Transit or occultation?     

Daniel M. Popper 《Astrophysics and Space Science》1976,45(2):391-396

Published discussions of photometric observations of VZ Hya and CM Lac disagree as to whether the deeper eclipses are transits or occultations. The evolutionary histories of these systems are critically dependent on a resolution of the disagreement. The luminosity ratio of the components of VZ Hya, inferred from spectrograms, shows unequivocally that the deeper eclipse is a transit. The case of CM Lac is less definitive. While the photometric observations favor primary minimum as a transit, the ratio of the radii is close to unity, and neither the spectrograms nor the photometry leads to an unequivocal result. Some confusion in notation appearing in a discussion of the components of EI Cep is noted. In each of the three cases the observational evidence favors a straightforward evolutionary interpretation in which the more massive star is the larger.  相似文献   

Correlating fracture trends and hydraulic head using semivariogram cloud analysis     

Kurtzman D  Nativ R  Adar EM 《Ground water》2005,43(2):250-258

Fracture trends (defined as kilometer-scale linear features interpolated between field observations of fractures along their strikes) often have a dominant orientation. Finding a correlation between this orientation and hydraulic data could shed light on their hydraulic influence. A significant correlation between head residuals from first-order regional drift and the orientation of 2- to 4-km-long fracture trends was found in a study site in the Negev, Israel, using the semivariogram cloud analysis. Correlation of head residuals rather than the head itself implies that the orientation of the fracture trends controls the anisotropy and heterogeneity at this scale, mainly because the fracture trends define the orientation of blocks, which differ in their hydraulic properties. Preferential transmissive pathways are probably shorter than the full extent of the fracture trends, causing a relatively high head difference along the trends on the 2- to 4-km scale. Fracture trend density and additional data from short-range hydraulic tests helped characterize two blocks separated by a fault zone. The identification of hydraulic features on a kilometer scale is necessary for better modeling of regional ground water flow and transport. Hydraulic tests at this scale are not feasible, thereby rendering combined analyses of head and structural data, such as the one presented here, essential.  相似文献   

Compositional effects on element partitioning between Mg-silicate perovskite and silicate melts     

Christian?LiebskeEmail author  Alexandre?Corgne  Daniel?J.?Frost  David?C.?Rubie  Bernard?J.?Wood 《Contributions to Mineralogy and Petrology》2005,149(1):113-128

High-pressure melting experiments were performed at ~26 GPa and ~2,200–2,400°C on synthetic peridotite compositions with varying FeO and Al₂O₃ contents and on a synthetic CI chondrite analogue composition. Peridotite liquids show a crystallisation sequence of ferropericlase (Fp) followed down temperature by Mg-silicate perovskite (MgPv) + Fp, which contrasts a sequence of MgPv followed by MgPv + Fp observed in the chondritic composition. The difference in crystallisation sequence is a consequence of the different bulk Mg/Si ratios. MgPv/melt partition coefficients for major, minor and trace elements were determined by electron microprobe and secondary ion mass spectrometry. Partition coefficients of tri- and tetravalent elements increase with increasing Al concentration in MgPv. A lattice strain model indicates that Al³⁺ substitutes predominantly onto the Si-site in MgPv, whereas most elements substitute onto the Mg-site, which is consistent with a charge-compensating coupled substitution mechanism. MgPv/melt partition coefficients for Mg (D_Mg) and Si (D_Si) are related to the melt Mg/Si ratio such that D_Si becomes lower than D_Mg at low Mg/Si melt ratios. We use a crystal fractionation model, based on upper mantle refractory lithophile element ratios, to constrain the amount of MgPv and Ca-silicate perovskite (CaPv) that could have fractionated during a Hadean magma ocean event and could still be present as a chemically distinct heterogeneity in the lower mantle today. We show that a fractionated crystal pile composed of 96% MgPv and 4% CaPv could comprise up to 13 wt% of the entire mantle.  相似文献   

A Widebinary Solar Companion as a Possible Origin of Sedna-like Objects     

John J. Matese  Daniel P. Whitmire  Jack J. Lissauer 《Earth, Moon, and Planets》2005,97(3-4):459-470

Sedna is the first inner Oort cloud object to be discovered. Its dynamical origin remains unclear, and a possible mechanism is considered here. We investigate the parameter space of a hypothetical solar companion which could adiabatically detach the perihelion of a Neptune-dominated TNO with a Sedna-like semimajor axis. Demanding that the TNO’s maximum value of osculating perihelion exceed Sedna’s observed value of 76 AU, we find that the companion’s mass and orbital parameters (m _c , a _c , q _c , Q _c , i _c ) are restricted to $$m_c>rapprox 5\hskip.25em\hbox{M}_{\rm J}\left(\frac{Q_c}{7850\hbox{ AU}} \frac{q_c}{7850\hbox{ AU}}\right)^{3/2}$$ during the epoch of strongest perturbations. The ecliptic inclination of the companion should be in the range $45{\deg}\lessapprox i_c\lessapprox 135{\deg}$ if the TNO is to retain a small inclination while its perihelion is increased. We also consider the circumstances where the minimum value of osculating perihelion would pass the object to the dynamical dominance of Saturn and Jupiter, if allowed. It has previously been argued that an overpopulated band of outer Oort cloud comets with an anomalous distribution of orbital elements could be produced by a solar companion with present parameter values $$m_c\approx 5\hskip.25em\hbox{M}_{\rm J}\left(\frac{9000\hbox{ AU}}{a_c}\right)^{1/2}.$$ If the same hypothetical object is responsible for both observations, then it is likely recorded in the IRAS and possibly the 2MASS databases.  相似文献   

