1,092 research outputs found
Magnetically driven accretion in protoplanetary discs
We characterize magnetically driven accretion at radii between 1 au and 100
au in protoplanetary discs, using a series of local non-ideal
magnetohydrodynamic (MHD) simulations. The simulations assume a Minimum Mass
Solar Nebula (MMSN) disc that is threaded by a net vertical magnetic field of
specified strength. Confirming previous results, we find that the Hall effect
has only a modest impact on accretion at 30 au, and essentially none at 100 au.
At 1-10 au the Hall effect introduces a pronounced bi-modality in the accretion
process, with vertical magnetic fields aligned to the disc rotation supporting
a strong laminar Maxwell stress that is absent if the field is anti-aligned. In
the anti-aligned case, we instead find evidence for bursts of turbulent stress
at 5-10 au, which we tentatively identify with the non-axisymmetric Hall-shear
instability. The presence or absence of these bursts depends upon the details
of the adopted chemical model, which suggests that appreciable regions of
actual protoplanetary discs might lie close to the borderline between laminar
and turbulent behaviour. Given the number of important control parameters that
have already been identified in MHD models, quantitative predictions for disc
structure in terms of only radius and accretion rate appear to be difficult.
Instead, we identify robust qualitative tests of magnetically driven accretion.
These include the presence of turbulence in the outer disc, independent of the
orientation of the vertical magnetic fields, and a Hall-mediated bi-modality in
turbulent properties extending from the region of thermal ionization to 10 au.Comment: accepted to MNRAS after very minor revision
Evidence for universality in the initial planetesimal mass function
Planetesimals may form from the gravitational collapse of dense particle
clumps initiated by the streaming instability. We use simulations of
aerodynamically coupled gas-particle mixtures to investigate whether the
properties of planetesimals formed in this way depend upon the sizes of the
particles that participate in the instability. Based on three high resolution
simulations that span a range of dimensionless stopping time no statistically significant differences in the initial
planetesimal mass function are found. The mass functions are fit by a
power-law, , with and
errors of . Comparing the particle density fields prior
to collapse, we find that the high wavenumber power spectra are similarly
indistinguishable, though the large-scale geometry of structures induced via
the streaming instability is significantly different between all three cases.
We interpret the results as evidence for a near-universal slope to the mass
function, arising from the small-scale structure of streaming-induced
turbulence.Comment: 7 pages, 4 figures, accepted to ApJ Letters after minor
modifications, including two new figures and some new text that better
clarify our result
Why the public is torn over the contact-tracing app and how the government can maximize uptake
Drawing on a qualitative study consisting of five focus groups, Simon Williams, Christopher J Armitage, Tova Tampe and Kimberly Dienes find that people are currently torn over whether or not they will use the contract-tracing app when it is available. They discuss the main concerns that emerged from the research and offer some key recommendations for ensuring that there will be sufficient uptake
Turbulent Linewidths as a Diagnostic of Self-Gravity in Protostellar Discs
We use smoothed particle hydrodynamics simulations of massive protostellar
discs to investigate the predicted broadening of molecular lines from discs in
which self-gravity is the dominant source of angular momentum transport. The
simulations include radiative transfer, and span a range of disc-to-star mass
ratios between 0.25 and 1.5. Subtracting off the mean azimuthal flow velocity,
we compute the distribution of the in-plane and perpendicular peculiar velocity
due to large scale structure and turbulence induced by self-gravity. For the
lower mass discs, we show that the characteristic peculiar velocities scale
with the square root of the effective turbulent viscosity parameter, as
expected from local turbulent-disc theory. The derived velocities are
anisotropic, with substantially larger in-plane than perpendicular values. As
the disc mass is increased, the validity of the locally determined turbulence
approximation breaks down, and this is accompanied by anomalously large
in-plane broadening. There is also a high variance due to the importance of
low-m spiral modes. For low-mass discs, the magnitude of in-plane broadening
is, to leading order, equal to the predictions from local disc theory and
cannot constrain the source of turbulence. However, combining our results with
prior evaluations of turbulent broadening expected in discs where the
magnetorotational instability (MRI) is active, we argue that self-gravity may
be distinguishable from the MRI in these systems if it is possible to measure
the anisotropy of the peculiar velocity field with disc inclination.
Furthermore, for large mass discs, the dominant contribution of large-scale
modes is a distinguishing characteristic of self-gravitating turbulence versus
MRI driven turbulence.Comment: 8 pages, 13 figures, accepted for publication in MNRA
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