Showing papers in "Earth Moon and Planets in 1998"
TL;DR: In this article, a 1 cm diameter Leonid meteoroid is estimated to have a velocity of 72 km/s and a Knudsen number of 4 and a Mach number of 270.
Abstract: The flow field is computed around a 1 cm diameter Leonid meteoroid as it enters the Earth's atmosphere at an altitude of 95 km and a velocity of 72 km/s. These conditions correspond to a Knudsen number of 4 and a Mach number of 270. To accurately compute the gas flow, these extreme nonequilibrium conditions require application of a kinetic approach and the present work employs the direct simulation Monte Carlo method. A meteoroid ablation model is included in the computations and is found to play a significant role. The computational results predict that a large region of the flow field is affected by meteoroid ablation that produces an extended wake at high temperature in a sate of thermal equilibrium. These findings are in qualitative agreement with spectroscopic observations of the 1998 Leonid meteoroid shower. The computations indicate that the results are sensitive to the material properties assumed for the meteoroid.
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TL;DR: Comet simulation experiments are discussed in this paper, in the context of physical models and the results in cometary physics, gathered especially from the GIOTTO space mission to comet P'Halley.
Abstract: Comet simulation experiments are discussed, in the context of physical models and the results in cometary physics, gathered especially from the GIOTTO space mission to comet P’Halley. The “status of the today knowledge” about comets, the experiments could start from, is briefly reviewed. The setup of the KOSI (German = Kometen Simulation) — experiments and the techniques to produce cometary analogous material, on the basis of that knowledge are described in general, as for the different KOSI experiments. The limitations of the simulation of physical processes at the surface of real comets in an earth-bound laboratory are discussed, and the possibilities to receive common insights in cometary physics are shown. Methods and procedures are described, and the major results reviewed. As with attempting to reproduce any natural phenomenon in the laboratory, there are short-comings to these experiments, but there are possibly major new insights to be gained. Physical laws only have the same consequences under same experimental or environmental conditions. A number of small-scale comet simulation experiments have been performed, since the early 60ties in many laboratories, but the largest and most ambitious series of comet simulation experiments to date were performed between 1987 and 1993 using the German space agency’s (DLR) space hardware testing facilities in Cologne. These experiments were triggered by the scientific community after the comet P’Halley’s recurrence in 1986 and the many data gathered by the space missions in this year. Simulation experiments have proved valuable in developing methods for making cometary analogues, and for exploring specific properties of such materials in detail. These experiments provided new insights into the morphology and physical behavior of aggregates formed out of silicate- /water-ice -grains likely to exist in comets. The formation of a dust mantle on the surface, and a system of ice layers below the mantle from the different admixed materials, have been detected after the insolation of the artificial comet. The mechanisms for heat transfer between the comet’s surface and its interior, compositional, structural, and isotopic changes that occur near the comet’s surface, were described by modeling in accordance with the experimental results. The mechanisms of the ejection of dust and ice grains from the surface, and the importance of gas-drag in propelling grains were investigated by close-up video cameras.
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TL;DR: In this paper, the nature of carbon-containing molecules and carbonaceous solids present in meteorites, comets, and the interstellar medium is discussed, and a review is presented to build a bridge between astronomical spectroscopy and laboratory studies relevant to the investigation of cosmic carbon.
Abstract: In this review, the nature of carbon-containing molecules and carbonaceous solids present in meteorites, comets, and the interstellar medium is discussed. Carbon plays an active role in the lifecycle of stars and the interstellar medium. It is the basis of a rich interstellar chemistry and the main component of pre-biotic organic material in space. The aim of the review is to build a bridge between astronomical spectroscopy and laboratory studies relevant to the investigation of cosmic carbon. Special emphasis is given to the structural variety of carbon-containing species and their characterization by experimental techniques.
13 citations
TL;DR: In this paper, the expressions of variations of the dynamical ellipticity and the principal moments of inertia due to the deformations produced by the zonal part of the tidal potential are obtained.
