arXiv subjects

T. Birnstiel

Publications and source records attributed to T. Birnstiel.

At least 37 records · Page 2Linked to original sources

arXiv2017

The circumstellar disk HD$\,$169142: gas, dust and planets acting in concert?

HD$\,$169142 is an excellent target to investigate signs of planet-disk interaction due to the previous evidence of gap structures. We performed J-band (~1.2μm) polarized intensity imaging of HD169142 with VLT/SPHERE. We observe polarized scattered light down to 0.16" (~19 au) and find an inner gap with a significantly reduced scattered light flux. We confirm the previously detected double ring structure peaking at 0.18" (~21 au) and 0.56" (~66 au), and marginally detect a faint third gap at 0.70"-0.73" (~82-85 au). We explore dust evolution models in a disk perturbed by two giant planets, as well as models with a parameterized dust size distribution. The dust evolution model is able to reproduce the ring locations and gap widths in polarized intensity, but fails to reproduce their depths. It, however, gives a good match with the ALMA dust continuum image at 1.3 mm. Models with a parameterized dust size distribution better reproduce the gap depth in scattered light, suggesting that dust filtration at the outer edges of the gaps is less effective. The pile-up of millimeter grains in a dust trap and the continuous distribution of small grains throughout the gap likely require a more efficient dust fragmentation and dust diffusion in the dust trap. Alternatively, turbulence or charging effects might lead to a reservoir of small grains at the surface layer that is not affected by the dust growth and fragmentation cycle dominating the dense disk midplane. The exploration of models shows that extracting planet properties such as mass from observed gap profiles is highly degenerate.

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arXiv2017

Millimeter Spectral Indices and Dust Trapping By Planets in Brown Dwarf Disks

Disks around brown dwarfs (BDs) are excellent laboratories to study the first steps of planet formation in cold and low-mass disk conditions. The radial-drift velocities of dust particles in BD disks are higher than in disks around more massive stars. Therefore, BD disks are expected to be more depleted in millimeter-sized grains compared to disks around T Tauri or Herbig Ae/Be stars. However, recent millimeter observations of BD disks revealed low millimeter spectral indices, indicating the presence of large grains in these disks and challenging models of dust evolution. We present 3\,mm photometric observations carried out with the IRAM/Plateau de Bure Interferometer (PdBI) of three BD disks in the Taurus star forming region, which have been observed with ALMA at 0.89\,mm. The disks were not resolved and only one was detected with enough confidence ($\sim3.5σ$) with PdBI. Based on these observations, we obtain the values and lower limits of the spectral index and find low values ($α_{\rm{mm}}\lesssim 3.0$). We compare these observations in the context of particle trapping by an embedded planet, a promising mechanism to explain the observational signatures in more massive and warmer disks. We find, however, that this model cannot reproduce the current millimeter observations for BD disks, and multiple-strong pressure bumps globally distributed in the disk remain as a favorable scenario to explain observations. Alternative possibilities are that the gas masses in BD disk are very low ($\sim2\times10^{-3}\,M_{\rm{Jup}}$) such that the millimeter grains are decoupled and do not drift, or fast growth of fluffy aggregates.

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arXiv2017

Dust Density Distribution and Imaging Analysis of Different Ice Lines in Protoplanetary Disks

Recent high angular resolution observations of protoplanetary disks at different wavelengths have revealed several kinds of structures, including multiple bright and dark rings. Embedded planets are the most used explanation for such structures, but there are alternative models capable of shaping the dust in rings as it has been observed. We assume a disk around a Herbig star and investigate the effect that ice lines have on the dust evolution, following the growth, fragmentation, and dynamics of multiple dust size particles, covering from 1 $μ$m to 2 m sized objects. We use simplified prescriptions of the fragmentation velocity threshold, which is assumed to change radially at the location of one, two, or three ice lines. We assume changes at the radial location of main volatiles, specifically H$_2$O, CO$_2$, and NH$_3$. Radiative transfer calculations are done using the resulting dust density distributions in order to compare with current multiwavelength observations. We find that the structures in the dust density profiles and radial intensities at different wavelengths strongly depend on the disk viscosity. A clear gap of emission can be formed between ice lines and be surrounded by ring-like structures, in particular between the H$_2$O and CO$_2$ (or CO). The gaps are expected to be shallower and narrower at millimeter emission than at near-infrared, opposite to model predictions of particle trapping. In our models, the total gas surface density is not expected to show strong variations, in contrast to other gap-forming scenarios such as embedded giant planets or radial variations of the disk viscosity.

