15,688 research outputs found
A dimensional analysis of supersaturated total dissolved gas dissipation
Elevated levels of total dissolved gas (TDG) may occur downstream of dam discharges, leading to increased incidence of gas bubble disease in fish. Accelerating the dissipation of supersaturated TDG in the downstream river can mitigate this negative problem. However, developing effective mitigation techniques is hampered by limitations in present models of TDG dissipation processes. Furthermore, data useful for modelling the dissipation of supersaturated TDG through the
free surface in natural rivers are limited. Past studies indicated that the TDG dissipation process is quantitatively different from the reaeration process, and TDG behavior is quantitatively different from dissolved oxygen. However, a correct parameterization of the TDG dissipation process is still missing.
The paper presents a novel dimensional analysis of the dissipation of supersaturated TDG. This approach can provide a relationship between the TDG dissipation coefficient and some classical fluid mechanics index-numbers. This dimensional analysis considers some key parameters for the dissipation process both water and TDG properties as well as flow characteristics, including turbulence. These parameters are water kinematic viscosity, TDG molecular diffusivity and vertical turbulent diffusivity, and channel width. The application of dimensional analysis pointed out that the TDG dissipation coefficient is a function of the Schmidt number, the aspect ratio of the channel, and the shear Reynolds
number. The dimensional analysis was then verified using both field data collected in some large natural rivers and reservoirs in Sichuan and experimental data in laboratory flume at State Key Laboratory of Hydraulics and Mountain River Engineering of Sichuan University. The analysis revealed the key role of turbulence in controlling the TDG dissipation while the importance of gas/water characteristics remains still unclear and needs further investigations
H ortho-to-para conversion on grains: A route to fast deuterium fractionation in dense cloud cores?
Deuterium fractionation, i.e. the enhancement of deuterated species with
respect to the non-deuterated ones, is considered to be a reliable chemical
clock of star-forming regions. This process is strongly affected by the
ortho-to-para (o-p) H ratio. In this letter we explore the effect of the
o-p H conversion on grains on the deuteration timescale in fully depleted
dense cores, including the most relevant uncertainties that affect this complex
process. We show that (i) the o-p H conversion on grains is not strongly
influenced by the uncertainties on the conversion time and the sticking
coefficient and (ii) that the process is controlled by the temperature and the
residence time of ortho-H on the surface, i.e. by the binding energy. We
find that for binding energies in between 330-550 K, depending on the
temperature, the o-p H conversion on grains can shorten the deuterium
fractionation timescale by orders of magnitude, opening a new route to explain
the large observed deuteration fraction in dense molecular
cloud cores. Our results suggest that the star formation timescale, when
estimated through the timescale to reach the observed deuteration fractions,
might be shorter than previously proposed. However, more accurate measurements
of the binding energy are needed to better assess the overall role of this
process.Comment: Accepted for publication in ApJ Letter
Adiabatic connection at negative coupling strengths
The adiabatic connection of density functional theory (DFT) for electronic
systems is generalized here to negative values of the coupling strength
(with {\em attractive} electrons). In the extreme limit
a simple physical solution is presented and its implications
for DFT (as well as its limitations) are discussed. For two-electron systems (a
case in which the present solution can be calculated exactly), we find that an
interpolation between the limit and the opposite limit of
infinitely strong repulsion () yields a rather accurate
estimate of the second-order correlation energy E\cor\glt[\rho] for several
different densities , without using virtual orbitals. The same procedure
is also applied to the Be isoelectronic series, analyzing the effects of
near-degeneracy.Comment: 9 pages, submitted to PR
Alx1, a member of the Cart1/Alx3/Alx4 subfamily of Paired-class homeodomain proteins, is an essential component of the gene network controlling skeletogenic fate specification in the sea urchin embryo
In the sea urchin embryo, the large micromeres and their progeny function as a critical signaling center and execute a complex morphogenetic program. We have identified a new and essential component of the gene network that controls large micromere specification, the homeodomain protein Alx1. Alx1 is expressed exclusively by cells of the large micromere lineage beginning in the first interphase after the large micromeres are born. Morpholino studies demonstrate that Alx1 is essential at an early stage of specification and controls downstream genes required for epithelial-mesenchymal transition and biomineralization. Expression of Alx1 is cell autonomous and regulated maternally through ß-catenin and its downstream effector, Pmar1. Alx1 expression can be activated in other cell lineages at much later stages of development, however, through a regulative pathway of skeletogenesis that is responsive to cell signaling. The Alx1 protein is highly conserved among euechinoid sea urchins and is closely related to the Cart1/Alx3/Alx4 family of vertebrate homeodomain proteins. In vertebrates, these proteins regulate the formation of skeletal elements of the limbs, face and neck. Our findings suggest that the ancestral deuterostome had a population of biomineral-forming mesenchyme cells that expressed an Alx1-like protein
Perfect quantum transport in arbitrary spin networks
Spin chains have been proposed as wires to transport information between
distributed registers in a quantum information processor. Unfortunately, the
challenges in manufacturing linear chains with engineered couplings has
hindered experimental implementations. Here we present strategies to achieve
perfect quantum information transport in arbitrary spin networks. Our proposal
is based on the weak coupling limit for pure state transport, where information
is transferred between two end-spins that are only weakly coupled to the rest
of the network. This regime allows disregarding the complex, internal dynamics
of the bulk network and relying on virtual transitions or on the coupling to a
single bulk eigenmode. We further introduce control methods capable of tuning
the transport process and achieve perfect fidelity with limited resources,
involving only manipulation of the end-qubits. These strategies could be thus
applied not only to engineered systems with relaxed fabrication precision, but
also to naturally occurring networks; specifically, we discuss the practical
implementation of quantum state transfer between two separated nitrogen vacancy
(NV) centers through a network of nitrogen substitutional impurities.Comment: 5+7 page
X-ray Properties of Low-Mass Pre-Main Sequence Stars in the Orion Trapezium Cluster
The Chandra High Energy Transmission Gratings (HETG) Orion Legacy Project
(HOLP) is the first comprehensive set of observations of a very young massive
stellar cluster which provides high resolution X-ray spectra of very young
stars over a wide mass range (0.7 - 2.3 Msun). In this paper, we focus on the
six brightest X-ray sources with T Tauri stellar counterparts which are
well-characterized at optical and infra-red wavelengths. All stars show column
densities which are substantially smaller than expected from optical extinction
indicating that the sources are located on the near side of the cluster with
respect to the observer as well as that these stars are embedded in more dusty
environments. Stellar X-ray luminosities are well above erg/s, in
some cases exceeding erg/s for a substantial amount of time. The
stars during these observations show no flares but are persistently bright. The
spectra can be well fit with two temperature plasma components of 10 MK and 40
MK, of which the latter dominates the flux by a ratio 6:1 on average. The total
EMs range between 3 - 8 cm and are comparable to active
coronal sources. Limits on the forbidden to inter-combination line ratios in
the He-Like K-shell lines show that we observe a predominantely optically thin
plasma with electron densities below cm. Observed abundances
compare well with active coronal sources underlying the coronal nature of these
sources. The surface flux in this sample of 0.6 to 2.3 Msun classical T Tauri
stars shows that coronal activity and possibly coronal loop size increase
significantly between ages 0.1 to 10 Myrs.Comment: 13 pages, 12 figures, submitted to the Astrophysical Journa
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