122 research outputs found
Investigation of a generalized Obukhov Model for Turbulence
We introduce a generalization of Obukhov's model [A.M. Obukhov, Adv. Geophys.
6, 113 (1959)] for the description of the joint position-velocity statistics of
a single fluid particle in fully developed turbulence. In the presented model
the velocity is assumed to undergo a continuous time random walk. This takes
into account long time correlations. As a consequence the evolution equation
for the joint position-velocity probability distribution is a Fokker-Planck
equation with a fractional time derivative. We determine the solution of this
equation in the form of an integral transform and derive a relation for
arbitrary single time moments. Analytical solutions for the joint probability
distribution and its moments are given.Comment: 10 page
A Cross-Over in the Enstrophy Decay in Two-Dimensional Turbulence in a Finite Box
The numerical simulation of two-dimensional decaying turbulence in a large
but finite box presented in this paper uncovered two physically different
regimes of enstrophy decay. During the initial stage, the enstrophy, generated
by a random Gaussian initial condition, decays as t^{-gamma} with gamma
approximately 0.7-0.8. After that, the flow undergoes a transition to a gas or
fluid composed of distinct vortices. Simultaneously, the magnitude of the decay
exponent crosses over to gamma approximately 0.4. An exact relation for the
total number of vortices, N(t), in terms of the mean circulation of an
individual vortex is derived. A theory predicting that N(t) is proportional to
t^{-xi} and the magnitudes of exponents gamma=2/5 and xi=4/5 is presented and
the possibility of an additional very late-time cross-over to gamma=1/3 and
xi=2/3 is also discussed.Comment: 11 pages, 7 figure
The Pollution of Pristine Material in Compressible Turbulence
The first generation of stars had very different properties than later
stellar generations, as they formed from a "pristine" gas that was free of
heavy elements. Normal star formation took place only after the first stars
polluted the surrounding turbulent interstellar gas, increasing its local heavy
element concentration, Z, beyond a critical value, Z_c (10^-8 < Z_c <10^-5).
Motivated by this astrophysical problem, we investigate the fundamental physics
of the pollution of pristine fluid elements in isotropic compressible
turbulence. Turbulence stretches the pollutants, produces concentration
structures at small scales, and brings the pollutants and the unpolluted flow
in closer contact. Our theoretical approach employs the probability
distribution function (PDF) method for turbulent mixing. We adopt three PDF
closure models and derive evolution equations for the pristine fraction from
the models. To test and constrain the theoretical models, we conduct numerical
simulations for decaying passive scalars in isothermal turbulent flows with
Mach numbers of 0.9 and 6.2, and compute the mass fraction, P(Z_c, t), of the
flow with Z < Z_c. In the Mach 0.9 flow, the evolution of P(Z_c, t)$ is well
described by a continuous convolution model and dP(Z_c, t)/dt = P(Z_c, t)
ln[P(Z_c, t)]/tau_con, if the mass fraction of the polluted flow is larger than
~ 0.1. If the initial pollutant fraction is smaller than ~ 0.1, an early phase
exists during which the pristine fraction follows an equation from a nonlinear
integral model: dP(Z_c, t)/dt = P(Z_c, t) [P(Z_c, t)-1]/tau_int. The timescales
tau_con and tau_int are measured from our simulations. When normalized to the
flow dynamical time, the decay of P(Z_ c, t) in the Mach 6.2 flow is slower
than at Mach 0.9, and we show that P(Z_c, t) in the Mach 6.2 flow can be well
fit using a formula from a generalized version of the self-convolution model.Comment: 30 pages, 8 figure. Accepted by Journal of Fluid Mechanic
A Disk Shadow Around the Young Star ASR 41 in NGC 1333
We present images of the young stellar object ASR 41 in the NGC 1333 star
forming region at the wavelengths of H_alpha and [SII] and in the I, J, H, and
K-bands.
ASR 41 has the near-infrared morphology of an edge-on disk object, but
appears an order of magnitude larger than typical systems of this kind.
