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See detailEtude des produits de dissociation de H2O dans un échantillon de comètes d'origines variées
Decock, Alice ULg

Doctoral thesis (2014)

Les comètes sont connues pour contenir de grandes quantités d’eau et des molécules organiques en tout genre. Formées lors de la naissance de notre Système Solaire, il y a 4.6 milliards d’années, elles ... [more ▼]

Les comètes sont connues pour contenir de grandes quantités d’eau et des molécules organiques en tout genre. Formées lors de la naissance de notre Système Solaire, il y a 4.6 milliards d’années, elles n’ont ensuite pas beaucoup évolué, ce qui les rend témoins potentiels des processus physico-chimiques présents à cette période. L’étude des comètes permet donc d’en apprendre davantage sur leur propre nature encore potentiellement énigmatique, mais également sur notre Système Solaire lui-même, et tout particulièrement sa genèse. La mission européenne Rosetta, en orbite autour de la comète 67P/Churyumov-Gerasimenko témoigne de l’intérêt porté à ces petits corps gelés du Système Solaire. Cette mission est unique puisqu’elle va permettre pour la première fois de mesurer la composition chimique précise d’un noyau cométaire. Ce type de mission est par contre très coûteux et ne concerne qu’une comète en particulier. Avec des télescopes au sol, il est possible d’étudier un nombre plus important de comètes. Certes, le noyau n’est dans ce cas pas directement atteignable, mais la spectroscopie permet d’analyser l’atmosphère de la comète. Formée par la sublimation des glaces du noyau et la dissociation des molécules qui s’en échappent, la coma contient de nombreuses informations nous permettant d’accroître nos connaissances sur la composition chimique du noyau. L’objectif de cette thèse est l’étude des molécules liées à l’eau dans les comètes. Les glaces cométaires renferment en effet 80% d’eau. Etudier cette molécule est donc crucial pour définir la nature des comètes et comprendre les conditions physiques et chimiques régnant dans la coma. Toutefois, H2O n’est pas détectable dans le domaine de longueur d’onde visible. Sur base d’un ensemble de données spectroscopiques visibles acquises depuis le sol, nous proposons dans cette thèse l’analyse de deux produits de dissociation de la molécule d’eau observables dans l’atmosphère de la comète, l’oxygène atomique et le radical OH. Le premier volet de ce travail se concentre sur les trois raies interdites de l’oxygène localisées à 5577.339 Å pour la raie verte (O(1S)) et à 6300.304 Å et 6363.776 Å (O(1D)) pour les raies du doublet rouge en vue de déterminer la ou les molécules parentes à l’origine de ces atomes. Dans cette optique, nous avons créé un spectre synthétique de la molécule de C2 afin de décontaminer la raie verte des raies dues au C2. Ensuite, nous avons mesuré les rapports d’intensité et les largeurs intrinsèques des trois raies d’oxygène pour des comètes situées à différentes distances héliocentriques. La comparaison du rapport de l’intensité de la raie verte sur la somme des intensités des raies rouges (rapport G/R) avec les taux d’excitation fournis par la théorie montre que H2O est la molécule parente principale des atomes d’oxygène lorsque la comète est observée à r ∼ 1 ua. Par contre, lorsque la comète est loin du Soleil (>2.5 ua), les molécules de CO2 contribuent également à la production d’oxygène. La mesure des largeurs intrinsèques des raies montre que la raie verte est plus large que les raies rouges alors que la théorie prédit l’inverse. Découle de cette observation que la raie verte pourrait principalement provenir de la photodissociation du CO2 alors que les raies rouges seraient uniquement formées via H2O. En étudiant les raies d’oxygène à différentes distances du noyau cométaire, nous réalisons que la molécule parente de l’oxygène varie : le CO2 est le contributeur premier des atomes d’oxygène en deçà de ∼1000 km du noyau et laisse ensuite la place à H2O. Qui plus est, nous notons l’importance du quenching collisionnel produit par H2O dans la coma interne qui joue un rôle significatif dans la perte des atomes de O(1S) et O(1D). Un modèle d’émission est réalisé pour reproduire nos données observationnelles. En se penchant sur le comportement adopté par les raies d’oxygène près du noyau et sur l’ajustement fourni par le modèle, une estimation de l’abondance relative du CO2 est déterminée. Des lors, cette thèse présente une nouvelle méthode pour déterminer l’abon- dance CO2/H2O dans les comètes à partir de données obtenues depuis le sol alors qu’une mesure directe de la molécule de CO2 n’est jusqu’à ce jour possible que depuis l’Espace. La seconde partie de notre travail porte sur l’analyse des rapports isotopiques 16O/18O et D/H à partir des isotopes du radical OH. La connaissance des rapports isotopiques dans des comètes d’origines variées est importante car cela peut nous renseigner sur les conditions physiques et chimiques existantes lorsque la comète s’est formée. De plus, la mesure du D/H s’inscrit dans le débat actuel de l’origine des océans terrestres. Dans ce contexte, des spectres synthétiques de 16OH, 18OH et OD sont créés sur base d’un modèle de fluorescence. Le rapport 16O/18O est déduit pour la première fois par ce modèle pour la comète C/2012 F6 (Lemmon) et il établit le point de départ d’une longue série de mesures portées sur des comètes brillantes à venir. [less ▲]

