W. Lyra - Profile on Academia.edu (original) (raw)

Papers by W. Lyra

Turbulence and angular momentum transport in accretion disks remains a topic of debate. With the ... more Turbulence and angular momentum transport in accretion disks remains a topic of debate. With the realization that dead zones are robust features of protoplanetary disks, the search for hydrodynamical sources of turbulence continues. A possible source is the baroclinic instability (BI), which has been shown to exist in unmagnetized non-barotropic disks. We present shearing box simulations of baroclinicly unstable, magnetized,

EPJ Web of Conferences, 2013

Vortices are the fundamental units of turbulent flow. Understanding their stability properties th... more Vortices are the fundamental units of turbulent flow. Understanding their stability properties therefore provides fundamental insights on the nature of turbulence itself. In this contribution I briely review the phenomenological aspects of the instability of elliptic streamlines, in the hydro (elliptic instability) and hydromagnetic (magneto-elliptic instability) regimes. Vortex survival in disks is a balance between vortex destruction by these mechanisms, and vortex production by others, namely, the Rossby wave instability and the baroclinic instability. a

Planet formation through vortices in layered accretion flow

ABSTRACT Large scale anticyclonic vortices concentrate solid material in disks and may thus behav... more ABSTRACT Large scale anticyclonic vortices concentrate solid material in disks and may thus behave as a route for fast planet formation. The debate over their 3D stability in recent years has lead to the conclusion that they are stable to 3D perturbations provided there is a baroclinic feedback operating. In this contribution, I show how such baroclinic instability interacts with the magneto-rotational instability. Our results indicate that vortices do not survive magnetization, and therefore should exist only inside the dead zone.

Formation and Retention of Planets in Protoplanetary Disks

ABSTRACT This work presents 3D resistive magnetohydrodynamical models of the solar nebula, showin... more ABSTRACT This work presents 3D resistive magnetohydrodynamical models of the solar nebula, showing that the region between magnetically active and dead zones is prone to the excitation of vortices, that effectively form planetary embryos of Moon to Mars mass.

claimed that a hydrodynamical linear overstability exists in protoplanetary disks, powered by buo... more claimed that a hydrodynamical linear overstability exists in protoplanetary disks, powered by buoyancy in the presence of thermal relaxation. We analyse this claim, confirming it through rigorous compressible linear analysis. We model the system numerically, reproducing the linear growth rate for all cases studied. We also study the saturated properties of the overstability in the shearing box, finding that the saturated state produces finite amplitude fluctuations strong enough to trigger the subcritical baroclinic instability. Saturation leads to a fast burst of enstrophy in the box, and a large-scale vortex develops in the course of the next ≈100 orbits. The amount of angular momentum transport achieved is of the order of α ≈ 10 −3 , as in compressible SBI models. For the first time, a self-sustained 3D vortex is produced from linear amplitude perturbation of a quiescent base state.

Belt in our solar system. These "debris disks" show a variety of non-trivial structures attribute... more Belt in our solar system. These "debris disks" show a variety of non-trivial structures attributed to planetary perturbations and utilized to constrain the properties of the planets 1-3 . However, analyses of these systems have largely ignored the fact that, increasingly, debris disks are found to contain small quantities of gas 4-9 , a component all debris disks should contain at some level 10, 11 . Several debris disks have been measured with a dust-to-gas ratio around unity 4-9 where the effect of hydrodynamics on the structure of the disk cannot be ignored 12, 13 . Here we report that dust-gas interac-1 https://ntrs.nasa.gov/search.jsp?R=20150000163 2017-08-26T00:03:12+00:00Z tions can produce some of the key patterns seen in debris disks that were previously attributed to planets. Through linear and nonlinear modeling of the hydrodynamical problem, we find that a robust clumping instability exists in this configuration, organizing the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk 14 . The hypothesis that these disks might contain planets, though thrilling, is not necessarily required to explain these systems.

