127 research outputs found
On the ionisation fraction in protoplanetary disks III. The effect of X-ray flares on gas-phase chemistry
Context. Recent observations of the X-ray emission from T Tauri stars in the
Orion nebula have shown that they undergo frequent outbursts in their X-ray
luminosity. These X-ray flares are characterised by increases in luminosity by
two orders of magnitude, a typical duration of less than one day, and a
significant hardening of the X-ray spectrum.
Aims. It is unknown what effect these X-ray flares will have on the
ionisation fraction and dead-zone structure in protoplanetary disks. We present
the results of calculations designed to address this question.
Methods. We have performed calculations of the ionisation fraction in a
standard -disk model using two different chemical reaction networks. We
include in our models ionisation due to X-rays from the central star, and
calculate the time-dependent ionisation fraction and dead--zone structure for
the inner 10 AU of a protoplanetary disk model.
Results. We find that the disk response to X-ray flares depends on whether
the plasma temperature increases during flares and/or whether heavy metals
(such as magnesium) are present in the gas phase. Under favourable conditions
the outer disk dead--zone can disappear altogether,and the dead-zone located
between 0.5 < R < 2 AU can disappear and reappear in phase with the X-ray
luminosity.
Conclusions. X-ray flares can have a significant effect on the dead-zone
structure in protoplanetary disks. Caution is required in interpreting this
result as the duration of X-ray bursts is considerably shorter than the growth
time of MHD turbulence due to the magnetorotational instability.Comment: 12 pages, 8 figures, accepted by A &
On the Ionisation Fraction in Protoplanetary Disks II: The Effect of Turbulent Mixing on Gas--phase Chemistry
We calculate the ionisation fraction in protostellar disk models using two
different gas-phase chemical networks, and examine the effect of turbulent
mixing by modelling the diffusion of chemical species vertically through the
disk. The aim is to determine in which regions of the disk gas can couple to a
magnetic field and sustain MHD turbulence. We find that the effect of diffusion
depends crucially on the elemental abundance of heavy metals (magnesium)
included in the chemical model. In the absence of heavy metals, diffusion has
essentially no effect on the ionisation structure of the disks, as the
recombination time scale is much shorter than the turbulent diffusion time
scale. When metals are included with an elemental abundance above a threshold
value, the diffusion can dramatically reduce the size of the magnetically
decoupled region, or even remove it altogther. For a complex chemistry the
elemental abundance of magnesium required to remove the dead zone is 10(-10) -
10(-8). We also find that diffusion can modify the reaction pathways, giving
rise to dominant species when diffusion is switched on that are minor species
when diffusion is absent. This suggests that there may be chemical signatures
of diffusive mixing that could be used to indirectly detect turbulent activity
in protoplanetary disks. We find examples of models in which the dead zone in
the outer disk region is rendered deeper when diffusion is switched on. Overall
these results suggest that global MHD turbulence in protoplanetary disks may be
self-sustaining under favourable circumstances, as turbulent mixing can help
maintain the ionisation fraction above that necessary to ensure good coupling
between the gas and magnetic field.Comment: 11 pages, 7 figures; accepted for publication in A &
On the Ionisation Fraction in Protoplanetary Disks I: Comparing Different Reaction Networks
We calculate the ionisation fraction in protostellar disk models using a
number of different chemical reaction networks, including gas-phase and
gas-grain reaction schemes. The disk models we consider are conventional
alpha-disks, which include viscous heating and radiative cooling. The primary
source of ionisation is assumed to be X-ray irradiation from the central star.
We consider a number of gas-phase chemical networks. In general we find that
the simple models predict higher fractional ionisation levels and more
extensive active zones than the more complex models. When heavy metal atoms are
included the simple models predict that the disk is magnetically active
throughout. The complex models predict that extensive regions of the disk
remain magnetically uncoupled even with a fractional abundance of magnesium of
10(-8). The addition of submicron sized grains with a concentration of 10(-12)
causes the size of the dead zone to increase dramatically for all kinetic
models considered. We find that the simple and complex gas-grain reaction
schemes agree on the size and structure of the resulting dead zone. We examine
the effects of depleting the concentration of small grains as a crude means of
modeling the growth of grains during planet formation. We find that a depletion
factor of 10(-4) causes the gas-grain chemistry to converge to the gas-phase
chemistry when heavy metals are absent. 10(-8) is required when magnesium is
included. This suggests that efficient grain growth and settling will be
required in protoplanetary disks, before a substantial fraction of the disk
mass in the planet forming zone between 1 - 10 AU becomes magnetically active
and turbulent.Comment: 21 pages, 23 figures, accepted for publication in A & A Includes
correction to our implementation of the Umebayashi-Nakano reaction networ
On the growth and orbital evolution of giant planets in layered protoplanetary disks
We present the results of hydrodynamic simulations of the growth and orbital
evolution of giant planets embedded in a protoplanetary disk with a dead-zone.
