1,042 research outputs found
Water in Comet 2/2003 K4 (LINEAR) with Spitzer
We present sensitive 5.5 to 7.6 micron spectra of comet C/2003 K4 (LINEAR)
obtained on 16 July 2004 (r_{h} = 1.760 AU, Delta_{Spitzer} = 1.409 AU, phase
angle 35.4 degrees) with the Spitzer Space Telescope. The nu_{2} vibrational
band of water is detected with a high signal-to-noise ratio (> 50). Model
fitting to the best spectrum yields a water ortho-to-para ratio of 2.47 +/-
0.27, which corresponds to a spin temperature of 28.5^{+6.5}_{-3.5} K. Spectra
acquired at different offset positions show that the rotational temperature
decreases with increasing distance from the nucleus, which is consistent with
evolution from thermal to fluorescence equilibrium. The inferred water
production rate is (2.43 +/- 0.25) \times 10^{29} molec. s^{-1}. The spectra do
not show any evidence for emission from PAHs and carbonate minerals, in
contrast to results reported for comets 9P/Tempel 1 and C/1995 O1 (Hale-Bopp).
However, residual emission is observed near 7.3 micron the origin of which
remains unidentified.Comment: 33 pages, including 11 figures, 2 tables, ApJ 2007 accepte
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 MetropolisâHastings 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 modelâs 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 Methodology to Engineer and Validate Dynamic Multi-level Multi-agent Based Simulations
This article proposes a methodology to model and simulate complex systems,
based on IRM4MLS, a generic agent-based meta-model able to deal with
multi-level systems. This methodology permits the engineering of dynamic
multi-level agent-based models, to represent complex systems over several
scales and domains of interest. Its goal is to simulate a phenomenon using
dynamically the lightest representation to save computer resources without loss
of information. This methodology is based on two mechanisms: (1) the activation
or deactivation of agents representing different domain parts of the same
phenomenon and (2) the aggregation or disaggregation of agents representing the
same phenomenon at different scales.Comment: Presented at 3th International Workshop on Multi-Agent Based
Simulation, Valencia, Spain, 5th June 201
Recommended from our members
Very Slow Search and Reach: Failure to Maximize Expected Gain in an Eye-Hand Coordination Task
We examined an eye-hand coordination task where optimal visual search and hand movement strategies were inter-related. Observers were asked to find and touch a target among five distractors on a touch screen. Their reward for touching the target was reduced by an amount proportional to how long they took to locate and reach to it. Coordinating the eye and the hand appropriately would markedly reduce the search-reach time. Using statistical decision theory we derived the sequence of interrelated eye and hand movements that would maximize expected gain and we predicted how hand movements should change as the eye gathered further information about target location. We recorded human observers' eye movements and hand movements and compared them with the optimal strategy that would have maximized expected gain. We found that most observers failed to adopt the optimal search-reach strategy. We analyze and describe the strategies they did adopt.Psycholog
Rotation-stimulated structures in the CN and C3 comae of comet 103P/Hartley 2 around the EPOXI encounter
In late 2010 a Jupiter Family comet 103P/Hartley 2 was a subject of an
intensive world-wide investigation. On UT October 20.7 the comet approached the
Earth within only 0.12 AU, and on UT November 4.6 it was visited by NASA's
EPOXI spacecraft. We joined this international effort and organized an
observing campaign. The images of the comet were obtained through narrowband
filters using the 2-m telescope of the Rozhen National Astronomical
Observatory. They were taken during 4 nights around the moment of the EPOXI
encounter. Image processing methods and periodicity analysis techniques were
used to reveal transient coma structures and investigate their repeatability
and kinematics. We observe shells, arc-, jet- and spiral-like patterns, very
similar for the CN and C3 comae. The CN features expanded outwards with the
sky-plane projected velocities between 0.1 to 0.3 km/s. A corkscrew structure,
observed on November 6, evolved with a much higher velocity of 0.66 km/s.
