1,451 research outputs found
New fire diurnal cycle characterizations to improve fire radiative energy assessments made from low-Earth orbit satellites sampling
Accurate near real time fire emissions estimates are required for
air quality forecasts. To date, most approaches are based on
satellite-derived estimates of fire radiative power (FRP), which can
be converted to fire radiative energy (FRE) which is directly
related to fire emissions. Uncertainties in these FRE estimations
are often substantial. This is for a large part because the most
often used low-Earth orbit satellite-based instruments like the
MODerate-resolution Imaging Spectroradiometer (MODIS) have
a relatively poor sampling of the usually pronounced fire diurnal
cycle. In this paper we explore the spatial variation of this fire
diurnal cycle and its drivers. Specifically, we assess how
representing the fire diurnal cycle affects FRP and FRE estimations
when using data collected at MODIS overpasses. Using data
assimilation we explored three different methods to estimate hourly
FRE, based on an incremental sophistication of parameterizing the
fire diurnal cycle. We sampled data from the geostationary Meteosat
Spinning Enhanced Visible and Infrared Imager (SEVIRI) at MODIS
detection opportunities to drive the three approaches. The full
SEVIRI time-series, providing full coverage of the diurnal cycle,
were used to evaluate the results. Our study period comprised three
years (2010â2012), and we focussed on Africa and the Mediterranean
basin to avoid the use of potentially lower quality SEVIRI data
obtained at very far off-nadir view angles. We found that the fire
diurnal cycle varies substantially over the study region, and
depends on both fuel and weather conditions. For example, more
"intense" fires characterized by a fire diurnal cycle with high
peak fire activity, long duration over the day, and with nighttime
fire activity are most common in areas of large fire size (i.e.,
large burned area per fire event). These areas are most prevalent in
relatively arid regions. Ignoring the fire diurnal cycle as done
currently in some approaches caused structural errors, while
generally overestimating FRE. Including information on the
climatology of the fire diurnal cycle provided the most promising avenue
to improve FRE estimations. This approach also improved the
performance on relatively high spatiotemporal resolutions, although
only when aggregating model results to coarser spatial and/or
temporal scale good correlation was found with the full SEVIRI
hourly reference dataset. In general model performance was best in
areas of frequent fire and low errors of omission. We recommend the use
of regionally varying fire diurnal cycle information within the
Global Fire Assimilation System (GFAS) used in the Copernicus
Atmosphere Monitoring Services, which will improve FRE estimates and
may allow for further reconciliation of biomass burning emission
estimates from different inventories
Dust in 3C324
The results of a deep submillimetre observation using SCUBA of the powerful
radio galaxy 3C324, at redshift z=1.206, are presented. At 850 microns,
emission from the location of the host radio galaxy is marginally detected at
the 4.2 sigma level, 3.01 +/- 0.72 mJy, but there is no detection of emission
at 450 microns to a 3 sigma limit of 21 mJy. A new 32 GHz radio observation
using the Effelsberg 100m telescope confirms that the sub-millimetre signal is
not associated with synchrotron emission. These observations indicate that both
the mass of warm dust within 3C324, and the star formation rate, lie up to an
order of magnitude below the values recently determined for radio galaxies at z
= 3 to 4. The results are compared with dust masses and star formation rates
derived in other ways for 3C324.Comment: 5 pages LaTeX, including 1 figure. Accepted for publication in MNRA
Physical Conditions in Orion's Veil
Orion's veil consists of several layers of largely neutral gas lying between
us and the main ionizing stars of the Orion nebula. It is visible in 21cm H I
absorption and in optical and UV absorption lines of H I and other species.
Toward the Trapezium, the veil has two remarkable properties, high magnetic
field (~100 microGauss) and a surprising lack of molecular hydrogen given its
total hydrogen column density. Here we compute photoionization models of the
veil to establish its gas density and its distance from the Trapezium. We use a
greatly improved model of the hydrogen molecule that determines level
populations in ~1e5 rotational/vibrational levels and provides improved
estimates of molecular hydrogen destruction via the Lyman-Werner bands. Our
best fit photoionization models place the veil 1-3 pc in front of the star at a
density of 1e3-1e4 cubic centimeters. Magnetic energy dominates the energy of
non-thermal motions in at least one of the 21cm H I velocity components.