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arXiv2017

VLA observations of the disk around the young brown dwarf 2MASS J044427+2512

We present multi-wavelength radio observations obtained with the VLA of the protoplanetary disk surrounding the young brown dwarf 2MASS J04442713+2512164 (2M0444) in the Taurus star forming region. 2M0444 is the brightest known brown dwarf disk at millimeter wavelengths, making this an ideal target to probe radio emission from a young brown dwarf. Thermal emission from dust in the disk is detected at 6.8 and 9.1 mm, whereas the 1.36 cm measured flux is dominated by ionized gas emission. We combine these data with previous observations at shorter sub-mm and mm wavelengths to test the predictions of dust evolution models in gas-rich disks after adapting their parameters to the case of 2M0444. These models show that the radial drift mechanism affecting solids in a gaseous environment has to be either completely made inefficient, or significantly slowed down by very strong gas pressure bumps in order to explain the presence of mm/cm-sized grains in the outer regions of the 2M0444 disk. We also discuss the possible mechanisms for the origin of the ionized gas emission detected at 1.36 cm. The inferred radio luminosity for this emission is in line with the relation between radio and bolometric luminosity valid for for more massive and luminous young stellar objects, and extrapolated down to the very low luminosity of the 2M0444 brown dwarf.

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arXiv2017

A Multi-Wavelength Analysis of Dust and Gas in the SR 24S Transition Disk

We present new Atacama Large Millimeter/sub-millimeter Array (ALMA) 1.3 mm continuum observations of the SR 24S transition disk with an angular resolution $\lesssim0.18"$ (12 au radius). We perform a multi-wavelength investigation by combining new data with previous ALMA data at 0.45 mm. The visibilities and images of the continuum emission at the two wavelengths are well characterized by a ring-like emission. Visibility modeling finds that the ring-like emission is narrower at longer wavelengths, in good agreement with models of dust trapping in pressure bumps, although there are complex residuals that suggest potentially asymmetric structures. The 0.45 mm emission has a shallower profile inside the central cavity than the 1.3 mm emission. In addition, we find that the $^{13}$CO and C$^{18}$O (J=2-1) emission peaks at the center of the continuum cavity. We do not detect either continuum or gas emission from the northern companion to this system (SR 24N), which is itself a binary system. The upper limit for the dust disk mass of SR 24N is $\lesssim 0.12\,M_{\bigoplus}$, which gives a disk mass ratio in dust between the two components of $M_{\mathrm{dust, SR\,24S}}/M_{\mathrm{dust, SR\,24N}}\gtrsim840$. The current ALMA observations may imply that either planets have already formed in the SR 24N disk or that dust growth to mm-sizes is inhibited there and that only warm gas, as seen by ro-vibrational CO emission inside the truncation radii of the binary, is present.

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arXiv2017

A Surface Density Perturbation in the TW Hydrae Disk at 95 au Traced by Molecular Emission

We present ALMA Cycle~2 observations at $0.5^{\prime\prime}$ resolution of TW Hya of CS $J=5-4$ emission. The radial profile of the integrated line emission displays oscillatory features outwards of $1.5^{\prime\prime}$ ($\approx 90$ au). A dip-like feature at $1.6^{\prime\prime}$ is coincident in location, depth and width with features observed in dust scattered light at near-infrared wavelengths. Using a thermochemical model indicative of TW Hya, gas-grain chemical modelling and non-LTE radiative transfer, we demonstrate that such a feature can be reproduced with a surface density depression, consistent with the modelling performed for scattered light observations of TW Hya. We further demonstrate that a gap in the dust distribution and dust opacity only cannot reproduce the observed CS feature. The outer enhancement at $3.1^{\prime\prime}$ is identified as a region of intensified desorption due to enhanced penetration of the interstellar FUV radiation at the exponential edge of the disk surface density, which intensifies the photochemical processing of gas and ices.