We also present detailed models of the scattering and radiative transfer in
systems consisting of a young star surrounded by a proto-planetary disk, and
the whole system being embedded in either an infalling envelope or a uniform
molecular cloud. The best fit to the observed morphology can be achieved with a
disk of approx. 200 AU diameter, immersed in a low density cloud. The low cloud
density is necessary to stay below the sub-mm flux upper limits and to preserve
the shadow cast by the disk via single scattering.
The results demonstrate that ASR 41 is probably not inherently different from
typical edge-on disk objects, and that its large apparent size is due to the
shadow of a much smaller disk being projected into the surrounding dusty
molecular material
Multi-Generational Star Formation in L1551
The L1551 molecular cloud contains two small clusters of Class 0 and I
protostars, as well as a halo of more evolved Class II and III YSOs, indicating
a current and at least one past burst of star formation. We present here new,
sensitive maps of 850 and 450 um dust emission covering most of the L1551
cloud, new CO J=2-1 data of the molecular cloud, and a new, deep, optical image
of [SII] emission. No new Class 0/I YSOs were detected. Compact sub-millimetre
emitters are concentrated in two sub-clusters: IRS5 and L1551NE, and the
HL~Tauri group. Both stellar groups show significant extended emission and
outflow/jet activity. A jet, terminating at HH 265 and with a very weak
associated molecular outflow, may originate from LkHa 358, or from a binary
companion to another member of the HL Tauri group. Several Herbig Haro objects
associated with IRS5/NE were clearly detected in the sub-mm, as were faint
ridges of emission tracing outflow cavity walls. We confirm a large-scale
molecular outflow originating from NE parallel to that from IRS5, and suggest
that the "hollow shell" morphology is more likely due to two interacting
outflows. We confirm the presence of a prestellar core (L1551-MC) of mass 2-3
Mo north-west of IRS5. The next generation cluster may be forming in this core.
The L1551 cloud appears cometary in morphology, and appears to be illuminated
and eroded from the direction of Orion, perhaps explaining the multiple
episodes of star formation in this cloud. The full paper (including figures)
can be downloaded at http://www.jach.hawaii.edu/~gms/l1551/l1551-apj641.pdf, or
viewed at http://www.jach.hawaii.edu/~gms/l1551/.Comment: Accepted for publication in The Astrophysical Journal, April 2006
(vol. 641). 27 pages, 17 figure
Fluxes and gas transfer rates of the biogenic trace gas DMS derived from atmospheric gradients
Author Posting. © American Geophysical Union, 2004. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research 109 (2004): C08S10, doi:10.1029/2003JC001795.Gas transfer rates were determined from vertical profile measurements of atmospheric dimethylsulfide (DMS) gradients over the equatorial Pacific Ocean obtained during the GasEx-2001 cruise. A quadratic relationship between gas transfer velocity and wind speed was derived from the DMS flux measurements; this relationship was in close agreement with a parameterization derived from relaxed eddy accumulation measurements of DMS over the northeastern Pacific Ocean. However, the GasEx-2001 relationship results in gas transfer rates that are a factor 2 higher than gas transfer rates calculated from a parameterization that is based on coincident eddy correlation measurements of CO2 flux. The measurement precision of both the profiling and eddy correlation techniques applied during GasEx-2001 is comparable; the two gas transfer data sets are in agreement within their uncertainty. Differences in the number of samples and the wind speed range over which CO2 and DMS fluxes were measured are likely causes for the observed discrepancy.Funding for this work came
from the Netherlands Organization for Scientific Research (NWO) and from
the NOP project 951203: ââMicrometeorology of air/sea fluxes of carbon
dioxide. This work was supported by the Global Carbon Cycle project of
the NOAA Office of Global Programs grant NA17RJ1223, National
Science Foundation grant OCE-9986724, and NSF grant ATM-0120569
Measurements of momentum and heat transfer across the airâsea interface
Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 1054â1072, doi:10.1175/2007JPO3739.1.This study makes direct measurements of turbulent fluxes in the mixed layer in order to close heat and momentum budgets across the airâsea interface and to assess the ability of rigid-boundary turbulence models to predict mean vertical gradients beneath the oceanâs wavy surface. Observations were made at 20 Hz at nominal depths of 2.2 and 1.7 m in 16 m of water. A new method is developed to estimate the fluxes and the length scales of dominant flux-carrying eddies from cospectra at frequencies below the wave band. The results are compared to independent estimates of those quantities, with good agreement between the two sets of estimates. The observed temperature gradients were smaller than predicted by standard rigid-boundary closure models, consistent with the suggestion that wave breaking and Langmuir circulation increase turbulent diffusivity in the upper ocean. Similarly, the MoninâObukhov stability function Ïh was smaller in the authorsâ measurements than the stability functions used in rigid-boundary applications of the MoninâObukhov similarity theory. The dominant horizontal length scales of flux-carrying turbulent eddies were found to be consistent with observations in the bottom boundary layer of the atmosphere and from laboratory experiments in three ways: 1) in statically unstable conditions, the eddy sizes scaled linearly with distance to the boundary; 2) in statically stable conditions, length scales decreased with increasing downward buoyancy flux; and 3) downwind length scales were larger than crosswind length scales.We are
grateful to the Office of Naval Research for funding
this work as a part of CBLAST-Low
Genome wide analysis of gene dosage in 24,092 individuals estimates that 10,000 genes modulate cognitive ability
International audienceGenomic copy number variants (CNVs) are routinely identified and reported back to patients with neuropsychiatric disorders, but their quantitative effects on essential traits such as cognitive ability are poorly documented. We have recently shown that the effect size of deletions on cognitive ability can be statistically predicted using measures of intolerance to haploinsufficiency. However, the effect sizes of duplications remain unknown. It is also unknown if the effect of multigenic CNVs are driven by a few genes intolerant to haploinsufficiency or distributed across tolerant genes as well. Here, we identified all CNVsâ>â50 kilobases in 24,092 individuals from unselected and autism cohorts with assessments of general intelligence. Statistical models used measures of intolerance to haploinsufficiency of genes included in CNVs to predict their effect size on intelligence. Intolerant genes decrease general intelligence by 0.8 and 2.6 points of intelligence quotient when duplicated or deleted, respectively. Effect sizes showed no heterogeneity across cohorts. Validation analyses demonstrated that models could predict CNV effect sizes with 78% accuracy. Data on the inheritance of 27,766 CNVs showed that deletions and duplications with the same effect size on intelligence occur de novo at the same frequency. We estimated that around 10,000 intolerant and tolerant genes negatively affect intelligence when deleted, and less than 2% have large effect sizes. Genes encompassed in CNVs were not enriched in any GOterms but gene regulation and brain expression were GOterms overrepresented in the intolerant subgroup. Such pervasive effects on cognition may be related to emergent properties of the genome not restricted to a limited number of biological pathways
A survey of SiO 5-4 emission towards outflows from low-luminosity protostellar candidates
We have observed the SiO J=5--4 line towards a sample of 25 low-luminosity
(L* < 10^3 Lsun) protostellar outflow systems. The line was detected towards 7
of the 25 sources, a detection rate of 28 per cent. The majority (5 out of 7)
of sources detected were of class 0 status. We detected a higher fraction of
class 0 sources compared with the class I and II sources, although given the
small numbers involved the significance of this result should be regarded as
tentative. Most of the detected sources showed emission either at or close to
the central position, coincident with the protostar. In four cases (HH211,
HH25MMS, V-380 OriNE and HH212) emission was also detected at positions away
from the center, and was stronger than that observed at the centre position.
SiO abundances of 10^(-8) to 8x10^(-7) are derived from LTE analysis. For 2
sources we have additional transitions which we use to conduct statistical
equilibrium modeling to estimate the gas density in the SiO-emitting regions.
For HH25MMS these results suggest that the SiO emission arises in a
higher-density region than the methanol previously observed. We find that the
most likely explanation for the preferential detection of SiO emission towards
class 0 sources is the greater density of those environments, reinforced by
higher shock velocities. We conclude that while not all class 0 sources exhibit
SiO emission, SiO emission is a good signpost for the presence of class 0
sources.Comment: 21 pages, inc 10 B&W figures. Uses emulateapj5.sty. Accepted for
publication in ApJ, scheduled for 1 March 2004 issu
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