The Astrophysical Journal, 2012

It has been suggested that the transition between magnetorotationally active and dead zones in pr... more It has been suggested that the transition between magnetorotationally active and dead zones in protoplanetary disks should be prone to the excitation of vortices via Rossby wave instability (RWI). However, the only numerical evidence for this has come from alpha disk models, where the magnetic field evolution is not followed, and the effect of turbulence is parameterized by Laplacian viscosity. We aim to establish the phenomenology of the flow in the transition in three-dimensional resistive-magnetohydrodynamical models. We model the transition by a sharp jump in resistivity, as expected in the inner dead zone boundary, using the Pencil Code to simulate the flow. We find that vortices are readily excited in the dead side of the transition. We measure the mass accretion rate finding similar levels of Reynolds stress at the dead and active zones, at the α ≈ 10 −2 level. The vortex sits in a pressure maximum and does not migrate, surviving until the end of the simulation. A pressure maximum in the active zone also triggers the RWI. The magnetized vortex that results should be disrupted by parasitical magneto-elliptic instabilities, yet it subsists in high resolution. This suggests that either the parasitic modes are still numerically damped or that the RWI supplies vorticity faster than they can destroy it. We conclude that the resistive transition between the active and dead zones in the inner regions of protoplanetary disks, if sharp enough, can indeed excite vortices via RWI. Our results lend credence to previous works that relied on the alpha-disk approximation, and caution against the use of overly reduced azimuthal coverage on modeling this transition.

Nature, 2013

show a variety of non-trivial structures attributed to planetary perturbations and used to constr... more show a variety of non-trivial structures attributed to planetary perturbations and used to constrain the properties of the planets 1-3 . However, these analyses have largely ignored the fact that some debris disks are found to contain small quantities of gas 4-9 , a component that all such disks should contain at some level 10, 11 . Several debris disks have been measured with a dust-to-gas ratio around unity 4-9 at which the effect of hydrodynamics on the structure of the disk cannot be ignored 12, 13 . Here we report linear and nonlinear modelling that shows that dust-gas interactions can produce some of 1 arXiv:1307.5916v2 [astro-ph.EP] 2 Aug 2013 the key patterns attributed to planets. We find a robust clumping instability that organizes the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk 14 . The conclusion that such disks might contain planets is not necessarily required to explain these systems.

Journal of Fluid Mechanics, 2012

It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case o... more It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case of the magneto-elliptic instability (MEI). This limit is obtained for horizontal modes in the presence of rotation and an external vertical magnetic field, when the aspect ratio of the elliptic streamlines tends to infinite. In this paper we unveil the link between these previously unconnected mechanisms, explaining both the MEI and the MRI as different manifestations of the same Magneto-Elliptic-Rotational Instability (MERI). The growth rates are found and the influence of the magnetic and rotational effects is explained, in particular the effect of the magnetic field on the range of negative Rossby numbers at which the horizontal instability is excited. Furthermore, we show how the horizontal rotational MEI in the rotating shear flow limit links to the MRI by the use of the local shearing box model, typically used in the study of accretion discs. In such limit the growth rates of the two instability types coincide for any power-type background angular velocity radial profile with negative exponent corresponding to the value of the Rossby number of the rotating shear flow. The MRI requirement for instability is that the background angular velocity profile is a decreasing function of the distance from the centre of the disk which corresponds to the horizontal rotational MEI requirement of negative Rossby numbers. Finally a physical interpretation of the horizontal instability, based on a balance between the strain, the Lorentz force and the Coriolis force is given.

Astronomy and Astrophysics, 2005

A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complem... more A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complementing the works previously done on this subject . The chromospheric diagnostic was built with a statistically significant sample, covering nine years of observations, and including 175 solar neighborhood stars. Regarding the age indicator, the presence of stars for which very accurate ages are determined, such as those belonging to clusters and kinematic groups, lends confidence to our analysis. We also investigate the possibility that stars of the same age might have gone through different tracks of chromospheric decay, identifying -within the same age range -effects of metallicity and mass. These parameters, however, as well as age, seem to be significant only for dwarf stars, losing their meaning when we analyze stars in the subgiant branch. This result suggests that, in these evolved stars, the emission mechanism cannot be magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al. 2002c, and references therein). The Sun is found to be a typical star in its Hα chromospheric flux, for its age, mass and metallicity. As a byproduct of this work, we developed an automatic method to determine temperatures from the wings of Hα, which means the suppression of the error inherent to the visual procedure used in the literature.