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arXiv2016

Testing dust trapping in the circumbinary disk around GG Tau A

The protoplanetary disk around the GGTau A binary system is so far one of the most studied young circumbinary disk. Observations of the dust continuum emission at sub-mm/mm wavelengths detected a dust ring located between 200AU and 300AU from the center of mass of the system. If the disk and the binary orbit are coplanar, given the observed projected separation of the stars in the binary system, the classical theory of tidal interaction between a binary system and its circumbinary disk predicts a truncation radius which is significantly smaller than the measured inner radius of the mm-sized dust ring. We investigate the origin of this dust ring structure in the GG Tau A disk, test whether the interaction between the binary and the disk can produce a dust trap at the location of the observed ring, and discuss an alternative scenario which invokes a misalignment between the disk and the stellar orbital planes. We run a set of hydrodynamical simulations for an orbit consistent with the astrometric solutions for the GG Tau A stellar proper motions, and for different disk temperature and viscosities. We then apply a dust evolution model in post-processing in order to to retrieve the distribution of mm-sized grains. Comparing models and observations, we show that, if the binary orbit and the disk were coplanar,the tidal truncation of the circumbinary disk would occur at a radius that is too small compared to the inner edge inferred by the dust observations, and that the pressure bump and the dust ring in the models would be located at <150AU from the center of mass of the stellar system. This shows that the GG Tau A disk cannot be coplanar with the binary orbital plane. We also discuss the viability of the misaligned disk scenario, suggesting that in order for dust trapping to occur at the observed radius, the disk and orbital plane must be misaligned by an angle of about 25 degrees.

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arXiv2016

Investigating dust trapping in transition disks with millimeter-wave polarization

Spatially resolved polarized (sub-)mm emission has been observed for example in the protoplanetary disk around HL Tau. Magnetically aligned grains are commonly interpreted as the source of polarization. However, self-scattering by large dust grains with a high enough albedo is another polarization mechanism, becoming a compelling method independent of the spectral index to constrain the dust grain size in protoplanetary disks. We study the dust polarization at mm wavelength in the dust trapping scenario proposed for transition disks, when a giant planet opens a gap in the disk. We investigate the characteristic polarization patterns and their dependence on disk inclination, dust size evolution, planet position, and observing wavelength. We combine two-dimensional hydrodynamical simulations of planet-disk interactions with self-consistent dust growth models. These size-dependent dust density distributions are used for follow-up three-dimensional radiative transfer calculations to predict the polarization degree at ALMA bands due to scattered thermal emission. We find that the polarization pattern of a disk with a planetary gap after 1 Myr of dust evolution shows a distinctive three ring structure. Two narrow inner rings are located at the planet gap edges. For increasing observing wavelengths all three rings slightly change their position, where the innermost and outermost rings move inward. This distance is detectable comparing the results at ALMA bands 3, 6 and 7. Within the highest polarized intensity regions the polarization vectors are oriented in the azimuthal direction. For an inclined disk there is an interplay between polarization originating from a flux gradient and inclination-induced quadrupole polarization. For intermediate inclined transition disks the polarization degree is as high as ~ 2% at band 3, which is well above the detection limit of future ALMA observations.

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arXiv2016

Dust Evolution and the Formation of Planetesimals

The solid content of circumstellar disks is inherited from the interstellar medium: dust particles of at most a micrometer in size. Protoplanetary disks are the environment where these dust grains need to grow at least 13 orders of magnitude in size. Our understanding of this growth process is far from complete, with different physics seemingly posing obstacles to this growth at various stages. Yet, the ubiquity of planets in our galaxy suggests that planet formation is a robust mechanism. This chapter focuses on the earliest stages of planet formation, the growth of small dust grains towards the gravitationally bound "planetesimals", the building blocks of planets. We will introduce some of the key physics involved in the growth processes and discuss how they are expected to shape the global behavior of the solid content of disks. We will consider possible pathways towards the formation of larger bodies and conclude by reviewing some of the recent observational advances in the field.

