arXiv subjects

Richard A. Booth

Publications and source records attributed to Richard A. Booth.

77 records · Page 5Linked to original sources

arXiv2016

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.

astro-ph.EP↗

arXiv2016

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 submm continuum images and demonstrate how these images can be used to measure planet masses to within a factor of about two. To this end we perform multi-fluid gas and dust simulations of discs containing low mass planets, generating simulated observations at $1.65 μ$m, $10 μ$m and $850 μ$m. We show that the minimum planet mass that produces a detectable signature is $\sim 15 M_\oplus$: 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 $\sim 20 M_\oplus$ 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 outward 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 sub-mm images. We emphasise 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.

astro-ph.EP↗

arXiv2016

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, $St \sim 1$); 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 disc's 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 order the disc scale height. 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 $St \sim 10^{-2}$ or $10^{-1}$ ($10\,\mathrm{cm}$ to $\mathrm{m}$ sizes at $30\,\mathrm{au}$), such Stokes numbers being insufficient to allow concentration of solids in spiral features. However, for icy solids (which may survive collisions up to several $10\,\mathrm{m\,s}^{-1}$), growth to $St \sim 1$ ($10\,\mathrm{m}$ size) may be possible beyond $30\,\mathrm{au}$ 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.

astro-ph.EP↗

arXiv2016

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 $5 \times 10^{-7}\,\mathrm{M}_{\odot}\,\mathrm{yr}^{-1}$ from the red giant is required. The simulations also produce a polar accretion flow, which may explain the broad wings of the quiescent H α 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, $\dot{M} \sim 10^{-6}$ to $10^{-5}\,\mathrm{M_\odot}\,\mathrm{yr}^{-1}$, to explain the variability in SN 2006X.

astro-ph.HE↗

arXiv2015

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 $α \sim 0.1$, can generate a large velocity noise up to ${σ_v} \lesssim 0.3 c_s$ 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.

astro-ph.EP↗