806 research outputs found
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
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
Observations of the 18-cm OH lines of comet 103P/Hartley 2 at Nan\c{c}ay in support to the EPOXI and Herschel missions
The 18-cm radio lines of the OH radical were observed in comet 103P/Hartley 2
with the Nan\c{c}ay radio telescope in support to its flyby by the EPOXI
mission and to observations with the Herschel Space Observatory. The OH lines
were detected from 24 September to 15 December 2010. These observations are
used to estimate the gas expansion velocity within the coma to 0.83 \pm 0.08
km/s in October 2010. The water production increased steeply but progressively
before perihelion, and reached 1.9 \pm 0.3 X 10E28 s-1 just before the EPOXI
flyby.Comment: Accepted for publication in Icarus (6 pages, 4 figures
Water in Comets 71P/Clark and C/2004 B1 (LINEAR) with Spitzer
We present 5.5 to 7.6 micron spectra of comets 71P/Clark (2006 May 27.56 UT,
r_h = 1.57 AU pre-perihelion) and C/2004 B1 (LINEAR) (2005 October 15.22 UT,
r_h = 2.21 AU pre-perihelion and 2006 May 16.22 UT, r_h = 2.06 AU
post-perihelion) obtained with the Spitzer Space Telescope. The nu_2
vibrational band of water is detected with a signal-to-noise ratio of 11 to 50.
Fitting the spectra using a fluorescence model of water emission yields a water
rotational temperature of < 18 K for 71P/Clark and approximately less than or
equivalent to 14 +/- 2 K (pre-perihelion) and 23 +/- 4 K (post-perihelion) for
C/2004 B1 (LINEAR). The water ortho-to-para ratio in C/2004 B1 (LINEAR) is
measured to be 2.31 +/- 0.18, which corresponds to a spin temperature of
26^{+3}_{-2} K. Water production rates are derived. The agreement between the
water model and the measurements is good, as previously found for Spitzer
spectra of C/2003 K4 (LINEAR). The Spitzer spectra of these three comets 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).Comment: 16 pages, 1 table, 7 figures, ApJ accepted 200
Gas and dust productions of Comet 103P/Hartley 2 from millimetre observations: Interpreting rotation-induced time variations
Comet 103P/Hartley 2 made a close approach to the Earth in October 2010. It was the target of an extensive observing campaign including ground- and orbit-based observatories and was visited by the Deep Impact spacecraft in the framework of its mission extension EPOXI. We present observations of HCN and CH_3OH emission lines conducted with the IRAM Plateau de Bure interferometer on 22–23, 28 October and 4, 5 November 2010 at 1.1, 1.9 and 3.4 mm wavelengths. The thermal emission from the dust coma and nucleus is detected simultaneously. Interferometric images with unprecedented spatial resolution of ∼100 to ∼500 km are obtained. A sine–wave like variation of the thermal continuum is observed in the 23 October data, that we associate with the nucleus thermal light curve. The nucleus contributes up to 30–55% of the observed continuum emission. The dust thermal emission is used to measure the dust production rate. The inferred large dust-to-gas ratio (in the range 2–6) can be explained by the unusual activity of the comet for its size, which allows decimeter size particles and large boulders to be entrained by the gas due to the small nucleus gravity. The rotational temperature of CH_3OH is measured with beam radii from ∼150 km to ∼1500 km. We attribute the increase from ∼35 K to ∼46 K with increasing beam size to radiative processes. The HCN production rate displays strong rotation-induced temporal variations, varying from ∼0.3 × 10^(25) s^(−1) to ∼2.0 × 10^(25) s^(−1) in the 4–5 November period. The HCN production curve, as well as the CO_2 and H_2O production curves measured by EPOXI, are interpreted with a geometric model which takes into account the complex rotational state of 103P/Hartley 2 and its shape. The HCN and H_2O production curves are in phase, showing that these molecules have common sources. The ∼1.7 h delay, in average, of the HCN and H_2O production curves with respect to the CO_2 production curve suggests that HCN and H_2O are mainly produced by subliming icy grains. The scale length of production of HCN is determined to be on the order of 500–1000 km, implying a mean velocity of 100–200 m s^(−1) for the icy grains producing HCN. From the time evolution of the insolation of the nucleus, we show that the CO_2 production is modulated by the insolation of the small lobe of the nucleus. The three-cycle pattern of the production curves reported earlier is best explained by an overactivity of the small lobe in the longitude range 0–180°. The good correlation between the insolation of the small lobe and CO_2 production is consistent with CO_2 being produced from small depths below the surface. The time evolution of the velocity offset of the HCN lines, as well as the displacement of the HCN photocenter in the interferometric maps, are overall consistent with this interpretation. Other localized sources of gas on the nucleus surface are also suggested
Ethyl alcohol and sugar in comet C/2014 Q2 (Lovejoy)
