4,025 research outputs found
Generation of density inhomogeneities by magnetohydrodynamic waves in two dimensions
Using two dimensional simulations, we study the formation of structures with
a high-density contrast by magnetohydrodynamic waves in regions in which the
ratio of thermal to magnetic pressure is small. The initial state is a uniform
background perturbed by fast-mode wave. Our most significant result is that
dense structures persist for far longer in a two-dimensional simulation than in
the one-dimensional case. Once formed, these structures persist as long as the
fast-mode amplitude remains high.Comment: 6 pages, 7 figures, accepted by MNRA
Shock-triggered formation of magnetically-dominated clouds
To understand the formation of a magnetically dominated molecular cloud out
of an atomic cloud, we follow the dynamical evolution of the cloud with a
time-dependent axisymmetric magnetohydrodynamic code. A thermally stable warm
atomic cloud is initially in static equilibrium with the surrounding hot
ionised gas. A shock propagating through the hot medium interacts with the
cloud. As a fast-mode shock propagates through the cloud, the gas behind it
becomes thermally unstable. The value of the gas also becomes much
smaller than the initial value of order unity. These conditions are ideal for
magnetohydrodynamic waves to produce high-density clumps embedded in a rarefied
warm medium. A slow-mode shock follows the fast-mode shock. Behind this shock a
dense shell forms, which subsequently fragments. This is a primary region for
the formation of massive stars. Our simulations show that only weak and
moderate-strength shocks can form cold clouds which have properties typical of
giant molecular clouds.Comment: 7 pages, 6 figures, accepted by Astronomy and Astrophysic
Preparation of Subradiant States using Local Qubit Control in Circuit QED
Transitions between quantum states by photon absorption or emission are
intimately related to symmetries of the system which lead to selection rules
and the formation of dark states. In a circuit quantum electrodynamics setup,
in which two resonant superconducting qubits are coupled through an on-chip
cavity and driven via the common cavity field, one single-excitation state
remains dark. Here, we demonstrate that this dark state can be excited using
local phase control of individual qubit drives to change the symmetry of the
driving field. We observe that the dark state decay via spontaneous emission
into the cavity is suppressed, a characteristic signature of subradiance. This
local control technique could be used to prepare and study highly correlated
quantum states of cavity-coupled qubits.Comment: 5 pages, 4 figure
Removal and transformation of hexavalent chromium in sequencing batch reactor
The objectives of this study are to evaluate the efficiency of removal of hexavalent chromium (Cr(VI)) in a sequencing batch reactor (SBR) and to ascertain the fate of Cr(VI) in the treatment process. An SBR was operated with the FILL, REACT, SETTLE, DRAW and IDLE periods in the time ratio of 2:12:2:1.5:6.5 for a cycle time of 24 h. The study was divided into 5 phases with the addition of 0.5, 2.0, 3.0 and 5.0 mg/ℓ of Cr(VI) in Phases II, III, IV and V for a duration of 46, 75, 43 and 16 operational cycles, respectively. The Cr(VI) removal efficiencies for SBR were found to be 79.8, 88.4 and 99.8% in Phases III, IV and V, respectively. The results revealed that Cr(VI) removal efficiency improved with acclimated activated sludge. Determination of Cr in the suspended sludge showed that around 95% of the Cr species were Cr(III). Determination of Cr concentration profiles during the FILL and REACT periods showed that the predominant species was Cr(III) as Cr(VI) was bio-reduced. The proposed Cr(VI) removal mechanism involves bioreduction to Cr(III) which was subsequently precipitated and adsorbed by activated sludge. Precipitation rather than sorption is envisaged to be the main path of removal of Cr(III) from the solution.Keywords: Sequencing batch reactor, hexavalent chromium, removal, transformation, mechanis
Magnetic Fields and Star Formation
Research performed in the 1950s and 1960s by Leon Mestel on the roles of
magnetic fields in star formation established the framework within which he and
other key figures have conducted subsequent investigations on the subject. This
short tribute to Leon contains a brief summary of some, but not all, of his
ground breaking contributions in the area. It also mentions of some of the
relevant problems that have received attention in the last few years. The
coverage is not comprehensive, and the authors have drawn on their own results
more and touched more briefly on those of others than they would in a normal
review. Theirs is a personal contribution to the issue honouring Leon, one of
the truly great gentlemen, wits, and most insightful of astrophysicists.Comment: 13 pages, no figures, to be published in Astronomy and Geophysic
Dust in Interstellar Clouds, Evolved Stars and Supernovae
Outflows of pre-main-sequence stars drive shocks into molecular material
within 0.01 - 1 pc of the young stars. The shock-heated gas emits infrared,
millimeter and submillimeter lines of many species including. Dust grains are
important charge carriers and play a large role in coupling the magnetic field
and flow of neutral gas. Some effects of the dust on the dynamics of oblique
shocks began to emerge in the 1990s. However, detailed models of these shocks
are required for the calculation of the grain sputtering contribution to gas
phase abundances of species producing observed emissions. We are developing
such models. Some of the molecular species introduced into the gas phase by
sputtering in shocks or by thermally driven desorption in hot cores form on
grain surfaces. Recently laboratory studies have begun to contribute to the
understanding of surface reactions and thermally driven desorption important
for the chemistry of star forming clouds. Dusty plasmas are prevalent in many
evolved stars just as well as in star forming regions. Radiation pressure on
dust plays a significant role in mass loss from some post-main-sequence stars.
The mechanisms leading to the formation of carbonaceous dust in the stellar
outflows are similar to those important for soot formation in flames. However,
nucleation in oxygen-rich outflows is less well understood and remains a
challenging research area. Dust is observed in supernova ejecta that have not
passed through the reverse shocks that develop in the interaction of ejecta
with ambient media. Dust is detected in high redshift galaxies that are
sufficiently young that the only stars that could have produced the dust were
so massive that they became supernovae. Consequently, the issue of the survival
of dust in strong supernova shocks is of considerable interest.Comment: 4 pages, to be published in the proceedings of Fifth International
Conference on Physics of Dusty Plasma
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