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G B Valsecchi - One of the best experts on this subject based on the ideXlab platform.
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planetary perturbations for oort cloud Comets iii evolution of the cloud and production of centaurs and halley type Comets
Icarus, 2014Co-Authors: Marc Fouchard, H Rickman, Ch Froeschle, G B ValsecchiAbstract:Abstract We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b]. Icarus, in press), the planetary perturbations experienced each time the Comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million Comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. In the companion paper we studied in detail how observable Comets are injected from the Oort cloud, when account is taken of the planetary perturbations. In the present paper we concentrate on how the cloud may evolve in the long term and also on the production of decoupled Comets, which evolve into semi-major axes less than 1000 AU. Concerning the long-term evolution, we find that the largest stellar perturbations that may statistically be expected during the age of the Solar System induce a large scale migration of Comets within the cloud. Thus, Comets leave the inner parts, but the losses from the outer parts are even larger, so at the end of our simulations the Oort cloud is more centrally condensed than at the beginning. The decoupled Comets, which form a source of centaurs and Halley type Comets (roughly in the proportions of 70% and 30%, respectively), are mainly produced by planetary perturbations, Jupiter and Saturn being the most efficient. This effect is dependent on synergies with the Galactic tide and stellar encounters, bringing the perihelia of Oort cloud Comets into the planetary region. The star-planet synergy has a large contribution due to the strong encounters that produce major comet showers. However, outside these showers a large majority of decouplings may be attributed to the tide-planet synergy.
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planetary perturbations for oort cloud Comets ii implications for the origin of observable Comets
Icarus, 2014Co-Authors: Marc Fouchard, H Rickman, Ch Froeschle, G B ValsecchiAbstract:Abstract We present Monte Carlo simulations of the dynamical history of the Oort cloud, where in addition to the main external perturbers (Galactic tides and stellar encounters) we include, as done in a companion paper (Fouchard, M., Rickman, H., Froeschle, Ch., Valsecchi, G.B. [2013b]. Icarus, in press), the planetary perturbations experienced each time the Comets penetrate to within 50 AU of the Sun. Each simulation involves an initial sample of four million Comets and extends over a maximum of 5 Gyr. For better understanding of the outcomes, we supplement the full dynamical model by others, where one or more of the effects are left out. We concentrate on the production of observable Comets, reaching for the first time a perihelion within 5 AU of the Sun. We distinguish between four categories, depending on whether the comet jumps across, or creeps through, the Jupiter–Saturn barrier (perihelion distances between 5 and 15 AU), and whether the orbit leading to the observable perihelion is preceded by a major planetary perturbation or not. For reasons explained in the paper, we call the strongly perturbed Comets “Kaib–Quinn Comets”. We thus derive a synthetic picture of the Oort spike, from which we draw two main conclusions regarding the full dynamical model. One is that 2/3 of the observable Comets are injected with the aid of a planetary perturbation at the previous perihelion passage, and about half of the observable Comets are of the Kaib–Quinn type. The other is that the creepers dominate over the jumpers. Due to this fact, the spike peaks at only 31 000 AU, and the majority of new Comets have semi-major axes less than this value. The creepers show a clear preference for retrograde orbits as a consequence of the need to avoid untimely, planetary ejection before becoming observable. Thus, the new Comets should have a 60/40 preference for retrograde against prograde orbits in apparent conflict with observations. However, both these and other results depend on our model assumptions regarding the initial structure of the Oort cloud, which is isotropic in shape and has a relatively steep energy distribution. We also find that they depend on the details of the past history of external perturbations including GMC encounters, and we provide special discussions of those issues.
Michael F Ahearn - One of the best experts on this subject based on the ideXlab platform.