Abstract: In this paper, the expressions of variations of the dynamical ellipticity and the principal moments of inertia due to the deformations produced by the zonal part of the tidal potential are obtained. Starting from these expressions, we have studied from equations related to Hamiltonian theory, their effects on the nutation and finally we have evaluated numerically such influences, with a level of truncation at 0.1 μas. Thus we have shown that some coefficients are quite large with respect to the usual accuracy of up-to-date observations.
13 citations
TL;DR: In this article, the existence and properties of exoplanetary systems have been investigated using optical-to-submillimeter imaging of 7 main-sequence stars within 50 parsecs of the Sun.
Abstract: Since 1984, roughly 100 main sequence stars within 50 parsecs of the Sun have been identified as possibly possessing replenished, circumstellar dust disks. Optical to submillimeter imaging has resolved disk-like structure around 7 main sequence stars. We review these results, and discuss how they elucidate the existence and properties of exoplanetary systems.
12 citations
12 citations
TL;DR: The 1998 Leonid meteor shower was observed at the Gaomeigu station of Yunnan Observatory during five successive days in November as mentioned in this paper, and the visual records indicate that the number of meteors increased suddenly, from a ZHR of about 140 to over 400, in the early morning of November 17th, Beijing time. But it decreased slowly in the following two days.
Abstract: The 1998 Leonid meteor shower was observed at the Gaomeigu station of Yunnan Observatory during five successive days in November. The visual records indicate that the number of meteors increased suddenly, from a ZHR of about 140 to over 400, in the early morning of November 17th, Beijing time. But it decreased slowly in the following two days. During the maximum there was a high proportion - about 10 percent - of very bright fireballs with enduring trains. The brightest one was about -10 magnitude with a smoke train fading about three minutes after.
TL;DR: In this article, the authors quantified the maximum possible amount of CO2 that can be out gassed via a single massive episode of resurfacing of the planet and suggested that the equivalent of eight global resurfacing episodes would be required to account for the present values of CO 2 atmosphere.
Abstract: In absence of other mechanisms, the main input of CO2into the Venusian atmosphere is via volcanic out gassing. Since Venus can be regarded as a planet-wide large igneous province, we can expect large quantities of CO2 being transferred into its atmosphere via volcanic out gassing. We have quantified the maximum possible amount of CO2 that can be out gassed via a single massive episode of resurfacing of the planet. This figure (5.6 × 1019 kg of CO2) is about 8 times smaller than the total CO2 present in the Venusian atmosphere (4.55 × 1020 kg CO2). The lack of planet-wide, efficient mechanisms for the recycling of CO2 on Venus indicates that CO2 has progressively accumulated in the atmosphere. Based on these considerations we suggest that the “equivalent” to eight global resurfacing episodes would be required to account for the present values of CO2 atmosphere.
TL;DR: In this paper, the authors investigate the response of the particles in a protoplanetary disk with radius RD = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300-1600 AU) orbit.
Abstract: Most main sequence stars are binaries or higher multiplicity Systems and it appears that at birth most stars have circumstellar disks. It is commonly accepted that planetary systems arise from the material of these disks; consequently, binary and multiple systems may have a main role in planet formation. In this paper, we study the stage of planetary formation during which the particulate material is still dispersed as centimetre-to-metre sized primordial aggregates. We investigate the response of the particles, in a protoplanetary disk with radius RD = 100 AU around a solar-like star, to the gravitational field of bound perturbing companions in a moderately wide (300–1600 AU) orbit. For this purpose, we have carried out a series of simulations of coplanar hierarchical configurations using a direct integration code that models gravitational and viscous forces. The massive protoplanetary disk is around one of the components of the binary. The evolution in time of the dust sub-disk depends mainly on the nature (prograde or retrograde) of the relative revolution of the stellar companion, and on the temperature and mass of the circumstellar disk. Our results show that for binary companions near the limit of tidal truncation of the disk, the perturbation leads to an enhanced accretion rate onto the primary, decreasing the lifetime of the particles in the protoplanetary disk with respect to the case of a single star. As a consequence of an enhanced accretion rate the mass of the disk decreases faster, which leads to a longer resultant lifetime for particles in the disk. On the other hand, binary companions may induce tidal arms in the dust phase of protoplanetary disks. Spiral perturbations with m = 1 may increase in a factor 10 or more the dust surface density in the neighbourhood of the arm, facilitating the growth of the particles. Moreover, in a massive disk (0.01M⊙) the survival time of particles is significantly shorter than in a less massive nebula (0.001M⊙) and the temperature of the disk severely influences the spiral-in time of particles. The rapid evolution of the dust component found in post T Tauri stars can be explained as a result of their binary nature. Binarity may also influence the evolution of circumpulsar disks.