Calibração da Linha Hα como Diagnóstico Cromosférico e Indicador de Idade em Estrelas de Tipo Solar

mpia.mpg.de

... e Indicador de Idade em Estrelas de Tipo Solar Wladimir Lyra Orientador: Dr. Gustavo Frederic... more ... e Indicador de Idade em Estrelas de Tipo Solar Wladimir Lyra Orientador: Dr. Gustavo FredericoPorto de Mello (UFRJ/CCMN/OV/Departamento de Astronomia) Projeto Final de Curso para obtenção do título de Astrônomo Rio de Janeiro – julho de 2003 Page 2. 2 Page 3. 3 ...

The Astrophysical Journal, 2015

We analyse the concentration of solid particles in vortices created and sustained by radial buoya... more We analyse the concentration of solid particles in vortices created and sustained by radial buoyancy in protoplanetary disks, i.e. baroclinic vortex growth. Besides the gas drag acting on particles we also allow for back-reaction from dust onto the gas. This becomes important when the local dustto-gas ratio approaches unity. In our 2D, local, shearing sheet simulations we see high concentrations of grains inside the vortices for a broad range of Stokes numbers, St. An initial dust-to-gas ratio of 1:100 can easily be reversed to 100:1 for St = 1. The increased dust-to-gas ratio triggers the streaming instability, thus counter-intuitively limiting the maximal achievable overdensities. We find that particle trapping inside vortices opens the possibility for gravity-assisted planetesimal formation even for small particles (St = 0.01) and low initial dust-to-gas ratios (1:10 4 ).

Global

models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and t... more models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and transport of solids

Planet

formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks

Global models of turbulence in protoplanetary disks

Forming Planetary Cores in a Turbulent Non-Isothermal Disk

ABSTRACT Non-Isothermal disks have been shown to have regions where the net torque on a planet is... more ABSTRACT Non-Isothermal disks have been shown to have regions where the net torque on a planet is positive, leading to outward migration of the planet. When a region with negative torque is directly exterior to this, planets in the inner region migrate outwards and planets in the outer region migrate inwards, converging where the torque is zero. We incorporate the torques from an evolving non-isothermal disk into an N-body simulation, and find that the bodies do converge to the zero torque region, but effects of neighbouring planets prevents the planets from merging. Though N-body interactions prevent complete merging to form one core, the addition of a weak stochastic force to simulate turbulence in the disk allows for orbit crossings and mergers near the convergence zone. In this way, it is possible to move from the sub-Earth mass regime into the 10 Earth mass planetary core regime in 2-3 million years.

Compact Dust Concentration in the MWC 758 Protoplanetary Disk

The Astrophysical Journal, 2015

ABSTRACT The formation of planetesimals requires that primordial dust grains grow from micron- to... more ABSTRACT The formation of planetesimals requires that primordial dust grains grow from micron- to km-sized bodies. Dust traps caused by gas pressure maxima have been proposed as regions where grains can concentrate and grow fast enough to form planetesimals, before radially migrating onto the star. We report new VLA Ka & Ku observations of the protoplanetary disk around the Herbig Ae/Be star MWC 758. The Ka image shows a compact emission region in the outer disk indicating a strong concentration of big dust grains. Tracing smaller grains, archival ALMA data in band 7 continuum shows extended disk emission with an intensity maximum to the north-west of the central star, which matches the VLA clump position. This segregation of grains sizes is expected in the context of dust trapping, where big grains are trapped more easily than smaller grains in gas pressure maxima. We develop a non-axisymmetric parametric model inspired by a steady state vortex solution which reproduces the observations, including the spectral energy distribution. Finally, we compare the radio continuum with SPHERE scattered light data. The ALMA continuum spatially coincides with a region devoid of scattered polarised emission and the VLA clump is offset to the north of the north-western spiral-like feature, indicating moderate or no flaring in the outer disk.