Therefore, the veil is the first interstellar environment where magnetic
dominance appears to exist. We find that the low ratio of molecular to atomic
hydrogen (< 1e-4) is a consequence of high UV flux incident upon the veil due
to its proximity to the Trapezium stars and the absence of small grains in the
region.Comment: 45 pages, 20 figures, accepted for publication in Ap
Physical Conditoins in Orion's Veil II: A Multi-Component Study of the Line of Sight Toward the Trapezium
Orion's Veil is an absorbing screen that lies along the line of sight to the
Orion H II region. It consists of two or more layers of gas that must lie
within a few parsecs of the Trapezium cluster. Our previous work considered the
Veil as a whole and found that the magnetic field dominates the energetics of
the gas in at least one component. Here we use high-resolution STIS UV spectra
that resolve the two velocity components in absorption and determine the
conditions in each. We derive a volume hydrogen density, 21 cm spin
temperature, turbulent velocity, and kinetic temperature, for each. We combine
these estimates with magnetic field measurements to find that magnetic energy
significantly dominates turbulent and thermal energies in one component, while
the other component is close to equipartition between turbulent and magnetic
energies. We observe molecular hydrogen absorption for highly excited v, J
levels that are photoexcited by the stellar continuum, and detect blueshifted S
III and P III. These ions must arise from ionized gas between the mostly
neutral portions of the Veil and the Trapezium and shields the Veil from
ionizing radiation. We find that this layer of ionized gas is also responsible
for He I absorption in the Veil, which resolves a 40-year-old debate on the
origin of He I absorption towards the Trapezium. Finally, we determine that the
ionized and mostly atomic layers of the Veil will collide in less than 85,000
years.Comment: 43 pages, 15 figures, to be published in Ap
Molecular gas in the northern nucleus of Mrk273: Physical and chemical properties of the disk and its outflow
Aiming to characterise the properties of the molecular gas in the
ultraluminous infrared galaxy Mrk273 and its outflow, we used the NOEMA
interferometer to image the dense gas molecular tracers HCN, HCO+, HNC, HOC+
and HC3N at 86GHz and 256GHz with angular resolutions of 4.9x4.5 arcsec
(3.7x3.4 kpc) and 0.61x0.55 arcsec (460x420 pc). We also modelled the flux of
several H2O lines observed with Herschel using a radiative transfer code that
includes excitation by collisions as well as by far-infrared photons. The disk
of the Mrk273 north nucleus has two components with decoupled kinematics. The
gas in the outer parts (1.5 kpc) rotates with a south-east to north-west
direction, while in the inner disk (300 pc) follows a north-east to south-west
rotation. The central 300 pc, which hosts a compact starburst region, is filled
with dense and warm gas, contains a dynamical mass of (4-5)x10^9M_sun, a
luminosity of L'_HCN=(3-4)x10^8 K km/s pc^2, and a dust temperature of 55 K. At
the very centre, a compact core with R~50 pc has a luminosity of
L_IR=4x10^11L_sun (30% of the total infrared luminosity), and a dust
temperature of 95 K. The core is expanding at low velocities ~50-100 km/s,
probably affected by the outflowing gas. We detect the blue-shifted component
of the outflow, while the red-shifted counterpart remains undetected in our
data. Its cold and dense phase reaches fast velocities up to ~1000 km/s, while
the warm outflowing gas has more moderate maximum velocities of ~600 km/s. The
outflow is detected as far as 460 pc from the centre in the northern direction,
and has a mass of dense gas <8x10^8M_sun. The difference between the position
angles of the inner disk (~70 degree) and the outflow (~10 degree) indicates
that the outflow is likely powered by the AGN, and not by the starburst.
Regarding the chemistry, we measure an extremely low HCO+/HOC+ ratio of 10+-5
in the inner disk of Mrk273.Comment: Accepted for publication in A&A. 21 pages, 17 figures, 7 tables, and
a lot of interesting tex
Improved coronary calcium detection and quantification with low-dose full field-of-view photon-counting CT:a phantom study
OBJECTIVE: The aim of the current study was to systematically assess coronary artery calcium (CAC) detection and quantification for spectral photon-counting CT (SPCCT) in comparison to conventional CT and, in addition, to evaluate the possibility of radiation dose reduction. METHODS: Routine clinical CAC CT protocols were used for data acquisition and reconstruction of two CAC containing cylindrical inserts which were positioned within an anthropomorphic thorax phantom. In addition, data was acquired at 50% lower radiation dose by reducing tube current, and slice thickness was decreased. Calcifications were considered detectable when three adjacent voxels exceeded the CAC scoring threshold of 130 Hounsfield units (HU). Quantification of CAC (as volume and mass score) was assessed by comparison with known physical quantities. RESULTS: In comparison with CT, SPCCT detected 33% and 7% more calcifications for the small and large phantoms, respectively. At reduced radiation