TL;DR: In this article, the spectral energy distribution (SED) and IR polarization near perhelion passage of a comet Hale-Bopp were analyzed and it was shown that the spectrum of the coma was dominated by scattering and thermal emission from sub-micron sized dust grains.
Abstract: Comets, such as C/1995 O1 (Hale-Bopp), are important to studies of the origins of the solar system because they are believed to be frozen reservoirs of the most primitive pre-solar dust grains and ices. Here, we report 1.2–18.5 μm infrared (IR) spectrophotometric and polarimetric observations of comet Hale-Bopp. Our measurements of the spectral energy distribution (SED) and IR polarization near perhelion passage suggest that emission from the coma was dominated by scattering and thermal emission from sub-micron sized dust grains. Hale-Bopp's surprising brightness may have been largely a result of the properties of its coma grains rather than the size of its nucleus. The thermal emission continuum from the grains had a superheat of S = Tcolor/TBB ≥ 1.7, the peak of the 10 μm silicate emission feature was 1.7 mags above the carbon grain continuum, and the albedo (reflectivity) of the grains was ≥ 0.4 at a scattering angles, θ ≥ 135°
TL;DR: In this article, the main goals of experimental simulation in the laboratory of a planetary atmosphere are to feed the theoretical models, and to help the treatment of observations, through the production of laboratory analogues of gaseous or solid phases.
Abstract: The main goals of experimental simulation in the laboratory of a planetary atmosphere are to feed the theoretical models, and to help the treatment of observations. This type of simulation permits the direct study of objects that space missions can't study or have not studied yet, through the production of laboratory analogues of gaseous or solid phases. But the representativity of these laboratory analogues is of crucial importance.
TL;DR: In this paper, the authors present a selection of results obtained with detailed simulations reproducing the process of data acquisition and analysis for the global astrometry satellite GAIA, a space mission concept currently under study by ESA as part of Horizon 2000 Plus program.
Abstract: We present a selection of results obtained with detailed simulations reproducing the process of data acquisition and analysis for the global astrometry satellite GAIA, a space mission concept currently under study by ESA as part of Horizon 2000 Plus program
TL;DR: In this paper, the point source of neutral gas undergoing ionization and expanding into an uniform magnetic field is considered, and the possible relation of these jets to cometary rays is discussed.
Abstract: The point source of neutral gas undergoing ionization and expanding into an uniform magnetic field is considered. Friction between the neutral and ionized particles results in the formation of the magnetic field barrier and diamagnetic cavity surrounding the source. At least one neutral point inevitably arises at the boundary of the cavity. When the neutral gas production rate grows, two neutral points may arise at this boundary. In the vicinity of these points magnetic field lines converge, along with the plasma flow which is magnetic field aligned in the steady state. As a result, two plasma jets originate from the neutral points. Possible relation of these jets to cometary rays is discussed.
TL;DR: The possibility of searching for extrasolar planets from the observations during Russian space astrometry mission Struve and from the Solar Stereoscope Observatory (SSO) is discussed in this paper.
Abstract: The possibility of search for extrasolar planets from the observations during Russian space astrometry mission Struve and from the Solar Stereoscope Observatory (SSO) is discussed here.
