470 research outputs found
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
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
Radio observations of Jupiter-family comets
Radio observations from decimetric to submillimetric wavelengths are now a
basic tool for the investigation of comets. Spectroscopic observations allow us
i) to monitor the gas production rate of the comets, by directly observing the
water molecule, or by observing secondary products (e.g., the OH radical) or
minor species (e.g., HCN); ii) to investigate the chemical composition of
comets; iii) to probe the physical conditions of cometary atmospheres: kinetic
temperature and expansion velocity. Continuum observations probe large-size
dust particles and (for the largest objects) cometary nuclei. Comets are
classified from their orbital characteristics into two separate classes: i)
nearly-isotropic, mainly long-period comets and ii) ecliptic, short-period
comets, the so-called Jupiter-family comets. These two classes apparently come
from two different reservoirs, respectively the Oort cloud and the
trans-Neptunian scattered disc. Due to their different history and - possibly -
their different origin, they may have different chemical and physical
properties that are worth being investigated. The present article reviews the
contribution of radio observations to our knowledge of the Jupiter-family
comets (JFCs). The difficulty of such a study is the commonly low gas and dust
productions of these comets. Long-period, nearly-isotropic comets from the Oort
cloud are better known from Earth-based observations. On the other hand,
Jupiter-family comets are more easily accessed by space missions. However,
unique opportunities to observe Jupiter-family comets are offered when these
objects come by chance close to the Earth. About a dozen JFCs were successfully
observed by radio techniques up to now. No obvious evidence for different
properties between JFCs and other families of comets is found.Comment: Accepted for publication in Planetary and Space Science (special
issue on Jupiter-family comets). 18 pages including 12 figures and 7 table
Cometary water expansion velocity from OH line shapes
We retrieve the H_2O expansion velocity in a number of comets, using the
18-cm line shapes of the OH radical observed with the Nan\c{c}ay radio
telescope. The H_2O velocity is derived from the large base of a trapezium
fitted to the observed spectra. This method, which was previously applied to 9
comets, is now extended to 30 further comets. This allows us to study the
evolution of their water molecule outflow velocity over a large range of
heliocentric distances and gas production rates. Our analysis confirms and
extends previous analyses. The retrieved expansion velocities increases with
increasing gas production rates and decreasing heliocentric distances.
Heuristic laws are proposed, which could be used for the interpretation of
observations of cometary molecules and as a touchstone for hydrodynamical
models. The expansion velocities retrieved from 18 cm line shapes are larger
than those obtained from millimetric observations of parent molecules with
smaller fields of view, which demonstrates the acceleration of the gas with
cometocentric distance. Our results are in reasonable quantitative agreement
with current hydrodynamical models of cometary atmospheres.Comment: Accepted for publication in Astronomy & Astrophysic
Interferometric imaging of carbon monoxide in comet C/1995 O1 (Hale-Bopp): evidence for a strong rotating jet
Observations of the CO J(1-0) 115 GHz and J(2-1) 230 GHz lines in comet
C/1995 O1 (Hale-Bopp) were performed with the IRAM Plateau de Bure
interferometer on 11 March, 1997. The observations were conducted in both
single-dish (ON-OFF) and interferometric modes with 0.13 km s-1 spectral
resolution. Images of CO emission with 1.7 to 3" angular resolution were
obtained. The ON-OFF and interferometric spectra show a velocity shift with
sinusoidal time variations related to the Hale-Bopp nucleus rotation of 11.35
h. The peak position of the CO images moves perpendicularly to the spin axis
direction in the plane of the sky. This suggests the presence of a CO jet,
which is active night and day at about the same extent, and is spiralling with
nucleus rotation. The high quality of the data allows us to constrain the
characteristics of this CO jet. We have developed a 3-D model to interpret the
temporal evolution of CO spectra and maps. The CO coma is represented as the
combination of an isotropic distribution and a spiralling gas jet, both of
nucleus origin. Spectra and visibilities (the direct output of interferometric
data) analysis shows that the CO jet comprises ~40% the total CO production and
is located at a latitude ~20 degrees North on the nucleus surface. Our
inability to reproduce all observational characteristics shows that the real
structure of the CO coma is more complex than assumed, especially in the first
thousand kilometres from the nucleus. The presence of another moving CO
structure, faint but compact and possibly created by an outburst, is
identified.Comment: 20 pages, 26 figures. 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
Ethylene glycol in comet C/1995 O1 (Hale-Bopp)
We report the detection of ethylene glycol (HOCH_2CH_2OH) in comet C/1995 O1 (Hale-Bopp) from the analysis of archival radio spectra. Its production rate is â0.25% that of water, making it one of the most abundant organic molecules in cometary ices. This detection strengthens the similarity between interstellar and cometary material
The composition of ices in comet C/1995 O1 (Hale-Bopp) from radio spectroscopy - Further results and upper limits on undetected species
From radio spectroscopic observations of comets, more than 22 molecules, radicals and ions, plus several isotopologues, were detected, the majority of them being recently revealed in comets C/1996 B2 (Hyakutake) and C/1995 O1 (Hale-Bopp). Among them, 6 molecules were detected for the first time (BockelĂ©e-Morvan et al. [CITE]) in the course of a spectral survey conducted at radio wavelengths in comet Hale-Bopp with the CSO, the IRAM 30-m telescope and Plateau de Bure interferometer. In addition, many species were searched for unsuccessfully, some of them with stringent upper limits. We present here a review of these observations and further analysis of their results. This include: (i) confirmed detection of acetaldehyde (CH_3CHO); (ii) limits on small molecules such as ketene (H_2CCO) or methanimine (CH_2NH); (iii) limits on the abundance ratios in homologous series such as HC_5N/HC_3N, ethanol/methanol, acetic acid/formic acid; (iv) searches for precursors of key cometary species such as atomic Na and HNC; (v) constraints on more exotic species ranging from water dimer (H_2O)_2 to glycine; (vi) detection of the H_2^(34)S isotopic species and independent observations of HDO and DCN; (vii) limits on several other deuterated species; (viii) limits on several radicals and ions and a tentative detection of the C_2H radical; (ix) the presence of unidentified lines. Typical abundance upper limits of 2â5 x 10^(-4) relative to water are achieved for many species. Better upper limits are obtained for some linear molecules with high dipole moments. But more complex molecules such as dimethyl ether or glycine are poorly constrained. These results should give important clues to the chemical composition of cometary ices, to the formation mechanisms of cometary material, and to the chemical processes which occur in the inner coma
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