Evolutionary status of T Tauri stars     

Olga Eretnova  Alexander Dudorov 《天文和天体物理学研究(英文版)》2018,(8)

The problem of determining the masses and ages of T Tauri star(TTS) using their evolutionary status is discussed. We test four pre-main sequence evolutionary models using well determined observational parameters of 12 binary TTSs and two binary red dwarfs. It is shown that the masses derived using the tracks of all models are in good agreement with the masses obtained from the observations of TTSs with masses M 0.7 M_⊙(mean error ε ~ 10%). Low-mass stars with M ≤ 0.7 M_⊙have significantly greater mean error: ε ~ 50% for the tracks of Bressan et al. and Chen et al., and ε ~ 30% for the other tracks. The isochrones of all tested evolutionary models diverge for stars with masses M ≤ 0.7 M_⊙. The difference increases with the mass decrease and can reach 10% of KelvinHelmholtz time for stars with mass M = 0.2 M_⊙. The ages of most of the considered TTSs are smaller than the Kelvin-Helmholtz time. This confirms their evolutionary status of being pre-main sequence stars.  相似文献   

Martian cratering 12. Utilizing primary crater clusters to study crater populations and meteoroid properties     

William K. Hartmann  Ingrid J. Daubar  Olga Popova  Emily C. S. Joseph 《Meteoritics & planetary science》2018,53(4):672-686

Images from Mars Global Surveyor and later images from Mars Reconnaissance Orbiter reveal that roughly half of the meteoroids striking Mars (at meter to few decameter crater diameters) fragment in the Martian atmosphere, producing small clusters of primary impact craters. Statistics of these “primary clusters” yield valuable information about important Martian phenomena and properties of interplanetary bodies, including meteoroid behavior in the Martian atmosphere, bulk strengths of bodies striking Mars, and the fraction of Martian “field secondary” craters, a datum that would improve crater count chronometry. Many Martian impactors fragment at altitudes significantly higher than 18 km above the mean surface of Mars, and we find that most bodies striking Mars and Earth have low bulk strengths, consistent with crumbly or highly fractured objects. Applying statistics of primary clusters at various elevations and independent diameter bins, we describe a technique to estimate the percentage of semirandomly scattered “field secondary” craters. Our provisional estimate of this percentage, in the diameter range ~250 m down to ~22 m, is ~40% to ~80% of the total impacts, with the higher percentages at smaller diameters. Our data argue against earlier suggestions of overwhelming dominance by either primaries or secondaries in this diameter range.  相似文献   

Timing,magnitude and geochemistry of major Southeast Asian volcanic eruptions: identifying tephrochronologic markers     

Caroline Bouvet De Maisonneuve  Olga Bergal-Kuvikas 《第四纪科学杂志》2020,35(1-2):272-287

We review the current knowledge about Southeast Asian volcanoes and their eruption histories, and focus on identifying tephrochronologic markers representing major explosive eruptions in order to further future palaeoclimate and volcanological studies. Forty-one volcanic edifices in Southeast Asia have been classified as large calderas by Whelley et al. (2015) and thus have, or are likely to have, produced large explosive eruptions with a Volcanic Explosivity Index (VEI) of 6–8. Unfortunately, only 20 such eruptions have known ages, spanning from 1.2 Ma to 1991 ad , and fewer have geochemical data that can be used for tephrostratigraphic correlations. Volcanic products from different geodynamic regions and different sources can generally be distinguished on major element plots (e.g. K₂O versus CaO) of matrix glass composition. However, the distinction of multiple eruptions from the same source often requires additional data such as trace element compositions of matrix glass and/or mineral compositions. Biotite, but also magnetite compositions (MgO and TiO₂ content in particular) appear to be very discriminating. Up to nine tuffs in addition to the three to four Toba tuffs can be utilised as widespread tephrochronologic markers and span a range from 1.2 to 1.6 Ma to recent. As only a few Holocene major eruptions have been well characterised and dated, many large calderas are still unstudied, and many distal tephra layers are still lacking a source, more tephrochronologic markers can certainly be defined in the future.  相似文献   

Determination of Trace Elements (Ag, B, Ge, Mo, Sn, W, Tl) in Reference Samples of Soils and Bottom Sediments by Atomic Emission Spectrometry: A Traceability and Fitness-for-Purpose Study     

Albina I. Kuznetsova  Olga V. Zarubina  Olga A. Sklyarova 《Geostandards and Geoanalytical Research》2007,31(3):251-259

Reference samples of soils from the Institute of Applied Physics, Irkutsk (RIAP), the Institute of Geochemistry, Irkutsk (IGI) and the United States Geological Survey, Reston (USGS) were analysed with the aim of determining Ag, B, Ge, Mo, Sn, Tl and W abundances by an atomic emission method with air-stabilised D.C. arc excitation. Two series of reference samples of soils and bottom sediments, GSS-1-8 and GSD-1-12 (IGGE), were used to ensure the traceability link for the analytical results. Traceability was also demonstrated through the comparison of measured results by AES and ICP-MS methods. It is shown that the reference samples GSS-1-8 and GSD-1-12 satisfied the "fitness-for-purpose" criterion (uncertainty U of the certified value should be one-third to one-tenth the magnitude of routine laboratory data uncertainty S (S/U > 3-10)) and can be applied for calibrating AES techniques.  相似文献   

Recent changes in flood risk in the Gran La Plata,Buenos Aires province,Argentina: causes and management strategy     

