Can dead zones create structures like a transition disk?

[Abridged] Regions of low ionisation where the activity of the magneto-rotational instability is suppressed, the so-called dead zones, have been suggested to explain gaps and asymmetries of transition disks. We investigate the gas and dust evolution simultaneously assuming simplified prescriptions for a dead zone and a magnetohydrodynamic (MHD) wind acting on the disk. We explore whether the resulting gas and dust distribution can create signatures similar to those observed in transition disks. For the dust evolution, we included the transport, growth, and fragmentation of dust particles. To compare with observations, we produced synthetic images in scattered optical light and in thermal emission at mm wavelengths. In all models with a dead zone, a bump in the gas surface density is produced that is able to efficiently trap large particles ($\gtrsim 1$ mm) at the outer edge of the dead zone. The gas bump reaches an amplitude of a factor of $\sim5$, which can be enhanced by the presence of an MHD wind that removes mass from the inner disk. While our 1D simulations suggest that such a structure can be present only for $\sim$1 Myr, the structure may be maintained for a longer time when more realistic 2D/3D simulations are performed. In the synthetic images, gap-like low-emission regions are seen at scattered light and in thermal emission at mm wavelengths, as previously predicted in the case of planet-disk interaction. As a conclusion, main signatures of transition disks can be reproduced by assuming a dead zone in the disk, such as gap-like structure in scattered light and millimetre continuum emission, and a lower gas surface density within the dead zone. Previous studies showed that the Rossby wave instability can also develop at the edge of such dead zones, forming vortices and also creating asymmetries.Comment: Minor changes after language edition. Accepted for publication in A&

First 3-D grid-based gas-dust simulations of circumstellar disks with an embedded planet

Substructures are ubiquitous in high resolution (sub-)millimeter continuum observations of circumstellar disks. They are possibly caused by forming planets embedded in the disk. To investigate the relation between observed substructures and young planets, we perform novel three-dimensional two-fluid (gas+1-mm-dust) hydrodynamic simulations of circumstellar disks with embedded planets (Neptune-, Saturn-, Jupiter-, 5 Jupiter-mass) at different orbital distances from the star (5.2AU, 30AU, 50AU). We turn these simulations into synthetic (sub-)millimeter ALMA images. We find that all but the Neptune-mass planet open annular gaps in both the gas and the dust component of the disk. We find that the temporal evolution of the dust density distribution is distinctly different of the gas'. For example, the planets cause significant vertical stirring of the dust in the circumstellar disk which opposes the vertical settling. This creates a thicker dust disk than disks without a planet. We find that this effect greatly influences the dust masses derived from the synthetic ALMA images. Comparing the dust disk masses in the 3D simulations and the ones derived from the 2D ALMA synthetic images, we find the former to be a factor of a few (up to 10) larger, pointing to that real disks might be significantly more massive than previously thought based on ALMA continuum images using the optically thin assumption and equation. Finally, we analyze the synthetic ALMA images and provide an empirical relationship between the planet mass and the width of the gap in the ALMA images including the effects of the beam size.Comment: 21 pages, 11 figures, accepted for publication in MNRA

Comets and Planetesimal Formation

In this chapter, we review the processes involved in the formation of planetesimals and comets. We will start with a description of the physics of dust grain growth and how this is mediated by gas-dust interactions in planet-forming disks. We will then delve into the various models of planetesimal formation, describing how these planetesimals form as well as their resulting structure. In doing so, we focus on and compare two paradigms for planetesimal formation: the gravitational collapse of particle over-densities (which can be produced by a variety of mechanisms) and the growth of particles into planetesimals via collisional and gravitational coagulation. Finally, we compare the predictions from these models with data collected by the Rosetta and New Horizons missions and that obtained via observations of distant Kuiper Belt Objects.Comment: Planetesimal Formation Review accepted for publication in Comets II

The impact of dynamic pressure bumps on the observational properties of protoplanetary disks

Over the last years, large (sub-)millimetre surveys of protoplanetary disks have well constrained the demographics of disks, such as their millimetre luminosities, spectral indices, and disk radii. Additionally, several high-resolution observations have revealed an abundance of substructures in the disks dust continuum. The most prominent are ring like structures, likely due to pressure bumps trapping dust particles. The origins and characteristics of these bumps, nevertheless, need to be further investigated. The purpose of this work is to study how dynamic pressure bumps affect observational properties of protoplanetary disks. We further aim to differentiate between the planetary- versus zonal flow-origin of pressure bumps. We perform one-dimensional gas and dust evolution simulations, setting up models with varying pressure bump features. We subsequently run radiative transfer calculations to obtain synthetic images and the different quantities of observations. We find that the outermost pressure bump determines the disks dust size across different millimetre wavelengths. Our modelled dust traps need to form early (< 0.1 Myr), fast (on viscous timescales), and must be long lived (> Myr) to obtain the observed high millimetre luminosities and low spectral indices of disks. While the planetary bump models can reproduce these observables irrespectively of the opacity prescription, the highest opacities are needed for the zonal flow bump model to be in line with observations. Our findings favour the planetary- over the zonal flow-origin of pressure bumps and support the idea that planet formation already occurs in early class 0-1 stages of circumstellar disks. The determination of the disks effective size through its outermost pressure bump also delivers a possible answer to why disks in recent low-resolution surveys appear to have the same sizes across different millimetre wavelengths.Comment: 22 pages, 15 figures. To be published in Astronomy & Astrophysic

