W. Lyra | California State University, Northridge (original) (raw)

Papers by W. Lyra

Research paper thumbnail of The baroclinic instability in the context of layered accretion. Self-sustained vortices and their magnetic stability in local compressible unstratified models of protoplanetary 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,

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Research paper thumbnail of Elliptic and magneto-elliptic instabilities

EPJ Web of Conferences, 2013

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Research paper thumbnail of 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.

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Research paper thumbnail of 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.

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Research paper thumbnail of Convective overstability in accretion disks: 3D linear analysis and nonlinear saturation

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Research paper thumbnail of Sharp eccentric rings in planetless hydrodynamical models of debris disks

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Research paper thumbnail of Rossby Wave Instability at Dead Zone Boundaries in Three-Dimensional Resistive Magnetohydrodynamical Global Models of Protoplanetary Disks

The Astrophysical Journal, 2012

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Research paper thumbnail of Formation of sharp eccentric rings in debris disks with gas but without planets

Nature, 2013

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Research paper thumbnail of On the connection between the magneto-elliptic and magneto-rotational instabilities

Journal of Fluid Mechanics, 2012

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Research paper thumbnail of Fine structure of the chromospheric activity in Solar-type stars ? The H? line

Astronomy and Astrophysics, 2005

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Research paper thumbnail of 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 ...

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Research paper thumbnail of Particle Trapping and Streaming Instability in Vortices in Protoplanetary Disks

The Astrophysical Journal, 2015

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Research paper thumbnail of 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

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Research paper thumbnail of Planet

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

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Research paper thumbnail of Global models of turbulence in protoplanetary disks

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Research paper thumbnail of 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.

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Research paper thumbnail of 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.

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Research paper thumbnail of 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.

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Research paper thumbnail of The Driving of Decretion by Maxwell Stress in Disks

ABSTRACT

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Research paper thumbnail of 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.

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Research paper thumbnail of The baroclinic instability in the context of layered accretion. Self-sustained vortices and their magnetic stability in local compressible unstratified models of protoplanetary 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,

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Research paper thumbnail of Elliptic and magneto-elliptic instabilities

EPJ Web of Conferences, 2013

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Research paper thumbnail of 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.

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Research paper thumbnail of 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.

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Research paper thumbnail of Convective overstability in accretion disks: 3D linear analysis and nonlinear saturation

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Research paper thumbnail of Sharp eccentric rings in planetless hydrodynamical models of debris disks

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Rossby Wave Instability at Dead Zone Boundaries in Three-Dimensional Resistive Magnetohydrodynamical Global Models of Protoplanetary Disks

The Astrophysical Journal, 2012

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Research paper thumbnail of Formation of sharp eccentric rings in debris disks with gas but without planets

Nature, 2013

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Research paper thumbnail of On the connection between the magneto-elliptic and magneto-rotational instabilities

Journal of Fluid Mechanics, 2012

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Research paper thumbnail of Fine structure of the chromospheric activity in Solar-type stars ? The H? line

Astronomy and Astrophysics, 2005

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Research paper thumbnail of 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 ...

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Research paper thumbnail of Particle Trapping and Streaming Instability in Vortices in Protoplanetary Disks

The Astrophysical Journal, 2015

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Research paper thumbnail of 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

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Planet

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

Bookmarks Related papers MentionsView impact

Research paper thumbnail of Global models of turbulence in protoplanetary disks

Bookmarks Related papers MentionsView impact

Research paper thumbnail of 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.

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Research paper thumbnail of 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.

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Research paper thumbnail of 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.

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Research paper thumbnail of The Driving of Decretion by Maxwell Stress in Disks

ABSTRACT

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Research paper thumbnail of 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.

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