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51.
U. de Angelis L. De Cesare A. Forlani G. Platania 《Astrophysics and Space Science》1974,27(1):117-135
The consequences of gas-liquid phase transitions in the core of hot white dwarf stars are discussed. Expressions for the latent heat and the liquefaction curveT
l
=T
l
(Q) are obtained. Then amodel for a hot white dwarf is introduced and the corresponding liquefaction sequences are built on the H-R diagram; relations luminosity-central temperature and effective temperature-central temperature are also given for liquefying white dwarfs.Finally the cooling curves are obtained for such stars taking into account the effect of latent heat emission.Our results seem to suggest a possible identification of the central stars of planetary nebulae as hot liquefying white dwarfs. 相似文献
52.
Roberto Avigliano Giulio Di Anastasio Salvatore Improta Marco Peresani Cesare Ravazzi 《第四纪科学杂志》2000,15(8):789-803
A late glacial to early Holocene lacustrine and peat succession, rich in conifer remains and including some palaeolithic flint artefacts, has been investigated in the Palughetto intermorainic basin (Venetian Pre‐Alps). The geomorphological and stratigraphical relationships, 14C dates and pollen analyses allow a reconstruction of the environmental history of the basin and provide significant insights into the reforestation and peopling of the Pre‐Alps. The onset of peat accumulation is dated to 14.4–14.1 kyr cal. BP, coinciding with reforestation at middle altitudes that immediately post‐dates the immigration of Larix decidua and Picea abies subsp. europaea. Plant macrofossils point to the expansion of spruce about 14.3 kyr cal. BP, so far one of the earliest directly dated in the late glacial period of southern Europe. The previous hypothesis of an early Holocene spruce immigration in the Southern Alps from Slovenia needs reconsideration. Organic sedimentation stopped at the end of the Younger Dryas and was followed by the evolution of hydromorphic soils containing lithic artefacts, anthropic structures and wood charcoal. The typological features of the flint implements refer human occupation of the site to the end of the recent Epigravettian. Charcoals yielded dates either consistent with, or younger than, the archaeological chronology, in the early and middle Holocene. Copyright © 2000 John Wiley & Sons, Ltd. 相似文献
53.
Fluid-present anatexis of metapelites at El Joyazo (SE Spain): constraints from Raman spectroscopy of graphite 总被引:2,自引:0,他引:2
The garnet-biotite-sillimanite anatectic xenoliths in the Neogene dacite dome of El Joyazo (also called Cerro de Hoyazo,
SE Spain) contain four types of graphite (I to IV), distinguished on the basis of grain size and texture. Structural characterization
of graphite by Laser Raman spectroscopy (LRS) shows systematic differences in the degree of ordering among the four types:
only type III is fully consistent with the granulite-facies conditions reached by the xenoliths during partial melting, the
others indicate metamorphic temperatures covering amphibolite-facies conditions, with only a few examples of granulite-grade
crystallinity. All graphite crystallized before or during the anatectic event, indicating that a large fraction of the graphite
did not equilibrate at peak temperatures. The mm-scale coexistence of different types and degrees of ordering in the graphite
suggests different origins, i.e. of biogenic derivation and “fluid-deposited”, and is explained in terms of fluid-melt-graphite
interaction during the anatectic event. Disequilibrium behaviour during high-temperature metamorphism and anatexis is typical
of types I, II and particularly of IV, and is attributed to sluggish kinetics of solid-state graphitization, mainly owing
to the limited time of the process and carbon saturation of the intergranular fluid. The coexisting, well-ordered type III
graphite is the product of melting in the presence of a graphite-saturated fluid, a process that would account for the deposition
of new graphite. The LRS results, together with petrologic observations, suggest that it is possible that high melt fractions
can be generated by fluid-present melting of a metasedimentary protolith also in a closed system. Although this contradicts
the commonly accepted hypothesis that, due to limited rock porosities, extensive fluid-present melting is precluded unless
infiltration occurs, it is a possible end-member model in anatectic settings characterized by rapid heating rates and low-grade
source rocks.
Received: 14 July 1998 / Accepted: 16 November 1998 相似文献
54.
Cesare Aureli 《Pure and Applied Geophysics》1947,10(1-2):181-191
Riassunto Si calcolano — mediante la rappresentazione gratica degli archi diurni descritti dal Sole nel suo apparente moto diurno al disopra dell'orizzonte topografico visto dall'Osservatorio Meteorologico del Villaggio Sanatoriale di Sondalo — gli azimut, le altezze, gli istanti del sorgere e tramontare astronomico e topografico del Sole, mettendo in evidenza la decurtazione che il soleggiamento viene a subire a causa dei rilievi montuosi della località.Si determina, valendosi della diretta osservazione, la distribuzione del soleggiamento effettivamente possibile in diversi punti del Villaggio.Si esaminano infine i coefficienti di soleggiamento mettendoli in rapporto con la nebulosità osservata sulla località nel periodo di tempo 1940–44.
Zusammenfassung Der Verf. hat die effektiv mögliche Sonnenscheindauer in verschiedenen Punkten des «Villaggio Sanatoriale di Sondalo» berechnet und auch direkt gemessen. Die Ergebnissen werden verglichen und erörtet. Folgt eine Prüfung der Zusammenhang mit der Verteilung der Bewölkung.相似文献
55.
