1,583 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
The chemical diversity of comets
A fundamental question in cometary science is whether the different dynamical
classes of comets have different chemical compositions, which would reflect
different initial conditions. From the ground or Earth orbit, radio and
infrared spectroscopic observations of a now significant sample of comets
indeed reveal deep differences in the relative abundances of cometary ices.
However, no obvious correlation with dynamical classes is found. Further
results come, or are expected, from space exploration. Such investigations, by
nature limited to a small number of objects, are unfortunately focussed on
short-period comets (mainly Jupiter-family). But these in situ studies provide
"ground truth" for remote sensing. We discuss the chemical differences in
comets from our database of spectroscopic radio observations, which has been
recently enriched by several Jupiter-family and Halley-type comets.Comment: In press in Earth, Moon and Planets (proceedings of the workshop
"Future Ground-based Solar System Research: Synergies with Space Probes and
Space Telescopes", Portoferraio, Isola d'Elba, Livorno (Italy), 8-12
September 2008). 6 pages with 2 figure
No compelling evidence of distributed production of CO in comet C/1995 O1 (Hale-Bopp) from millimeter interferometric data and a reanalysis of near-IR lines
Based on long-slit infrared spectroscopic observations, it has been suggested
that half of the carbon monoxide present in the atmosphere of comet C/1995 O1
(Hale-Bopp) close to perihelion was released by a distributed source in the
coma, whose nature (dust or gas) remains unidentified. We re-assess the origin
of CO in Hale-Bopp's coma from millimeter interferometric data and a
re-analysis of the IR lines.
Simultaneous observations of the CO J(1-0) (115 GHz) and J(2-1) (230 GHz)
lines were undertaken with the IRAM interferometer in single-dish and
interferometric modes. The diversity of angular resolutions (from 1700 to 42000
km diameter at the comet) is suitable to study the radial distribution of CO
and detect the extended source observed in the infrared. We used excitation and
radiative transfer models to simulate the observations. Various CO density
distributions were considered, including 3D time-dependent hydrodynamical
simulations which reproduce a CO rotating jet. The CO J(1-0) and J(2-1)
observations can be consistently explained by a nuclear production of CO.
Composite 50:50 nuclear/extended productions with characteristic scale lengths
of CO parent L_p > 1500 km are rejected.
Based on similar radiation transfer calculations, we show that the CO v = 1-0
ro-vibrational lines observed in comet Hale-Bopp at heliocentric distances less
than 1.5 AU are severely optically thick. The broad extent of the CO brightness
distribution in the infrared is mainly due to optical depth effects. Additional
factors can be found in the complex structure of the CO coma, and non-ideal
slit positioning caused by the anisotropy of dust IR emission.
We conclude that both CO millimeter and infrared lines do not provide
compelling evidence for a distributed source of CO in Hale-Bopp's atmosphere.Comment: Accepted for publication in Icarus (55 pages, 13 figures
Observations of cometary parent molecules with the IRAM radio telescope
Several rotational transitions of HCN, H2S, H2CO, and CH3OH were detected in comets P/Brorsen-Metcalf 1989 X, Austin (1989c1) and Levy (1990c) with the Institute for Millimeter Radioastronomy (IRAM) 30-m radio telescope. This allows us to determine the production rates of these molecules and to probe the physical conditions of the coma
On the magnitude of spheres, surfaces and other homogeneous spaces
In this paper we define the magnitude of metric spaces using measures rather
than finite subsets as had been done previously and show that this agrees with
earlier work with Leinster in arXiv:0908.1582. An explicit formula for the
magnitude of an n-sphere with its intrinsic metric is given. For an arbitrary
homogeneous Riemannian manifold the leading terms of the asymptotic expansion
of the magnitude are calculated and expressed in terms of the volume and total
scalar curvature of the manifold. In the particular case of a homogeneous
surface the form of the asymptotics can be given exactly up to vanishing terms
and this involves just the area and Euler characteristic in the way conjectured
for subsets of Euclidean space in previous work.Comment: 21 pages. Main change from v1: details added to proof of Theorem
Interferometric mapping of the 3.3-mm continuum emission of comet 17P/Holmes after its 2007 outburst
Comet 17P/Holmes underwent a dramatic outburst in October 2007, caused by the
sudden fragmentation of its nucleus and the production of a large quantity of
grains scattering sunlight. We report on 90 GHz continuum observations carried
out with the IRAM Plateau de Bure interferometer on 27.1 and 28.2 October 2007
UT, i.e., 4-5 days after the outburst. These observations probed the thermal
radiation of large dust particles, and therefore provide the best constraints
on the mass in the ejecta debris. The thermal emission of the debris was
modelled and coupled to a time-dependent description of their expansion after