The presence of numerous complex organic molecules (COMs; defined as those containing six or more atoms) around protostars shows that star formation is accompanied by an increase of molecular complexity. These COMs may be part of the material from which planetesimals and, ultimately, planets formed. Comets represent some of the oldest and most primitive material in the solar system, including ices, and are thus our best window into the volatile composition of the solar protoplanetary disk. Molecules identified to be present in cometary ices include water, simple hydrocarbons, oxygen, sulfur, and nitrogen-bearing species, as well as a few COMs, such as ethylene glycol and glycine. We report the detection of 21 molecules in comet C/2014 Q2 (Lovejoy), including the first identification of ethyl alcohol (ethanol, C_2H_5OH) and the simplest monosaccharide sugar glycolaldehyde (CH_2OHCHO) in a comet. The abundances of ethanol and glycolaldehyde, respectively 5 and 0.8% relative to methanol (0.12 and 0.02% relative to water), are somewhat higher than the values measured in solar-type protostars. Overall, the high abundance of COMs in cometary ices supports the formation through grain-surface reactions in the solar system protoplanetary disk
The water regime of dwarf planet (1) Ceres
The traditional view of minor bodies in the (inner) Solar System is that they are split into icy comets and rocky asteroids. However this has been challenged by recent results, such as the discovery of comets on asteroidal orbits in the outer asteroid belt (between Mars and Jupiter) and the detection of water ice frost on the surface of asteroid (24) Themis. The discovery of water ice on the surface of asteroids has profound implications for how the Solar System formed, and challenges our ideas about the stability of ice in the inner Solar System. The study of volatiles in the asteroid belt places strong constraints on the temperature and composition distribution in the proto-planetary disk,and on possible sources of terrestrial water, and strongly constrains formation models of the early Solar System
Leveraging the ALMA Atacama Compact Array for Cometary Science: An Interferometric Survey of Comet C/2015 ER61 (PanSTARRS) and Evidence for a Distributed Source of Carbon Monosulfide
We report the first survey of molecular emission from cometary volatiles
using standalone Atacama Compact Array (ACA) observations of the Atacama Large
Millimeter/Submillimeter Array (ALMA) toward comet C/2015 ER61 (PanSTARRS)
carried out on UT 2017 April 11 and 15, shortly after its April 4 outburst.
These measurements of HCN, CS, CHOH, HCO, and HNC (along with continuum
emission from dust) probed the inner coma of C/2015 ER61, revealing asymmetric
outgassing and discerning parent from daughter/distributed source species. This
work presents spectrally integrated flux maps, autocorrelation spectra,
production rates, and parent scale lengths for each molecule, and a stringent
upper limit for CO. HCN is consistent with direct nucleus release in C/2015
ER61, whereas CS, HCO, HNC, and potentially CHOH are associated with
distributed sources in the coma. Adopting a Haser model, parent scale lengths
determined for HCO (L 2200 km) and HNC (L 3300 km)
are consistent with previous work in comets, whereas significant extended
source production (L 2000 km) is indicated for CS, suggesting
production from an unknown parent in the coma. The continuum presents a
point-source distribution, with a flux density implying an excessively large
nucleus, inconsistent with other estimates of the nucleus size. It is best
explained by the thermal emission of slowly-moving outburst ejectas, with total
mass 5--8 10 kg. These results demonstrate the power of the ACA
for revealing the abundances, spatial distributions, and locations of molecular
production for volatiles in moderately bright comets such as C/2015 ER61
Low NH/HO ratio in comet C/2020 F3 (NEOWISE) at 0.7 au from the Sun
A lower-than-solar elemental nitrogen content has been demonstrated for
several comets, including 1P/Halley and 67P/C-G with independent in situ
measurements of volatile and refractory budgets. The recently discovered
semi-refractory ammonium salts in 67P/C-G are thought to be the missing
nitrogen reservoir in comets. The thermal desorption of ammonium salts from
cometary dust particles leads to their decomposition into ammonia and a
corresponding acid. The NH/HO ratio is expected to increase with
decreasing heliocentric distance with evidence for this in near-infrared
observations. NH has been claimed to be more extended than expected for a
nuclear source. Here, the aim is to constrain the NH/HO ratio in
comet C/2020 F3 (NEOWISE) during its July 2020 passage. OH emission from comet
C/2020 F3 (NEOWISE) was monitored for 2 months with NRT and observed from GBT
on 24 July and 11 August 2020. Contemporaneously with the 24 July 2020 OH
observations, the NH hyperfine lines were targeted with GBT. The
concurrent GBT and NRT observations allowed the OH quenching radius to be
determined at km on 24 July 2020, which
is important for accurately deriving . C/2020 F3 (NEOWISE) was a
highly active comet with molec
s one day before perihelion. The upper limit for
is at au from the
Sun. The obtained NH/HO ratio is a factor of a few lower than
measurements for other comets at such heliocentric distances. The abundance of
NH may vary strongly with time depending on the amount of water-poor dust
in the coma. Lifted dust can be heated, fragmented, and super-heated; whereby,
ammonium salts, if present, can rapidly thermally disintegrate and modify the
NH/HO ratio.Comment: Accepted for publication in A&A; 18 pages, 8 figures, 6 table
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