Rossby wave instability does not require sharp resistivity gradients

ABSTRACT Rossby wave instability (RWI) at dead zone boundaries may play an important role in plan... more ABSTRACT Rossby wave instability (RWI) at dead zone boundaries may play an important role in planet formation. Viscous hydrodynamics results suggest RWI is excited only when the viscosity changes over a radial distance less than two density scale heights. However in the disks around Solar-mass T Tauri stars, it is not viscosity but magnetic forces that provide the accretion stress beyond about 10 AU, where surface densities are low enough so stellar X-rays and interstellar cosmic rays can penetrate. Here we aim to explore the conditions for RWI in the smooth transition with increasing distance, from resistive and magnetically-dead to conducting and magnetically-active. We perform 3D unstratified MHD simulations with the Pencil Code, using static resistivity profiles. As a result, we find that in MHD, contrary to viscous models, the RWI is triggered even with a gradual change in resistivity extending from 10 to 40 AU (i.e., spanning 15 scale heights for aspect ratio 0.1). This is because magneto-rotational turbulence sets in abruptly when the resistivity reaches a threshold level. At higher resistivities the longest unstable wavelength is quenched, resulting in a sharp decline of the Maxwell stress towards the star. The sharp gradient in the magnetic forces leads to a localized density bump, that is in turn Rossby wave unstable. We conclude that even weak gradients in the resistivity can lead to sharp transitions in the Maxwell stress. The upshot is that the RWI is more easily activated in the outer disk than previously thought. Rossby vortices at the outer dead zone boundary thus could underlie the dust asymmetries seen in the outer reaches of transition disks.

The Driving of Decretion by Maxwell Stress in Disks

ABSTRACT

Making Intermediate mass black holes around Supermassive black holes: like making Jupiters around stars

ABSTRACT Stellar mass black hole seeds can rapidly grow into intermediate mass black holes (IMBH)... more ABSTRACT Stellar mass black hole seeds can rapidly grow into intermediate mass black holes (IMBH) in accretion disks around supermassive black holes. Initial seed growth is dominated by collisions with stars and later seed growth is dominated by gas accretion. IMBH grow much faster in this model than in globular cluster. There are strong theoretical and observational analogies with the growth of Jupiters in disks around stars. I discuss the growth and observational signatures of IMBH in AGN disks.

Turbulence and angular momentum transport in accretion disks remains a topic of debate. With the ... more Turbulence and angular momentum transport in accretion disks remains a topic of debate. With the realization that dead zones are robust features of protoplanetary disks, the search for hydrodynamical sources of turbulence continues. A possible source is the baroclinic instability (BI), which has been shown to exist in unmagnetized non-barotropic disks. We present shearing box simulations of baroclinicly unstable, magnetized,

EPJ Web of Conferences, 2013

Vortices are the fundamental units of turbulent flow. Understanding their stability properties th... more Vortices are the fundamental units of turbulent flow. Understanding their stability properties therefore provides fundamental insights on the nature of turbulence itself. In this contribution I briely review the phenomenological aspects of the instability of elliptic streamlines, in the hydro (elliptic instability) and hydromagnetic (magneto-elliptic instability) regimes. Vortex survival in disks is a balance between vortex destruction by these mechanisms, and vortex production by others, namely, the Rossby wave instability and the baroclinic instability. a

Planet formation through vortices in layered accretion flow

ABSTRACT Large scale anticyclonic vortices concentrate solid material in disks and may thus behav... more ABSTRACT Large scale anticyclonic vortices concentrate solid material in disks and may thus behave as a route for fast planet formation. The debate over their 3D stability in recent years has lead to the conclusion that they are stable to 3D perturbations provided there is a baroclinic feedback operating. In this contribution, I show how such baroclinic instability interacts with the magneto-rotational instability. Our results indicate that vortices do not survive magnetization, and therefore should exist only inside the dead zone.

Formation and Retention of Planets in Protoplanetary Disks

ABSTRACT This work presents 3D resistive magnetohydrodynamical models of the solar nebula, showin... more ABSTRACT This work presents 3D resistive magnetohydrodynamical models of the solar nebula, showing that the region between magnetically active and dead zones is prone to the excitation of vortices, that effectively form planetary embryos of Moon to Mars mass.

claimed that a hydrodynamical linear overstability exists in protoplanetary disks, powered by buo... more claimed that a hydrodynamical linear overstability exists in protoplanetary disks, powered by buoyancy in the presence of thermal relaxation. We analyse this claim, confirming it through rigorous compressible linear analysis. We model the system numerically, reproducing the linear growth rate for all cases studied. We also study the saturated properties of the overstability in the shearing box, finding that the saturated state produces finite amplitude fluctuations strong enough to trigger the subcritical baroclinic instability. Saturation leads to a fast burst of enstrophy in the box, and a large-scale vortex develops in the course of the next ≈100 orbits. The amount of angular momentum transport achieved is of the order of α ≈ 10 −3 , as in compressible SBI models. For the first time, a self-sustained 3D vortex is produced from linear amplitude perturbation of a quiescent base state.

