186 research outputs found
A modeling approach of the relationship between nitrous oxide fluxes from soils and the water-filled pore space
International audienceNitrous oxide (N2O) fluxes can increase significantly following small increases in soil water-filled pore space (WFPS). Thus, it is essential to improve our knowledge of this crucial relationship to better model N2O emissions by soils. We studied how much the addition of a gas transport and a gas–liquid equilibrium module to the model of N2O emissions NOE could improve simulation results. A sensitivity analysis of the modified model (NOEGTE: gas transport and equilibrium) was first performed, and then the model was tested with published data of a wetting–drying experiment. Simulated N2O fluxes plotted against WFPS appeared to be bell-shaped during the 7 days simulated, combining the effects of the low N2O production for WFPS 0.95. The WFPS generating the maximum simulated N2O fluxes shifted with time, from 0.76 after 12 h, to 0.79 after 168 h, because of an increase over time of the gas concentration gradient between the soil surface and the atmosphere. NOEGTE appeared able to capture the pattern of N2O emissions monitored in the experimental data. In particular, N2O peaks during drying were well reproduced in terms of timing, but their magnitudes were often overestimated. They were attributed to the increasing gas diffusivity and N2O exchanges from the liquid phase to the gaseous phase
Bayesian calibration of the nitrous oxide emission module of an agro-ecosystem model
Nitrous oxide (N2O) is the main biogenic greenhouse gas contributing to the global warming potential
(GWP) of agro-ecosystems. Evaluating the impact of agriculture on climate therefore requires a capacity
to predict N2O emissions in relation to environmental conditions and crop management. Biophysical
models simulating the dynamics of carbon and nitrogen in agro-ecosystems have a unique potential to
explore these relationships, but are fraught with high uncertainties in their parameters due to their
variations over time and space. Here, we used a Bayesian approach to calibrate the parameters of the N2O
submodel of the agro-ecosystem model CERES-EGC. The submodel simulates N2O emissions from the
nitrification and denitrification processes, which are modelled as the product of a potential rate with
three dimensionless factors related to soil water content, nitrogen content and temperature. These
equations involve a total set of 15 parameters, four of which are site-specific and should be measured on
site, while the other 11 are considered global, i.e. invariant over time and space. We first gathered prior
information on the model parameters based on the literature review, and assigned them uniform
probability distributions. A Bayesian method based on the Metropolis–Hastings algorithm was
subsequently developed to update the parameter distributions against a database of seven different
field-sites in France. Three parallel Markov chains were run to ensure a convergence of the algorithm.
This site-specific calibration significantly reduced the spread in parameter distribution, and the
uncertainty in the N2O simulations. The model’s root mean square error (RMSE) was also abated by 73%
across the field sites compared to the prior parameterization. The Bayesian calibration was subsequently
applied simultaneously to all data sets, to obtain better global estimates for the parameters initially
deemed universal. This made it possible to reduce the RMSE by 33% on average, compared to the
uncalibrated model. These global parameter values may be used to obtain more realistic estimates of
N2O emissions from arable soils at regional or continental scales
A rare early-type star revealed in the Wing of the Small Magellanic Cloud
Sk 183 is the visually-brightest star in the N90 nebula, a young star-forming
region in the Wing of the Small Magellanic Cloud (SMC). We present new optical
spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of
the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the
basis of its nitrogen spectrum, the star also displays broadened He I
absorption which suggests a later type. We propose that Sk 183 has a composite
spectrum and that it is similar to another star in the SMC, MPG 324. This
brings the number of rare O2- and O3-type stars known in the whole of the SMC
to a mere four. We estimate physical parameters for Sk 183 from analysis of its
spectrum. For a single-star model, we estimate an effective temperature of
46+/-2 kK, a low mass-loss rate of ~10^-7 Msun yr^-1, and a spectroscopic mass
of 46^+9_-8 Msun (for an adopted distance modulus of 18.7 mag to the young
population in the SMC Wing). An illustrative binary model requires a slightly
hotter temperature (~47.5 kK) for the primary component. In either scenario, Sk
183 is the earliest-type star known in N90 and will therefore be the dominant
source of hydrogen-ionising photons. This suggests Sk 183 is the primary
influence on the star formation along the inner edge of the nebula.Comment: Accepted by ApJ, 10 pages, 7 figures, v2 after proof
The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136
Detailed studies of resolved young massive star clusters are necessary to
determine their dynamical state and evaluate the importance of gas expulsion
and early cluster evolution. In an effort to gain insight into the dynamical
state of the young massive cluster R136 and obtain the first measurement of its
velocity dispersion, we analyse multi-epoch spectroscopic data of the inner
regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of
the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the
variability, we use the radial velocities of non-variable sources to place an
upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within
a projected distance of 5 pc from the centre of the cluster. After accounting
for the contributions of undetected binaries and measurement errors through
Monte Carlo simulations, we conclude that the true velocity dispersion is
likely between 4 and 5 km/s given a range of standard assumptions about the
