3,585 research outputs found
Nitrogen dynamics in the shallow groundwater of a riparian wetland zone of the Garonne, SW France: nitrate inputs, bacterial densities, organic matter supply and denitrification measurements
This study highlights the role of interactions between surface and sub-surface water of the riparian zone of a large river (the Garonne, SW
France). Information is given about the role of surface water in supplying Dissolved Organic Carbon (DOC ) to the riparian zone for nitrate
removal processes. The densities of bacteria (up to 3.3106 cell m L-1) in groundwater are strongly conditioned by the water moving during
flood events. Total bacterial densities in groundwater were related to surface water bacterial densities. In sediment, total bacteria are attached
mainly to fine particles (90 % in the fraction < 1 mm). Spatial variations in organic carbon and nitrate content in groundwater at the site
studied are correlated with exchanges between the groundwater and the river, from the upstream to the downstream part of the meander. Total
bacterial densities, nitrate and decressing organic carbon concentrations follow the same pattern. These results suggest that, in this kind of
riparian wetland, nitrate from alluvial groundwater influenced by agricultural practices may be denitrified by bacteria in the presence of
organic carbon from river surface water
A standardised method for measuring in situ denitrification in shallow aquifers: numerical validation and measurements in riparian wetlands
A tracer test to examine in situ denitrification in shallow groundwater by a piezometer with a packer system used bromide as a tracer of dilution and acetylene (10%) to block the denitrification process at the nitrous oxide stage. During the test, dissolved oxygen, nitrate (NO3-), bromide (Br-), nitrous oxide (N2O) and dissolved organic carbon (DOC) were measured. To calibrate the experimental method, comparison with numerical simulations of the groundwater transfer were carried out, taking into account the environmental characteristics. The method was tested by measurements undertaken in different environmental conditions (geology, land use and hydrology) in two riparian wetlands. Denitrification rates measured by this method ranged from 5.7 10-6 g N-NO3-L-1 h-1 to 1.97 10-3 g N-NO3-L-1 h-1 The method is applicable in shallow aquifers with a permeability from 10-2 to 10-4m s-1
3D SIMULATION OF A 500KG UO2 MELT IN A COLD CRUCIBLE INDUCTION FURNACE
International audienc
The Nature of Radio Continuum Emission in the Dwarf Starburst Galaxy NGC 625
We present new multi-frequency radio continuum imaging of the dwarf starburst
galaxy NGC 625 obtained with the Very Large Array. Data at 20, 6, and 3.6 cm
reveal global continuum emission dominated by free-free emission, with only
mild synchrotron components. Each of the major HII regions is detected; the
individual spectral indices are thermal for the youngest regions (showing
strongest H Alpha emission) and nonthermal for the oldest. We do not detect any
sources that appear to be associated with deeply embedded, dense, young
clusters, though we have discovered one low-luminosity, obscured source that
has no luminous optical counterpart and which resides in the region of highest
optical extinction. Since NGC 625 is a Wolf-Rayet galaxy with strong recent
star formation, these radio properties suggest that the youngest star formation
complexes have not yet evolved to the point where their thermal spectra are
significantly contaminated by synchrotron emission. The nonthermal components
are associated with regions of older star formation that have smaller ionized
gas components. These results imply a range of ages of the HII regions and
radio components that agrees with our previous resolved stellar population
analysis, where an extended burst of star formation has pervaded the disk of
NGC 625 over the last ~ 50 Myr. We compare the nature of radio continuum
emission in selected nearby dwarf starburst and Wolf-Rayet galaxies,
demonstrating that thermal radio continuum emission appears to be more common
in these systems than in typical HII galaxies with less recent star formation
and more evolved stellar clusters.Comment: ApJ, in press; 27 pages, 5 figures. Full-resolution version may be
obtained at http://www.astro.umn.edu/~cannon/n625.vla.p
Extended mid-infrared emission from VV 114: probing the birth of a ULIRG
We present our 5-16 micron spectro-imaging observations of VV114, an infrared
luminous early-stage merger, taken with the ISOCAM camera on-board ISO. We find
that only 40% of the mid-infrared (MIR) flux is associated with a compact
nuclear region, while the rest of the emission originates from a rather diffuse
component extended over several kpc. This is in stark contrast with the very
compact MIR starbursts usually seen in luminous infrared galaxies. A secondary
peak of MIR emission is associated with an extra-nuclear star forming region
which displays the largest Halpha equivalent width in the whole system.
Comparing our data with the distribution of the molecular gas and cold dust, as
well as with radio observations, it becomes evident that the conversion of
molecular gas into stars can be triggered over large areas at the very first
stages of an interaction. The presence of a very strong continuum at 5 microns
in one of the sources indicates that an enshrouded active galactic nucleus may
contribute to 40% of its MIR flux. We finally note that the relative variations
in the UV to radio spectral properties between the merging galaxies provide
evidence that the extinction-corrected star formation rate of similar objects
at high z, such as those detected in optical deep surveys, can not be
accurately derived from their rest-frame UV properties.Comment: 14 pages, 5 figures, accepted for publication in A&
Star formation triggered by the Galactic HII region RCW 120: First results from the Herschel Space Observatory
By means of different physical mechanisms, the expansion of HII regions can
promote the formation of new stars of all masses. RCW 120 is a nearby Galactic
HII region where triggered star formation occurs. This region is well-studied -
there being a wealth of existing data - and is nearby. However, it is
surrounded by dense regions for which far infrared data is essential to obtain
an unbiased view of the star formation process and in particular to establish
whether very young protostars are present. We attempt to identify all Young
Stellar Objects (YSOs), especially those previously undetected at shorter
wavelengths, to derive their physical properties and obtain insight into the
star formation history in this region. We use Herschel-PACS and -SPIRE images
to determine the distribution of YSOs observed in the field. We use a spectral
energy distribution fitting tool to derive the YSOs physical properties.
