Smoothed particle hydrodynamics simulations of gas and dust mixtures

We present a 'two-fluid' implementation of dust in smoothed particle hydrodynamics (SPH) in the test particle limit. The scheme is able to handle both short and long stopping times and reproduces the short friction time limit, which is not properly handled in other implementations. We apply novel tests to verify its accuracy and limitations, including multi-dimensional tests that have not been previously applied to the drag-coupled dust problem and which are particularly relevant to self-gravitating protoplanetary discs. Our tests demonstrate several key requirements for accurate simulations of gas-dust mixtures. Firstly, in standard SPH particle jitter can degrade the dust solution, even when the gas density is well reproduced. The use of integral gradients, a Wendland kernel and a large number of neighbours can control this, albeit at a greater computational cost. Secondly, when it is necessary to limit the artificial viscosity we recommend using the Cullen & Dehnen (2010) switch, since the alternative, using ${\alpha} \sim 0.1$, can generate a large velocity noise up to ${{\sigma}_v} \lesssim 0.3 c_s$ in the dust particles. Thirdly, we find that an accurate dust density estimate requires $>400$ neighbours, since, unlike the gas, the dust particles do not feel regularization forces. This density noise applies to all particle-based two-fluid implementations of dust, irrespective of the hydro solver and could lead to numerically induced fragmentation. Although our tests show accurate dusty gas simulations are possible, care must be taken to minimize the contribution from numerical noise

Judged in a foreign language : A Chinese-Spanish court interpreting case study

Recent legislation in Spain has transposed Directive 2010/64/EU, which recognises interpretation as an essential tool for safeguarding fairness in criminal proceedings, in particular, for preventing any state of defencelessness. Previous research, however, has suggested important deficiencies in court interpreting in this country. This article analyses court interpreting from Chinese to Spanish, based on a case study of a recording of a criminal trial that took place in Barcelona in February 2015. The trial was transcribed verbatim and annotated in accordance with Cecilia Wadensjö's distinction between "talk as text" and "talk as activity." The analysis focuses on examples of errors of interpretation (non-translated speech acts, omissions, and additions), speech style and non-renditions. The results are compared with those of a corpus of 55 court proceedings in which the interpretation was from Spanish to English and from French to Romanian. The article concludes with a discussion of the factors-such as lack of specialised training, lack of deontological codes or general unawareness of the interpreter's role-that may have affected the quality of the interpretation in the trial analysed. The low proportion of interpretation during that court session (only 17.6% of the total duration) is perhaps the most striking result of the case stud

A dusty origin for the correlation between protoplanetary disc accretion rates and dust masses

ABSTRACT Recent observations have uncovered a correlation between the accretion rates (measured from the UV continuum excess) of protoplanetary discs and their masses inferred from observations of the submm continuum. While viscous evolution models predict such a correlation, the predicted values are in tension with data obtained from the Lupus and Upper Scorpius star-forming regions; for example, they underpredict the scatter in accretion rates, particularly in older regions. Here, we argue that since the submm observations trace the discs’ dust, by explicitly modelling the dust grain growth, evolution, and emission, we can better understand the correlation. We show that for turbulent viscosities with α ≲ 10−3, the depletion of dust from the disc due to radial drift means we can reproduce the range of masses and accretion rates seen in the Lupus and Upper Sco data sets. One consequence of this model is that the upper locus of accretion rates at a given dust mass does not evolve with the age of the region. Moreover, we find that internal photoevaporation is necessary to produce the lowest accretion rates observed. In order to replicate the correct dust masses at the time of disc dispersal, we favour relatively low photoevaporation rates ≲ 10−9 M⊙ yr−1 for most sources but cannot discriminate between EUV or X-ray-driven winds. A limited number of sources, particularly in Lupus, are shown to have higher masses than predicted by our models which may be evidence for variations in the properties of the dust or dust trapping induced in substructures.</jats:p

