2,032 research outputs found
Evaluation of a global aerosol microphysics model against size-resolved particle statistics in the marine atmosphere
A statistical synthesis of marine aerosol measurements from experiments in four different oceans is used to evaluate a global aerosol microphysics model (GLOMAP). We compare the model against observed size resolved particle concentrations, probability distributions, and the temporal persistence of different size particles. We attempt to explain the observed sub-micrometre size distributions in terms of sulfate and sea spray and quantify the possible contributions of anthropogenic sulfate and carbonaceous material to the number and mass distribution. The model predicts a bimodal size distribution that agrees well with observations as a grand average over all regions, but there are large regional differences. Notably, observed Aitken mode number concentrations are more than a factor 10 higher than in the model for the N Atlantic but a factor 7 lower than the model in the NW Pacific. We also find that modelled Aitken mode and accumulation mode geometric mean diameters are generally smaller in the model by 10–30%. Comparison with observed free tropospheric Aitken mode distributions suggests that the model underpredicts growth of these particles during descent to the marine boundary layer (MBL). Recent observations of a substantial organic component of free tropospheric aerosol could explain this discrepancy. We find that anthropogenic continental material makes a substantial contribution to N Atlantic MBL aerosol, with typically 60–90% of sulfate across the particle size range coming from anthropogenic sources, even if we analyse air that has spent an average of >120 h away from land. However, anthropogenic primary black carbon and organic carbon particles (at the emission size and quantity assumed here) do not explain the large discrepancies in Aitken mode number. Several explanations for the discrepancy are suggested. The lack of lower atmospheric particle formation in the model may explain low N Atlantic particle concentrations. However, the observed and modelled particle persistence at Cape Grim in the Southern Ocean, does not reveal a diurnal cycle consistent with a photochemically driven local particle source. We also show that a physically based cloud drop activation scheme better explains the observed change in accumulation mode geometric mean diameter with particle number
Associations between daily sitting time and the combinations of lifestyle risk factors in men
Background: Understanding the reciprocal role that multiple problematic behaviours play in men's health is important for intervention delivery and for reducing the healthcare burden. Data regarding the concurrence of problematic health behaviours is currently limited but offers insights into risk profiles, and should now include total time spent sitting/day. Methods: Self-reported data on lifestyle health behaviours was collected from 232 men aged ≥18 years who engaged in a men's health promotion programme delivered by 16 English Premier League Clubs. Results: Men at risk due to high sitting display multiple concurrent lifestyle risk factors, 88.6% displayed at least two ancillary risk factors and were three times more likely to report ≥2 lifestyle risk factors (OR. =3.13, 95% confidence interval (CI). =1.52-6.42) than those with low sitting risk. Significant differences in the mean number of risk factors reported between those participants in the higher risk (2.43. ±. 0.90) and lower risk (2.13. ±. 0.96) sitting categories were also found (P=0.015). Conclusions: Hard-to-reach men displayed multiple problematic concurrent behaviours, strongly linked to total sitting time. © 2012 WPMH GmbH
Growth and migration of solids in evolving protostellar disks I: Methods and Analytical tests
This series of papers investigates the early stages of planet formation by
modeling the evolution of the gas and solid content of protostellar disks from
the early T Tauri phase until complete dispersal of the gas. In this first
paper, I present a new set of simplified equations modeling the growth and
migration of various species of grains in a gaseous protostellar disk evolving
as a result of the combined effects of viscous accretion and photo-evaporation
from the central star. Using the assumption that the grain size distribution
function always maintains a power-law structure approximating the average
outcome of the exact coagulation/shattering equation, the model focuses on the
calculation of the growth rate of the largest grains only. The coupled
evolution equations for the maximum grain size, the surface density of the gas
and the surface density of solids are then presented and solved
self-consistently using a standard 1+1 dimensional formalism. I show that the
global evolution of solids is controlled by a leaky reservoir of small grains
at large radii, and propose an empirically derived evolution equation for the
total mass of solids, which can be used to estimate the total heavy element
retention efficiency in the planet formation paradigm. Consistency with
observation of the total mass of solids in the Minimum Solar Nebula augmented
with the mass of the Oort cloud sets strong upper limit on the initial grain
size distribution, as well as on the turbulent parameter \alphat. Detailed
comparisons with SED observations are presented in a following paper.Comment: Submitted to ApJ. 23 pages and 13 figure
P-glycoprotein and metallothionein expression and resistance to chemotherapy in osteosarcoma.
The expression of the drug resistance (DR) mediators P-glycoprotein (P-gp) and the metallothioneins (MT) was assessed immunohistochemically in biopsy material from patients with high-grade malignant osteosarcoma (OS). No significant difference was found in survival rate between expressors of both P-gp and MT and non-expressors. Thus, it was concluded that lack of expression of these two drug resistance-related proteins does not appear to confer any advantage in terms of patient survival in osteosarcoma
Radio Continuum Evidence for Outflow and Absorption in the Seyfert 1 Galaxy Markarian 231
The VLBA and the VLA have been used to image the continuum radio emission
from Mrk 231, a Seyfert 1 galaxy and the brightest infrared galaxy in the local
universe. The smallest scales reveal a double source less than 2 pc in extent.
The components of this central source have minimum brightness temperatures of
10^9 to 10^{10} K, spectral turnovers between 2 and 10 GHz, and appear to
define the galaxy nucleus plus the inner regions of a jet. The components may
be free-free absorbed or synchtrotron self-absorbed.
On larger scales, the images confirm a previously known north-south triple
source extending 40 pc and elongated perpendicular to a 350-pc starburst disk.
