Variable dynamics in the inner disk of HD 135344B revealed with multi-epoch scattered light imaging

Variable dynamics in the inner disk of HD 135344B revealed with multi-epoch scattered light imaging

 

Authors:

Stolker et al

Abstract:

We present multi-epoch VLT/SPHERE observations of the protoplanetary disk around HD 135344B (SAO 206462). The J-band scattered light imagery reveal, with high spatial resolution (∼41 mas, 6.4 au), the disk surface beyond ∼20 au. Temporal variations are identified in the azimuthal brightness distributions of all epochs, presumably related to the asymmetrically shading dust distribution in the inner disk. These shadows manifest themselves as narrow lanes, cast by localized density enhancements, and broader features which possibly trace the larger scale dynamics of the inner disk. We acquired visible and near-infrared photometry which shows variations up to 10% in the JHK bands, possibly correlated with the presence of the shadows. Analysis of archival VLTI/PIONIER H-band visibilities constrain the orientation of the inner disk to i=18.2deg+3.4−4.1 and PA=57.3deg±5.7deg, consistent with an alignment with the outer disk or a minor disk warp of several degrees. The latter scenario could explain the broad, quasi-stationary shadowing in N-NW direction in case the inclination of the outer disk is slightly larger. The correlation between the shadowing and the near-infrared excess is quantified with a grid of radiative transfer models. The variability of the scattered light contrast requires extended variations in the inner disk atmosphere (H/r≲0.2). Possible mechanisms that may cause asymmetric variations in the optical depth (Δτ≲1) through the atmosphere of the inner disk include turbulent fluctuations, planetesimal collisions, or a dusty disk wind, possibly enhanced by a minor disk warp. A fine temporal sampling is required to follow day-to-day changes of the shadow patterns which may be a face-on variant of the UX Orionis phenomenon.

Shadows cast on the transition disk of HD 135344B

Shadows cast on the transition disk of HD 135344B
Multiwavelength VLT/SPHERE polarimetric differential imaging
Author:

Stolker et al

Abstract:

Context.

The protoplanetary disk around the F-type star HD 135344B (SAO 206462) is in a transition stage and shows many intriguing structures both in scattered light and thermal (sub-)millimeter emission which are possibly related to planet formation processes.

Aims.

We aim to study the morphology and surface brightness of the disk in scattered light to gain insight into the innermost disk regions, the formation of protoplanets, planet-disk interactions traced in the surface and midplane layers, and the dust grain properties of the disk surface.

Methods.

We have carried out high-contrast polarimetric differential imaging (PDI) observations with VLT/SPHERE and obtained polarized scattered light images with ZIMPOL in the R and I-bands and with IRDIS in the Y and J-bands. The scattered light images and surface brightness profiles are used to study in detail structures in the disk surface and brightness variations. We have constructed a 3D radiative transfer model to support the interpretation of several detected shadow features.

Results.

The scattered light images reveal with unprecedented angular resolution and sensitivity the spiral arms as well as the 25 au cavity of the disk. Multiple shadow features are discovered on the outer disk with one shadow only being present during the second observation epoch. A positive surface brightness gradient is observed in the stellar irradiation corrected (r²-scaled) images in southwest direction possibly due to an azimuthally asymmetric perturbation of the temperature and/or surface density by the passing spiral arms. The disk integrated polarized flux, normalized to the stellar flux, shows a positive trend towards longer wavelengths which we attribute to large (2πa ≳ λ) aggregate dust grains in the disk surface. Part of the non-azimuthal polarization signal in the U_φ image of the J-band observation can be attributed to multiple scattering in the disk.

Conclusions.

The detected shadow features and their possible variability have the potential to provide insight into the structure of and processes occurring in the innermost disk regions. Possible explanations for the presence of the shadows include a 22° misaligned inner disk, a warped disk region that connects the inner disk with the outer disk, and variable or transient phenomena such as a perturbation of the inner disk or an asymmetric accretion flow. The spiral arms are best explained by one or multiple protoplanets in the exterior of the disk although no gap is detected beyond the spiral arms up to 1.′′0.

Self-Sustained Recycling in the Inner Dust Ring of Pre-Transitional Disks

Self-Sustained Recycling in the Inner Dust Ring of Pre-Transitional Disks

Authors:

Husmann et al

Abstract:

Observations of pre-transitional disks show a narrow inner dust ring and a larger outer one. They are separated by a cavity with no or only little dust. We propose an efficient recycling mechanism for the inner dust ring which keeps it in a steady-state. No major particle sources are needed for replenishment. Dust particles and pebbles drift outwards by radiation pressure and photophoresis. The pebbles grow during outward drift until they reach a balanced position where residual gravity compensates photophoresis. While still growing larger they reverse their motion and drift inwards. Eventually, their speed is fast enough that they get destroyed in collisions with other pebbles and drift outwards again. We quantify the force balance and drift velocities for the disks LkCa15 and HD135344B. We simulate single particle evolution and show that this scenario is viable. Growth and drift timescales are on the same order and a steady state can be established in the inner dust ring.