Evidence of a past disc-disc encounter: HV and DO Tau

© 2018 The Author(s). Published by Oxford University Press on behalf of The Royal Astronomical Society. Theory and observations suggest that star formation occurs hierarchically due to the fragmentation of giant molecular clouds. In this case we would expect substructure and enhanced stellar multiplicity in the primordial cluster. This substructure is expected to decay quickly in most environments, however historic stellar encounters might leave imprints in a protoplanetary disc (PPD) population. In a low-density environment such as Taurus, tidal tails from violent star-disc or disc-disc encounters might be preserved over timescales sufficient to be observed. In this work, we investigate the possibility that just such an event occurred between HV Tau C (itself a component of a triple system) and DO Tau ~0.1 Myr ago, as evidenced by an apparent 'bridge' structure evident in the 160 μ m emission. By modelling the encounter using smoothed particle hydrodynamics (SPH) we reproduce the main features of the observed extended structure ('V'-shaped emission pointing west of HV Tau and a tail-like structure extending east of DO Tau). We suggest that HV Tau and DO Tau formed together in a quadruple system on a scale of ~5000 au (0.025 pc)

On the Complexity of an Unregulated Traffic Crossing

The steady development of motor vehicle technology will enable cars of the near future to assume an ever increasing role in the decision making and control of the vehicle itself. In the foreseeable future, cars will have the ability to communicate with one another in order to better coordinate their motion. This motivates a number of interesting algorithmic problems. One of the most challenging aspects of traffic coordination involves traffic intersections. In this paper we consider two formulations of a simple and fundamental geometric optimization problem involving coordinating the motion of vehicles through an intersection. We are given a set of $n$ vehicles in the plane, each modeled as a unit length line segment that moves monotonically, either horizontally or vertically, subject to a maximum speed limit. Each vehicle is described by a start and goal position and a start time and deadline. The question is whether, subject to the speed limit, there exists a collision-free motion plan so that each vehicle travels from its start position to its goal position prior to its deadline. We present three results. We begin by showing that this problem is NP-complete with a reduction from 3-SAT. Second, we consider a constrained version in which cars traveling horizontally can alter their speeds while cars traveling vertically cannot. We present a simple algorithm that solves this problem in $O(n \log n)$ time. Finally, we provide a solution to the discrete version of the problem and prove its asymptotic optimality in terms of the maximum delay of a vehicle

A super-resolution analysis of the DSHARP survey: Substructure is common in the inner 30 au

The DSHARP survey evidenced the ubiquity of substructure in the mm dust distribution of large, bright protoplanetary discs. Intriguingly, these datasets have yet higher resolution information that is not recovered in a CLEAN image. We first show that the intrinsic performance of the CLEAN algorithm is resolution-limited. Then analyzing all 20 DSHARP sources using the 1D, super-resolution code Frankenstein (frank), we accurately fit the 1D visibilities to a mean factor of 4.3 longer baseline than the Fourier transform of the CLEAN images and a factor of 3.0 longer baseline than the transform of the CLEAN component models. This yields a higher resolution brightness profile for each source, identifying new substructure interior to 30 au in multiple discs; resolving known gaps to be deeper, wider, and more structured; and known rings to be narrower and brighter. Across the survey, high contrast gaps are an average 14% wider and 44% deeper in the frank profiles relative to CLEAN, and high contrast rings are an average 26% narrower. Categorizing the frank brightness profiles into trends, we find that the relative scarcity of features interior to 30 au in the survey's CLEAN images is an artifact of resolving power, rather than an intrinsic rarity of inner disc (or compact disc) substructure. Finally the rings in the frank profiles are narrower than the previously inferred deconvolved widths, indicating smaller alpha / St ratios in the local gas disc

The origin of the eccentricity of the hot Jupiter in CI Tau

Following the recent discovery of the first radial velocity planet in a star still possessing a protoplanetary disc (CI Tau), we examine the origin of the planet's eccentricity (e $\sim 0.3$). We show through long timescale ($10^5$ orbits) simulations that the planetary eccentricity can be pumped by the disc, even when its local surface density is well below the threshold previously derived from short timescale integrations. We show that the disc may be able to excite the planet's orbital eccentricity in $<$ a Myr for the system parameters of CI Tau. We also perform two planet scattering experiments and show that alternatively the observed planet may plausibly have acquired its eccentricity through dynamical scattering of a migrating lower mass planet, which has either been ejected from the system or swallowed by the central star. In the latter case the present location and eccentricity of the observed planet can be recovered if it was previously stalled within the disc's magnetospheric cavity.This work has been supported by the DISCSIM project, grant agreement 341137 funded by the European Research Council under ERC-2013-ADG, and from STFC through grant ST/L000636/1. This work used the DIRAC Shared Memory Processing and Data Analytic systems, both at the University of Cambridge and operated, respectively, by the COSMOS Project at the Department of Applied Mathematics and Theoretical Physics and the Cambridge High Performance Computing Service, on behalf of the STFC DiRAC HPC Facility (www.dirac.ac.uk). This equipment was funded by BIS National E-infrastructure capital grants ST/J005673/1 and ST/K001590/1, STFC capital grants ST/H008586/1, ST/H008861/1 and ST/H00887X/1, STFC DiRAC Operations grant ST/K00333X/1, and STFC DiRAC Operations grant ST/K00333X/1. DiRAC is part of the National E-Infrastructure.This is the final version of the article. It first appeared from Oxford University Press via https://doi.org/10.1093/mnrasl/slw18