Belt in our solar system. These "debris disks" show a variety of non-trivial structures attribute... more Belt in our solar system. These "debris disks" show a variety of non-trivial structures attributed to planetary perturbations and utilized to constrain the properties of the planets 1-3 . However, analyses of these systems have largely ignored the fact that, increasingly, debris disks are found to contain small quantities of gas 4-9 , a component all debris disks should contain at some level 10, 11 . Several debris disks have been measured with a dust-to-gas ratio around unity 4-9 where the effect of hydrodynamics on the structure of the disk cannot be ignored 12, 13 . Here we report that dust-gas interac-1 https://ntrs.nasa.gov/search.jsp?R=20150000163 2017-08-26T00:03:12+00:00Z tions can produce some of the key patterns seen in debris disks that were previously attributed to planets. Through linear and nonlinear modeling of the hydrodynamical problem, we find that a robust clumping instability exists in this configuration, organizing the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk 14 . The hypothesis that these disks might contain planets, though thrilling, is not necessarily required to explain these systems.

The Astrophysical Journal, 2012

It has been suggested that the transition between magnetorotationally active and dead zones in pr... more It has been suggested that the transition between magnetorotationally active and dead zones in protoplanetary disks should be prone to the excitation of vortices via Rossby wave instability (RWI). However, the only numerical evidence for this has come from alpha disk models, where the magnetic field evolution is not followed, and the effect of turbulence is parameterized by Laplacian viscosity. We aim to establish the phenomenology of the flow in the transition in three-dimensional resistive-magnetohydrodynamical models. We model the transition by a sharp jump in resistivity, as expected in the inner dead zone boundary, using the Pencil Code to simulate the flow. We find that vortices are readily excited in the dead side of the transition. We measure the mass accretion rate finding similar levels of Reynolds stress at the dead and active zones, at the α ≈ 10 −2 level. The vortex sits in a pressure maximum and does not migrate, surviving until the end of the simulation. A pressure maximum in the active zone also triggers the RWI. The magnetized vortex that results should be disrupted by parasitical magneto-elliptic instabilities, yet it subsists in high resolution. This suggests that either the parasitic modes are still numerically damped or that the RWI supplies vorticity faster than they can destroy it. We conclude that the resistive transition between the active and dead zones in the inner regions of protoplanetary disks, if sharp enough, can indeed excite vortices via RWI. Our results lend credence to previous works that relied on the alpha-disk approximation, and caution against the use of overly reduced azimuthal coverage on modeling this transition.

Nature, 2013

show a variety of non-trivial structures attributed to planetary perturbations and used to constr... more show a variety of non-trivial structures attributed to planetary perturbations and used to constrain the properties of the planets 1-3 . However, these analyses have largely ignored the fact that some debris disks are found to contain small quantities of gas 4-9 , a component that all such disks should contain at some level 10, 11 . Several debris disks have been measured with a dust-to-gas ratio around unity 4-9 at which the effect of hydrodynamics on the structure of the disk cannot be ignored 12, 13 . Here we report linear and nonlinear modelling that shows that dust-gas interactions can produce some of 1 arXiv:1307.5916v2 [astro-ph.EP] 2 Aug 2013 the key patterns attributed to planets. We find a robust clumping instability that organizes the dust into narrow, eccentric rings, similar to the Fomalhaut debris disk 14 . The conclusion that such disks might contain planets is not necessarily required to explain these systems.