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See detailToward a Unique Nitrogen Isotopic Ratio in Cometary Ices
Rousselot, Philippe; Pirali, Olivier; Jehin, Emmanuel ULg et al

in Astrophysical Journal Letters (2014), 780

Determination of the nitrogen isotopic ratios in different bodies of the solar system provides important information regarding the solar system's origin. We unambiguously identified emission lines in ... [more ▼]

Determination of the nitrogen isotopic ratios in different bodies of the solar system provides important information regarding the solar system's origin. We unambiguously identified emission lines in comets due to the [SUP]15[/SUP]NH[SUB]2[/SUB] radical produced by the photodissociation of [SUP]15[/SUP]NH[SUB]3[/SUB]. Analysis of our data has permitted us to measure the [SUP]14[/SUP]N/[SUP]15[/SUP]N isotopic ratio in comets for a molecule carrying the amine (-NH) functional group. This ratio, within the error, appears similar to that measured in comets in the HCN molecule and the CN radical, and lower than the protosolar value, suggesting that N[SUB]2[/SUB] and NH[SUB]3[/SUB] result from the separation of nitrogen into two distinct reservoirs in the solar nebula. This ratio also appears similar to that measured in Titan's atmospheric N[SUB]2[/SUB], supporting the hypothesis that, if the latter is representative of its primordial value in NH[SUB]3[/SUB], these bodies were assembled from building blocks sharing a common formation location. [less ▲]

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See detailStudy of the Forbidden Oxygen Lines in Comets at Different Heliocentric and Nucleocentric Distances
Decock, Alice ULg; Rousselot, P.; Jehin, Emmanuel ULg et al

in Bulletin of the American Astronomical Society (2013, October 01), 45

Oxygen is an important element in the chemistry of the solar system objects given its abundance and its presence in many molecules including H2O 80% of cometary ices). The analysis of oxygen atoms in ... [more ▼]

Oxygen is an important element in the chemistry of the solar system objects given its abundance and its presence in many molecules including H2O 80% of cometary ices). The analysis of oxygen atoms in comets can provide information not only on the comets themselves but also on the solar system. These atoms have been analyzed using the 3 forbidden oxygen lines [OI] observed in emission in the optical region at 5577.339 Å (the green line), 6300.304 Å and 6363.776 Å (the red lines) (Swings, 1962). Our analysis is based on a sample of 12 comets of various origins. The observing material is made of 53 high signal-to-noise spectra obtained with the high-resolution UVES spectrograph at the ESO VLT from 2002 to 2012 (Manfroid et al, 2009). After noticing that the green line is blended with one C2 line, we built synthetic spectra of C2 for each observing circumstances and we subtracted its contribution to the cometary spectra in order to ensure the decontamination of the 5577 Å line. Then, we measured the intensity of the 3 [OI] lines at different heliocentric distances. By comparing the green to red lines ratio (G/R) with the Bhardwaj & Raghuram (2012) effective excitation rates, we found that H2O is the main parent molecule when the comet is observed at 1 au. When the comet is located beyond 2.5 au from the Sun, CO2 also contributes to the production of oxygen. Studying forbidden oxygen lines could be a new way to estimate the abundances of CO2 in comets, a very difficult task from the ground (Decock et al. 2013). In order to estimate the effect of the quenching on our results, we analyzed the evolution of the G/R ratio at different nucleocentric distances. For nearby comets, we divided the extended 2D spectrum into several zones in order to analyze the oxygen lines as close as possible to the nucleus (down to ~10 km for the closest comets). Their analysis will allow us to study the link of the oxygen lines with the nucleocentric distance. We found a clear variation of the G/R ratio close to the comet nucleus that is in agreement with a contribution from CO2 as predicted by Raghuram & Bhardwaj (2013). [less ▲]