dose and reduced slice thickness, small phantom CAC detection increased by 108% and 150% for CT and SPCCT, respectively. For the large phantom size, noise levels interfered with CAC detection. Although comparable between CT and SPCCT, routine protocols CAC quantification showed large deviations (up to 134%) from physical CAC volume. At reduced radiation dose and slice thickness, physical volume overestimations decreased to 96% and 72% for CT and SPCCT, respectively. In comparison with volume scores, mass score deviations from physical quantities were smaller. CONCLUSION: CAC detection on SPCCT is superior to CT, and was even preserved at a reduced radiation dose. Furthermore, SPCCT allows for improved physical volume estimation. KEY POINTS: âą In comparison with conventional CT, increased coronary artery calcium detection (up to 156%) for spectral photon-counting CT was found, even at 50% radiation dose reduction. âą Spectral photon-counting CT can more accurately measure physical volumes than conventional CT, especially at reduced slice thickness and for high-density coronary artery calcium. âą For both conventional and spectral photon-counting CT, reduced slice thickness reconstructions result in more accurate physical mass approximation
The Color Magnitude Distribution of Field Galaxies to z~3: the evolution and modeling of the blue sequence
Using deep NIR VLT/ISAAC and optical HST/WFPC2 imaging in the fields of the
HDFS and MS1054-03, we study the rest-frame UV-to-optical colors and magnitudes
of galaxies to z~3. While there is no evidence for a red sequence at z~3, there
does appear to be a well-defined color-magnitude relation (CMR) for blue
galaxies at all redshifts, with more luminous galaxies having redder U-V
colors. The slope of the blue CMR is independent of redshift d(U-V)/dMV = -0.09
(0.01) and can be explained by a correlation of dust-reddening with luminosity.
The average color at fixed luminosity reddens strongly \Delta(U-V) = 0.75 from
z~3 to z=0, much of which can be attributed to aging of the stars. The color
scatter of the blue sequence is relatively small sigma(U-V) = 0.25 (0.03) and
constant to z~3, but notably asymmetrical with a sharp blue ridge and a wing
towards redder colors. We explore sets of star formation histories to study the
constraints placed by the shape of the scatter at z=2-3. One particular set of
models, episodic star formation, reproduces the detailed properties very well.
For a two-state model with high and low star formation, the duty cycle is
constrained to be > 40% and the contrast between the states must be a factor >
5 (or a scatter in log(SFR) of > 0.35 dex around the mean). However, episodic
models do not explain the observed tail of very red galaxies, primarily Distant
Red Galaxies (DRGs), which may have ceased star formation altogether or are
more heavily obscured. Finally, the relative number density of red, luminous MV
< -20.5 galaxies increases by a factor of ~ 6 from z = 2.7 to z = 0.5, as does
their contribution to the total rest-frame V-band luminosity density. We are
likely viewing the progressive formation of red, passively evolving galaxies.Comment: 29 pages, 24 figures, in emulateapj style. Abstract is abridged. Some
postscript figures are compressed. accepted for publication in ApJ (scheduled
for August 20, 2007, v665n 2 issue
GSH23.0-0.7+117, a neutral hydrogen shell in the inner Galaxy
GSH23.0-0.7+117 is a well-defined neutral hydrogen shell discovered in the
VLA Galactic Plane Survey (VGPS). Only the blueshifted side of the shell was
detected. The expansion velocity and systemic velocity were determined through
the systematic behavior of the HI emission with velocity. The center of the
shell is at (l,b,v)=(23.05,-0.77,+117 km/s). The angular radius of the shell is
6.8', or 15 pc at a distance of 7.8 kpc. The HI mass divided by the volume of
the half-shell implies an average density n_H = 11 +/- 4 cm^{-3} for the medium
in which the shell expanded. The estimated age of GSH23.0-0.7+117 is 1 Myr,
with an upper limit of 2 Myr. The modest expansion energy of 2 * 10^{48} erg
can be provided by the stellar wind of a single O4 to O8 star over the age of
the shell. The 3 sigma upper limit to the 1.4 GHz continuum flux density
(S_{1.4} < 248 mJy) is used to derive an upper limit to the Lyman continuum
luminosity generated inside the shell. This upper limit implies a maximum of
one O9 star (O8 to O9.5 taking into account the error in the distance) inside
the HI shell, unless most of the incident ionizing flux leaks through the HI
shell. To allow this, the shell should be fragmented on scales smaller than the
beam (2.3 pc). If the stellar wind bubble is not adiabatic, or the bubble has
burst (as suggested by the HI channel maps), agreement between the energy and
ionization requirements is even less likely. The limit set by the non-detection
in the continuum provides a significant challenge for the interpretation of
GSH23.0-0.7+117 as a stellar wind bubble. A similar analysis may be applicable
to other Galactic HI shells that have not been detected in the continuum.Comment: 18 pages, 6 figures. Figures 1 and 4 separately in GIF format.
Accepted for publication in Astrophysical Journa
- âŠ