Journal of Fluid Mechanics, 2012

It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case o... more It has been recently suggested that the magneto-rotational instability (MRI) is a limiting case of the magneto-elliptic instability (MEI). This limit is obtained for horizontal modes in the presence of rotation and an external vertical magnetic field, when the aspect ratio of the elliptic streamlines tends to infinite. In this paper we unveil the link between these previously unconnected mechanisms, explaining both the MEI and the MRI as different manifestations of the same Magneto-Elliptic-Rotational Instability (MERI). The growth rates are found and the influence of the magnetic and rotational effects is explained, in particular the effect of the magnetic field on the range of negative Rossby numbers at which the horizontal instability is excited. Furthermore, we show how the horizontal rotational MEI in the rotating shear flow limit links to the MRI by the use of the local shearing box model, typically used in the study of accretion discs. In such limit the growth rates of the two instability types coincide for any power-type background angular velocity radial profile with negative exponent corresponding to the value of the Rossby number of the rotating shear flow. The MRI requirement for instability is that the background angular velocity profile is a decreasing function of the distance from the centre of the disk which corresponds to the horizontal rotational MEI requirement of negative Rossby numbers. Finally a physical interpretation of the horizontal instability, based on a balance between the strain, the Lorentz force and the Coriolis force is given.

Astronomy and Astrophysics, 2005

A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complem... more A calibration of Hα as both a chromospheric diagnostic and an age indicator is presented, complementing the works previously done on this subject . The chromospheric diagnostic was built with a statistically significant sample, covering nine years of observations, and including 175 solar neighborhood stars. Regarding the age indicator, the presence of stars for which very accurate ages are determined, such as those belonging to clusters and kinematic groups, lends confidence to our analysis. We also investigate the possibility that stars of the same age might have gone through different tracks of chromospheric decay, identifying -within the same age range -effects of metallicity and mass. These parameters, however, as well as age, seem to be significant only for dwarf stars, losing their meaning when we analyze stars in the subgiant branch. This result suggests that, in these evolved stars, the emission mechanism cannot be magnetohydrodynamical in nature, in agreement with recent models (Fawzy et al. 2002c, and references therein). The Sun is found to be a typical star in its Hα chromospheric flux, for its age, mass and metallicity. As a byproduct of this work, we developed an automatic method to determine temperatures from the wings of Hα, which means the suppression of the error inherent to the visual procedure used in the literature.

Calibração da Linha Hα como Diagnóstico Cromosférico e Indicador de Idade em Estrelas de Tipo Solar

mpia.mpg.de

... e Indicador de Idade em Estrelas de Tipo Solar Wladimir Lyra Orientador: Dr. Gustavo Frederic... more ... e Indicador de Idade em Estrelas de Tipo Solar Wladimir Lyra Orientador: Dr. Gustavo FredericoPorto de Mello (UFRJ/CCMN/OV/Departamento de Astronomia) Projeto Final de Curso para obtenção do título de Astrônomo Rio de Janeiro – julho de 2003 Page 2. 2 Page 3. 3 ...

The Astrophysical Journal, 2015

We analyse the concentration of solid particles in vortices created and sustained by radial buoya... more We analyse the concentration of solid particles in vortices created and sustained by radial buoyancy in protoplanetary disks, i.e. baroclinic vortex growth. Besides the gas drag acting on particles we also allow for back-reaction from dust onto the gas. This becomes important when the local dustto-gas ratio approaches unity. In our 2D, local, shearing sheet simulations we see high concentrations of grains inside the vortices for a broad range of Stokes numbers, St. An initial dust-to-gas ratio of 1:100 can easily be reversed to 100:1 for St = 1. The increased dust-to-gas ratio triggers the streaming instability, thus counter-intuitively limiting the maximal achievable overdensities. We find that particle trapping inside vortices opens the possibility for gravity-assisted planetesimal formation even for small particles (St = 0.01) and low initial dust-to-gas ratios (1:10 4 ).