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See detailA Search For 15NH2 Emission Lines In Comets
Rousselot, Philippe; Pirali, O.; Jehin, Emmanuel ULg et al

in Bulletin of the American Astronomical Society (2013, October 01), 45

The determination of nitrogen isotopic ratios in solar system objects is important for a good understanding of their origin. The measurements of [SUB]14[/SUB]N/[SUB]15[/SUB]N isotopic ratio done so far in ... [more ▼]

The determination of nitrogen isotopic ratios in solar system objects is important for a good understanding of their origin. The measurements of [SUB]14[/SUB]N/[SUB]15[/SUB]N isotopic ratio done so far in various solar system objects and molecules have revealed a great diversity (from 50 to 441), all of them, except Jupiter, being enriched in [SUB]15[/SUB]N compared to the protosolar nebula. Different explanations have been proposed to explain this enrichement. One of them suggests that these differences reflect the different interstellar N reservoirs from which N-bearing molecules are originating (Hily-Blant et al., 2013). These authors, from observations of H[SUB]13[/SUB]CN and HC[SUB]15[/SUB]N in two prestellar cores, suggest that the molecules carrying the nitrile- (-CN) functional group would be more enriched in [SUB]15[/SUB]N than the molecules carrying the amine (-NH) functional group. Comets are interesting targets to test this theory because they contain both HCN and NH[SUP]3[/SUP] molecules. So far the [SUB]14[/SUB]N/[SUB]15[/SUB]N ratio has only been measured in CN (Arpigny et al., 2003; Manfroid et al., 2009) and HCN (Bockelée-Morvan et al., 2005, 2008) in comets, leading for both species to [SUB]14[/SUB]N/[SUB]15[/SUB]N ≈ 150. Our work aimed at measuring the [SUB]14[/SUB]N/[SUB]15[/SUB]N isotopic ratio in NH[SUP]2[/SUP], which comes from NH[SUP]3[/SUP]. We have determined accurately the wavelengths of [SUB]15[/SUB]NH[SUP]2[/SUP] emission lines with the AILES beamline spectrometer at synchrotron SOLEIL by Fourier transform spectroscopy. The analysis of this spectrum has permitted to extract the [SUB]15[/SUB]NH[SUP]2[/SUP] emission lines wavelengths and to search for [SUB]15[/SUB]NH[SUP]2[/SUP] cometary emission lines. Thanks to a collection of spectra of 12 different comets obtained from 2002 to 2011 with the UVES spectrometer at the VLT ESO 8-m telescope (Manfroid et al., 2009), it has been possible to search for [SUB]15[/SUB]NH[SUP]2[/SUP] emission lines with a high sensitivity. We will present the results obtained from these data. Arpigny et al., Science, 301, 1522-1525, 2003 Bockelée-Morvan et al., in Comets II, ed. M. C. Festou, H. U. Keller, & H. A. Weaver (Tucson: Univ. Arizona Press), 391-423, 2005 Bockelée-Morvan et al., ApJ, 679, L49-L52, 2008 Hily-Blant et al., Icarus 223, 582-590, 2013 Manfroid et al., A&A, 503, 613-624, 2009 [less ▲]

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See detailStudy of the forbidden oxygen lines in comets at different heliocentric and nucleocentric distances
Decock, Alice ULg; Jehin, Emmanuel ULg; Rousselot, Philippe et al

in EPSC Abstract 2013 (2013, September 12), 8

Oxygen is an important element in the chemistry of the Solar System objects given its abundance and its presence in many molecules including H2O, which constitutes 80% of cometary ices. The analysis of ... [more ▼]

Oxygen is an important element in the chemistry of the Solar System objects given its abundance and its presence in many molecules including H2O, which constitutes 80% of cometary ices. The analysis of oxygen atoms in comets can provide information not only on the comets themselves but also on our Solar System. These atoms have been analyzed using the three forbidden oxygen lines [OI] observed in emission in the optical region at 5577 Å (the green line), 6300 Å and 6364 Å (the red lines) [1]. These lines are difficult to analyze because their detection requires high spectral and spatial resolutions. The oxygen analysis is interesting because it allows the determination of its parent molecules. [less ▲]

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See detailForbidden oxygen lines in comets at various heliocentric distances
Decock, Alice ULg; Jehin, Emmanuel ULg; Hutsemekers, Damien ULg et al

in Astronomy and Astrophysics (2013), 555

We present a study of the three forbidden oxygen lines [OI] located in the optical region - i.e., 5577.339 Å (the green line), 6300.304 Å and 6363.776 Å (the two red lines) - in order to better understand ... [more ▼]

