growth.\n

Context. The final stages of a protoplanetary disk are essential for our understanding of the formation and evolution of planets. Photoevaporation is an important mechanism that contributes to the dispersal of an accretion disk and has significant consequences for the disk’s lifetime. However, the combined effects of photoevaporation and star-disk interaction have not been investigated in previous studies. Aims. A photoevaporative disk evolution model including a detailed formulation of the inner star-disk interaction region will improve the understanding of the final stages of disk evolution. Methods. We combined an implicit disk evolution model with a photoevaporative mass-loss profile. By including the innermost disk regions down to 0.01 AU, we could calculate the star-disk interaction, the stellar spin evolution, and the transition from an accreting disk to the propeller regime self-consistently. Starting from an early Class II star-disk system (with an age of 1 Myr), we calculated the long-term evolution of the system until the disk becomes almost completely dissolved. Results. Photoevaporation has a significant effect on disk structure and evolution. The radial extent of the dead zone decreases, and the number of episodic accretion events (outbursts) is reduced by high stellar X-ray luminosities. Reasonable accretion rates (>10−8 M⊙yr) in combination with photoevaporative gaps are possible for a dead zone that is still massive enough to develop episodic accretion events. Furthermore, the stellar spin evolution during the Class II evolution is less affected by the star-disk interaction in the case of high X-ray luminosities. Conclusions. Our results suggest that the formation of planets, especially habitable planets, in the dead zone is strongly impaired in the case of strong X-ray luminosities. Additionally, the importance of the star-disk interaction during the Class II phase with respect to the stellar spin evolution is reduced.

Aims. The central region of a circumstellar disk is difficult to resolve in global numerical simulations of collapsing cloud cores, but its effect on the evolution of the entire disk can be significant. Methods. We used numerical hydrodynamics simulations to model the long-term evolution of self-gravitating and viscous circumstellar disks in the thin-disk limit. Simulations start from the gravitational collapse of pre-stellar cores of 0.5–1.0 M⊙ and both gaseous and dusty subsystems were considered, including a model for dust growth. The inner unresolved 1.0 au of the disk is replaced with a central smart cell (CSC), a simplified model that simulates physical processes that may occur in this region. Results. We found that the mass transport rate through the CSC has an appreciable effect on the evolution of the entire disk. Models with slow mass transport form more massive and warmer disks, and are more susceptible to gravitational instability and fragmentation, including a newly identified episodic mode of disk fragmentation in the T Tauri phase of disk evolution. Models with slow mass transport through the CSC feature episodic accretion and luminosity bursts in the early evolution, while models with fast transport are characterized by a steadily declining accretion rate with low-amplitude flickering. Dust grows to a larger, decimeter size in the slow transport models and efficiently drifts in the CSC, where it accumulates and reaches the limit where a streaming instability becomes operational. We argue that gravitational instability, together with a streaming instability likely operating in the inner disk regions, constitute two concurrent planet-forming mechanisms, which may explain the observed diversity of exoplanetary orbits. Conclusions. We conclude that sophisticated models of the inner unresolved disk regions should be used when modeling the formation and evolution of gaseous and dusty protoplanetary disks.

Planetary systems are thought to be born in protoplanetary disks. Isotope ratios are a powerful tool for investigating the material origin and evolution from molecular clouds to planetary systems via protoplanetary disks. However, it is challenging to measure the isotope (isotopologue) ratios, especially in protoplanetary disks, because the emission lines of major species are saturated. We developed a new method to overcome these challenges by using optically thin line wings induced by thermal broadening. As a first application of the method, we analyzed two carbon monoxide isotopologue lines, 12CO 3–2 and 13CO 3–2, from archival observations of a protoplanetary disk around TW Hya with the Atacama Large Millimeter/submillimeter Array. The 12CO/13CO ratio was estimated to be 21 ± 5 at disk radii of 70–110 au, which is significantly smaller than the value observed in the local interstellar medium, ∼69. It implies that an isotope exchange reaction occurs in a low-temperature environment with C/O > 1. In contrast, it is suggested that 12CO/13CO is higher than ∼84 in the outer disk (r > 130 au), which can be explained by the difference in the binding energy of the isotopologues on dust grains and the CO gas depletion processes. Our results imply that the gas-phase 12CO/13CO can vary by a factor of >4 even inside a protoplanetary disk and therefore can be used to trace material evolution in disks.

