17 research outputs found
Ambipolar diffusion in smoothed particle magnetohydrodynamics
In partially ionised plasmas, the magnetic field can become decoupled from
the neutral gas and diffuse through it in a process known as ambipolar
diffusion. Although ambipolar diffusion has been implemented in several grid
codes, we here provide an implementation in smoothed particle
magnetohydrodynamics (SPMHD). We use the strong coupling approximation in which
the ion density is negligible, allowing a single fluid approach. The equations
are derived to conserve energy, and to provide a positive definite contribution
to the entropy. We test the implementation in both a simple 1D SPMHD code and
the fully 3D code PHANTOM. The wave damping test yields agreement within 0.03-2
per cent of the analytical result, depending on the value of the collisional
coupling constant. The oblique C-shocks test yields results that typically
agree within 4 per cent of the semi-analytical result. Our algorithm is
therefore suitable for exploring the effect ambipolar diffusion has on physical
processes, such as the formation of stars from molecular clouds.Comment: Accepted for publication by MNRA
The growth and hydrodynamic collapse of a protoplanet envelope
We have conducted three-dimensional self-gravitating radiation hydrodynamical
models of gas accretion onto high mass cores (15-33 Earth masses) over hundreds
of orbits. Of these models, one case accretes more than a third of a Jupiter
mass of gas, before eventually undergoing a hydrodynamic collapse. This
collapse causes the density near the core to increase by more than an order of
magnitude, and the outer envelope to evolve into a circumplanetary disc. A
small reduction in the mass within the Hill radius (R_H) accompanies this
collapse as a shock propagates outwards. This collapse leads to a new
hydrostatic equilibrium for the protoplanetary envelope, at which point 97 per
cent of the mass contained within the Hill radius is within the inner 0.03 R_H
which had previously contained less than 40 per cent. Following this collapse
the protoplanet resumes accretion at its prior rate. The net flow of mass
towards this dense protoplanet is predominantly from high latitudes, whilst at
the outer edge of the circumplanetary disc there is net outflow of gas along
the midplane. We also find a turnover of gas deep within the bound envelope
that may be caused by the establishment of convection cells.Comment: 16 pages, 16 figures. Accepted for publication in MNRA
On the relative motions of dense cores and envelopes in star-forming molecular clouds
Hydrodynamical simulations of star formation indicate that the motions of
protostars through their natal molecular clouds may be crucial in determining
the properties of stars through competitive accretion and dynamical
interactions. Walsh, Myers & Burton recently investigated whether such motions
might be observable in the earliest stages of star formation by measuring the
relative shifts of line-centre velocities of low- and high-density tracers of
low-mass star-forming cores. They found very small (~0.1 km/s) relative
motions. In this paper, we analyse the hydrodynamical simulation of Bate,
Bonnell & Bromm and find that it also gives small relative velocities between
high-density cores and low-density envelopes, despite the fact that competitive
accretion and dynamical interactions occur between protostars in the
simulation. Thus, the simulation is consistent with the observations in this
respect. However, we also find some differences between the simulation and the
observations. Overall, we find that the high-density gas has a higher velocity
dispersion than that observed by Walsh et al. We explore this by examining the
dependence of the gas velocity dispersion on density and its evolution with
time during the simulation. We find that early in the simulation the gas
velocity dispersion decreases monotonically with increasing density, while
later in the simulation, when the dense cores have formed multiple objects, the
velocity dispersion of the high-density gas increases. Thus, the simulation is
in best agreement with the observations early on, before many objects have
formed in each dense core.Comment: 8 pages, 7 figures. Accepted for publication in MNRA
Gas accretion by planetary cores
We present accretion rates obtained from three-dimensional self-gravitating
radiation hydrodynamical models of giant planet growth. We investigate the
dependence of accretion rates upon grain opacity and core/protoplanet mass. The
accretion rates found for low mass cores are inline with the results of
previous one-dimensional models that include radiative transfer.Comment: To be published in American Institute of Physics; Conference
proceedings - Exoplanets and Disks: Their Formation and Diversity. 4 pages, 3
figure
On the accumulation of planetesimals near disc gaps created by protoplanets
We have performed three-dimensional two-fluid (gas-dust) hydrodynamical
models of circumstellar discs with embedded protoplanets (3 - 333 M\oplu) and
small solid bodies (radii 10cm to 10m). We find that high mass planets (\gtrsim
Saturn mass) open sufficiently deep gaps in the gas disc such that the density
maximum at the outer edge of the gap can very efficiently trap metre-sized
solid bodies. This allows the accumulation of solids at the outer edge of the
gap as solids from large radii spiral inwards to the trapping region. This
process of accumulation occurs fastest for those bodies that spiral inwards
most rapidly, typically metre-sized boulders, whilst smaller and larger objects
will not migrate sufficiently rapidly in the discs lifetime to benefit from the
process. Around a Jupiter mass planet we find that bound clumps of solid
material, as large as several Earth masses, may form, potentially collapsing
under self-gravity to form planets or planetesimals. These results are in
agreement with Lyra et al. (2009), supporting their finding that the formation
of a second generation of planetesimals or of terrestrial mass planets may be
triggered by the presence of a high mass planet.Comment: 14 pages, 10 figures. Accepted for publication in MNRA
Planet migration: self-gravitating radiation hydrodynamical models of protoplanets with surfaces
We calculate radial migration rates of protoplanets in laminar minimum mass
solar nebula discs using three-dimensional self-gravitating radiation
hydrodynamical (RHD) models. The protoplanets are free to migrate, whereupon
their migration rates are measured. For low mass protoplanets (10-50 M_\oplus)
we find increases in the migration timescales of up to an order of magnitude
between locally-isothermal and RHD models. In the high-mass regime the
migration rates are changed very little. These results are arrived at by
calculating migration rates in locally-isothermal models, before sequentially
introducing self-gravity, and radiative transfer, allowing us to isolate the
effects of the additional physics. We find that using a locally-isothermal
equation of state, without self-gravity, we reproduce the migration rates
obtained by previous analytic and numerical models. We explore the impact of
different protoplanet models, and changes to their assumed radii, upon
migration. The introduction of self-gravity gives a slight reduction of the
migration rates, whilst the inertial mass problem, which has been proposed for
high mass protoplanets with circumplanetary discs, is reproduced. Upon
introducing radiative transfer to models of low mass protoplanets (\approx 10
M_\oplus), modelled as small radius accreting point masses, we find outward
migration with a rate of approximately twice the analytic inward rate. However,
when modelling such a protoplanet in a more realistic manner, with a surface
which enables the formation of a deep envelope, this outward migration is not
seen.Comment: 21 pages, 21 figure
Circumplanetary disc properties obtained from radiation hydrodynamical simulations of gas accretion by protoplanets
We investigate the properties of circumplanetary discs formed in
three-dimensional, self-gravitating radiation hydrodynamical models of gas
accretion by protoplanets. We determine disc sizes, scaleheights, and density
and temperature profiles for different protoplanet masses, in solar nebulae of
differing grain opacities.