HD 135344B's Protoplanetary Disk Shows Evidence of at Least one Gas Giant Protoplanet

Shadows cast on the transition disk of HD 135344B

Authors:

Stolker et al

Abstract:

The protoplanetary disk of the F-type star HD 135344B (SAO 206462) is in a transition stage and shows many intriguing structures both in scattered light and thermal millimeter emission which are possibly related to planet formation processes and planet-disk interactions. We have carried out high-contrast polarimetric differential imaging (PDI) observations with VLT/SPHERE and obtained the first optical polarized scattered light images with the sub-instrument ZIMPOL in R- and I-band. Additionally, near-infrared polarimetric observations were done with IRDIS in Y- and J-band. We will use the scattered light images, surface brightness profiles and color to study in detail disk structures and brightness variations. The scattered light images reveal with unprecedented sensitivity and angular resolution the spiral arm structure of the disk as well as the inner disk cavity of 25 au in all filters. Multiple shadow features are discovered on the outer disk and the observations of the two epochs show indications of variability of one shadow. A positive surface brightness gradient is observed in the r^2-scaled images in south-west direction due to an azimuthally asymmetric perturbation of the temperature and/or surface density by the passing spiral arms. The scattering efficiency in polarized light shows a positive linear trend towards longer wavelengths presumably because of large/aggregate dust grains (2pi a greater than or equal lambda) in the disk surface. The shadows on the outer disk of HD 135344B could be cast by an inner dust belt which is 22 degrees inclined with respect to the outer disk, a warped disk region which connects the inner disk with the cavity and an accretion funnel flow from the inner disk onto the star. The wide open spiral arms indicate the presence of one or multiple massive protoplanets, a local disk instability beyond the dust cavity or a combination of the two.

Particle trapping in Transition Disks

Testing particle trapping in transition disks with ALMA

Authors:

Pinilla et al

Abstract:

We present new ALMA continuum observations at 336 GHz of two transition disks, SR 21 and HD 135344B. In combination with previous ALMA observations from Cycle 0 at 689 GHz, we compare the visibility profiles at the two frequencies and calculate the spectral index (αmm). The observations of SR 21 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 SR 21 with values of αmm∼3.8−4.2 in the inner region (r≲35 AU) and αmm∼2.6−3.0 outside. An axisymmetric ring ("ring model") or a ring with the addition of an azimuthal Gaussian profile, for mimicking a vortex structure ("vortex model"), is assumed for fitting the disk morphology. For SR 21, the ring model better fits the emission at 336 GHz, conversely the vortex model better fits the 689 GHz emission. For HD 135344B, neither a significant shift in the null of the visibilities nor radial variations of αmm are detected. Furthermore, for HD 135344B, 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 αmm 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 SR 21 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 HD 135344B, although it cannot be excluded either.

Observing the Structure of HD 135344B's Circumstellar Disk

Constraining the structure of the transition disk HD 135344B (SAO 206462) by simultaneous modeling of multi-wavelength gas and dust observations

Authors:

Carmona et al

Abstract:

HD 135344B is an accreting (pre-) transition disk which displays emission of warm CO extending tens of AU inside its 30 AU dust cavity. We employ the dust radiative transfer code MCFOST and the thermo-chemical code ProDiMo to derive the disk structure from the simultaneous modeling of the spectral energy distribution (SED), VLT/CRIRES CO P(10) 4.75 micron, Herschel/PACS [O I] 63 micron, Spitzer-IRS, and JCMT 12CO J=3-2 spectra, VLTI/PIONIER H-band visibilities, and constraints from (sub-)mm continuum interferometry and near-IR imaging. We found a disk model able to describe simultaneously the current observations. This disk has the following structure: (1) to reproduce the SED, the near-IR interferometry data, and the CO ro-vibrational emission, refractory grains (we suggest carbon) are present inside the silicate sublimation radius (0.08 less than R less than 0.2 AU); (2) the dust cavity (R less than 30 AU) is filled with gas, the surface density of this gas must increase with radius to fit the CO P(10) line profile, a small gap of a few AU in the gas is compatible with current data, a large gap in the gas is not likely; (4) the gas/dust ratio inside the cavity is greater than 100 to account for the 870 micron continuum upper limit and the CO P(10) line flux; (5) the gas/dust ratio at 30 less than R less than 200 AU is less than 10 to simultaneously describe the [O I] 63 micron line flux and the CO P(10) line profile; (6) in the outer disk most of the mass should be located in the mid-plane and a significant fraction of the dust is in large grains. Conclusions: Simultaneous modeling of the gas and dust it is required to break the model degeneracies and constrain the disk structure. An increasing gas surface density with radius in the inner dust cavity echoes the effect of a migrating jovian planet. The global low gas mass (a few MJupiter) in the HD 135344B's disk suggests that it is an evolved disk that has already lost a large fraction of its mass.