The Atacama Large Millimeter/submillimeter Array (ALMA) large program AGE-PRO explores protoplanetary disk evolution by studying gas and dust across various ages. This work focuses on 10 evolved disks in Upper Scorpius, observed in dust continuum emission, CO and its isotopologues, and N2H+ with ALMA Bands 6 and 7. Disk radii, from the radial location enclosing 68% of the flux, are comparable to those in the younger Lupus region for both gas and dust tracers. However, solid masses are about an order of magnitude below those in Lupus and Ophiuchus, while the dust spectral index suggests some level of dust evolution. These empirical findings align with a combination of radial drift, dust trapping, and grain growth into larger bodies. A moderate correlation between CO and continuum fluxes suggests a link between gas and dust content, through the increased scatter compared to younger regions, possibly due to age variations, gas-to-dust ratio differences, or CO depletion. Additionally, the correlation between C18O and N2H+ fluxes observed in Lupus persists in Upper Scorpius, indicating a relatively stable CO gas abundance over the Class II stage of disk evolution. In conclusion, the AGE-PRO survey of Upper Scorpius disks reveals intriguing trends in disk evolution. The findings point toward potential gas evolution and the presence of dust traps in these older disks. Future high-resolution observations are needed to confirm these possibilities and further refine our understanding of disk evolution and planet formation in older environments.

The gas dominates the mass and dynamics of protoplanetary disks.However, very few observational constraints exist about the physical properties of the gas in the planet forming region of the disks. This thesis presents an ensemble of new observational projects aimed at studying the gas in the disks of selected nearby Herbig Ae/Be stars (HAEBES) and classical T Tauri stars (CTTS). In the first chapter we present a search for CO 4.7 micron ro-vibrational emission from HAEBES using ISAAC, ESO's first generation VLT near-infrared spectrograph. In the second chapter, we describe a project in which we intended to probe the outer cold gas of protoplanetary disks, by measuring gas absorption features of the disk superimposed on the optical spectra of close (< 1.5") visual companions of nearby HAEBES utilizing FORS2, ESO's VLT optical spectrograph. In the third chapter, we present a large observational effort to detect H2 fundamental rotational emission at 12.278 and 17.035 $\\mu$m from HAEBES employing VISIR, ESO's new high-resolution mid-infrared spectrograph. In the fourth chapter, we describe the first results of a sensitive search for near-infrared H2 ro-vibrational emission at 2.1218, 2.2233 and 2.2477 micron in the CTTS LkHa 264 and the debris disk 49 Cet using CRIRES, ESO's new VLT near-infrared high-resolution spectrograph. From our observations (detections and non-detections), we derive important constraints on the physical properties of the studied disks (e.g. mass, column density, temperature, age, excitation mechanism, inclination). We show that high-resolution infrared spectroscopy is a crucial tool for future studies of the structure of protoplanetary disks.

The evolution of protoplanetary discs and the related process of planet formation is regulated by angular momentum transport and mass-loss processes. Over the past decade, the paradigm of viscosity has been challenged and MHD disc winds appear as a compelling scenario to account for disc accretion. In this work, we aim to construct the equivalent of the widely used analytical description of viscous evolution for the MHD wind case. The transport of angular momentum and mass induced by the wind is parametrized by an α-like parameter and by the magnetic lever arm parameter λ. Extensions of the paradigmatic Lynden-Bell and Pringle similarity solutions to the wind case are presented. We show that wind-driven accretion leads to a steeper decrease in the disc mass and accretion rate than in viscous models due to the absence of disc spreading. If the decline of the magnetic field strength is slower than that of the gas surface density, the disc is dispersed after a finite time. The evolution of the disc in the $\dot{M}_*-M_{D}$ plane is sensitive to the wind and turbulence parameters. A disc population evolving under the action of winds can exhibit a correlation between $\dot{M}_*$ and MD depending on the initial conditions. The simplified framework proposed in this work opens to a new avenue to test the effectiveness of wind-driven accretion from the observed disc demographics and constitutes an important step to include wind-driven accretion in planet population synthesis models.

