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

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 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

Observations of OH in comet Levy with the Nancay radio telescope

Due to extremely favorable excitation conditions, comet Levy (1990c) exhibited in August-September 1990 the strongest OH 18-cm signal ever recorded in a comet at the Nancay radio telescope. This unique opportunity was used to measure the OH satellite lines at 1612 and 1721 MHz, to perform extensive mapping of the OH radio emission and to make a sensitive evaluation of the cometary magnetic field, of the H2O outflow velocity and of the OH production rate

Physical phenomena governing the behaviour of wildfires : numerical simulation of crown fires in boreal fores

This paper presents the physical phenomena contributing to the behaviour of wildfires. The problem was formulated using a multiphase approach, including the set of balance equations governing the coupled system formed by the vegetation and the surrounding atmosphere in the vicinity of the fire front. Some numerical simulations carried out for a crown fire in boreal forest are compared to data collected during an experimental campaign conducted in the North West territories in Canada

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

Deuterium Fractionation: the Ariadne's Thread from the Pre-collapse Phase to Meteorites and Comets today

The Solar System formed about 4.6 billion years ago from a condensation of matter inside a molecular cloud. Trying to reconstruct what happened is the goal of this chapter. For that, we put together our understanding of Galactic objects that will eventually form new suns and planetary systems, with our knowledge on comets, meteorites and small bodies of the Solar System today. Our specific tool is the molecular deuteration, namely the amount of deuterium with respect to hydrogen in molecules. This is the Ariadne's thread that helps us to find the way out from a labyrinth of possible histories of our Solar System. The chapter reviews the observations and theories of the deuterium fractionation in pre-stellar cores, protostars, protoplanetary disks, comets, interplanetary dust particles and meteorites and links them together trying to build up a coherent picture of the history of the Solar System formation. We emphasise the interdisciplinary nature of the chapter, which gathers together researchers from different communities with the common goal of understanding the Solar System history.Comment: Accepted for publication as a chapter in Protostars and Planets VI, University of Arizona Press (2014), eds. H. Beuther, R. Klessen, C. Dullemond, Th. Hennin

Effet de la teneur en eau sur la propagation d'un feu de surface à travers une strate de végétation homogène

Cette étude porte sur le comportement d’un feu de surface se propageant à travers une strate de végétation homogène. Ce problème a été étudié par simulation numérique, à partir d’une formulation CFD multiphasique, qui consiste à résoudre les équations de bilan (masse, mouvement, énergie) du système couplé formé par la strate de végétation et la couche atmosphérique environnante. Parmi les phénomènes physiques pris en compte, la combustion en phase gazeuse est résolue en supposant que le taux de réaction est piloté par la vitesse de mélange des produits de pyrolyse avec l’air environnant. Le couplage turbulence/rayonnement est partiellement intégré à partir d’un modèle basé sur d’une hypothèse d’épaisseur optique des structures turbulentes petite devant l’unité (‘Optically Thin Fluctuation Approximation’). L’étude est focalisée sur l’effet de la teneur en eau du combustible sur la dynamique de propagation du feu. Les résultats sont analysés en termes de temps de résidence et profondeur et vitesse de propagation du front, courbe de distribution des fractions massiques d’H2O et d’O2, pour deux valeurs de la vitesse du vent. Les résultats numériques ont permis de montrer que la teneur en eau du combustible pouvait affecter la propagation du front de deux manières : par contraction de la profondeur du front de combustion (propagation limitée par la pyrolyse) et par dilution des gaz frais qui alimentent en oxygène le foyer. Ces deux mécanismes pouvant conduire chacun à l’arrêt de la propagation et à l’extinction du front d’incendie

France South Pilot Centre - Main activities of the MSNM-GP laboratory

ERCOFTAC Bulletin 77Presentation of the main activities of the MSNM-GP laborator

Assessment of the plume theory predictions of crown scorch using transport models

The aim of our work is to study numerically crown scorch as the effectsof a fire line spreading through surface fuel under a tree canopy. Theobjective was to assess the usual assumptions made when one uses the Van Wagner criteria, which are indeed simple predictive models for crown scorch height, to estimate crown scorch. For this purpose the FIRESTAR 2D and FIRETEC wildfire simulators are used. We simulated the fire line by a heat source at ground level and mainly investigated the temperature field. As a first step, we ran computations of thermal plumes with no-wind and with no canopy, for first comparison to plume theory. The influence of crown existence on the temperature field above the heat source, as well as on crown scorch, was then investigated. As a second step, the effect of a wind to the plume, as well as to crown scorch, was shown for the no-canopy and canopy cases