An Evolutionary Turbulent Model of Saturn's Subnebula: Implications for the Origin of the Atmosphere of Titan     

Olivier Mousis  Daniel GautierDominique Bockelée-Morvan 《Icarus》2002,156(1):162-175

We have elaborated an evolutionary turbulent model of the subnebula of Saturn derived from that of Dubrulle (1993, Icarus106, 59-76) for the solar nebula, which is valid for a geometrically thin disk. We demonstrate that if carbon and nitrogen were in the form of CO and N₂, respectively, in the early subnebula, these molecules were not subsequently converted into CH₄ and NH₃ during the evolution of the disk, contrary to the current scenario initially proposed by Prinn and Fegley (1981, Astrophys. J., 249, 308-317). However, if the early subnebula contained some CH₄ and NH₃, these gases were not subsequently converted into CO and N₂. We argue that Titan must have been formed from planetesimals migrating from the outer part of the subnebula to the present orbit of the satellite. These planetesimals were relics of those embedded in the feeding zone of Saturn prior to the completion of the planet and contained hydrates of NH₃ and clathrate hydrates of CH₄. It is shown that, for plausible abundances of CH₄ and NH₃ in the solar nebula at 10 AU, the masses of methane and nitrogen trapped in Titan were higher than the estimate of masses of these components in the primitive atmosphere of the satellite. If our scenario is valid and if our turbulent model properly describes the structure and the evolution of the actual subnebula of Saturn, the Xe/C ratio should be six times higher in Titan's atmosphere today than in the Sun, while the current scenario would probably result in a quasi solar Xe/C ratio. The mass spectrometer and gas chromatograph instrument aboard the Huygens Titan probe of the Cassini mission has the capability of measuring this ratio in 2004, thus permitting us to discriminate between the current scenario and the one proposed in this report.  相似文献   

A possible new molecular mechanism of thundercloud electrification   总被引：1，自引：1，他引：1  

Pavel Jungwirth  Daniel Rosenfeld  Victoria Buch 《Atmospheric Research》2005,76(1-4):190

Thunderclouds are electrified when charge is transferred between small and large ice particles colliding in a cloud that contains strong updrafts. The small ice particles rise with one type of charge and the large ice particles fall and carry with them downward the other type of charge, which is most often negative, so that normally lightning lowers negative charge from cloud to the ground. While the collisional mechanism of thundercloud charging is well established, the nature of the charge transfer between the colliding ice particles is not very well understood on the atomic level, and no present theory can explain it in full detail. Here we propose a new charge separation mechanism that is based on molecular dynamics simulations of particle surfaces and collisions, keeping track of the individual charges as they move in the form of salt ions from one ice particle to another. Under normal conditions, when sulfates dominate as cloud condensation nuclei, this ionic mechanism is consistent with the prevailing negative charging of graupels in thunderclouds. Moreover, with dearth of sulfate anions, the present mechanism predicts a shift towards positive charging. This fits well to a large range of observations of enhanced positive lightning, connected with smoke rich in chlorides and nitrates, that could not be explained satisfactorily previously.  相似文献   

Precipitation and evaporation budgets over the Baltic Proper: observations and modelling     

Ann-Sofi Smedman  Sven-Erik Gryning  Karl Bumke  Ulf H  gstr  m  Anna Rutgersson  Ekaterina Batchvarova  Gerhard Peters  Barbara Hennemuth  Bengt Tammelin  Reijo Hyv  nen  Anders Omstedt  Daniel Michelson  Tage Andersson  Marco Clemens 《Journal of Atmospheric & Ocean Science》2005,10(3):163-191