the outburst. The analysis was performed in the Fourier plane. Visibilities
were computed for the two observing dates and compared to the data to measure
their velocity and mass. Optical data and 250-GHz continuum measurements
published in the literature were used to further constrain the dust kinematics
and size distribution. Two distinct dust components in terms of kinematic
properties are identified in the data. The large-velocity component, with
typical velocities V0 of 50-100 m/s for 1 mm particles, displays a steep size
distribution with a size index estimated to q = -3.7 (\pm0.1), assuming a
minimum grain size of 0.1 \mum. It corresponds to the fast expanding shell
observed in optical images. The slowly-moving "core" component (V0 = 7-9 m/s)
detected near the nucleus has a size index |q| < 3.4 and contains a higher
proportion of large particles than the shell. The dust mass in the core is in
the range 0.1-1 that of the shell. Using optical constants pertaining to porous
grains (50% porosity) made of astronomical silicates mixed with water ice (48%
in mass), the total dust mass Mdust injected by the outburst is estimated to
4-14 x 10**11 kg, corresponding to 3-9% the nucleus mass.Comment: 15 pages with 11 figures and 7 tables. Accepted for publication in
Astronomy & Astrophysic
Antifreeze in the hot core of Orion - First detection of ethylene glycol in Orion-KL
Comparison of their chemical compositions shows, to first order, a good
agreement between the cometary and interstellar abundances. However, a complex
O-bearing organic molecule, ethylene glycol (CHOH), seems to depart
from this correlation because it was not easily detected in the interstellar
medium although it proved to be rather abundant with respect to other O-bearing
species in comet Hale-Bopp. Ethylene glycol thus appears, together with the
related molecules glycolaldehyde CHOHCHO and ethanol CHCHOH,
as a key species in the comparison of interstellar and cometary ices as well as
in any discussion on the formation of cometary matter. We focus here on the
analysis of ethylene glycol in the nearest and best studied hot core-like
region, Orion-KL. We use ALMA interferometric data because high spatial
resolution observations allow us to reduce the line confusion problem with
respect to single-dish observations since different molecules are expected to
exhibit different spatial distributions. Furthermore, a large spectral
bandwidth is needed because many individual transitions are required to
securely detect large organic molecules. Confusion and continuum subtraction
are major issues and have been handled with care. We have detected the aGg'
conformer of ethylene glycol in Orion-KL. The emission is compact and peaks
towards the Hot Core close to the main continuum peak, about 2" to the
south-west; this distribution is notably different from other O-bearing
species. Assuming optically thin lines and local thermodynamic equilibrium, we
derive a rotational temperature of 145 K and a column density of 4.6 10
cm. The limit on the column density of the gGg' conformer is five times
lower.Comment: 19 pages, 10 figures, A&A 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 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
Nitrogen isotopic ratios in Barnard 1: a consistent study of the N2H+, NH3, CN, HCN and HNC isotopologues
The 15N isotopologue abundance ratio measured today in different bodies of
the solar system is thought to be connected to 15N-fractionation effects that
would have occured in the protosolar nebula. The present study aims at putting
constraints on the degree of 15N-fractionation that occurs during the
prestellar phase, through observations of D, 13C and 15N-substituted
isotopologues towards B1b. Both molecules from the nitrogen hydride family,
i.e. N2H+ and NH3, and from the nitrile family, i.e. HCN, HNC and CN, are
considered in the analysis. As a first step, we model the continuum emission in
order to derive the physical structure of the cloud, i.e. gas temperature and
H2 density. These parameters are subsequently used as an input in a non-local
radiative transfer model to infer the radial abundances profiles of the various
molecules. Our modeling shows that all the molecules are affected by depletion
onto dust grains, in the region that encompasses the B1-bS and B1-bN cores.
While high levels of deuterium fractionation are derived, we conclude that no
fractionation occurs in the case of the nitrogen chemistry. Independently of
the chemical family, the molecular abundances are consistent with 14N/15N~300,
a value representative of the elemental atomic abundances of the parental gas.
The inefficiency of the 15N-fractionation effects in the B1b region can be
linked to the relatively high gas temperature ~17K which is representative of
the innermost part of the cloud. Since this region shows signs of depletion
onto dust grains, we can not exclude the possibility that the molecules were
previously enriched in 15N, earlier in the B1b history, and that such an
enrichment could have been incorporated into the ice mantles. It is thus
necessary to repeat this kind of study in colder sources to test such a
possibility.Comment: accepted in A&
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