We find that the analytical prediction of circumplanetary disc radii in an
evacuated gap (R_Hill/3) from Quillen & Trilling (1998) yields a good estimate
for discs formed by high mass protoplanets. The radial density profiles of the
circumplanetary discs may be described by power-laws between r^-2 and r^-3/2.
We find no evidence for the ring-like density enhancements that have been found
in some previous models of circumplanetary discs. Temperature profiles follow a
~r^-7/10 power-law regardless of protoplanet mass or nebula grain opacity. The
discs invariably have large scaleheights (H/r > 0.2), making them thick in
comparison with their encompassing circumstellar discs, and they show no
flaring.Comment: 9 pages, 6 figures, accepted for publication in MNRA
Gas accretion onto planetary cores: three-dimensional self-gravitating radiation hydrodynamical calculations
We present results from three-dimensional, self-gravitating radiation
hydrodynamical models of gas accretion by planetary cores. In some cases, the
accretion flow is resolved down to the surface of the solid core -- the first
time such simulations have been performed. We investigate the dependence of the
gas accretion rate upon the planetary core mass, and the surface density and
opacity of the encompassing protoplanetary disc. Accretion of planetesimals is
neglected.
We find that high-mass protoplanets are surrounded by thick circumplanetary
discs during their gas accretion phase but, contrary to locally-isothermal
calculations, discs do not form around accreting protoplanets with masses ~<
50M_Earth when radiation hydrodynamical simulations are performed, even if the
grain opacity is reduced from interstellar values by a factor of 100. We find
that the opacity of the gas plays a large role in determining the accretion
rates for low-mass planetary cores. For example, reducing the opacities from
interstellar values by a factor of 100 leads to roughly an order of magnitude
increase in the accretion rates for 10-20M_Earth protoplanets. The dependence
on opacity becomes less important in determining the accretion rate for more
massive cores where gravity dominates the effects of thermal support and the
protoplanet is essentially accreting at the runaway rate. Finally, for low-mass
planetary cores (~< 20M_Earth), we obtain accretion rates that are in agreement
with previous one-dimensional quasi-static models. This indicates that
three-dimensional hydrodynamical effects may not significantly alter the gas
accretion timescales that have been obtained from quasi-static models.Comment: 16 pages, 15 figures, accepted for publication in MNRAS. V2 includes
small corrections to the radiation hydrodynamical accretion rates for a
Jupiter mass core, including an updated figure 8; conclusions are unaffecte
Intrapopulation Variability Shaping Isotope Discrimination and Turnover: Experimental Evidence in Arctic Foxes
Tissue-specific stable isotope signatures can provide insights into the trophic ecology of consumers and their roles in food webs. Two parameters are central for making valid inferences based on stable isotopes, isotopic discrimination (difference in isotopic ratio between consumer and its diet) and turnover time (renewal process of molecules in a given tissue usually measured when half of the tissue composition has changed). We investigated simultaneously the effects of age, sex, and diet types on the variation of discrimination and half-life in nitrogen and carbon stable isotopes (δ15N and δ13C, respectively) in five tissues (blood cells, plasma, muscle, liver, nail, and hair) of a top predator, the arctic fox Vulpes lagopus. We fed 40 farmed foxes (equal numbers of adults and yearlings of both sexes) with diet capturing the range of resources used by their wild counterparts. We found that, for a single species, six tissues, and three diet types, the range of discrimination values can be almost as large as what is known at the scale of the whole mammalian or avian class. Discrimination varied depending on sex, age, tissue, and diet types, ranging from 0.3‰ to 5.3‰ (mean = 2.6‰) for δ15N and from 0.2‰ to 2.9‰ (mean = 0.9‰) for δ13C. We also found an impact of population structure on δ15N half-life in blood cells. Varying across individuals, δ15N half-life in plasma (6 to 10 days) was also shorter than for δ13C (14 to 22 days), though δ15N and δ13C half-lives are usually considered as equal. Overall, our multi-factorial experiment revealed that at least six levels of isotopic variations could co-occur in the same population. Our experimental analysis provides a framework for quantifying multiple sources of variation in isotopic discrimination and half-life that needs to be taken into account when designing and analysing ecological field studies