Herschel-PACS and -SPIRE images confirm the existence of a young source and
allow us to determine its nature as a high-mass (8-10 MSun) Class 0 object
(whose emission is dominated by a massive envelope) towards the massive
condensation 1 observed at (sub)-millimeter wavelengths. This source was not
detected at 24 micron and only barely seen in the MISPGAL 70 micron data.
Several other red sources are detected at Herschel wavelengths and coincide
with the peaks of the millimeter condensations. SED fitting results for the
brightest Herschel sources indicate that, apart from the massive Class 0 that
forms in condensation 1, young low mass stars are forming around RCW 120. The
YSOs observed on the borders of RCW 120 are younger than its ionizing star,
which has an age of about 2.5 Myr.Comment: 5 pqges, 3 figures, accepted by A&A (Special issue on the Herschel
first results
Numerical simulations in the development of the French radioactive waste vitrification processes using induction furnace
International audienceFor many years, the CEA (Commissariat à l’Énergie Atomique et aux Énergies Alternatives) Marcoule France has developed various processes dedicated to radioactive waste confinement, especially vitrification processes for HLLW. For 15 years now, the numerical simulation has become an important tool for research and developement projects held in the CEA-AREVA Joint Vitrification Laboratory (LCV). Induction heating, fluid mechanics and thermal simulations take part of all new R&D projects. The apports of such simulations are, first, the enhancement of the working knowledge of existing process. Those data are very useful to define optimisation choices, for example upgrades made on the hot metallic melter used since the 90s at LaHague facility. Second, the simulations are, of course, also used at the conception stage of new processes as a tool allowing wide ranges parametric tests. This has been extensively used in the design of the cold crucible inductive melter (CCIM) commissioned in 2010 at La Hague plant. Finally, it is a powerful and relatively cheap tool for prospective studies for processes of the future. Whatever the purpose, the potential benefits are gains on the reliability, the output capacity and the life time
Dust in an extremely metal-poor galaxy: mid-infrared observations of SBS 0335-052
The metal deficient (Z = Z_sun/41) Blue Compact Dwarf Galaxy (BCD) SBS
0335-052 was observed with ISOCAM between 5 and 17 mic. With a L_12mic/L_B
ratio of 2.15, the galaxy is unexpectedly bright in the mid-infrared for such a
low-metallicity object. The mid-infrared spectrum shows no sign of the
Unidentified Infrared Bands, which we interpret as an effect of the destruction
of their carriers by the very high UV energy density in SBS 0335-052. The
spectral energy distribution (SED) is dominated by a very strong continuum
which makes the ionic lines of [SIV] and [NeIII] very weak. From 5 to 17 mic,
the SED can be fitted with a grey-body spectrum, modified by an extinction law
similar to that observed toward the Galactic Center, with an optical depth of
A_V~19-21 mag. Such a large optical depth implies that a large fraction (as
much as ~ 75%) of the current star-formation activity in SBS 0335-052 is hidden
by dust with a mass between 3x10^3 M_sun and 5x10^5 M_sun. Silicate grains are
present as silicate extinction bands at 9.7 and 18 mic can account for the
unusual shape of the MIR spectrum of SBS 0335-052. It is remarkable that such a
nearly primordial environment contains as much dust as galaxies which are 10
times more metal-rich. If the hidden star formation in SBS 0335-052 is typical
of young galaxies at high redshifts, then the cosmic star formation rate
derived from UV/optical fluxes would be underestimated.Comment: 13 pages, 4 figures, requires aaspp4.sty, accepted in Ap
Tropospheric ozone over Equatorial Africa: regional aspects from the MOZAIC data
We analyze ozone observations recorded over Equatorial Africa between April 1997 and March 2003 by the MOZAIC programme, providing the first ozone climatology deriving from continental in-situ data over this region. Three-dimensional streamlines strongly suggests connections between the characteristics of the ozone monthly mean vertical profiles, the most persistent circulation patterns in the troposphere over Equatorial Africa (on a monthly basis) such as the Harmattan, the African Easterly Jet, the Trades and the regions of ozone precursors emissions by biomass burning. During the biomass burning season in each hemisphere, the lower troposphere exhibits layers of enhanced ozone (i.e. 70 ppbv over the coast of Gulf of Guinea in December-February and 85 ppbv over Congo in June-August). The characteristics of the ozone monthly mean vertical profiles are clearly connected to the regional flow regime determined by seasonal dynamic forcing. The mean ozone profile over the coast of Gulf of Guinea in the burning season is characterized by systematically high ozone below 650hPa ; these are due to the transport by the Harmattan and the AEJ of the pollutants originating from upwind fires. The confinement of high ozone to the lower troposphere is due to the high stability of the Harmattan and the blocking Saharan anticyclone which prevents efficient vertical mixing. In contrast, ozone enhancements observed over Central Africa during the local dry season (June-August) are not only found in the lower troposphere but throughout the troposphere. Moreover, this study highlights a connection between the regions of the coast of Gulf of Guinea and regions of Congo to the south that appears on a semi annual basis. Vertical profiles in wet-season regions exhibit ozone enhancements in the lower troposphere due to biomass burning products transport from fires situated in the opposite dry-season hemisphere
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