On the millimetre continuum flux–radius correlation of proto-planetary discs

A correlation between proto-planetary disc radii and sub-mm fluxes has been recently reported. In this Letter we show that the correlation is a sensitive probe of grain growth processes. Using models of grain growth and drift, we have shown in a companion paper that the observed disc radii trace where the dust grains are large enough to have a significant sub-mm opacity. We show that the observed correlation emerges naturally if the maximum grain size is set by radial drift, implying relatively low values of the viscous $\alpha$ parameter $\lesssim 0.001$. In this case the relation has an almost universal normalisation, while if the grain size is set by fragmentation the flux at a given radius depends on the dust-to-gas ratio. We highlight two observational consequences of the fact that radial drift limits the grain size. The first is that the dust masses measured from the sub-mm could be overestimated by a factor of a few. The second is that the correlation should be present also at longer wavelengths (e.g. 3mm), with a normalisation factor that scales as the square of the observing frequency as in the optically thick case

The time evolution of dusty protoplanetary disc radii: Observed and physical radii differ

Proto-planetary disc surveys conducted with ALMA are measuring disc radii in multiple star forming regions. The disc radius is a fundamental quantity to diagnose whether discs undergo viscous spreading, discriminating between viscosity or angular momentum removal by winds as drivers of disc evolution. Observationally, however, the sub-mm continuum emission is dominated by the dust, which also drifts inwards, complicating the picture. In this paper we investigate, using theoretical models of dust grain growth and radial drift, how the radii of dusty viscous proto-planetary discs evolve with time. Despite the existence of a sharp outer edge in the dust distribution, we find that the radius enclosing most of the dust $\textit{mass}$ increases with time, closely following the evolution of the gas radius. This behaviour arises because, although dust initially grows and drifts rapidly onto the star, the residual dust retained on Myr timescales is relatively well coupled to the gas. Observing the expansion of the dust disc requires using definitions based on high fractions of the disc $\textit{flux}$ (e.g. 95 per cent) and very long integrations with ALMA, because the dust grains in the outer part of the disc are small and have a low sub-mm opacity. We show that existing surveys lack the sensitivity to detect viscous spreading. The disc radii they measure do not trace the mass radius or the sharp outer edge in the dust distribution, but the outer limit of where the grains have significant sub-mm opacity. We predict that these observed radii should shrink with time

On the Origin of the Spiral Morphology in the Elias 2-27 Circumstellar Disk

The young star Elias 2-27 has recently been observed to posses a massive circumstellar disk with two prominent large-scale spiral arms. In this Letter, we perform three-dimensional Smoothed Particle Hydrodynamics simulations, radiative transfer modeling, synthetic ALMA imaging, and an unsharped masking technique to explore three possibilities for the origin of the observed structures - an undetected companion either internal or external to the spirals, and a self-gravitating disk. We find that a gravitationally unstable disk and a disk with an external companion can produce morphology that is consistent with the observations. In addition, for the latter, we find that the companion could be a relatively massive planetary-mass companion (≲10-13 M Jup ) and located at large radial distances (between ≈300-700 au). We therefore suggest that Elias 2-27 may be one of the first detections of a disk undergoing gravitational instabilities, or a disk that has recently undergone fragmentation to produce a massive companion.We acknowledge support from the DISCSIM project, grant agreement 341137 under ERC-2013-ADG. F.M. acknowledges support from The Leverhulme Trust. This Letter uses the following ALMA data: ADS/JAO.ALMA# 2013.1.00498.S. This work used the Darwin DiRAC HPC cluster at the University of Cambridge and was undertaken on the Cambridge COSMOS SMP system, part of the STFC DiRAC HPC Facility supported by BIS NeI capital grant ST/J005673/1 and STFC grants ST/H008586/1, ST/K00333X/1