7,670 research outputs found
Stellar escapers from M67 can reach solar-like Galactic orbits
We investigate the possibility that the Sun could have been born in M67 by
carrying out -body simulations of M67-like clusters in a time-varying
Galactic environment, and following the galactic orbits of stars that escape
from them. We find that model clusters that occupy similar orbits to M67 today
can be divided up into three groups. Hot clusters are born with a high initial
-velocity, depleted clusters are born on cold orbits but are destroyed by
GMC encounters in the Galactic disc, and scattered clusters are born on cold
orbits and survive with more than 1000 stars at an age of 4.6 Gyr. We find that
all cluster models in all three cluster groups have stellar escapers that are
kinematicaly similar to the Sun. Hot clusters having the lowest such fraction
%, whilst depleted clusters have the highest fraction,
%. We calculate that clusters that are destroyed in the
Galactic disc have a specific frequency of escapers that end up on solar-like
orbits that is 2 times that of escapers from clusters that survive their
journey
Arcsecond resolution images of the chemical structure of the low-mass protostar IRAS 16293-2422
It remains a key challenge to establish the molecular content of different
components of low-mass protostars, like their envelopes and disks, and how this
depends on the evolutionary stage and/or environment of the young stars.
Observations at submillimeter wavelengths provide a direct possibility to study
the chemical composition of low-mass protostars through transitions probing
temperatures up to a few hundred K in the gas surrounding these sources. This
paper presents a large molecular line survey of the deeply embedded
protostellar binary IRAS 16293-2422 from the Submillimeter Array (SMA) -
including images of individual lines down to approximately 1.5-3" (190-380 AU)
resolution. More than 500 individual transitions are identified related to 54
molecular species (including isotopologues) probing temperatures up to about
550 K. Strong chemical differences are found between the two components in the
protostellar system with a separation between, in particular, the sulfur- and
nitrogen-bearing species and oxygen-bearing complex organics. The action of
protostellar outflow on the ambient envelope material is seen in images of CO
and SiO and appear to influence a number of other species, including
(deuterated) water, HDO. The effects of cold gas-phase chemistry is directly
imaged through maps of CO, N2D+ and DCO+, showing enhancements of first DCO+
and subsequently N2D+ in the outer envelope where CO freezes-out on dust
grains.Comment: Accepted for publication in A&A, 30 pages, 22 figure
Ecology of Thioploca spp.: Nitrate and sulfur storage in relation to chemical microgradients and influence of Thioploca spp. on the sedimentary nitrogen cycle
Microsensors, including a recently developed NO3 − biosensor, were applied to measure O2 and NO3 − profiles in marine sediments from the upwelling area off central Chile and to investigate the influence of Thioploca spp. on the sedimentary nitrogen metabolism. The studies were performed in undisturbed sediment cores incubated in a small laboratory flume to simulate the environmental conditions of low O2, high NO3 −, and bottom water current. On addition of NO3 −and NO2 −, Thioploca spp. exhibited positive chemotaxis and stretched out of the sediment into the flume water. In a core densely populated with Thioploca, the penetration depth of NO3 − was only 0.5 mm and a sharp maximum of NO3 − uptake was observed 0.5 mm above the sediment surface. In sediments with only fewThioploca spp., NO3 − was detectable down to a depth of 2 mm and the maximum consumption rates were observed within the sediment. No chemotaxis toward nitrous oxide (N2O) was observed, which is consistent with the observation that Thioploca does not denitrify but reduces intracellular NO3 − to NH4 +. Measurements of the intracellular NO3 − and S0 pools inThioploca filaments from various depths in the sediment gave insights into possible differences in the migration behavior between the different species. Living filaments containing significant amounts of intracellular NO3 − were found to a depth of at least 13 cm, providing final proof for the vertical shuttling of Thioploca spp. and nitrate transport into the sediment
On C*-algebras generated by pairs of q-commuting isometries
We consider the C*-algebras O_2^q and A_2^q generated, respectively, by
isometries s_1, s_2 satisfying the relation s_1^* s_2 = q s_2 s_1^* with |q| <
1 (the deformed Cuntz relation), and by isometries s_1, s_2 satisfying the
relation s_2 s_1 = q s_1 s_2 with |q| = 1. We show that O_2^q is isomorphic to
the Cuntz-Toeplitz C*-algebra O_2^0 for any |q| < 1. We further prove that
A_2^{q_1} is isomorphic to A_2^{q_2} if and only if either q_1 = q_2 or q_1 =
complex conjugate of q_2. In the second part of our paper, we discuss the
complexity of the representation theory of A_2^q. We show that A_2^q is *-wild
for any q in the circle |q| = 1, and hence that A_2^q is not nuclear for any q
in the circle.Comment: 18 pages, LaTeX2e "article" document class; submitted. V2 clarifies
the relationships between the various deformation systems treate
On particles in the Arctic stratosphere
Soon after the discovery of the Antarctic ozone hole it became clear that particles in the polar stratosphere had
an infl uence on the destruction of the ozone layer. Two major types of particles, sulphate aerosols and Polar
Stratospheric Clouds (PSCs), provide the surfaces where fast heterogeneous chemical reactions convert inactive
halogen reservoir species into potentially ozone-destroying radicals. Lidar measurements have been used to classify
the PSCs. Following the Mt. Pinatubo eruption in June 1991 it was found that the Arctic stratosphere was loaded
with aerosols, and that aerosols observed with lidar and ozone observed with ozone sondes displayed a layered
structure, and that the aerosol and ozone contents in the layers frequently appeared to be negatively correlated.