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arXiv2016

Constraining turbulence mixing strength in transitional discs with planets using SPHERE and ALMA

We investigate the effect that the turbulent mixing strength parameter $α_{\rm{turb}}$ plays on near-infrared polarimetric and sub-millimetre interferometric imaging observations of transitional discs (TDs) with a gap carved by a planet. We generate synthetic observations of these objects with ALMA and VLT/SPHERE-ZIMPOL by combining hydrodynamical, dust evolution, radiative transfer and instrument models for values of $α_{\rm{turb}}=[10^{-4}, 10^{-3}, 10^{-2}]$. We find that, through a combination of effects on the viscosity of the gas, the turbulent mixing and dust evolution processes, $α_{\rm{turb}}$ strongly affects the morphology of the dust distribution that can be traced with these observations. We constrain the value of $α_{\rm{turb}}$ to be within an order of magnitude of $10^{-3}$ in TD sources that show cavities in sub-mm continuum images while featuring continuous distribution of dust or smaller cavities in NIR-polarimetric images.

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arXiv2016

The shadow of the Flying Saucer: A very low temperature for large dust grains

Dust determines the temperature structure of protoplanetary disks. However, dust temperature determinations almost invariably rely on a complex modeling of the Spectral Energy Distribution. We attempt a direct determination of the temperature of large grains emitting at mm wavelengths.} We observe the edge-on dust disk of the Flying Saucer, which appears in silhouette against the CO J=2-1 emission from a background molecular cloud in $ρ$ Oph. The combination of velocity gradients due to the Keplerian rotation of the disk and intensity variations in the CO background as a function of velocity allows us to directly measure the %absorbing dust temperature. The dust opacity can then be derived from the emitted continuum radiation. The dust disk absorbs the radiation from the CO clouds at several velocities. We derive very low dust temperatures, 5 to 7 K at radii around 100 au, which is much lower than most model predictions. The dust optical depth is $> 0.2$ at 230 GHz, and the scale height at 100 au is at least 8 au (best fit 13 au). However, the dust disk is very flat (flaring index -0.35), which is indicative of dust settling in the outer parts.

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arXiv2015

Variability and dust filtration in the transition disk J160421.7-213028 observed in optical scattered light

Context. Some of transition disks show asymmetric structures in thermal sub-millimetre emission and optical scattered light. These structures can be the result of planet(s) or companions embedded in the disk. Aims. We aim to detect and analyse the scattered light of the transition disk J160421.7-213028, identify disk structures, and compare the results with previous observations of this disk at other wavelengths. Methods. We obtained and analysed new polarised intensity observations of the transition disk J160421.7-213028 with VLT/SPHERE using the visible light instrument ZIMPOL at $R'$ band (0.626$μ$m). We probe the disk gap down to a radius of confidence of 0.1'' (${\sim}15$ AU at 145 pc). We interpret the results in the context of dust evolution when planets interact with the parental disk. Results. We observe a gap from 0.1 to 0.3'' (${\sim}15$ to 40AU) and a bright annulus as previously detected by HiCIAO $H$ band observations at $1.65μ$m. The radial width of the annulus is around $40$AU, and its centre is at ${\sim}61$AU from the central star. The peak of the reflected light at 0.626$μ$m is located 20 AU inward of the cavity detected in the submillimetre. In addition, we detect a dip at a position angle of ${\sim}46.2 \pm 5.4^\circ$. A dip was also detected with HiCIAO, but located at ${\sim}85^\circ$. If the dip observed with HiCIAO is the same, this suggests an average dip rotation of ${\sim}12^\circ/$year, which is inconsistent with the local Keplerian angular velocity of $\sim$0.8$^\circ$/yr at $\sim$61AU. Conclusions. The spatial discrepancy in the radial emission in J160421.7-213028 at different wavelengths is consistent with dust filtration at the outer edge of a gap carved by a massive planet. The dip rotation can be interpreted by fast variability of the inner disk and/or the presence of a warp or circumplanetary material of a planet at ${\sim}9.6$ AU.

