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Register a new userReview of gas and dust in debris discs
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Authors
Quentin Kral
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This proceeding summarises a talk given on the state-of-the-art of debris disc modelling. We first review the basics of debris disc physics, which is followed by a short overview of the state-of-the-art in terms of modelling dust and gas in debris disc systems.
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We have conducted a search for optical circumstellar absorption lines in the spectra of 16 debris disc host stars. None of the stars in our sample showed signs of emission line activity in either H$_{alpha}$, Ca II or Na I, confirming their more evolved nature. Four stars were found to exhibit narrow absorption features near the cores of the photospheric Ca II and Na I D lines (when Na I D data were available). We analyse the characteristics of these spectral features to determine whether they are of circumstellar or interstellar origins. The strongest evidence for circumstellar gas is seen in the spectrum of HD110058, which is known to host a debris disc observed close to edge-on. This is consistent with a recent ALMA detection of molecular gas in this debris disc, which shows many similarities to the $beta$ Pictoris system.
We present 3D smoothed particle hydrodynamics simulations of protoplanetary discs undergoing a flyby by a stellar perturber on a parabolic orbit lying in a plane inclined relative to the disc mid-plane. We model the disc as a mixture of gas and dust, with grains ranging from 1 {mu}m to 10 cm in size. Exploring different orbital inclinations, periastron distances and mass ratios, we investigate the disc dynamical response during and after the flyby. We find that flybys induce evolving spiral structure in both gas and dust which can persist for thousands of years after periastron. Gas and dust structures induced by the flyby differ because of drag-induced effects on the dust grains. Variations in the accretion rate by up to an order of magnitude occur over a time-scale of order 10 years or less, inducing FU Orionis-like outbursts. The remnant discs are truncated and warped. The dust disc is left more compact than the gas disc, both because of disc truncation and accelerated radial drift of grains induced by the flyby.
We present far-IR/sub-mm imaging and spectroscopy of 49 Ceti, an unusual circumstellar disk around a nearby young A1V star. The system is famous for showing the dust properties of a debris disk, but the gas properties of a low-mass protoplanetary disk. The data were acquired with the Herschel Space Observatory PACS and SPIRE instruments, largely as part of the Gas in Protoplanetary Systems (GASPS) Open Time Key Programme. Disk dust emission is detected in images at 70, 160, 250, 350, and 500 mu m; 49 Cet is significantly extended in the 70 mu m image, spatially resolving the outer dust disk for the first time. Spectra covering small wavelength ranges centered on eight atomic and molecular emission lines were obtained, including [OI] 63 mu m and [CII] 158 mu m. The CII line was detected at the 5sigma level - the first detection of atomic emission from the disk. No other emission lines were seen, despite the fact that the OI line is the brightest one observed in Herschel protoplanetary disk spectra (Meeus et al. 2012; Dent et al. 2013). We present an estimate of the amount of circumstellar atomic gas implied by the CII emission. The new far-IR/sub-mm data fills in a large gap in the previous spectral energy distribution (SED) of 49 Cet. A simple model of the new SED confirms the two-component structure of the disk: warm inner dust and cold outer dust that produces most of the observed excess. Finally, we discuss preliminary thermochemical modeling of the 49 Cet gas/dust disk and our attempts to match several observational results simultaneously. Although we are not yet successful in doing so, our investigations shed light on the evolutionary status of the 49 Cet gas, which might not be primordial gas but rather secondary gas coming from comets.
In most debris discs, dust grain dynamics is strongly affected by stellar radiation pressure. As this mechanism is size-dependent, we expect dust grains to be spatially segregated according to their sizes. However, because of the complex interplay between radiation pressure, collisions and dynamical perturbations, this spatial segregation of the particle size distribution (PSD) has proven difficult to investigate with numerical models. We propose to explore this issue using a new-generation code that can handle some of the coupling between dynamical and collisional effects. We investigate how PSDs behave in both unperturbed discs at rest and in discs pertubed by planetary objects. We use the DyCoSS code of Thebault(2012) to investigate the coupled effect of collisions, radiation pressure and dynamical perturbations in systems having reached a steady state. We consider 2 setups: a narrow ring perturbed by an exterior planet, and an extended disc into which a planet is embedded. For both setups we consider an additional unperturbed case with no planet. We also investigate how possible spatial size segregation affect disc images at different wavelengths. We find that PSDs are always strongly spatially segregated. The only case for which they follow a standard dn/dr = C.r**(-3.5) law is for an unperturbed narrow ring, but only within the parent body ring itself. For all other configurations, the PSD can strongly depart from such power laws and have strong spatial gradients. As an example, the geometrical cross section of the disc is rarely dominated by the smallest grains on bound orbits, as it is expected to be in standard PSDs in s**q with q<-3. Although the exact profiles and spatial variations of PSDs are a complex function of the considered set-up, we are however able to derive some robust results that should be useful for image-or-SED-fitting models of observed discs.
We study the linear perturbations of collisionless near-Keplerian discs. Such systems are models for debris discs around stars and the stellar discs surrounding supermassive black holes at the centres of galaxies. Using a finite-element method, we solve the linearized collisionless Boltzmann equation and Poissons equation for a wide range of disc masses and rms orbital eccentricities to obtain the eigenfrequencies and shapes of normal modes. We find that these discs can support large-scale `slow modes, in which the frequency is proportional to the disc mass. Slow modes are present for arbitrarily small disc mass so long as the self-gravity of the disc is the dominant source of apsidal precession. We find that slow modes are of two general types: parent modes and hybrid child modes, the latter arising from resonant interactions between parent modes and singular van Kampen modes. The most prominent slow modes have azimuthal wavenumbers $m=1$ and $m=2$. We illustrate how slow modes in debris discs are excited during a fly-by of a neighbouring star. Many of the non-axisymmetric features seen in debris discs (clumps, eccentricity, spiral waves) that are commonly attributed to planets could instead arise from slow modes; the two hypotheses can be distinguished by long-term measurements of the pattern speed of the features.
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