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Register a new userModeling the delivery of dust from discs to ionized winds
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A necessary first step for dust removal in protoplanetary disc winds is the delivery of dust from the disc to the wind. In the case of ionized winds, the disc and wind are sharply delineated by a narrow ionization front where the gas density and temperature vary by more than an order of magnitude. Using a novel method that is able to model the transport of dust across the ionization front in the presence of disc turbulence, we revisit the problem of dust delivery. Our results show that the delivery of dust to the wind is determined by the vertical gas flow through the disc induced by the mass loss, rather than turbulent diffusion (unless the turbulence is strong, i.e. $alpha gtrsim 0.01$). Using these results we provide a simple relation between the maximum size of particle that can be delivered to the wind and the local mass-loss rate per unit area from the wind. This relation is independent of the physical origin of the wind and predicts typical sizes in the 0.01 -- $1,mu m$ range for EUV or X-ray driven winds. These values are a factor $sim 10$ smaller than those obtained when considering only whether the wind is able to carry away the grains.
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X-ray- and EUV- (XEUV-) driven photoevaporative winds acting on protoplanetary disks around young T-Tauri stars may crucially impact disk evolution, affecting both gas and dust distributions. We investigate the dust entrainment in XEUV-driven photoevaporative winds and compare our results to existing MHD and EUV-only models. For an X-ray luminosity of $L_X = 2 cdot 10^{30},mathrm{erg/s}$ emitted by a $M_* = 0.7,mathrm{M}_odot$ star, corresponding to a wind mass-loss rate of $dot{M}_mathrm{w} simeq 2.6 cdot 10^{-8} ,mathrm{M_odot/yr}$, we find dust entrainment for sizes $a_0 lesssim 11,mu$m ($9,mu$m) from the inner $25,$AU ($120,$AU). This is an enhancement over dust entrainment in less vigorous EUV-driven winds with $dot{M}_mathrm{w} simeq 10^{-10},mathrm{M_odot/yr}$. Our numerical model also shows deviations of dust grain trajectories from the gas streamlines even for $mu$m-sized particles. In addition, we find a correlation between the size of the entrained grains and the maximum height they reach in the outflow.
When imaged at high-resolution, many proto-planetary discs show gaps and rings in their dust sub-mm continuum emission profile. These structures are widely considered to originate from local maxima in the gas pressure profile. The properties of the underlying gas structures are however unknown. In this paper we present a method to measure the dust-gas coupling $alpha/St$ and the width of the gas pressure bumps affecting the dust distribution, applying high-precision techniques to extract the gas rotation curve from emission lines data-cubes. As a proof-of-concept, we then apply the method to two discs with prominent sub-structure, HD163296 and AS 209. We find that in all cases the gas structures are larger than in the dust, confirming that the rings are pressure traps. Although the grains are sufficiently decoupled from the gas to be radially concentrated, we find that the degree of coupling of the dust is relatively good ($alpha/St sim 0.1$). We can therefore reject scenarios in which the disc turbulence is very low and the dust has grown significantly. If we further assume that the dust grain sizes are set by turbulent fragmentation, we find high values of the $alpha$ turbulent parameter ($alpha sim 10^{-2}$). Alternatively, solutions with smaller turbulence are still compatible with our analysis if another process is limiting grain growth. For HD163296, recent measurements of the disc mass suggest that this is the case if the grain size is 1mm. Future constraints on the dust spectral indices will help to discriminate between the two alternatives.
We present a 3D semi-analytic model of self-gravitating discs, and include a prescription for dust trapping in the disc spiral arms. Using Monte-Carlo radiative transfer we produce synthetic ALMA observations of these discs. In doing so we demonstrate that our model is capable of producing observational predictions, and able to model real image data of potentially self-gravitating discs. For a disc to generate spiral structure that would be observable with ALMA requires that the discs dust mass budget is dominated by millimetre and centimetre-sized grains. Discs in which grains have grown to the grain fragmentation threshold may satisfy this criterion, thus we predict that signatures of gravitational instability may be detectable in discs of lower mass than has previously been suggested. For example, we find that discs with disc-to-star mass ratios as low as $0.10$ are capable of driving observable spiral arms. Substructure becomes challenging to detect in discs where no grain growth has occurred or in which grain growth has proceeded well beyond the grain fragmentation threshold. We demonstrate how we can use our model to retrieve information about dust trapping and grain growth through multi-wavelength observations of discs, and using estimates of the opacity spectral index. Applying our disc model to the Elias 27, WaOph 6 and IM Lup systems we find gravitational instability to be a plausible explanation for the observed substructure in all 3 discs, if sufficient grain growth has indeed occurred.
MHD turbulence plays a crucial role in the dust dynamics of protoplanetary discs. It affects planet formation, vertical settling and is one possible origin of the large scale axisymmetric structures, such as rings, recently imaged by ALMA and SPHERE. Among the variety of MHD processes, the magnetorotational instability (MRI) has raised particular interest since it provides a source of turbulence and potentially organizes the flow into radial structures. However, the weak ionization of discs prevents the MRI from being excited beyond 1 AU. The strong sedimentation of millimetre dust measured in T-Tauri discs is also in contradiction with predictions based on ideal MRI turbulence. In this paper, we study the effects of non-ideal MHD and winds on the dynamics and sedimentation of dust grains. We consider a weakly ionized plasma subject to ambipolar diffusion characterizing the disc outer regions (>1 AU). For that, we perform numerical MHD simulations in the stratified shearing box, using the PLUTO code. Our simulations show that the mm-cm dust is contained vertically in a very thin layer, with typical heightscale ~0.4 AU at 30 AU, compatible with recent ALMA observations. Horizontally, the grains are trapped within pressure maxima induced by ambipolar diffusion, leading to the formation of dust rings. For micrometer grains, dust and gas scaleheights are similar. In this regime, the settling cannot be explained by a simple 1D diffusion theory but results from a large scale 2D circulation induced by both MHD winds and zonal flows. Overall, our results show that non-ideal MHD effects and their related winds play a major role in shaping the radial and vertical distribution of dust in protoplanetary discs. Leading to substantial accretion efficiency, non-ideal effects also a promising avenue to reconcile the low turbulent activity measured in discs with their relatively high accretion rates.
We present a novel mechanism for the outward transport of crystalline dust particles: the outward radial drift of pebbles. The dust ring structure is frequently observed in protoplanetary disks. One of the plausible mechanisms of the formation of dust rings is the accumulation of pebbles around the pressure maximum, which is formed by the mass loss due to magnetically driven disk winds. In evolving protoplanetary disks due to magnetically driven disk winds, dust particles can migrate outwardly from the crystallization front to the pressure maximum by radial drift. We found that the outward radial drift process can transport crystalline dust particles efficiently when the radial drift timescale is shorter than the advection timescale. Our model predicts that the crystallinity of silicate dust particles could be as high as 100% inside the dust ring position.
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