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arXiv2015

Testing particle trapping in transition disks with ALMA

We present new Atacama Large Millimeter/submillimeter Array (ALMA) continuum observations at 336GHz of two transition disks, SR21 and HD135344B. In combination with previous ALMA observations from Cycle 0 at 689GHz, we compare the visibility profiles at the two frequencies and calculate the spectral index ($α_{\rm{mm}}$). The observations of SR21 show a clear shift in the visibility nulls, indicating radial variations of the inner edge of the cavity at the two wavelengths. Notable radial variations of the spectral index are also detected for SR21 with values of $α_{\rm{mm}}{\sim}3.8-4.2$ in the inner region ($r<35$ AU) and $α_{\rm{mm}}{\sim}2.6-3.0$ outside. An axisymmetric ring (which we call the ring model) or a ring with the addition of an azimuthal Gaussian profile, for mimicking a vortex structure (which we call the vortex model), is assumed for fitting the disk morphology. For SR21, the ring model better fits the emission at 336GHz, conversely the vortex model better fits the 689GHz emission. For HD135344B, neither a significant shift in the null of the visibilities nor radial variations of $α_{\rm{mm}}$ are detected. Furthermore, for HD135344B, the vortex model fits both frequencies better than the ring model. However, the azimuthal extent of the vortex increases with wavelength, contrary to model predictions for particle trapping by anticyclonic vortices. For both disks, the azimuthal variations of $α_{\rm{mm}}$ remain uncertain to confirm azimuthal trapping. The comparison of the current data with a generic model of dust evolution that includes planet-disk interaction suggests that particles in the outer disk of SR21 have grown to millimetre sizes and have accumulated in a radial pressure bump, whereas with the current resolution there is not clear evidence of radial trapping in HD135344B, although it cannot be excluded either.

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arXiv2015

Sequential planet formation in the HD 100546 protoplanetary disk?

Context. The disk around the Herbig Ae star, HD 100546, shows structures that suggest the presence of two companions in the disk at $\sim10$ and $\sim70$ AU. The outer companion seems to be in the act of formation. Aims. Our aims are to provide constraints on the age of the planets in HD 100546 and to explore the potential evidence for sequential planet formation in transition disks such as HD 100546. Methods. We compare the recent resolved continuum observations of the disk around HD 100546 with the results of dust evolution simulations using an analytical prescription for the shapes of gaps carved by massive planets. Results. An inner pressure bump must have been present since early in the disk lifetime to have good agreement between the dust evolution models and the continuum observations of HD 100546. This pressure bump may have resulted from the presence of a very massive planet ($\sim20 M_{\rm{Jup}}$), which formed early in the inner disk ($r\sim$10 AU). If only this single planet exists, the disk is likely to be old, comparable to the stellar age ($\sim$5-10 Myr). Another possible explanation is an additional massive planet in the outer disk ($r\sim70$ AU): either a low-mass outer planet ($\lesssim5 M_{\rm{Jup}}$) injected at early times, or a higher mass outer planet ($\gtrsim15 M_{\rm{Jup}}$) formed very recently, traps the right amount of dust in pressure bumps to reproduce the observations. In the latter case, the disk could be much younger ($\sim3.0$ Myr). Conclusions. In the case in which two massive companions are embedded in the disk around HD 100546, as suggested in the literature, the outer companion could be at least $\gtrsim$2.5 Myr younger than the inner companion.

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arXiv2015

Asymmetric features in the protoplanetary disk MWC758

The study of dynamical processes in protoplanetary disks is essential to understand planet formation. In this context, transition disks are prime targets because they are at an advanced stage of disk clearing and may harbor direct signatures of disk evolution. In this paper, we aim to derive new constraints on the structure of the transition disk MWC 758, to detect non-axisymmetric features and understand their origin. We obtained infrared polarized intensity observations of the protoplanetary disk MWC 758 with SPHERE/VLT at 1.04 microns to resolve scattered light at a smaller inner working angle (0.093") and a higher angular resolution (0.027") than previously achieved. We observe polarized scattered light within 0.53" (148 au) down to the inner working angle (26 au) and detect distinct non-axisymmetric features but no fully depleted cavity. The two small-scale spiral features that were previously detected with HiCIAO are resolved more clearly, and new features are identified, including two that are located at previously inaccessible radii close to the star. We present a model based on the spiral density wave theory with two planetary companions in circular orbits. The best model requires a high disk aspect ratio (H/r~0.20 at the planet locations) to account for the large pitch angles which implies a very warm disk. Our observations reveal the complex morphology of the disk MWC758. To understand the origin of the detected features, the combination of high-resolution observations in the submillimeter with ALMA and detailed modeling is needed.