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the outburst triggered by the deep impact collision with comet tempel 1
Monthly Notices of the Royal Astronomical Society, 2011Co-Authors: S I Ipatov, Michael F AhearnAbstract:Time variations in velocities and relative amount of observed particles (mainly icy particles with diameter d<3 μm) ejected from Comet 9P/Tempel 1 are studied based on analysis of the images made by Deep Impact (DI) cameras during the first 13 minutes after the collision of the DI impactor with the comet. Analysis of maxima or minima of plots of the time variations in distances of contours of constant brightness from the place of ejection allowed us to estimate the characteristic velocities of particles at several moments in time te of ejection after impact for te≤115 s. Other approaches for estimates of the velocities were also used. All these estimates are in accordance with the same exponential decrease in velocity. The estimates of time variations in the relative amount of ejected particles were based also on results of the analysis of time variations in the size of the bright region of ejected material. At te~10 s, the morphology of the ejecta (e.g. the location and brightness of the brightest pixel) changed and the rate rte of ejection of observed material increased. Between 1 and 3 seconds after the impact and between 8 and 60 seconds after the impact, more small bright particles were ejected than expected from crater excavation alone. An outburst triggered by the impact could cause such a difference. The sharp (by a factor of 1.6) decrease in the rate of ejection at 55
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photometric study of the kreutz Comets observed by soho from 1996 to 2005
The Astronomical Journal, 2010Co-Authors: Matthew M. Knight, Michael F Ahearn, Philippe Lamy, D A Biesecker, G Faury, Douglas P Hamilton, A LlebariaAbstract:We present analysis of the photometry of more than 900 Kreutz Comets observed by SOHO from 1996 to 2005. The Kreutz Comets have "sungrazing" orbits with q ≈ 1-2 R� , high inclinations (i ≈ 143 ◦ ), and periods of 500-1000 years. We find that they do not have a bimodal distance of peak brightness as previously reported, but instead peak from 10.5 Rto 14 R� (prior to perihelion), suggesting there is a continuum of compositions rather than two distinct subpopulations. The light curves have two rates of brightening, typically ∝ r −7.3±2.0 when first observed by SOHO (at distances of 30-35 R� ) then rapidly transitioning to ∝ r −3.8±0.7 between 20 Rand 30 R� . It is unclear at what distance the steeper slope begins, but it likely does not extend much beyond the SOHO field of view. We derive nuclear sizes up to ∼50 m in radius for the SOHO-observed Comets, with a cumulative size distribution of N(>R) ∝ R −2.2 for Comets larger than 5 m in radius. This size distribution cannot explain the largest members of the family seen from the ground, suggesting that either the size distribution does not extend to the largest sizes or that the distribution is not uniform around the orbit. The total mass of the distribution up to the largest expected size (∼500 m) is ∼4 × 10 14 g, much less than the estimated masses of the largest ground-observed members. After correcting for the changing discovery circumstances, the flux of Comets reaching perihelion has increased since 1996, and the increase is seen in Comets of all sizes. Comparison of the SOHO Comets with the Solwind and Solar Maximum Mission discoveries suggests there may have been an overabundance of bright Comets arriving from 1979 to 1989, possibly indicative of a changing distribution around the Kreutz orbit.
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the outburst triggered by the deep impact collision with comet tempel 1
arXiv: Astrophysics, 2008Co-Authors: S I Ipatov, Michael F AhearnAbstract:Time variations in velocities and relative amount of observed particles (mainly icy particles with diameter d<3 {\mu}m) ejected from Comet 9P/Tempel 1 are studied based on analysis of the images made by Deep Impact (DI) cameras during the first 13 minutes after the collision of the DI impactor with the comet. Analysis of maxima or minima of plots of the time variations in distances of contours of constant brightness from the place of ejection allowed us to estimate the characteristic velocities of particles at several moments in time te of ejection after impact for te $\leq$ 115 s. Other approaches for estimates of the velocities were also used. All these estimates are in accordance with the same exponential decrease in velocity. The estimates of time variations in the relative amount of ejected particles were based also on results of the analysis of time variations in the size of the bright region of ejected material. At te~10 s, the morphology of the ejecta (e.g. the location and brightness of the brightest pixel) changed and the rate rte of ejection of observed material increased. Between 1 and 3 seconds after the impact and between 8 and 60 seconds after the impact, more small bright particles were ejected than expected from crater excavation alone. An outburst triggered by the impact could cause such a difference. The sharp (by a factor of 1.6) decrease in the rate of ejection at 55
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the ensemble properties of Comets results from narrowband photometry of 85 Comets 1976 1992
Icarus, 1995Co-Authors: Michael F Ahearn, Robert C Millis, David O Schleicher, David J Osip, Peter V BirchAbstract:Abstract We present the results of narrowband photometry of 85 Comets observed over a period of 17 years. The data have been reduced homogeneously to molecular production rates and a proxy for the dust production rate. We confirm previous investigations, both our own and those of others, showing that there is no differentiation with depth in the cometary nucleus, that most Comets are very similar to each other in chemical composition, and that the dust-to-gas ratio does not vary with the dynamical age of the comet. There is little variation of relative abundances with heliocentric distance, implying that for the species we observe the role of density-dependent processes in the coma is small. There is also little variation from one apparition to the next for most short-period Comets. We show, however, that there are significant compositional groupings of Comets, apparently related to place of formation. Specifically, the majority of Comets originating in the Kuiper belt appear to be significantly depleted in the carbon-chain molecules that lead to the observed C2 and C3. Kuiper belt Comets also exhibit smaller active areas, steeper slopes, and greater asymmetries about perihelion than do other Comets, implying smaller active fractions. We argue that at least the compositional effect is primordial rather than evolutionary. We also show a significant but unexplained correlation of some properties, notably the dust-to-gas ratio, with perihelion distance. We provide strong evidence that most CN, some C2, and virtually no NH are produced from grains in the coma rather than from nuclear ices. Evidence suggests that the mixing between water and other ices occurs at the level of grains rather than at the molecular level. We also show that there must be a large population of Comets having very low activity. The data appear to require mixing of cometesimals with different compositions into individual nuclei, with some nuclei containing nearly the full range of compositions seen in the ensemble while others contain a much smaller range of compositions.