The properties of the inner disk around HL Tau: Multi-wavelength modeling of the dust emission

We conducted a detailed radiative transfer modeling of the dust emission from the circumstellar disk around HL Tau. The goal of our study is to derive the surface density profile of the inner disk and its structure. In addition to the Atacama Large Millimeter/submillimeter Array images at Band 3 (2.9mm), Band 6 (1.3mm), and Band 7 (0.87mm), the most recent Karl G. Jansky Very Large Array (VLA) observations at 7mm were included in the analysis. A simulated annealing algorithm was invoked to search for the optimum model. The radiative transfer analysis demonstrates that most radial components (i.e., >6AU) of the disk become optically thin at a wavelength of 7mm, which allows us to constrain, for the first time, the dust density distribution in the inner region of the disk. We found that a homogeneous grain size distribution is not sufficient to explain the observed images at different wavelengths simultaneously, while models with a shallower grain size distribution in the inner disk work well. We found clear evidence that larger grains are trapped in the first bright ring. Our results imply that dust evolution has already taken place in the disk at a relatively young (i.e., ~1Myr) age. We compared the midplane temperature distribution, optical depth, and properties of various dust rings with those reported previously. Using the Toomre parameter, we briefly discussed the gravitational instability as a potential mechanism for the origin of the dust clump detected in the first bright ring via the VLA observations.Comment: Accepted for publication in A&A (10 pages

A Major Asymmetric Dust Trap in a Transition Disk

The statistics of discovered exoplanets suggest that planets form efficiently. However, there are fundamental unsolved problems, such as excessive inward drift of particles in protoplanetary disks during planet formation. Recent theories invoke dust traps to overcome this problem. We report the detection of a dust trap in the disk around the star Oph IRS 48 using observations from the Atacama Large Millimeter/submillimeter Array (ALMA). The 0.44-millimeter-wavelength continuum map shows high-contrast crescent-shaped emission on one side of the star originating from millimeter-sized grains, whereas both the mid-infrared image (micrometer-sized dust) and the gas traced by the carbon monoxide 6-5 rotational line suggest rings centered on the star. The difference in distribution of big grains versus small grains/gas can be modeled with a vortex-shaped dust trap triggered by a companion.Comment: 25 pages, 7 figures (accepted version prior to language editing

Millimeter emission in photoevaporating disks is determined by early substructures

[abridged]Photoevaporation and dust-trapping are individually considered to be important mechanisms in the evolution and morphology of protoplanetary disks. We studied how the presence of early substructures affects the evolution of the dust distribution and flux in the millimeter continuum of disks that are undergoing photoevaporative dispersal. We also tested if the predicted properties resemble those observed in the population of transition disks. We used the numerical code Dustpy to simulate disk evolution considering gas accretion, dust growth, dust-trapping at substructures, and mass loss due to X-ray and EUV (XEUV) photoevaporation and dust entrainment. Then, we compared how the dust mass and millimeter flux evolve for different disk models. We find that, during photoevaporative dispersal, disks with primordial substructures retain more dust and are brighter in the millimeter continuum than disks without early substructures, regardless of the photoevaporative cavity size. Once the photoevaporative cavity opens, the estimated fluxes for the disk models that are initially structured are comparable to those found in the bright transition disk population ($F_\textrm{mm} > 30\, \textrm{mJy}$), while the disk models that are initially smooth have fluxes comparable to the transition disks from the faint population ($F_\textrm{mm} < 30\, \textrm{mJy}$), suggesting a link between each model and population. Our models indicate that the efficiency of the dust trapping determines the millimeter flux of the disk, while the gas loss due to photoevaporation controls the formation and expansion of a cavity, decoupling the mechanisms responsible for each feature. In consequence, even a planet with a mass comparable to Saturn could trap enough dust to reproduce the millimeter emission of a bright transition disk, while its cavity size is independently driven by photoevaporative dispersal.Comment: Accepted for publication in A&