1,479 research outputs found
The chemical composition of CO-rich comet C/2009 P1 (Garradd) at Rh = 2.4 and 2.0 AU before perihelion
We quantified ten parent volatiles in comet C/2009 P1 (Garradd) before
perihelion, through high-dispersion infrared spectra acquired with CRIRES at
ESO's VLT on UT 2011 August 07 (Rh = 2.4 AU) and September 17-21 (Rh = 2.0 AU).
On August 07, water was searched but not detected at an upper limit (3{\sigma})
of 2.1 \times 10^28 s-1, while ethane was detected with a production rate of
6.1 \times 10^26 s-1 (apparent mixing ratio > 2.90%). On September 17-21, the
mean production rate for water was 8.4 \times 10^28 s-1, and abundance ratios
(relative to water) of detected trace species were: CO (12.51%), CH3OH (3.90%),
CH4 (1.24%), C2H6 (1.01%) and HCN (0.36%). Upper limits (3{\sigma}) to
abundances for four minor species were: NH3 (1.55%), C2H2 (0.13%), HDO (0.89%)
and OCS (0.20%). Given the relatively large heliocentric distance, we explored
the effect of water not being fully sublimated within our FOV and identified
the 'missing' water fraction needed to reconcile the retrieved abundance ratios
with the mean values found for "organics-normal". The individual spatial
profiles of parent volatiles and the continuum displayed rather asymmetric
outgassing. Indications of H2O and CO gas being released in different
directions suggest different active vents and/or the possible existence of
polar and apolar ice aggregates in the nucleus. The high fractional abundance
of CO identifies comet C/2009 P1 as a CO-rich comet.Comment: To appear in: The Astrophysical Journal Letters (LET27048R
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
Detection of parent H2O and CO2 molecules in the 2.5--5 micron spectrum of comet C/2007 N3 (Lulin) observed with AKARI
Comet C/2007 N3 (Lulin) was observed with the Japanese infrared satellite
AKARI in the near-infrared at a post-perihelion heliocentric distance of 1.7
AU. Observations were performed with the spectroscopic (2.5--5.0 micron) and
imaging (2.4, 3.2, and 4.1 micron) modes on 2009 March 30 and 31 UT,
respectively. AKARI images of the comet exhibit a sunward crescent-like shape
coma and a dust tail extended toward the anti-solar direction. The 4.1 micron
image (CO/CO2 and dust grains) shows a distribution different from the 2.4 and
3.2 micron images (H2O and dust grains). The observed spectrum shows distinct
bands at 2.66 and 4.26 micron, attributed to H2O and CO2, respectively. This is
the fifth comet in which CO2 has been directly detected in the near-infrared
spectrum. In addition, CO at 4.67 micron and a broad 3.2--3.6 micron emission
band from C-H bearing molecules were detected in the AKARI spectrum. The
relative abundance ratios CO2/H2O and CO/H2O derived from the molecular
production rates are \sim 4%--5% and < 2%, respectively. Comet Lulin belongs to
the group that has relatively low abundances of CO and CO2 among the comets
observed ever.Comment: 14 pages, 2 tables, 2 figures, accepted for publication in The
Astrophysical Journal Letter
Comets at radio wavelengths
Comets are considered as the most primitive objects in the Solar System.
Their composition provides information on the composition of the primitive
solar nebula, 4.6 Gyr ago. The radio domain is a privileged tool to study the
composition of cometary ices. Observations of the OH radical at 18 cm
wavelength allow us to measure the water production rate. A wealth of molecules
(and some of their isotopologues) coming from the sublimation of ices in the
nucleus have been identified by observations in the millimetre and
submillimetre domains. We present an historical review on radio observations of
comets, focusing on the results from our group, and including recent
observations with the Nan\c{c}ay radio telescope, the IRAM antennas, the Odin
satellite, the Herschel space observatory, ALMA, and the MIRO instrument aboard
the Rosetta space probe.Comment: Proceedings of URSI France scientific days, "Probing Matter with
Electromagnetic Waves", 24-25 March 2015, Paris. To be published in C. R.
Physiqu
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
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