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TOPIC: Circumstellar disks
Blobrana


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RE: Circumstellar disks
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Title: The formation of planets in circumbinary disks
Authors: F. I. Pelupessy, S. Portegies Zwart

We examine the formation of planets around binary stars in light of the recently discovered systems Kepler 16, 34 and 35. We conduct hydrodynamical simulations of self gravitating disks around binary systems. The selected binary and disk parameters are chosen consistent with observed systems. The disks are evolved until they settle in a quasi-equilibrium and the resulting systems are compared with the parameters of Kepler 16, 34 and 35. We find a close correspondence of the peak density at the inner disk gap and the orbit of the observed planets. We conclude, based on our simulations, that the orbits of the observed Kepler planets are determined by the size of the inner disk gap which for these systems results from the binary driving. This mediates planet formation either through the density enhancement or through planetary trapping at the density gradient inversion in the inner disk. For all three systems the current eccentricity of the planetary orbit is less than the disk eccentricity in the simulations. This, together with the long term stability of the orbits argues against in situ formation (e.g. a direct collapse scenario of the material in the ring). Conducting additional simulations of systems with a wider range of parameters (taken from a survey of eclipsing binaries), we find that the planet semi-major axis and binary eccentricity in such a scenario should be tightly correlated providing an observational test of this formation mechanism.

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Blobrana


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Posts: 131433
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Title: Coplanar Circumbinary Debris Disks
Authors: G. M. Kennedy, M. C. Wyatt, B. Sibthorpe, N. M. Phillips, B. Matthews, J. S. Greaves

We present resolved Herschel images of circumbinary debris disks in the alpha CrB (HD139006) and beta Tri (HD13161) systems. We find that both disks are consistent with being aligned with the binary orbital planes. Though secular perturbations from the binary can align the disk, in both cases the alignment time at the distances at which the disk is resolved is greater than the stellar age, so we conclude that the coplanarity was primordial. Neither disk can be modelled as a narrow ring, requiring extended radial distributions. To satisfy both the Herschel and mid-IR images of the alpha CrB disk, we construct a model that extends from 1-300AU, whose radial profile is broadly consistent with a picture where planetesimal collisions are excited by secular perturbations from the binary. However, this model is also consistent with stirring by other mechanisms, such as the formation of Pluto-sized objects. The beta Tri disk model extends from 50-400AU. A model with depleted (rather than empty) inner regions also reproduces the observations and is consistent with binary and other stirring mechanisms. As part of the modelling process, we find that the Herschel PACS beam varies by as much as 10% at 70um and a few % at 100um. The 70um variation can therefore hinder image interpretation, particularly for poorly resolved objects. The number of systems in which circumbinary debris disk orientations have been compared with the binary plane is now four. More systems are needed, but a picture in which disks around very close binaries (alpha CrB, beta Tri, and HD 98800, with periods of a few weeks to a year) are aligned, and disks around wider binaries (99 Her, with a 50 yr period) are misaligned, may be emerging. This picture is qualitatively consistent with the expectation that the protoplanetary disks from which the debris emerged are more likely to be aligned if their binaries have shorter periods.

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Blobrana


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Posts: 131433
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Title: Observing planet-disk interaction in debris disks
Authors: Steve Ertel, Sebastian Wolf, Jens Rodmann

Context. Structures in debris disks induced by planetdisk interaction are promising to provide valuable constraints on the existence and properties of embedded planets.
Aims. We investigate the observability of structures in debris disks induced by planet-disk interaction.
Methods. The observability of debris disks with the Atacama Large Millimetre/submillimetre Array (ALMA) is studied on the basis of a simple analytical disk model. Furthermore, N-body simulations are used to model the spatial dust distribution in debris disks under the influence of planet-disk interaction. Images at optical scattered light to millimetre thermal re-emission are computed. Available information about the expected capabilities of ALMA and the James Webb Space Telescope (JWST) are used to investigate the observability of characteristic disk structures through spatially resolved imaging.
Results. Planet-disk interaction can result in prominent structures. This provides the opportunity of detecting and characterising extrasolar planets in a range of masses and radial distances from the star that is not accessible to other techniques. Facilities that will be available in the near future are shown to provide the capabilities to spatially resolve and characterise structures in debris disks. Limitations are revealed and suggestions for possible instrument setups and observing strategies are given. In particular, ALMA is limited by its sensitivity to surface brightness, which requires a trade-off between sensitivity and spatial resolution. Space-based midinfrared observations will be able to detect and spatially resolve regions in debris disks even at a distance of several tens of AU from the star, where the emission from debris disks in this wavelength range is expected to be low.