The layered structure was probably due to modulation induced by the dynamics at the edge of the polar vortex.
Lidar observations of the Mt. Pinatubo aerosols were in several cases accompanied by balloon-borne backscatter
soundings, whereby backscatter measurements in three different wavelengths made it possible to obtain information
about the particle sizes. An investigation of the infl uence of synoptic temperature histories on the physical properties
of PSC particles has shown that most of the liquid type 1b particles were observed in the process of an ongoing,
relatively fast, and continuous cooling from temperatures clearly above the nitric acid trihydrate condensation
temperature (TNAT). On the other hand, it appeared that a relatively long period, with a duration of at least 1-2 days,
at temperatures below TNAT provide the conditions which may lead to the production of solid type 1a PSCs
Transport and reduction of nitrate in clayey till underneath forest and arable land.
Transport and reduction of nitrate in a typically macroporous clayey till were examined at variable flow rate and nitrate flux. The experiments were carried out using saturated, large diameter (0.5 m), undisturbed soil columns (LUC), from a forest and nearby agricultural sites. Transport of nitrate was controlled by flow along the macropores (fractures and biopores) in the columns. Nitrate reduction (denitrification) determined under active flow mainly followed first order reactions with half-lives (t1/2) increasing with depth (1.5–3.5 m) from 7 to 35 days at the forest site and 1–7 h at the agricultural site. Nitrate reduction was likely due to microbial degradation of accumulated organic matter coupled with successive consumption of O2 and NO3− in the macropore water followed by reductive dissolution of Fe and Mn from minerals along the macropores. Concentrations of total organic carbon measured in soil samples were near identical at the two study sites and consequently not useful as indicator for the observed differences in nitrate reduction. Instead the high reduction rates at the agricultural site were positively correlated with elevated concentration of water-soluble organic carbon and nitrate-removing bacteria relative to the forest site. After high concentrations of water-soluble organic carbon in the columns from the agricultural site were leached they lost their elevated reduction rates, which, however, was successfully re-established by infiltration of new reactive organics represented by pesticides. Simulations using a calibrated discrete fracture matrix diffusion (DFMD) model could reasonably reproduce the denitrification and resulting flux of nitrate observed during variable flow rate from the columns
A deeply embedded young protoplanetary disk around L1489 IRS observed by the submillimeter array
Circumstellar disks are expected to form early in the process that leads to
the formation of a young star, during the collapse of the dense molecular cloud
core. It is currently not well understood at what stage of the collapse the
disk is formed or how it subsequently evolves. We aim to identify whether an
embedded Keplerian protoplanetary disk resides in the L1489 IRS system. Given
the amount of envelope material still present, such a disk would respresent a
very young example of a protoplanetary disk. Using the Submillimeter Array
(SMA) we have observed the HCO 3--2 line with a resolution of about
1. At this resolution a protoplanetary disk with a radius of a few hundred
AUs should be detectable, if present. Radiative transfer tools are used to
model the emission from both continuum and line data. We find that these data
are consistent with theoretical models of a collapsing envelope and Keplerian
circumstellar disk. Models reproducing both the SED and the interferometric
continuum observations reveal that the disk is inclined by 40 which is
significantly different to the surrounding envelope (74). This
misalignment of the angular momentum axes may be caused by a gradient within
the angular momentum in the parental cloud or if L1489 IRS is a binary system
rather than just a single star. In the latter case, future observations looking
for variability at sub-arcsecond scales may be able to constrain these
dynamical variations directly. However, if stars form from turbulent cores, the
accreting material will not have a constant angular momentum axis (although the
average is well defined and conserved) in which case it is more likely to have
a misalignment of the angular momentum axes of the disk and the envelope.Comment: 11 pages, 13 figures, accepted by A&
Can on-farm bioenergy production make organic farming more sustainable? - A model for energy balance, nitrogen losses, and green house gas emissions in a 1000 ha energy catchment with organic dairy farming and integrated bioenergy production
Can biogas and bioethanol production make organic farming more sustainable?