binary distribution. This result is consistent with what is expected if the
cluster is in virial equilibrium, suggesting that gas expulsion has not altered
its dynamics. We find that the velocity dispersion would be ~25 km/s if
binaries were not identified and rejected, confirming the importance of the
multi-epoch strategy and the risk of interpreting velocity dispersion
measurements of unresolved extragalactic young massive clusters.Comment: 18 pages, 7 figures, accepted by A&
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
Studying the kinematics of the giant star-forming region 30 Doradus. I. The data
We present high-quality VLT-FLAMES optical spectroscopy of the nebular gas in
the giant star-forming region 30 Doradus. In this paper, the first of a series,
we introduce our observations and discuss the main kinematic features of 30
Dor, as revealed by the spectroscopy of the ionized gas in the region. The
primary data set consists of regular grid of nebular observations, which we
used to produce a spectroscopic datacube of 30 Dor, centered on the massive
star cluster R136 and covering a field-of-view of 10'x10'. The main emission
lines present in the datacube are from Halpha and [NII]6548,6584. The Halpha
emission-line profile varies across the region from simple single-peaked
emission to complex, multiple-component profiles, suggesting that different
physical mechanisms are acting on the excited gas. To analyse the gas
kinematics we fit Gaussian profiles to the observed Halpha features.
Unexpectedly, the narrowest Halpha profile in our sample lies close to the
supernova remnant 30 Dor B. We present maps of the velocity field and velocity
dispersion across 30 Dor, finding five previously unclassified expanding
structures. These maps highlight the kinematic richness of 30 Dor (e.g.
supersonic motions), which will be analysed in future papers.Comment: 24 pages, 12 figures. Accepted for publication in Astronomy and
Astrophysic
The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence
The evolution and fate of very massive stars (VMS) is tightly connected to
their mass-loss properties. Their initial and final masses differ significantly
as a result of mass loss. VMS have strong stellar winds and extremely high
ionising fluxes, which are thought to be critical sources of both mechanical
and radiative feedback in giant Hii regions. However, how VMS mass-loss
properties change during stellar evolution is poorly understood. In the
framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss
transition region from optically thin O to denser WNh star winds, thereby
testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and
WNh stars, an unprecedented sample of stars with the highest masses and
luminosities known. We perform a spectral analysis of optical VFTS as well as
near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN
to obtain stellar and wind parameters. For the first time, we observationally
resolve the transition between optically thin O and optically thick WNh star
winds. Our results suggest the existence of a kink between both mass-loss
regimes, in agreement with recent MC simulations. For the optically thick
regime, we confirm the steep dependence on the Eddington factor from previous
theoretical and observational studies. The transition occurs on the MS near a
luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find
that - even when accounting for moderate wind clumping (with f = 0.1) - wind
mass-loss rates are enhanced with respect to standard prescriptions currently
adopted in stellar evolution calculations. We also show that this results in
substantial helium surface enrichment. Based on our spectroscopic analyses, we
are able to provide the most accurate ionising fluxes for VMS known to date,
confirming the pivotal role of VMS in ionising and shaping their environments.Comment: Accepted for publication in A&A, 19 pages, 14 figures, 6 tables, (74
pages appendix, 68 figures, 4 tables
A closer look at the binary content of NGC 1850
Studies of young clusters have shown that a large fraction of O-/early B-type stars are in binary systems, where the binary fraction increases with mass. These massive stars are present in clusters of a few Myr, but gradually disappear for older clusters. The lack of detailed studies of intermediate-age clusters has meant that almost no information is available on the multiplicity properties of stars with M < 4 M☉. In this study we present the first characterization of the binary content of NGC 1850, a 100 Myr-old massive star cluster in the Large Magellanic Cloud, relying on a VLT/MUSE multi-epoch spectroscopic campaign. By sampling stars down to M = 2.5 M☉, we derive a close binary fraction of 24 ± 5 per cent in NGC 1850, in good agreement with the multiplicity frequency predicted for stars of this mass range. We also find a trend with stellar mass (magnitude), with higher mass (brighter) stars having higher binary fractions. We modelled the radial velocity curves of individual binaries using THE JOKER and constrained the orbital properties of 27 systems, ∼17 per cent of all binaries with reliable radial velocities in NGC 1850. This study has brought to light a number of interesting objects, such as four binaries showing mass functions f(M) > 1.25 M☉. One of these, star #47, has a peculiar spectrum, explainable with the presence of two discs in the system, around the visible star and the dark companion, which is a black hole candidate. These results confirm the importance and urgency of studying the binary content of clusters of any age.</p
Ion irradiation triggers the formation of the precursors of complex organics in space - The case of formaldehyde and acetaldehyde
Context. Cosmic rays and solar energetic particles induce changes in the composition of compounds frozen onto dust grains in the interstellar medium (ISM), in comets, and on the surfaces of atmosphere-less small bodies in the outer Solar System. This induces the destruction of pristine compounds and triggers the formation of various species, including the precursors of complex organics.
