Richard Anthony Booth

The minimum mass of detectable planets in protoplanetary discs and the derivation of planetary masses from high resolution observations

We investigate the minimum planet mass that produces observable signatures in infrared scattered light and submillimetre (submm) continuum images and demonstrate how these images can be used to measure planet masses to within a factor of about 2. To this end, we perform multi-fluid gas and dust simulations of discs containing low-mass planets, generating simulated observations at 1.65, 10 and 850 μm. We show that the minimum planet mass that produces a detectable signature is ∼15 M⊕: this value is strongly dependent on disc temperature and changes slightly with wavelength (favouring the submm). We also confirm previous results that there is a minimum planet mass of ∼20 M⊕ that produces a pressure maximum in the disc: only planets above this threshold mass generate a dust trap that can eventually create a hole in the submm dust. Below this mass, planets produce annular enhancements in dust outwards of the planet and a reduction in the vicinity of the planet. These features are in steady state and can be understood in terms of variations in the dust radial velocity, imposed by the perturbed gas pressure radial profile, analogous to a traffic jam. We also show how planet masses can be derived from structure in scattered light and submm images. We emphasize that simulations with dust need to be run over thousands of planetary orbits so as to allow the gas profile to achieve a steady state and caution against the estimation of planet masses using gas-only simulations.

Mathematical modelling of fibre-enhanced perfusion inside a tissue-engineering bioreactor

We develop a simple mathematical model for forced flow of culture medium through a porous scaffold in a tissue-engineering bioreactor. Porous-walled hollow fibres penetrate the scaffold and act as additional sources of culture medium. The model, based on Darcys law, is used to examine the nutrient and shear-stress distributions throughout the scaffold. We consider several configurations of fibres and inlet and outlet pipes. Compared with a numerical solution of the full Navier-Stokes equations within the complex scaffold geometry, the modelling approach is cheap, and does not require knowledge of the detailed microstructure of the particular scaffold being used. The potential of this approach is demonstrated through quantification of the effect the additional flow from the fibres has on the nutrient and shear-stress distribution.

Grand Challenges in Protoplanetary Disc Modelling

The Protoplanetary Discussions conference—held in Edinburgh, UK, from 2016 March 7th–11th—included several open sessions led by participants. This paper reports on the discussions collectively concerned with the multi-physics modelling of protoplanetary discs, including the self-consistent calculation of gas and dust dynamics, radiative transfer, and chemistry. After a short introduction to each of these disciplines in isolation, we identify a series of burning questions and grand challenges associated with their continuing development and integration. We then discuss potential pathways towards solving these challenges, grouped by strategical, technical, and collaborative developments. This paper is not intended to be a review, but rather to motivate and direct future research and collaboration across typically distinct fields based on community-driven input, to encourage further progress in our understanding of circumstellar and protoplanetary discs.

Collision velocity of dust grains in self-gravitating protoplanetary discs

We have conducted the first comprehensive numerical investigation of the relative velocity distribution of dust particles in self-gravitating protoplanetary discs with a view to assessing the viability of planetesimal formation via direct collapse in such environments. The viability depends crucially on the large sizes that are preferentially collected in pressure maxima produced by transient spiral features (Stokes numbers, St ∼ 1); growth to these size scales requires that collision velocities remain low enough that grain growth is not reversed by fragmentation. We show that, for a single-sized dust population, velocity driving by the discs gravitational perturbations is only effective for St > 3, while coupling to the gas velocity dominates otherwise. We develop a criterion for understanding this result in terms of the stopping distance being of the order of the disc scaleheight. Nevertheless, the relative velocities induced by differential radial drift in multi-sized dust populations are too high to allow the growth of silicate dust particles beyond St ∼ 10− 2 or 10−1 (10 cm to m sizes at 30 au), such Stokes numbers being insufficient to allow concentration of solids in spiral features. However, for icy solids (which may survive collisions up to several 10 m s−1), growth to St ∼ 1 (10 m size) may be possible beyond 30 au from the star. Such objects would be concentrated in spiral features and could potentially produce larger icy planetesimals/comets by gravitational collapse. These planetesimals would acquire moderate eccentricities and remain unmodified over the remaining lifetime of the disc.

Modelling the circumstellar medium in RS Ophiuchi and its link to Type Ia supernovae

Recent interpretations of narrow, variable absorption lines detected in some Type Ia supernovae suggest that their progenitors are surrounded by dense, circumstellar material. Similar variations detected in the symbiotic recurrent nova system RS Oph, which undergoes thermonuclear outbursts every ~20 years, making it an ideal candidate to investigate the origin of these lines. To this end, we present simulations of multiple mass transfer-nova cycles in RS Oph. We find that the quiescent mass transfer produces a dense, equatorial outflow, i.e., concentrated towards the binary orbital plane, and an accretion disc forms around the white dwarf. The interaction of a spherical nova outburst with these aspherical circumstellar structures produces a bipolar outflow, similar to that seen in HST imaging of the 2006 outburst. In order to produce an ionization structure that is consistent with observations, a mass-loss rate of

Callipeltosides A, B and C: Total Syntheses and Structural Confirmation

5 \times 10^{-7}\,\mathrm{M}_{\odot}\,\mathrm{yr}^{-1}

gandalf – Graphical Astrophysics code for N-body Dynamics And Lagrangian Fluids

from the red giant is required. The simulations also produce a polar accretion flow, which may explain the broad wings of the quiescent H {\alpha} line and hard X-rays. By comparing simulated absorption line profiles to observations of the 2006 outburst, we are able to determine which components arise in the wind and which are due to the novae. We explore the possible behaviour of absorption line profiles as they may appear should a supernova occur in a system like RS Oph. Our models show similarities to supernovae like SN 2006X, but require a high mass-loss rate,

The chemistry of protoplanetary fragments formed via gravitational instabilities

\dot{M} \sim 10^{-6}

Protoplanetary Disc Response to Distant Tidal Encounters in Stellar Clusters

to

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

10^{-5}\,\mathrm{M_\odot}\,\mathrm{yr}^{-1}