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Blobrana


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Posts: 131433
Date:
Young Circumstellar Disks
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Title: First Detection of Near-Infrared Line Emission from Organics in Young Circumstellar Disks
Authors: A. M. Mandell, J. Bast, E. F. van Dishoeck, G. A. Blake, C. Salyk, M. J. Mumma, G. Villanueva

We present an analysis of high-resolution spectroscopy of several bright T Tauri stars using the VLT/CRIRES and Keck/NIRSPEC spectrographs, revealing the first detections of emission from HCN and C2H2 in circumstellar disks at near-infrared wavelengths. Using advanced data reduction techniques we achieve a dynamic range with respect to the disk continuum of ~500 at 3 microns, revealing multiple emission features of H2O, OH, HCN, and C2H2. We also present stringent upper limits for two other molecules thought to be abundant in the inner disk, CH4 and NH3. Line profiles for the different detected molecules are broad but centrally peaked in most cases, even for disks with previously determined inclinations of greater than 20 degrees, suggesting that the emission has both a Keplerian and non-Keplerian component as observed previously for CO emission. We apply two different modelling strategies to constrain the molecular abundances and temperatures: we use a simplified single-temperature LTE slab model with a Gaussian line profile to make line identifications and determine a best-fit temperature and initial abundance ratios, and we compare these values with constraints derived from a detailed disk radiative transfer model assuming LTE excitation but utilizing a realistic temperature and density structure. Abundance ratios from both sets of models are consistent with each other and consistent with expected values from theoretical chemical models, and analysis of the line shapes suggests the molecular emission originates from within a narrow region in the inner disk (R < 1 AU).

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Blobrana


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Posts: 131433
Date:
RE: Circumstellar disks
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Title: A hot compact dust disk around a massive young stellar object
Authors: Stefan Kraus, Karl-Heinz Hofmann, Karl Menten, Dieter Schertl, Gerd Weigelt, Friedrich Wyrowski, Anthony Meilland, Karine Perraut, Romain Petrov, Sylvie Robbe-Dubois, Peter Schilke, Leonardo Testi

Circumstellar disks are an essential ingredient of the formation of low-mass stars. It is unclear, however, whether the accretion-disk paradigm can also account for the formation of stars more massive than about 10 solar masses, in which strong radiation pressure might halt mass infall. Massive stars may form by stellar merging, although more recent theoretical investigations suggest that the radiative-pressure limit may be overcome by considering more complex, nonspherical infall geometries. Clear observational evidence, such as the detection of compact dusty disks around massive young stellar objects, is needed to identify unambiguously the formation mode of the most massive stars. Here we report near-infrared interferometric observations that spatially resolve the astronomical unit-scale distribution of hot material around a high-mass (approx. 20 solar masses) young stellar object. The image shows an elongated structure with a size of about 13 x 19 astronomical units, consistent with a disk seen at an inclination angle of 45 degree. Using geometric and detailed physical models, we found a radial temperature gradient in the disk, with a dust-free region less than 9.5 astronomical units from the star, qualitatively and quantitatively similar to the disks observed in low-mass star formation. Perpendicular to the disk plane we observed a molecular outflow and two bow shocks, indicating that a bipolar outflow emanates from the inner regions of the system.

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Blobrana


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Posts: 131433
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Title: Tracing the potential planet-forming regions around seven pre-main-sequence stars
Authors: A. A. Schegerer, S. Wolf, C. A. Hummel, S. P. Quanz, A. Richichi

We investigate the nature of the innermost regions of seven circumstellar disks around pre-main-sequence stars. Our object sample contains disks apparently at various stages of their evolution. Both single stars and spatially resolved binaries are considered. In particular, we search for inner disk gaps as proposed for several young stellar objects. When analysing the underlying dust population in the atmosphere of circumstellar disks, the shape of the 10um feature is investigated. We performed interferometric observations in N band 8-13um with MIDI using baseline lengths of between 54m and 127m. The data analysis is based on radiative-transfer simulations using the Monte Carlo code MC3D by modelling simultaneously the SED, N band spectra, and interferometric visibilities. Correlated and uncorrelated N band spectra are compared to investigate the radial distribution of the dust composition of the disk atmosphere. Spatially resolved mid-infrared emission was detected in all objects. For four objects, the observed N band visibilities and corresponding SEDs could be simultaneously simulated using a parameterised active disk-model. For the more evolved objects of our sample, a purely passive disk-model provides the closest fit. The visibilities inferred for one source allow the presence of an inner disk gap. For another object, one of two visibility measurements could not be simulated by our modelling approach. All uncorrelated spectra reveal the 10um silicate emission feature. In contrast to this, some correlated spectra of the observations of the more evolved objects do not show this feature, indicating a lack of small silicates in the inner versus the outer regions of these disks. We conclude from this observational result that more evolved dust grains can be found in the more central disk regions.

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