Aims. We investigate the role of energetic ions in the formation of formaldehyde (H2CO) and acetaldehyde (CH3CHO), which are observed in the ISM and in comets, and which are thought to be the precursors of more complex compounds such as hexamethylenete-tramine (HMT), which is found in carbonaceous chondrites and in laboratory samples produced after the irradiation and warm-up of astrophysical ices.
Methods. We performed ion irradiation of water, methanol, and ammonia mixtures at 14–18 K. We bombarded frozen films with 40–200 keV H+ that simulate solar energetic particles and low-energy cosmic rays. Samples were analysed by infrared transmission spectroscopy.
Results. Among other molecules, we observe the formation of H2CO and CH3CHO, and we find that their abundance depends on the dose and on the stoichiometry of the mixtures. We find that the H2CO abundance reaches the highest value after a dose of 10 eV/16u and then it decreases as the dose increases.
Conclusions. The data suggest that surfaces exposed to high doses are depleted in H2CO. This explains why the amount of HMT in organic residues and that formed after irradiation of ices depends on the dose deposited in the ice. Because the H2CO abundance decreases at doses higher than 10 eV/16u, a lower quantity of H2CO is available to form HMT during the subsequent warm-up. The H2CO abundances caused by ion bombardment are insufficient to explain the ISM abundances, but ion bombardment can account for the abundance of CH3CHO towards the ISM and comets
The VLT-FLAMES Tarantula Survey XIX. B-type Supergiants - Atmospheric parameters and nitrogen abundances to investigate the role of binarity and the width of the main sequence
TLUSTY non-LTE model atmosphere calculations have been used to determine
atmospheric parameters and nitrogen (N) abundances for 34 single and 18 binary
B-type supergiants (BSGs). The effects of flux contribution from an unseen
secondary were considered for the binary sample. We present the first
systematic study of the incidence of binarity for a sample of BSGs across the
theoretical terminal age main sequence (TAMS). To account for the distribution
of effective temperatures of the BSGs it may be necessary to extend the TAMS to
lower temperatures. This is consistent with the derived distribution of mass
discrepancies, projected rotational velocities (vsini) and N abundances,
provided that stars cooler than this temperature are post RSG objects. For the
BSGs in the Tarantula and previous FLAMES surveys, most have small vsini. About
10% have larger vsini (>100 km/s) but surprisingly these show little or no N
enhancement. All the cooler BSGs have low vsini of <70km/s and high N abundance
estimates, implying that either bi-stability braking or evolution on a blue
loop may be important. A lack of cool binaries, possibly reflects the small
sample size. Single star evolutionary models, which include rotation, can
account for the N enhancement in both the single and binary samples. The
detailed distribution of N abundances in the single and binary samples may be
different, possibly reflecting differences in their evolutionary history. The
first comparative study of single and binary BSGs has revealed that the main
sequence may be significantly wider than previously assumed, extending to
Teff=20000K. Some marginal differences in single and binary atmospheric
parameters and abundances have been identified, possibly implying non-standard
evolution for some of the sample. This sample as a whole has implications for
several aspects of our understanding of the evolution of BSGs. Full abstract in
paperComment: 21 pages, 15 figures, 11 table
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