36 research outputs found
An opening criterion for dust gaps in protoplanetary discs
We aim to understand under which conditions a low mass planet can open a gap
in viscous dusty protoplanetary discs. For this purpose, we extend the theory
of dust radial drift to include the contribution from the tides of an embedded
planet and from the gas viscous forces. From this formalism, we derive i) a
grain size-dependent criterion for dust gap opening in discs, ii) an estimate
of the location of the outer edge of the dust gap and iii) an estimate of the
minimum Stokes number above which low-mass planets are able to carve gaps which
appear only in the dust disc. These analytical estimates are particularly
helpful to appraise the minimum mass of an hypothetical planet carving gaps in
discs observed at long wavelengths and high resolution. We validate the theory
against 3D SPH simulations of planet-disc interaction in a broad range of dusty
protoplanetary discs. We find a remarkable agreement between the theoretical
model and the numerical experiments.Comment: 17 pages, 13 figures. Accepted for publication in MNRA
Dust trapping by spiral arms in gravitationally unstable protostellar discs
In this paper we discuss the influence of gravitational instabilities in
massive protostellar discs on the dynamics of dust grains. Starting from a
Smoothed Particle Hydrodynamics (SPH) simulation, we have computed the
evolution of the dust in a quasi-static gas density structure typical of
self-gravitating disc. For different grain size distributions we have
investigated the capability of spiral arms to trap particles. We have run 3D
radiative transfer simulations in order to construct maps of the expected
emission at (sub-)millimetre and near-infrared wavelengths. Finally, we have
simulated realistic observations of our disc models at (sub-)millimetre and
near-infrared wavelengths as they may appear with the Atacama Large
Millimetre/sub-millimetre Array (ALMA) and the High-Contrast Coronographic
Imager for Adaptive Optics (HiCIAO) in order to investigate whether there are
observational signatures of the spiral structure. We find that the pressure
inhomogeites induced by gravitational instabilities produce a non-negligible
dynamical effect on centimetre sized particles leading to significant
overdensities in spiral arms. We also find that the spiral structure is readily
detectable by ALMA over a wide range of (sub-)millimetre wavelengths and by
HiCIAO in near-infrared scattered light for non-face-on discs located in the
Ophiucus star-forming region. In addition, we find clear spatial spectral index
variations across the disc, revealing that the dust trapping produces a
migration of large grains that can be potentially investigated through
multi-wavelenghts observations in the (sub-)millimetric. Therefore, the spiral
arms observed to date in protoplanetary disc might be interpreted as density
waves induced by the development of gravitational instabilities.Comment: 14 pages, 12 figures. Accepted for publication in MNRA
How to Detect the Signatures of Self-Gravitating Circumstellar Discs with the Atacama Large Millimetre/sub-millimetre Array
In this paper we present simulated Atacama Large Millimetre/sub-millimetre
Array (ALMA) observations of self-gravitating circumstellar discs with
different properties in size, mass and inclination, located in four of the most
extensively studied and surveyed star-forming regions. Starting from a Smoothed
Particle Hydrodynamics (SPH) simulation and representative dust opacities, we
have initially constructed maps of the expected emission at sub-mm wavelengths
of a large sample of discs with different properties. We have then simulated
realistic observations of discs as they may appear with ALMA using the Common
Astronomy Software Application ALMA simulator. We find that, with a proper
combination of antenna configuration and integration time, the spiral structure
characteristic of self-gravitating discs is readily detectable by ALMA over a
wide range of wavelengths at distances comparable to TW Hydrae (pc), Taurus - Auriga and Ophiucus (pc) star-forming regions.