Meromit Singer - One of the best experts on this subject based on the ideXlab platform.
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combinatorial prediction of gene marker panels from single cell transcriptomic data
bioRxiv, 2019Co-Authors: Conor Delaney, Alexandra Schnell, Louis V Cammarata, Aaron Yaosmith, Aviv Regev, Vijay K Kuchroo, Meromit SingerAbstract:Abstract Single-cell transcriptomic studies are identifying novel cell populations with exciting functional roles in various in vivo contexts, but identification of succinct gene-marker panels for such populations remains a challenge. In this work we introduce COMET, a computational framework for the identification of candidate marker panels consisting of one or more genes for cell populations of interest identified with single-cell RNA-seq data. We show that COMET outperforms other methods for the identification of single-gene panels, and enables, for the first time, prediction of multi-gene marker panels ranked by relevance. Staining by flow-cytometry assay confirmed the accuracy of COMET’s predictions in identifying marker-panels for cellular subtypes, at both the single- and multi-gene levels, validating COMET’s applicability and accuracy in predicting favorable marker-panels from transcriptomic input. COMET is a general non-parametric statistical framework and can be used as-is on various high-throughput datasets in addition to single-cell RNA-sequencing data. COMET is available for use via a web interface (http://www.Cometsc.com/) or a standalone software package (https://github.com/MSingerLab/CometsC).
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combinatorial prediction of gene marker panels from single cell transcriptomic data
bioRxiv, 2019Co-Authors: Conor Delaney, Alexandra Schnell, Louis V Cammarata, Aaron Yaosmith, Aviv Regev, Vijay K Kuchroo, Meromit SingerAbstract:Abstract Single-cell transcriptomic studies are identifying novel cell populations with exciting functional roles in various in vivo contexts, but identification of succinct gene-marker panels for such populations remains a challenge. In this work we introduce COMET, a computational framework for the identification of candidate marker panels consisting of one or more genes for cell populations of interest identified with single-cell RNA-seq data. We show that COMET outperforms other methods for the identification of single-gene panels, and enables, for the first time, prediction of multi-gene marker panels ranked by relevance. Staining by flow-cytometry assay confirmed the accuracy of COMET’s predictions in identifying marker-panels for cellular subtypes, at both the single- and multi-gene levels, validating COMET’s applicability and accuracy in predicting favorable marker-panels from transcriptomic input. COMET is a general non-parametric statistical framework and can be used as-is on various high-throughput datasets in addition to single-cell RNA-sequencing data. COMET is available for use via a web interface (http://www.Cometsc.com/) or a standalone software package (https://github.com/MSingerLab/CometsC).
Martin J. Duncan - One of the best experts on this subject based on the ideXlab platform.
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The Long-Term Dynamical Behavior of Short-Period Comets
Icarus, 1994Co-Authors: Harold F. Levison, Martin J. DuncanAbstract:We have developed and carefully tested a new computer code to follow the long-term dynamical evolution of a swarm of test particles in the solar system. This new integrator is approximately an order of magnitude faster than previously existing codes. The technique efficiently and accurately handles close approaches between test particles and planets while retaining the powerful features of recently developed mixed variable symplectic integrators. We use the new code to numerically integrate the orbits of the known short-period Comets (those with periods P 20 yr) orbits many times in their dynamical lifetimes. However, it is found that the Tisserand parameter, T, does not vary substantially for most Comets. Therefore, we adopt a classification originally suggested by Carusi et al. (1987) that defines Jupiter-family Comets (JFCs) as Comets with T > 2 and Halley-family Comets (HFCs) as those with T < 2. In this scheme, less than 8% of Comets change families during the integration and most of those that change tend to remain near the Tisserand dividing line throughout. Thus, the JFCs (as defined by the Tisserand parameter) are dynamically distinct from the HFCs. We find that in our forward integration, 92% of Comets are ejected from the solar system, and that ≈6% are destroyed by becoming sun-grazers. The number of sun-grazers is far more than would be expected from the existing analytic theories. The median lifetime of all known short-period Comets from the current time to ultimate destruction or ejection is approximately 4.5 × 105 years. The very flat inclination distribution of Jupiter-family Comets is found to become more distended as it ages. Since JFCs are dynamically distinct from HFCs, they must have an inclination distribution, when they first become visible, that is even flatter than that currently observed. For reasonable values of the physical lifetime before fading, we calculate that there should be roughly 5-20 times as many extinct JFCs as currently known JFCs. Our prediction for the mean cos (i) of the extinct JFCs is consistent with the existing data on these objects.