- Results from a model for the fossil energy balance, Nitrogen losses, and greenhouse gas emissions in a 1000 ha energy catchment with organic dairy farming and integrated biogas and bioethanol production.
Dalgaard T1, Pugesgaard S1, Jørgensen U1, Olesen JE1, Møller HB1 and Jensen ES2
1) Dept. Agroecology and Environment. Faculty of Agricultural Sciences (DJF), University of Aarhus. DK-8830 Tjele. Denmark. Contact: [email protected]
2) Biosystems Department, Risø DTU, The National Laboratory for Sustainable Energy, The Technical University of Denmark DK-4000 Roskilde, Denmark
The vision of organic farming systems, independent of fossil energy resources, with significantly lower nutrient losses, and no net contribution to the greenhouse gas emissions might be fulfilled via the integration of biogas production. This is an important hypothesis investigated in the www.bioconcens.elr.dk/uk/ research project.
This poster illustrates preliminary results from a model for the fossil energy balance, Nitrogen losses, and greenhouse gas emissions in a 1000 ha energy catchment with organic dairy farming and integrated biogas production in Denmark. The model will draw on results from previous models (e.g the farmGHG model), and includes a number of organic dairy farm type components, including information on livestock production, housing, manure storage, manure and fodder import/export, crop rotations, yield levels, and soil types. In addition, a biogas plant model component evaluates effects of the inclusion of variable amounts of manures and crop residues from the specified farm types, into the biogas energy production.
The model is intended to result in an overall catchment balance for the following three types of indicators: 1) the fossil energy use – i.e. the net fossil energy use minus the bioenergy production, 2) losses of Nitrogen in the form of nitrates, ammonia and nitrous oxide, and 3) the emission of the three main greenhouse gasses from agriculture: carbon dioxide, nitrous oxide and methane, measured in carbon dioxide equivalents. Moreover, these indicator values are specified for each of the farm types included in the model, and for the biogas plant component. Finally, selected model results are discussed in relation to the overall hypothesis of the research project, and it is discussed how the integration of biogas production in organic farming, can help to improve the self-sufficiency in Nitrogen, and thereby reduce the import of nutrients to the organic farming systems
On particles in the Arctic stratosphere
Soon after the discovery of the Antarctic ozone hole it became clear that particles in the polar stratosphere had
an infl uence on the destruction of the ozone layer. Two major types of particles, sulphate aerosols and Polar
Stratospheric Clouds (PSCs), provide the surfaces where fast heterogeneous chemical reactions convert inactive
halogen reservoir species into potentially ozone-destroying radicals. Lidar measurements have been used to classify
the PSCs. Following the Mt. Pinatubo eruption in June 1991 it was found that the Arctic stratosphere was loaded
with aerosols, and that aerosols observed with lidar and ozone observed with ozone sondes displayed a layered
structure, and that the aerosol and ozone contents in the layers frequently appeared to be negatively correlated.
The layered structure was probably due to modulation induced by the dynamics at the edge of the polar vortex.
Lidar observations of the Mt. Pinatubo aerosols were in several cases accompanied by balloon-borne backscatter
soundings, whereby backscatter measurements in three different wavelengths made it possible to obtain information
about the particle sizes. An investigation of the infl uence of synoptic temperature histories on the physical properties
of PSC particles has shown that most of the liquid type 1b particles were observed in the process of an ongoing,
relatively fast, and continuous cooling from temperatures clearly above the nitric acid trihydrate condensation
temperature (TNAT). On the other hand, it appeared that a relatively long period, with a duration of at least 1-2 days,
at temperatures below TNAT provide the conditions which may lead to the production of solid type 1a PSCs
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