Global

models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and t... more models of turbulence in protoplanetary disks I. A cylindrical potential on a Cartesian grid and transport of solids

Planet

formation bursts at the borders of the dead zone in 2D numerical simulations of circumstellar disks

Global models of turbulence in protoplanetary disks

Forming Planetary Cores in a Turbulent Non-Isothermal Disk

ABSTRACT Non-Isothermal disks have been shown to have regions where the net torque on a planet is... more ABSTRACT Non-Isothermal disks have been shown to have regions where the net torque on a planet is positive, leading to outward migration of the planet. When a region with negative torque is directly exterior to this, planets in the inner region migrate outwards and planets in the outer region migrate inwards, converging where the torque is zero. We incorporate the torques from an evolving non-isothermal disk into an N-body simulation, and find that the bodies do converge to the zero torque region, but effects of neighbouring planets prevents the planets from merging. Though N-body interactions prevent complete merging to form one core, the addition of a weak stochastic force to simulate turbulence in the disk allows for orbit crossings and mergers near the convergence zone. In this way, it is possible to move from the sub-Earth mass regime into the 10 Earth mass planetary core regime in 2-3 million years.

Compact Dust Concentration in the MWC 758 Protoplanetary Disk

The Astrophysical Journal, 2015

ABSTRACT The formation of planetesimals requires that primordial dust grains grow from micron- to... more ABSTRACT The formation of planetesimals requires that primordial dust grains grow from micron- to km-sized bodies. Dust traps caused by gas pressure maxima have been proposed as regions where grains can concentrate and grow fast enough to form planetesimals, before radially migrating onto the star. We report new VLA Ka & Ku observations of the protoplanetary disk around the Herbig Ae/Be star MWC 758. The Ka image shows a compact emission region in the outer disk indicating a strong concentration of big dust grains. Tracing smaller grains, archival ALMA data in band 7 continuum shows extended disk emission with an intensity maximum to the north-west of the central star, which matches the VLA clump position. This segregation of grains sizes is expected in the context of dust trapping, where big grains are trapped more easily than smaller grains in gas pressure maxima. We develop a non-axisymmetric parametric model inspired by a steady state vortex solution which reproduces the observations, including the spectral energy distribution. Finally, we compare the radio continuum with SPHERE scattered light data. The ALMA continuum spatially coincides with a region devoid of scattered polarised emission and the VLA clump is offset to the north of the north-western spiral-like feature, indicating moderate or no flaring in the outer disk.

Rossby wave instability does not require sharp resistivity gradients

ABSTRACT Rossby wave instability (RWI) at dead zone boundaries may play an important role in plan... more ABSTRACT Rossby wave instability (RWI) at dead zone boundaries may play an important role in planet formation. Viscous hydrodynamics results suggest RWI is excited only when the viscosity changes over a radial distance less than two density scale heights. However in the disks around Solar-mass T Tauri stars, it is not viscosity but magnetic forces that provide the accretion stress beyond about 10 AU, where surface densities are low enough so stellar X-rays and interstellar cosmic rays can penetrate. Here we aim to explore the conditions for RWI in the smooth transition with increasing distance, from resistive and magnetically-dead to conducting and magnetically-active. We perform 3D unstratified MHD simulations with the Pencil Code, using static resistivity profiles. As a result, we find that in MHD, contrary to viscous models, the RWI is triggered even with a gradual change in resistivity extending from 10 to 40 AU (i.e., spanning 15 scale heights for aspect ratio 0.1). This is because magneto-rotational turbulence sets in abruptly when the resistivity reaches a threshold level. At higher resistivities the longest unstable wavelength is quenched, resulting in a sharp decline of the Maxwell stress towards the star. The sharp gradient in the magnetic forces leads to a localized density bump, that is in turn Rossby wave unstable. We conclude that even weak gradients in the resistivity can lead to sharp transitions in the Maxwell stress. The upshot is that the RWI is more easily activated in the outer disk than previously thought. Rossby vortices at the outer dead zone boundary thus could underlie the dust asymmetries seen in the outer reaches of transition disks.

The Driving of Decretion by Maxwell Stress in Disks

ABSTRACT

Making Intermediate mass black holes around Supermassive black holes: like making Jupiters around stars

ABSTRACT Stellar mass black hole seeds can rapidly grow into intermediate mass black holes (IMBH)... more ABSTRACT Stellar mass black hole seeds can rapidly grow into intermediate mass black holes (IMBH) in accretion disks around supermassive black holes. Initial seed growth is dominated by collisions with stars and later seed growth is dominated by gas accretion. IMBH grow much faster in this model than in globular cluster. There are strong theoretical and observational analogies with the growth of Jupiters in disks around stars. I discuss the growth and observational signatures of IMBH in AGN disks.