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the long term dynamical behavior of short period Comets
Origins of Solar Systems Workshop: The Origin Evolution and Detectability of Short Period Comets, 1993Co-Authors: Harold F. Levison, Martin J. DuncanAbstract:The orbits of the known short-period Comets under the influence of the Sun and all the planets except Mercury and Pluto are numerically integrated. The calculation was undertaken in order to determine the dynamical lifetimes for these objects as well as explaining the current orbital element distribution. It is found that a comet can move between Jupiter-family and Halley-family Comets several times in its dynamical lifetime. The median lifetime of the known short-period Comets from the time they are first injected into a short-period comet orbit to ultimate ejection is approximately 50,000 years. The very flat inclination distribution of Jupiter-family Comets is observed to become more distended as it ages. The only possible explanation for the observed flat distribution is that the Comets become extinct before their inclination distribution can change significantly. It is shown that the anomalous concentration of the argument of perihelion of Jupiter-family Comets near 0 and 180 deg is a direct result of their aphelion distance being close to 5.2AU and the comet being recently perturbed onto a Jupiter-family orbit. Also the concentration of their aphelion near Jupiter's orbit is a result of the conservation of the Tisserand invariant during the capture process.
Andre Bieler - One of the best experts on this subject based on the ideXlab platform.
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sulphur bearing species in the coma of comet 67p churyumov gerasimenko
Monthly Notices of the Royal Astronomical Society, 2016Co-Authors: Ursina Calmonte, Andre Bieler, Kathrin Altwegg, H Balsiger, Jeanjacques Berthelier, Gael Cessateur, Frederik Dhooghe, E F Van Dishoeck, B FietheAbstract:Several sulphur-bearing species have already been observed in different families of Comets. However, the knowledge on the minor sulphur species is still limited. The comet's sulphur inventory is closely linked to the pre-solar cloud and holds important clues to the degree of reprocessing of the material in the solar nebula and during comet accretion. Sulphur in pre-solar clouds is highly depleted, which is quite puzzling as the S/O ratio in the diffuse interstellar medium is cosmic. This work focuses on the abundance of the previously known species H 2 S, OCS, SO, S 2 , SO 2 and CS 2 in the coma of comet 67P/Churyumov–Gerasimenko measured by Rosetta Orbiter Spectrometer for Ion and Neutral Analysis/Double Focusing Mass Spectrometer between equinox and perihelion 2015. Furthermore, we present the first detection of S 3 , S 4 , CH 3 SH and C 2 H 6 S in a comet, and we determine the elemental abundance of S/O in the bulk ice of (1.47 ± 0.05) × 10 −2 . We show that SO is present in the coma originating from the nucleus, but not CS in the case of 67P, and for the first time establish that S 2 is present in a volatile and a refractory phase. The derived total elemental sulphur abundance of 67P is in agreement with solar photospheric elemental abundances and shows no sulphur depletion as reported for dense interstellar clouds. Also the presence of S 2 at heliocentric distances larger than 3 au indicates that sulphur-bearing species have been processed by radiolysis in the pre-solar cloud and that at least some of the ice from this cloud has survived in Comets up the present.
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abundant molecular oxygen in the coma of comet 67p churyumov gerasimenko
Nature, 2015Co-Authors: Andre Bieler, Kathrin Altwegg, H Balsiger, A Barnun, Jeanjacques Berthelier, P Bochsler, Christelle Briois, Ursina Calmonte, Michael R Combi, J De KeyserAbstract:The composition of the neutral gas comas of most Comets is dominated by H2O,CO and CO2, typically comprising as much as 95 per cent of the total gas density1. In addition, cometary comas have been found to contain a rich array of other molecules, including sulfuric compounds and complex hydrocarbons. Molecular oxygen (O2), however, despite its detection on other icy bodies such as the moons of Jupiter and Saturn2,3, has remained undetected in cometary comas. Here we report in situ measurement of O2 in the coma of comet 67P/Churyumov–Gerasimenko, with local abundances ranging from one per cent to ten per cent relative to H2O and with a mean value of 3.8060.85 per cent. Our observations indicate that the O2/H2O ratio is isotropic in the coma and does not change systematically with heliocentric distance. This suggests that primordial O2 was incorporated into the nucleus during the comet’s formation, which is unexpected given the low upper limits from remote sensing observations4. Current Solar System formation models do not predict conditions that would allow this to occur.