The aim is to examine to what extent the presence of a dead-zone affects the
rates of mass accretion and migration for giant planets. We performed 3D
numerical simulations using a grid-based hydrodynamics code. In these
simulations of non-magnetised disks, the dead-zone is treated as a region where
the vertical profile of the viscosity depends on the distance from the
equatorial plane. We consider dead-zones with vertical sizes, H_dz, ranging
from 0 to H_dz=2.3H, where H is the disk scale-height. For all models, the
vertically integrated viscous stress, and the related mass flux through the
disk, have the same value, such that the simulations test the dependence of
planetary mass accretion and migration on the vertical distribution of the
viscous stress. For each model, an embedded 30 earth-masses planet on a fixed
circular orbit is allowed to accrete gas from the disk. Once the planet mass
becomes equal to that of Saturn or Jupiter, we allow the planet orbit to evolve
due to gravitational interaction with the disk. We find that the time scale
over which a protoplanet grows to become a giant planet is essentially
independent of the dead-zone size, and depends only on the total rate at which
the disk viscously supplies material to the planet. For Saturn-mass planets,
the migration rate depends only weakly on the size of the dead-zone for H_dz<
1.5H, but becomes slower when H_dz=2.3H. This effect is due to the desaturation
of corotation torques which originate from residual material in the partial-gap
region. For Jupiter-mass planets, there is a clear tendency for the migration
to proceed more slowly as the size of the dead-zone increases.Comment: Accepted for publication in A&A. 10 pages, 12 figure
Turbulent transport and its effect on the dead zone in protoplanetary discs
Protostellar accretion discs have cool, dense midplanes where externally
originating ionisation sources such as X-rays or cosmic rays are unable to
penetrate. This suggests that for a wide range of radii, MHD turbulence can
only be sustained in the surface layers where the ionisation fraction is
sufficiently high. A dead zone is expected to exist near the midplane, such
that active accretion only occurs near the upper and lower disc surfaces.
Recent work, however, suggests that under suitable conditions the dead zone may
be enlivened by turbulent transport of ions from the surface layers into the
dense interior.
In this paper we present a suite of simulations that examine where, and under
which conditions, a dead zone can be enlivened by turbulent mixing. We use
three-dimensional, multifluid shearing box MHD simulations, which include
vertical stratification, ionisation chemistry, ohmic resistivity, and
ionisation due to X-rays from the central protostar. We compare the results of
the MHD simulations with a simple reaction-diffusion model.
The simulations show that in the absence of gas-phase heavy metals, such as
magnesium, turbulent mixing has essentially no effect on the dead zone. The
addition of a relatively low abundance of magnesium, however, increases the
recombination time and allows turbulent mixing of ions to enliven the dead zone
completely beyond a distance of 5 AU from the central star, for our particular
disc model. During the late stages of protoplanetary disc evolution, when small
grains have been depleted and the disc surface density has decreased below its
high initial value, the structure of the dead zone may be significantly altered
by the action of turbulent transport.Comment: 20 pages, 11 figures, accepted for publication in A&A, high
resolution pdf available at
http://www.maths.qmul.ac.uk/~rpn/preprints/index.htm
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Subsurface fluids screening by an analytical system employing a diffusion-limited and implantable sampling module deployable with a cone penetrometer
An analytical system employing a diffusion-limited sampling module and a direct sampling ion trap for quantitative assessment of subsurface fluids was developed and field tested. The sampling module is deployable with a cone penetrometer. It can be retrieved and/or remain as an implant for an indefinite time period. The device geometry, comprised of two planar membranes enclosing a diffusion cell, provides good implant ruggedness and reliable service in the field. Also, the sampling module is protected within a push pipe housing to extend implant service life. Subsurface volatile organic compound (VOC) vapors, in nanoliter amounts, diffuse through the sampler membrane wall by a diffusion-limited process that is independent of the soil permeability. Sample vapors are transported to the surface for analysis by direct sampling ion trap, or other analytical devices. Metered pressurized or reduced pressure transport (carrier) gas is utilized for sample transport to the surface. The vapors obtained are a function only of the fluid partial pressure and the vapor conductance of the sampler. Thus, quantitative analytical data is obtained regardless of soil conditions. The sampling module was deployed in the field at Dover Air Force Base at depths of 5 to 8.5 feet by the US Army Site Characterization and Analysis Penetrometer System (SCAPS). Relatively small 1.75 inch diameter push pipe and the relatively small vapor samples extracted cause minimal soil disturbance which preserves the integrity of the sampler subsurface surroundings. Analytical results were obtained for the system sampler operating in real time and as an implant where equilibrium was obtained between sampler interior and the external surroundings
Cardiac Autonomic Effects of Acute Exposures to Airborne Particulates in Men and Women