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arXiv2014

Gas and dust structures in protoplanetary disks hosting multiple planets

Transition disks have dust-depleted inner regions and may represent an intermediate step of an on-going disk dispersal process, where planet formation is probably in progress. Recent millimetre observations of transition disks reveal radially and azimuthally asymmetric structures, where micron- and millimetre-sized dust particles may not spatially coexist. These properties can be the result of particle trapping and grain growth in pressure bumps originating from the disk interaction with a planetary companion. The multiple features observed in some transition disks, such as SR 21, suggest the presence of more than one planet. We study the gas and dust distributions of a disk hosting two massive planets as function of different disk and dust parameters. Observational signatures, such as the spectral energy distribution, sub-millimetre, and polarised images are simulated for the various parameters. We confirm that planets can lead to particle trapping, although for a disk with high viscosity ($α_{\rm{turb}}=10^{-2}$), the planet should be more massive than $5 M_{\rm{Jup}}$ and dust fragmentation should occur with low efficiency ($v_{f}\sim30\rm{m s}^{-1}$). This will lead to a ring-like feature as observed in transition disks in the millimetre. When trapping occurs, we find that a smooth distribution of micron sized grains throughout the disk, sometimes observed in scattered light, can only happen if the combination of planet mass and turbulence is such that small grains are not fully filtered out. A high disk viscosity ($α_{\rm{turb}}=10^{-2}$) ensures a replenishment of the cavity in micron-sized dust, while for lower viscosity ($α_{\rm{turb}}=10^{-3}$), the planet mass is constrained to be less than $5 M_{\rm{Jup}}$. In these cases, the gas distribution is likely to show low-amplitude azimuthal asymmetries caused by disk eccentricity rather than by long-lived vortices.

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arXiv2014

ALMA hints at the presence of two companions in the disk around HD 100546

HD 100546 is a well-studied Herbig Be star-disk system that likely hosts a close-in companion with compelling observational evidence for an embedded protoplanet at 68 AU. We present ALMA observations of the HD 100546 disk which resolve the gas and dust structure at (sub)mm wavelengths. The CO emission (at 345.795 GHz) originates from an extensive molecular disk (390+/-20 AU in radius) whereas the continuum emission is more compact (230+/-20 AU in radius) suggesting radial drift of the mm-sized grains. The CO emission is similar in extent to scattered light images indicating well-mixed gas and um-sized grains in the disk atmosphere. Assuming azimuthal symmetry, a single-component power-law model cannot reproduce the continuum visibilities. The visibilities and images are better reproduced by a double-component model: a compact ring with a width of 21 AU centered at 26 AU and an outer ring with a width of 75+/-3 AU centered at 190+/-3 AU. The influence of a companion and protoplanet on the dust evolution is investigated. The companion at 10 AU facilitates the accumulation of mm-sized grains within a compact ring, ~ 20 - 30 AU, by ~ 10 Myr. The injection of a protoplanet at 1 Myr hastens the ring formation (~ 1.2 Myr) and also triggers the development of an outer ring (~ 100 - 200 AU). These observations provide additional evidence for the presence of a close-in companion and hint at dynamical clearing by a protoplanet in the outer disk.

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arXiv2014

Millimetre spectral indices of transition disks and their relation to the cavity radius

Transition disks are protoplanetary disks with inner depleted dust cavities and excellent candidates to investigate the dust evolution under the existence of a pressure bump. A pressure bump at the outer edge of the cavity allows dust grains from the outer regions to stop their rapid inward migration towards the star and efficiently grow to millimetre sizes. Dynamical interactions with planet(s) have been one of the most exciting theories to explain the clearing of the inner disk. We look for evidence of the presence of millimetre dust particles in transition disks by measuring their spectral index with new and available photometric data. We investigate the influence of the size of the dust depleted cavity on the disk integrated millimetre spectral index. We present the 3mm photometric observations carried out with PdBI of four transition disks: LkHa330, UXTauA, LRLL31, and LRLL67. We use available values of their fluxes at 345GHz to calculate their spectral index, as well as the spectral index for a sample of twenty transition disks. We compare the observations with two kind of models. In the first set of models, we consider coagulation and fragmentation of dust in a disk in which a cavity is formed by a massive planet located at different positions. The second set of models assumes disks with truncated inner parts at different radius and with power-law dust size distributions, where the maximum size of grains is calculated considering turbulence as the source of destructive collisions. We show that the integrated spectral index is higher for transition disks than for regular protoplanetary disks. For transition disks, the probability that the measured spectral index is positively correlated with the cavity radius is 95%. High angular resolution imaging of transition disks is needed to distinguish between the dust trapping scenario and the truncated disk case.

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