Both lobes show evidence for free-free absorption near 2 GHz, probably due to
ionized gas with a density of 1-2 X 10^3 cm^{-3} in the innermost parts of the
starburst disk. The absorbing gas may be ionized by the active nucleus or by
local regions of enhanced star formation. The elongation of the 40-pc triple
differs by 65 deg from that of the 2-pc source. The different symmetry axes on
different scales imply strong curvature in the inner part of the radio jet.
The radio continuum from the 350-pc disk has a spectral index near -0.4 above
1.4 GHz and is plausibly energized by a massive burst of star formation. On VLA
scales, asymmetric and diffuse emission extends for more than 25 kpc. This
emission has a steep spectrum, linear polarization exceeding 50% at some
locations, and shares the symmetry axis of the 40-pc triple. The diffuse radio
source is probably generated by energy deposition from a slow-moving nuclear
jet, which conceivably could help energize the off-nuclear starburst as well.Comment: 34 pages, 7 Postscript figures, LaTeX file in AASTeX format, accepted
in ApJ, Vol. 516, May 1, 199
Warping and Precession of Accretion Disks Around Magnetic Stars: Nonlinear Evolution
The inner region of the accretion disk around a magnetized star (T Tauri
star, white dwarf or neutron star) is subjected to magnetic torques that induce
warping and precession of the disk. These torques arise from the interaction
between the stellar field and the induced electric currents in the disk. We
carry out numerical simulations of the nonlinear evolution of warped, viscous
accretion disks driven by the magnetic torques. We show that the disk can
develop into a highly warped steady state in which the disk attains a fixed
(warped) shape and precesses rigidly. The warp is most pronounced at the disk
inner radius (near the magnetosphere boundary). As the system parameters (such
as accretion rate) change, the disk can switch between a completely flat state
(warping stable) and a highly warped state. The precession of warped disks may
be responsible for a variety of quasi-periodic oscillations or radiation flux
variabilities observed in many different systems, including young stellar
objects and X-ray binaries.Comment: 16 pages, 7 figures; extended parameter searches, changes in
discussion; accepted for publication in Ap
Magnetically Torqued Thin Accretion Disks
We compute the properties of a geometrically thin, steady accretion disk
surrounding a central rotating, magnetized star. The magnetosphere is assumed
to entrain the disk over a wide range of radii. The model is simplified in that
we adopt two (alternate) ad hoc, but plausible, expressions for the azimuthal
component of the magnetic field as a function of radial distance. We find a
solution for the angular velocity profile tending to corotation close to the
central star, and smoothly matching a Keplerian curve at a radius where the
viscous stress vanishes. The value of this ''transition'' radius is nearly the
same for both of our adopted B-field models. We then solve analytically for the
torques on the central star and for the disk luminosity due to gravity and
magnetic torques. When expressed in a dimensionless form, the resulting
quantities depend on one parameter alone, the ratio of the transition radius to
the corotation radius. For rapid rotators, the accretion disk may be powered
mostly by spin-down of the central star. These results are independent of the
viscosity prescription in the disk. We also solve for the disk structure for
the special case of an optically thick alpha disk. Our results are applicable
to a range of astrophysical systems including accreting neutron stars,
intermediate polar cataclysmic variables, and T Tauri systems.Comment: 9 sharper figs, updated reference
Radiation-Driven Warping. II. Non-Isothermal Disks
Recent work by Pringle and by Maloney, Begelman & Pringle has shown that
geometrically thin, optically thick, accretion disks are unstable to warping
driven by radiation torque from the central source. In this paper we generalize
the study of radiation-driven warping to include general power-law surface
density distributions, . We consider the range
(isothermal disks) to , which corresponds to a
radiation-pressure-supported disk; this spans the range of surface density
distributions likely to be found in real astrophysical disks. There is a
critical minimum size for unstable disks. The critical radius and the
steady-state precession rate depend only weakly on . The case
divides the solutions into two qualitatively different regimes.
Nonlinear effects must be important if the warp extends to the disk inner edge
for , but for nonlinearity will be important only if
the warp amplitude is large at the origin. The effects of shadowing of the
central source by the warp will thus be very different in the two regimes of
In real accretion disks the outer boundary condition is likely to be
different from the zero-crossing condition that we have assumed. In accretion
disks around massive black holes in active galactic nuclei, the disk will
probably become optically thin before the outer disk boundary is reached, while
in X-ray binaries, there will be an outer disk region (outside the
circularization radius) in which the inflow velocity is zero but angular
momentum is still transported. We show that in both these cases the solutions
are similar to the zero-crossing eigenfunctions.Comment: 43 pages, 16 figures, to appear in The Astrophysical Journa
A Two-Fluid Thermally-Stable Cooling Flow Model
A new model for cooling flows in X-ray clusters, capable of naturally
explaining salient features observed, is proposed. The only requirement is that
a significant relativistic component, in the form of cosmic rays (CR), be
present in the intra-cluster medium and significantly frozen to the thermal
gas. Such an addition qualitatively alters the conventional isobaric thermal
instability criterion such that a fluid parcel becomes thermally stable when
its thermal pressure drops below a threshold fraction of its CR pressure.
Consequently, the lowest possible temperature at any radius is about one third
of the ambient temperature {\it at that radius}, exactly as observed, In
addition, we suggest that dissipation of internal gravity waves, excited by
radial oscillatory motions of inward drifting cooling clouds about their radial
equilibrium positions, may be responsible for heating up cooling gas. With the
ultimate energy source for powering the cooling X-ray luminosity and heating up
cooling gas being gravitational due to inward drifting cooling clouds as well
as the general inward flow, heating is spatially distributed and energetically
matched with cooling. One desirable property of this heating mechanism is that
heating energy is strongly centrally concentrated, providing the required
heating for emission-line nebulae.Comment: 13 pages, submitted to ApJ
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