TL;DR: A mean superheat value of 1.83 was derived in the post-perihelion phase which implies that a large fraction of the grain population are made up of small and hot grains with radii < 0.5 μm as discussed by the authors.
Abstract: During the recent apparition of comet Hale—Bopp (1995 O1) near infrared photometric observations were carried out in the J, H, K filter bands and also in the 3.0—3.4 μm region at the 1.2 m telescope at Gurushikkar, India. The effective temperature of the comet was substantially higher than the equilibrium blackbody temperature. A mean superheat value of 1.83 was derived in the post-perihelion phase which implies that a large fraction of the grain population are made up of small and hot grains with radii <0.5 μm. High albedo values of ∼0.4 were also derived in the scattering angle range 135° to 160° which could explain the unusual brightness of comet Hale—Bopp.
TL;DR: In this paper, the authors used a servo-controlled CCD spectrograph as a null detector to register the lines of the star on the same CCD pixels, thus, systematic errors linked to the Earth-induced large variations are cancelled.
Abstract: The method of stellar radial velocity variations has recently shown its capability by the first discovery of several extra-solar planets. Accuracies achieved today are in the range 3-10 m/s. The AAA (absolute astronomical accelerometer) is an instrument which aims to reach the photon noise limit for the measurement of velocity changes, with systematic errors of about 1 m/s, long term. The principle is to use a servo-controlled CCD spectrograph as a null detector, and to register always the lines of the star on the same CCD pixels. Thus, systematic errors linked to the Earth-induced large variations are cancelled. A tunable Fabry-Perot channelled spectrum is also following the star spectrum, while the FP thickness is measured by heterodyne detection of the beats between a tunable laser diode and a stabilized laser diode. A complete prototype of the instrument is operating with laboratory sources and the first results are presented. It is planned to use this system with a new spectrograph, to be coupled to the 152 cm telescope at Observatoire de Haute Provence.
TL;DR: In this paper, the formation and properties of long thin tails of young stellar objects in the Orion Nebula have been investigated using a variety of morphologies, including point-to-point structures.
Abstract: Recent observations of externally illuminated young stellar objects in the Orion Nebula show that they have a variety of morphologies. In particular, long thin tails which terminate in points have been observed. Mechanisms will be considered here to understand the formation and properties of these tails.
TL;DR: In this paper, the authors analyse different methods of searching for planets around neutron stars by timing observations of pulsars and demonstrate that such a model-independent approach leads to promising conclusions that can be useful when analysing timing observations to find planets or to prove that observed phenomena are of planetary origin.
Abstract: ABSTRA C T We analyse different methods of searching for planets around neutron stars by timing observations of pulsars. To this end, we study a few interesting models describing TOA residual variations that are observed, or could be observed, and which can mimic planets. We carry out a detailed theoretical analysis of the behaviour of these methods in the situations mentioned. We show that it is very helpful to look at these phenomena as some kind of quasi-periodic variations of residuals of time of arrival of pulsar pulses. We demonstrate that such a model-independent approach leads to promising conclusions that can be useful when analysing timing observations of pulsars to find planets or to prove that observed phenomena are of planetary origin.
TL;DR: Adaptive Ridge Regression (ARR) as discussed by the authors is an improved method of choosing the departure constant in ridge regression, which is applied in a case that the Earth orientation parameters (EOP) are determined by lunar laser ranging (LLR).
Abstract: The multicollinearity among regression variables is a common phenomenon in the reduction of astronomical data. The phenomenon of multicollinearity and the diagnostic factors are introduced first. As a remedy, a new method, called adaptive ridge regression (ARR), which is an improved method of choosing the departure constant θ in ridge regression, is suggested and applied in a case that the Earth orientation parameters (EOP) are determined by lunar laser ranging (LLR). It is pointed out, via a diagnosis, the variance inflation factors (VIFs), that there exists serious multicollinearity among the regression variables. It is shown that the ARR method is effective in reducing the multicollinearity and makes the regression coefficients more stable than that of using ordinary least squares estimation (LS), especially when there is serious multicollinearity.