However, for discs located in Orion complex (pc) only the largest
discs in our sample (outer radius of 100 au) show a spatially resolved
structure while the smaller ones (outer radius of 25 au) are characterized by a
spiral structure that is not conclusively detectable with ALMA.Comment: 12 pages, 10 figure
On planet formation in HL Tau
We explain the axisymmetric gaps seen in recent long-baseline observations of
the HL Tau protoplanetary disc with the Atacama Large Millimetre/Submillimetre
Array (ALMA) as being due to the different response of gas and dust to embedded
planets in protoplanetary discs. We perform global, three dimensional dusty
smoothed particle hydrodynamics calculations of multiple planets embedded in
dust/gas discs which successfully reproduce most of the structures seen in the
ALMA image. We find a best match to the observations using three embedded
planets with masses of 0.2, 0.27 and 0.55 in the three main gaps
observed by ALMA, though there remain uncertainties in the exact planet masses
from the disc model.Comment: 5 pages, 4 figures. Accepted for publication in MNRAS Letters. The
movie of the simulation is available at
https://www.youtube.com/watch?v=CKenxfslBMY and
https://www.youtube.com/watch?v=mGCfjrb5bT
Two mechanisms for dust gap opening in protoplanetary discs
We acknowledge an ARC Future Fellowship and Discovery Project. GD and G. Lodato acknowledge funding via PRINMIUR prot. 2010LY5N2T. G. Laibe is funded by ERC FP7 grant ECOGAL.We identify two distinct physical mechanisms for dust gap opening by embedded planets in protoplanetary discs based on the symmetry of the drag-induced motion around the planet: (I) a mechanism where low-mass planets, that do not disturb the gas, open gaps in dust by tidal torques assisted by drag in the inner disc, but resisted by drag in the outer disc and (II) the usual, drag-assisted, mechanism where higher mass planets create pressure maxima in the gas disc, which the drag torque then acts to evacuate further in the dust. The first mechanism produces gaps in dust but not gas, while the second produces partial or total gas gaps which are deeper in the dust phase. Dust gaps do not necessarily indicate gas gaps.Publisher PDFPeer reviewe
Signatures of an eccentric disc cavity: Dust and gas in IRS 48
We test the hypothesis that the disc cavity in the `transition disc' Oph IRS
48 is carved by an unseen binary companion. We use 3D dust-gas
smoothed-particle hydrodynamics simulations to demonstrate that marginally
coupled dust grains concentrate in the gas over-density that forms in in the
cavity around a low binary mass ratio binary. This produces high contrast ratio
dust asymmetries at the cavity edge similar to those observed in the disc
around IRS 48 and other transition discs. This structure was previously assumed
to be a vortex. However, we show that the observed velocity map of IRS 48
displays a peculiar asymmetry that is not predicted by the vortex hypothesis.
We show the unusual kinematics are naturally explained by the non-Keplerian
flow of gas in an eccentric circumbinary cavity. We further show that
perturbations observed in the isovelocity curves of IRS 48 may be explained as
the product of the dynamical interaction between the companion and the disc.
The presence of a 0.4 M companion at a 10 au separation
can qualitatively explain these observations. High spatial resolution line and
continuum imaging should be able to confirm this hypothesis.Comment: 9 pages, 7 figures, accepted for publication in MNRA
Is the spiral morphology of the Elias 2-27 circumstellar disc due to gravitational instability?
A recent ALMA observation of the Elias 2-27 system revealed a two-armed
structure extending out to ~300 au in radius. The protostellar disc surrounding
the central star is unusually massive, raising the possibility that the system
is gravitationally unstable. Recent work has shown that the observed morphology
of the system can be explained by disc self-gravity, so we examine the physical
properties of the disc necessary to detect self-gravitating spiral waves. Using
three-dimensional Smoothed Particle Hydrodynamics, coupled with radiative
transfer and synthetic ALMA imaging, we find that observable spiral structure
can only be explained by self-gravity if the disc has a low opacity (and
therefore efficient cooling), and is minimally supported by external
irradiation. This corresponds to a very narrow region of parameter space,
suggesting that, although it is possible for the spiral structure to be due to
disc self-gravity, other explanations, such as an external perturbation, may be
preferred.Comment: 12 pages, 5 figure
Flybys in protoplanetary discs: I. Gas and dust dynamics
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.Comment: 27 pages, 24 figures, accepted for publication in MNRA