The aim of this research was to investigate cardiac autonomic changes associated with acute exposures to airborne particulates. Methods: High fidelity 12-lead ECG (CardioSoft, Houston, TX) was acquired from 19 (10 male / 9 female) non-smoking volunteers (age 33.6 +/- 6.6 yrs) during 10 minutes pre-exposure, exposure and post-exposure to environmental tobacco smoke (ETS), cooking oil fumes, wood smoke and sham (water vapor). To control exposure levels, noise, subject activity, and temperature, all studies were conducted inside an environmental chamber. Results: The short-term fractal scaling exponent (Alpha-1) and the ratio of low frequency to high frequency Heart Rate Variability (HRV) powers (LF/HF, a purported sympathetic index) were both higher in males (p<0.017 and p<0.05, respectively) whereas approximate entropy (ApEn) and HF/(LF+HF) (a purported parasympathetic index) were both lower in males (p<0.036, and p<0.044, respectively). Compared to pre-exposure (p<0.0002) and sham exposure (p<0.047), male heart rates were elevated during early ETS post-exposure. Our data suggest that, in addition to tonic HRV gender differences, cardiac responses to some acute airborne particulates are gender related
Chemistry in Disks. II. -- Poor molecular content of the AB Aur disk
We study the molecular content and chemistry of a circumstellar disk
surrounding the Herbig Ae star AB Aur at (sub-)millimeter wavelengths. Our aim
is to reconstruct the chemical history and composition of the AB Aur disk and
to compare it with disks around low-mass, cooler T Tauri stars. We observe the
AB Aur disk with the IRAM Plateau de Bure Interferometer in the C- and D-
configurations in rotational lines of CS, HCN, C2H, CH3OH, HCO+, and CO
isotopes. Using an iterative minimization technique, observed columns densities
and abundances are derived. These values are further compared with results of
an advanced chemical model that is based on a steady-state flared disk
structure with a vertical temperature gradient, and gas-grain chemical network
with surface reactions. We firmly detect HCO+ in the 1--0 transition,
tentatively detect HCN, and do not detect CS, C2H, and CH3OH. The observed HCO+
and 13CO column densities as well as the upper limits to the column densities
of HCN, CS, C2H, and CH3OH are in good agreement with modeling results and
those from previous studies. The AB Aur disk possesses more CO, but is less
abundant in other molecular species compared to the DM Tau disk. This is
primarily caused by intense UV irradiation from the central Herbig A0 star,
which results in a hotter disk where CO freeze out does not occur and thus
surface formation of complex CO-bearing molecules might be inhibited.Comment: Accepted by A&
On the migration of protoplanets embedded in circumbinary disks
We present the results of hydrodynamical simulations of low mass protoplanets
embedded in circumbinary accretion disks. The aim is to examine the migration
and long term orbital evolution of the protoplanets, in order to establish the
stability properties of planets that form in circumbinary disks. Simulations
were performed using a grid--based hydrodynamics code. First we present a set
of calculations that study how a binary interacts with a circumbinary disk. We
evolve the system for 10^5 binary orbits, which is the time needed for the
system to reach a quasi-equilibrium state. From this time onward the apsidal
lines of the disk and the binary are aligned, and the binary eccentricity
remains essentially unchanged with a value of e_b ~ 0.08. Once this stationary
state is obtained, we embed a low mass protoplanet in the disk and let it
evolve under the action of the binary and disk forces. We consider protoplanets
with masses of 5, 10 and 20 Earth masses. In each case, we find that inward
migration of the protoplanet is stopped at the edge of the tidally truncated
cavity formed by the binary. This effect is due to positve corotation torques,
which can counterbalance the net negative Lindblad torques in disk regions
where the surface density profile has a sufficiently large positive gradient.
Halting of migration occurs in a region of long-term stability, suggesting that
low mass circumbinary planets may be common, and that gas giant circumbinary
planets should be able to form in circumbinary disks.Comment: 10 pages, 10 figures, accepted for publication in A&
Global MHD simulations of stratified and turbulent protoplanetary discs. I. Model properties
We present the results of global 3-D MHD simulations of stratified and
turbulent protoplanetary disc models. The aim of this work is to develop thin
disc models capable of sustaining turbulence for long run times, which can be
used for on-going studies of planet formation in turbulent discs. The results
are obtained using two codes written in spherical coordinates: GLOBAL and
NIRVANA. Both are time--explicit and use finite differences along with the
Constrained Transport algorithm to evolve the equations of MHD. In the presence
of a weak toroidal magnetic field, a thin protoplanetary disc in hydrostatic
equilibrium is destabilised by the magnetorotational instability (MRI). When
the resolution is large enough (25 vertical grid cells per scale height), the
entire disc settles into a turbulent quasi steady-state after about 300 orbits.
Angular momentum is transported outward such that the standard alpha parameter
is roughly 4-6*10^{-3}. We find that the initial toroidal flux is expelled from
the disc midplane and that the disc behaves essentially as a quasi-zero net
flux disc for the remainder of the simulation. As in previous studies, the disc
develops a dual structure composed of an MRI--driven turbulent core around its
midplane, and a magnetised corona stable to the MRI near its surface. By
varying disc parameters and boundary conditions, we show that these basic
properties of the models are robust. The high resolution disc models we present
in this paper achieve a quasi--steady state and sustain turbulence for hundreds
of orbits. As such, they are ideally suited to the study of outstanding
problems in planet formation such as disc--planet interactions and dust
dynamics.Comment: 19 pages, 29 figures, accepted in Astronomy & Astrophysic
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