Aims. Accretion bursts triggered by the magnetorotational instability (MRI) in the innermost disk regions were studied for protoplanetary gas-dust disks that formed from prestellar cores of a various mass Mcore and mass-to-magnetic flux ratio λ. Methods. Numerical magnetohydrodynamics simulations in the thin-disk limit were employed to study the long-term (~1.0 Myr) evolution of protoplanetary disks with an adaptive turbulent α-parameter, which explicitly depends on the strength of the magnetic field and ionization fraction in the disk. The numerical models also feature the co-evolution of gas and dust, including the back-reaction of dust on gas and dust growth. Results. A dead zone with a low ionization fraction of x≲10−13 and temperature on the order of several hundred Kelvin forms in the inner disk soon after its formation, extending from several to several tens of astronomical units depending on the model. The dead zone features pronounced dust rings that are formed due to the concentration of grown dust particles in the local pressure maxima. Thermal ionization of alkaline metals in the dead zone trigger the MRI and associated accretion burst, which is characterized by a sharp rise, small-scale variability in the active phase, and fast decline once the inner MRI-active region is depleted of matter. The burst occurrence frequency is highest in the initial stages of disk formation and is driven by gravitational instability (GI), but it declines with diminishing disk mass-loading from the infalling envelope. There is a causal link between the initial burst activity and the strength of GI in the disk fueled by mass infall from the envelope. We find that the MRI-driven burst phenomenon occurs for λ = 2–10, but diminishes in models with Mcore ≲ M⊙, suggesting a lower limit on the stellar mass for which the MRI-triggered burst can occur. Conclusions. The MRI-triggered bursts occur for a wide range of mass-to-magnetic flux ratios and initial cloud core masses. The burst occurrence frequency is highest in the initial disk formation stage and reduces as the disk evolves from a gravitationally unstable to a viscous-dominated state. The MRI-triggered bursts are intrinsically connected with the dust rings in the inner disk regions, and both can be a manifestation of the same phenomenon, that is to say the formation of a dead zone.

Superthermal gas giant planets or their progenitor cores are known to open deep gaps in protoplanetary disks, which stop large, drifting dust particles on their way to the inner disk. The possible separation of the disk into distinct reservoirs and the resulting dust depletion interior to the gap have important implications for planetesimal formation and the chemical and isotopic composition of the inner regions of protoplanetary disks. Dust fragmentation, however, maintains a reservoir of small grains that can traverse the gap. Dust evolution models are thus instrumental for studies of a gap’s filtration efficiency. We present 2D multifluid hydrodynamic simulations of planet–disk systems with dust coagulation and fragmentation. For the first time, we evolve a series of 2D simulations with dust coagulation over 45,000 planetary orbits and track the dust’s size evolution and origin by using the TriPoD dust coagulation method. We investigate the effects of different planetary masses, fragmentation velocities, and viscosities on the inner disk’s dust mass budget and composition, and highlight the advantages of multidimensional simulations over 1D models. Filtering can only be efficient for high planetary masses, high fragmentation velocities, and low diffusivities. Clear compositional distinctions between the inner and outer disk could not have been maintained by Jupiter’s core if the fragmentation velocity was low, even if α ≲ 5 × 10 −4 . Significant “contamination” of the inner disk by outer-disk dust occurs in much less than 2 × 10 5 yr in this case and even for more-massive objects. This either places tight constraints on the physical conditions in the solar nebula or mandates consideration of alternative explanations for the dichotomy between non-carbonaceous chondrites (NC) and carbonaceous chondrites (CC). Astrophysical constraints on the parameters could discriminate between these possibilities.