María Isabel Andrade  Olga Eugenia Scarpati 《GeoJournal》2007,70(4):245-250

The paper analyses the causes of flood occurrence in the Gran La Plata, Buenos Aires Province, Argentina, and the changes that have occurred since the 1970s. The area is characterized by serious deficiencies in the use and management of water resources. The main factors are: (1) flood risk information is not updated; (2) urban building organization does not consider the flood risk; (3) the topography and other physical features are not taken into account in urban development. This research considers some specific problems, especially in data availability, and suggests ways to solve them, including improved analytical methodology. One of the main objectives is to analyse flood risk in social terms, producing a map of flood risk from the “human social vulnerability” point of view. The results suggest that flood risk has increased since the 1980s and that this is associated with changes in precipitation patterns that have also been aggravated by lack of controls on urban development. The poorer areas with the lowest level of infrastructure and public services, many of them developed over the last quarter-century, are especially vulnerable.  相似文献   

Cenozoic graben facies in the Middle Amur sedimentary basin as inferred from seismic and electrical exploration     

T. V. Merkulova  E. P. Razvozzhaeva 《Russian Journal of Pacific Geology》2007,1(4):335-351

Most of the grabens of the Middle Amur sedimentary basin (MASB) are usually characterized by similar structures typical of continental rifts. Based on models elaborated for the sedimentary infill in well-studied rift basins, seismic facies developed in grabens of the MASB are first analyzed. The analysis revealed seismic facies characteristic of different parts of asymmetrical basins: facies of fans developed on steep slopes and facies of coastal shoals, alluvial plains, and deltas occurring on gentle slopes. Seismic facies peculiar of deep lacustrine settings that are considered promising with respect to oil-and gas-bearing source rocks is recognized. Correlation of seismic and drilling data confirms the correctness of the interpretations and shows that boreholes penetrated a thin sequence of deep-water lacustrine sediments. Using the vertical electrical sounding method, a low-resistivity sequence that is most promising for discovery of lacustrine sediments is defined in the southwestern and eastern parts of the MASB. The lowest values of the specific resistivity in this sequence (7–8 Ohm · m) are recorded in its southeastern part, where a borehole partly recovered a relatively thick silty-clayey sequence.  相似文献   

Neural network based daily precipitation generator (NNGEN-P)   总被引：1，自引：0，他引：1  

Jean-Philippe Boulanger  Fernando Martinez  Olga Penalba  Enrique Carlos Segura 《Climate Dynamics》2007,28(2-3):307-324

Daily weather generators are used in many applications and risk analyses. The present paper explores the potential of neural network architectures to design daily weather generator models. Focusing this first paper on precipitation, we design a collection of neural networks (multi-layer perceptrons in the present case), which are trained so as to approximate the empirical cumulative distribution (CDF) function for the occurrence of wet and dry spells and for the precipitation amounts. This approach contributes to correct some of the biases of the usual two-step weather generator models. As compared to a rainfall occurrence Markov model, NNGEN-P represents fairly well the mean and standard deviation of the number of wet days per month, and it significantly improves the simulation of the longest dry and wet periods. Then, we compared NNGEN-P to three parametric distribution functions usually applied to fit rainfall cumulative distribution functions (Gamma, Weibull and double-exponential). A data set of 19 Argentine stations was used. Also, data corresponding to stations in the United States, in Europe and in the Tropics were included to confirm the results. One of the advantages of NNGEN-P is that it is non-parametric. Unlike other parametric function, which adapt to certain types of climate regimes, NNGEN-P is fully adaptive to the observed cumulative distribution functions, which, on some occasions, may present complex shapes. On-going works will soon produce an extended version of NNGEN to temperature and radiation.  相似文献   

The IRI model residual difference and the new method of N(h)-profile determination     

Olga A. Maltseva  O. S. Poltavsky  A. S. Shlyupkin 《Acta Geophysica》2007,55(3):441-458

  相似文献   

Composition and Flux of Holocene Sediments on the Eastern Laptev Sea Shelf, Arctic Siberia     

Henning A. Bauch  Heidemarie Kassens  Olga D. Naidina  Martina Kunz-Pirrung  Jrn Thiede 《Quaternary Research》2001,55(3):344

A 467-cm-long core from the inner shelf of the eastern Laptev Sea provides a depositional history since 9400 cal yr. B.P. The history involves temporal changes in the fluvial runoff as well as postglacial sea-level rise and southward retreat of the coastline. Although the core contains marine fossils back to 8900 cal yr B.P., abundant plant debris in a sandy facies low in the core shows that a river influenced the study site until 8100 cal yr B.P. As sea level rose and the distance to the coast increased, this riverine influence diminished gradually and the sediment type changed, by 7400 cal yr B.P., from sandy silt to clayey silt. Although total sediment input decreased in a step-like fashion from 7600 to 4000 cal yr B.P., this interval had the highest average sedimentation rates and the greatest fluxes in most sedimentary components. While this maximum probably resulted from middle Holocene climate warming, the low input of sand to the site after 7400 cal yr B.P. probably resulted from further southward retreat of the coastline and river mouth. Since about 4000 cal yr B.P., total sediment flux has remained rather constant in this part of the Laptev Sea shelf due to a gradual stabilization of the depositional regime after completion of the Holocene sea-level rise.  相似文献   

Nuclear activity of NGC 5548 from UBVRI observations during 1990–2000     

V. T. Doroshenko  V. M. Lyuty  N. G. Bochkarev  N. I. Merkulova  S. G. Sergeev  V. V. Vlasyuk  V. P. Mikhailov  O. I. Spiridonova  O. V. Ezhkova  K. N. Grankin  S. Yu. Mel’nikov 《Astronomy Letters》2001,27(11):691-702

  相似文献