We present a study of the three forbidden oxygen lines [OI] located in the optical region - i.e., 5577.339 Å (the green line), 6300.304 Å and 6363.776 Å (the two red lines) - in order to better understand the production of these atoms in cometary atmospheres. The analysis is based on 48 high-resolution and high signal-to-noise spectra collected with UVES at the ESO VLT between 2003 and 2011 referring to 12 comets of different origins observed at various heliocentric distances. The flux ratio of the green line to the sum of the two red lines is evaluated to determine the parent species of the oxygen atoms by comparison with theoretical models. This analysis confirms that, at about 1 AU, H[SUB]2[/SUB]O is the main parent molecule producing oxygen atoms. At heliocentric distances >2.5 AU, this ratio changes rapidly, an indication that other molecules are starting to contribute. The most abundant species after H[SUB]2[/SUB]O in the coma, CO and CO[SUB]2[/SUB], are good candidates, and the ratio is used to estimate their abundances. We found that the CO[SUB]2[/SUB] abundance relative to H[SUB]2[/SUB]O in comet C/2001 Q4 (NEAT) observed at 4 AU can be as high as ~70%. The intrinsic widths of the oxygen lines were also measured. The green line is on average about 1 km s[SUP]-1[/SUP] broader than the red lines, while the theory predicts that the red lines are broader. This might be due to the nature of the excitation source or to a contribution of CO[SUB]2[/SUB] as the parent molecule of the 5577.339 Å line. At 4 AU, we found that the width of the green and red lines in comet C/2001 Q4 are the same, which could be explained if CO[SUB]2[/SUB] becomes the main contributor to the three [OI] lines at high heliocentric distances. Based on observations made with ESO Telescope at the La Silla Paranal Observatory under programs ID 268.C-5570, 270.C-5043, 073.C-0525, 274.C-5015, 075.C-0355, 080.C-0615, 280.C-5053, 086.C-0958, and 087.C-0929. [less ▲]

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See detailStudy of the forbidden oxygen lines in a dozen comets observed at the VLT (ESO)
Decock, Alice ULg; Jehin, Emmanuel ULg; Manfroid, Jean ULg et al

in EPSC Abstracts 2011 (2011)

The forbidden lines are difficult to analyse because their detection requires high spectral and spatial resolutions. Their analysis is however interesting because it allows the determination of the ... [more ▼]

The forbidden lines are difficult to analyse because their detection requires high spectral and spatial resolutions. Their analysis is however interesting because it allows the determination of the spatial distribution and the production rate of the parent molecules, supposedly H2O which doesn't have any feature in the optical range. But as shown by Cochran [2] [3], some issues remain about the nature of the parents of the oxygen atoms. Moreover the width of the green line was found larger than that of the red lines. One of the goals of this study is to determine the parent species that photo-dissociate to produce oxygen atoms and see how this process depends on the heliocentric distance. We present here the results of the analysis of a homogeneous set of high quality spectra of 13 different comets observed with UVES at the ESO VLT since 2002 [4] [5]. [less ▲]

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See detailA Multi-wavelength study with the ESO VLT of comet 103P/Hartley2 at the time of the EPOXI encounter
Jehin, Emmanuel ULg; Hutsemekers, Damien ULg; Manfroid, Jean ULg et al

in EPSC Abstracts 2011 (2011)

We report on simultaneous optical and infrared spectroscopic observations of the Jupiter Family comet 103P/Hartley2 performed with the UT-1 and UT-2 8-m Unit Telescopes of the ESO Very Large Telescope ... [more ▼]

We report on simultaneous optical and infrared spectroscopic observations of the Jupiter Family comet 103P/Hartley2 performed with the UT-1 and UT-2 8-m Unit Telescopes of the ESO Very Large Telescope (VLT). These coordinated observations were carried on during several nights (2010 Nov. 5, 9, 10 and 11 UT) around the NASA EPOXI encounter with the comet on Nov. 4 [1] and in support to the key program « Water and related chemistry in the Solar System » (HssO) [2] of the Herschel Space Observatory. From high resolution optical spectroscopy of the CN (0,0) 388 nm band using UVES at UT2 we determined the isotopic ratios 12C/13C = 95 ± 15 and 14N/15N = 155 ± 25 in the CN radical. From the NH2 (0,9,0) and the H2O+ bands around 600 nm, we derived a nuclear spin temperature of 33 ± 3 K for NH3 and 36 +7/-6 K for H2O. These values are similar to those found in Oort- Cloud and Jupiter Family comets. From lowresolution long-slit spectroscopy with FORS2 at UT1 we will determine the CN, C2 and C3 spatial profiles and their production rates. From the high-resolution near-IR spectra that we collected with CRIRES at UT1 we will measure simultaneously the production rates and mixing ratios of the parent molecules H2O, HCN, C2H6, and CH3OH that are well detected in our spectra and we will study the link to the daughter species. [less ▲]

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