Photometry of the inner coma of CN shows variability with a period of
18.32+/-0.30 h (valid for the middle moment of our run, UT 2010 Nov. 5.0835),
which we attribute to the nucleus rotation. This result is fully consistent
with independent determinations around the same time by other teams. The
pattern of repeatability is, however, not perfect, which is understendable
given the suggested excitation of the rotation state, and the variability
detected in CN correlates well with the cyclic changes in HCN, but only in the
active phases. The revealed coma structures, along with the snapshot of the
nucleus orientation obtained by EPOXI, let us estimate the spin axis
orientation. We obtained RA=122 deg, Dec=+16 deg (epoch J2000.0), neglecting at
this point the rotational excitation.Comment: 9 pages, 10 figures, submitted to Astron. Astrophy
Wide bandwidth instantaneous radio frequency spectrum analyzer based on nitrogen vacancy centers in diamond
We propose an original analog method to perform instantaneous and quantitative spectral analysis
of microwave signals. An ensemble of nitrogen-vacancy (NV) centers held in a diamond plate is
pumped by a 532 nm laser. Its photoluminescence is imaged through an optical microscope and
monitored by a digital camera. An incoming microwave signal is converted into a microwave field
in the area of the NV centers by a loop shaped antenna. The resonances induced by the magnetic
component of that field are detected through a decrease of the NV centers photoluminescence. A
magnetic field gradient induces a Zeeman shift of the resonances and transforms the frequency information
into spatial information, which allows for the simultaneous analysis of the microwave
signal in the entire frequency bandwidth of the device. The time dependent spectral analysis of an
amplitude modulated microwave signal is demonstrated over a bandwidth of 600 MHz, associated
to a frequency resolution of 7MHz , and a refresh rate of 4 ms. With such integration time, a field
of a few hundreds of lW can be detected. Since the optical properties of NV centers can be
maintained even in high magnetic field, we estimate that an optimized device could allow
frequency analysis in a range of 30 GHz, only limited by the amplitude of the magnetic field
gradient. In addition, an increase of the NV centers quantity could lead both to an increase of the
microwave sensitivity and to a decrease of the minimum refresh rate down to a few ls
Detection of CO and HCN in Pluto's atmosphere with ALMA
Observations of the Pluto-Charon system, acquired with the ALMA
interferometer on June 12-13, 2015, have yielded a detection of the CO(3-2) and
HCN(4-3) rotational transitions from Pluto, providing a strong confirmation of
the presence of CO, and the first observation of HCN, in Pluto's atmosphere.
The CO and HCN lines probe Pluto's atmosphere up to ~450 km and ~900 km
altitude, respectively. The CO detection yields (i) a much improved
determination of the CO mole fraction, as 515+/-40 ppm for a 12 ubar surface
pressure (ii) clear evidence for a well-marked temperature decrease (i.e.,
mesosphere) above the 30-50 km stratopause and a best-determined temperature of
70+/-2 K at 300 km, in agreement with recent inferences from New Horizons /
Alice solar occultation data. The HCN line shape implies a high abundance of
this species in the upper atmosphere, with a mole fraction >1.5x10-5 above 450
km and a value of 4x10-5 near 800 km. The large HCN abundance and the cold
upper atmosphere imply supersaturation of HCN to a degree (7-8 orders of
magnitude) hitherto unseen in planetary atmospheres, probably due to the slow
kinetics of condensation at the low pressure and temperature conditions of
Pluto's upper atmosphere. HCN is also present in the bottom ~100 km of the
atmosphere, with a 10-8 - 10-7 mole fraction; this implies either HCN
saturation or undersaturation there, depending on the precise stratopause
temperature. The HCN column is (1.6+/-0.4)x10^14 cm-2, suggesting a
surface-referred net production rate of ~2x10^7 cm-2s-1. Although HCN
rotational line cooling affects Pluto's atmosphere heat budget, the amounts
determined in this study are insufficient to explain the well-marked mesosphere
and upper atmosphere's ~70 K temperature. We finally report an upper limit on
the HC3N column density (< 2x10^13 cm-2) and on the HC15N / HC14N ratio (<
1/125).Comment: Revised version. Icarus, in press, Oct. 11, 2016. 57 pages, including
13 figures and 4 table
Isotopic ratios of H, C, N, O, and S in comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy)
The apparition of bright comets C/2012 F6 (Lemmon) and C/2014 Q2 (Lovejoy) in
March-April 2013 and January 2015, combined with the improved observational
capabilities of submillimeter facilities, offered an opportunity to carry out
sensitive compositional and isotopic studies of the volatiles in their coma. We
observed comet Lovejoy with the IRAM 30m telescope between 13 and 26 January
2015, and with the Odin submillimeter space observatory on 29 January - 3
February 2015. We detected 22 molecules and several isotopologues. The
HO and HO production rates measured with Odin follow a
periodic pattern with a period of 0.94 days and an amplitude of ~25%. The
inferred isotope ratios in comet Lovejoy are O/O = 499 24
and D/H = 1.4 0.4 in water, S/S = 24.7
3.5 in CS, all compatible with terrestrial values. The ratio
C/C = 109 14 in HCN is marginally higher than terrestrial
and N/N = 145 12 in HCN is half the Earth ratio. Several
upper limits for D/H or 12C/13C in other molecules are reported. From our
observation of HDO in comet C/2014 Q2 (Lovejoy), we report the first D/H ratio
in an Oort Cloud comet that is not larger than the terrestrial value. On the
other hand, the observation of the same HDO line in the other Oort-cloud comet,
C/2012 F6 (Lemmon), suggests a D/H value four times higher. Given the previous
measurements of D/H in cometary water, this illustrates that a diversity in the
D/H ratio and in the chemical composition, is present even within the same
dynamical group of comets, suggesting that current dynamical groups contain
comets formed at very different places or times in the early solar system.Comment: Accepted for publication in Astronomy and Astrophysic
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