Precipitation and evaporation budgets over the Baltic Sea were studied in a concerted project called PEP in BALTEX (Pilot study of Evaporation and Precipitation in the Baltic Sea), combining extensive field measurements and modelling efforts. Eddy-correlation-measurements of turbulent heat flux were made on a semi-continuous basis for a 12 month period at four well-exposed coastal sites in the Baltic Proper (the main basin of the Baltic Sea). Precipitation was measured at land-based sites with standard gauges and on four merchant ships travelling between Germany and Finland with the aid of specially designed ship rain gauges (SRGs). The evaporation and precipitation regime of the Baltic Sea was modelled for a 12 month period by applying a wide range of numerical models: the operational atmospheric High Resolution Limited Area Model (HIRLAM, Swedish and Finnish versions), the German atmospheric REgional-scale MOdel, REMO, the operational German Europe Model (only precipitation), the oceanographic model PROBE-Baltic, and two models that use interpolation of ground-based data, the Swedish MESAN model of SMHI and a German model of IFM-GEOMAR Kiel. Modelled precipitation was compared with SRG measurements on board the ships. A reasonable correlation was obtained, but the regional-scale models and MESAN gave some 20% higher precipitation over the sea than is measured. Bulk parameterisation schemes for evaporation were evaluated against measurements. A constant value of C_HN and C_EN with wind speed, underestimated large fluxes of both sensible and latent heat flux. The limited area models do not resolve the influence of the height of the marine boundary layer in coastal zones and the entrainment (on the surface fluxes), which may explain the observed low correlations between modelled and measured latent heat fluxes. Estimates of evaporation, E, and precipitation, P, for the entire Baltic Proper were made with several models for a 12 month period. While the annual variation was well represented by all predictions, there are still important differences in the annual means. Evaporation ranges from 509 to 625 mm year^-1 and precipitation between 624 and 805 mm year^-1 for this particular 12 month period. Taking the results of model verification from the present study into account, the best estimate of P-E is about 100 ± 50 mm for this particular 12 month period. But the annual mean of P-E varies considerably from year to year. This is reflected in simulations with the PROBE-Baltic model for an 18 year period, which gave 95 mm year^-1 for the 12 month period studied here and 32 mm year^-1 as an average for 18 years.  相似文献   

Stratospheric Photolysis Frequencies: Impact of an Improved Numerical Solution of the Radiative Transfer Equation     

Gaby Becker  Jens-Uwe Grooss  Daniel S. McKenna  Rolf Müller 《Journal of Atmospheric Chemistry》2000,37(3):217-229

Numerical schemes for the calculation of photolysis rates are usually employed in simulations of stratospheric chemistry. Here, we present an improvement of the treatment of the diffuse actinic flux in a widely used stratospheric photolysis scheme (Lary and Pyle, 1991). We discuss both the consequences of this improvement and the correction of an error present in earlier applications of this scheme on the calculation of stratospheric photolysis frequencies. The strongest impact of both changes to the scheme is for small solar zenith angles. The effect of the improved treatment of the diffuse flux is most pronounced in the lower stratosphere and in the troposphere. Overall, the change in the calculated photolysis frequencies in the region of interest in the stratosphere is below about 20%, although larger deviations are found for H₂O, O₂, NO, N₂O, and HCl.  相似文献   

The 1996 and 1998 subglacial eruptions beneath the Vatnajökull ice sheet in Iceland: contrasting geochemical and geophysical inferences on magma migration     

O. Sigmarsson  H. R. Karlsson  G. Larsen 《Bulletin of Volcanology》2000,61(7):468-476

87 Sr/⁸⁶Sr (0.70322) and δ ¹⁸O ( ∼2.9‰), whereas significantly lower and higher values, respectively, are found in samples from the Bárdarbunga volcanic system (0.70307 and 3.8‰). These results strongly indicate that the Gjálp magma originated from the Grímsv?tn magma system. The 1996 magma is of an intermediate composition, representing a basaltic icelandite formed by 50% fractional crystallization of a tholeiite magma similar in composition to that expelled by the 1998 Grímsv?tn eruption. The differentiation that produced the Gjálp magma may have taken place in a subsidiary magma chamber that last erupted in 1938 and would be located directly beneath the 1996 eruption site. This chamber was ruptured when a tectonic fracture propagated southward from Bárdarbunga central volcano, as indicated by the seismicity that preceded the eruption. Our geochemical results are therefore not in agreement with lateral magma migration feeding the 1996 Gjálp eruption. Moreover, the results clearly demonstrate that isotope ratios are excellent tracers for deciphering pathways of magma migration and permit a clear delineation of the volcanic systems beneath Vatnaj?kull ice sheet. Received: 1 April 1998 / Accepted: 17 August 1999  相似文献   

Black Sea chemocline oscillations during the Holocene: molecular and isotopic studies of marginal sediments     

Yongsong Huang  Katherine H. Freeman  Richard T. Wilkin  Michael A. Arthur  A. Daniel Jones 《Organic Geochemistry》2000,31(12):642

We measured δ¹³C values of free and sulfur-bound lipids and framboidal pyrite-size distributions in three sediment cores from the southern margins of the Black Sea. The margin cores show a marked difference in the occurrence of biomarkers from green sulfur bacteria compared with the deep-basin cores, as a result of deepening of the chemocline resulting from enhanced mixing and/or decreased light-penetration as a consequence of high turbidity and productivity in shelf waters. Quantitation of biomarkers suggests that photic-zone anoxia along the shelf margin was generally absent during the deposition of unit I, although occurred during the deposition of Unit IIb at two sites.  相似文献