Correlating Alpine glaciation with Adriatic sea‐level changes through lake and alluvial stratigraphy
Giovanni Monegato Roberta Pini Cesare Ravazzi Paula J. Reimer Lucia Wick 《第四纪科学杂志》2011,26(8):791-804
We compare lake and alluvial stratigraphy along a frame connecting the southern Alpine foothills and the Adriatic Sea, with the aim of matching the effects of Alpine glaciation and sea‐level changes on sedimentation during the last glacial cycle. The palynostratigraphy of Lake Fimon provided proxies for regional vegetation and climate change and was coupled with sediment petrography, loss on ignition and magnetic susceptibility, disentangling alluvial phases from fluvioglacial activity related to culminations of the southeastern Alpine glaciers. The Fimon area was not reached by alluvial fans during the penultimate glacial maximum, nor by the sea transgression during the last interglacial, but a closed lake soon developed at the Eemian onset due to enhanced rainfall. Sea‐level fall at glacial inception triggered the entrenchment of the drainage network in the plain reaching the outer Fimon Basin. Slow aggradation, but no sign of fluvioglacial activity, lasted to 38.2 ± 1.45 cal. ka BP, when a major forest withdrawal took place, coeval to the spread of alluvial fans. By 27.5 ± 0.5 cal. ka BP the Fimon Basin was dammed by the Brenta outwash system. The main step of of forest recovery commenced at around (15.8) cal. ka BP, when apex trenching of the outwash fans was triggered by the glacier's decay. Copyright © 2011 John Wiley & Sons, Ltd. 相似文献
56.
Madhusoodhan Satish-Kumar Hisayoshi Yurimoto Shoichi Itoh Bernardo Cesare 《Contributions to Mineralogy and Petrology》2011,162(4):821-834
High-spatial resolution carbon isotope analyses of natural graphite using secondary ion mass spectrometry (SIMS), together
with conventional mass spectrometry techniques, demonstrate isotopic heterogeneity within single graphite crystals precipitated
from a partially melted metamorphic rock. SIMS 13C/12C measurements were calibrated using an internal graphite standard previously analyzed by conventional isotope ratio mass
spectrometry, which gave a reproducibility of 0.3‰ (1σ) at a spatial resolution of 2–3 μm. This resolution helped to identify
an unusual carbon isotope distribution in a single graphite crystal from a metapelitic leucosome, showing remarkable core
to rim variations with sharp δ13C steps up to 10‰. The results suggest that the graphite crystal grew from one edge to other forming layers perpendicular
to the c-axis. The sharp isotopic steps indicate the presence of disequilibrium carbon isotope zoning in graphite and points to the
possible existence of carbon isotope sector zoning. Intra-crystalline carbon isotope disequilibrium in graphite is believed
to have resulted from the difference in diffusivity between 12C and 13C in the growth medium to the interface of graphite precipitation in different growth sectors. 相似文献
57.
18O/16O and14C analysis of deep-sea cores from the Gulf of Mexico revealed a 2.4% isotopic anomaly between 12,000 and 11,000 years ago. This value, together with estimates of the oxygen isotopic composition of ice meltwater and rates of mixing within the Gulf of Mexico indicates that the average yearly discharge of the Mississippi River was between 100,000 and 230,000 m3/s (as compared to 57,000 m3/s for the peak flood of February 17, 1937), with probable yearly peak floods twice as large. Corresponding sea-level rise was between 1 and 2 m per century. 相似文献
58.
Nicola Gretter Ausonio Ronchi Antonio Langone Cesare R. Perotti 《International Journal of Earth Sciences》2013,102(5):1181-1202
The Val Daone Conglomerate (VDC) is a continental clastic unit that crops out eastwards of the central Southern Alps, from the NE sector of the Collio Basin to the W as far as the Tione Basin to the E. This significant but as yet relatively unknown formation lies just above the regional unconformity that marks the boundary between the two Permian major tectono-sedimentary cycles (TSU1 and TSU2) and grades upwards paraconformably (?) to the fluvial red beds of Verrucano Lombardo/Val Gardena Sandstone, generally associated with Late Permian times. Recent palynological investigations on the VDC suggested a Guadalupian age (late Roadian–early Wordian), owing to the remarkable presence of diversified pollen associations; therefore, this sedimentary unit is to date the first one ascribed, on a palaeontological basis, to the Middle Permian in the entire Southern Alps domain. A detailed facies analysis of VDC shows deposition in amalgamated alluvial fan-braided and fluvial environments with wide channels and longitudinal bars. In its type area and Val Rendena, the VDC rests unconformably above the last volcanic episode of the TSU1. LA ICP-MS U–Pb dating on zircon from two samples of such topmost Lower Permian volcanic rocks, known as Ponte Murandin dacitic lava and Malga Plan rhyodacitic Ignimbrites (Tione Basin), provided Concordia ages of 278 ± 2 Ma (MSWD = 0.01) and 279 ± 2 Ma (MSWD = 0.16), respectively. As well as allowing us to better define the duration of the time gap between the two Permian megacycles in the central Southern Alps in almost 10 Ma, these radiometric age determinations are also significant because they enable us to regionally link the coeval volcanic bodies which crop out in the Collio Basin to the SW and in the “Athesian Volcanic Group” to the NE, respectively. 相似文献
59.
60.
Cesare Cecchi-Pestellini & David A. Williams 《Monthly notices of the Royal Astronomical Society》1998,296(2):414-418
High brightness temperatures ( T b ) implied by quasar intraday variability may be explained by coherent emission, or else by physically implausible bulk relativistic Lorentz factors Γ ≥ 100. Previous theory asserts that various absorption mechanisms will block escape of such coherent, high-brightness sources. Yet this same theory fails to account for laboratory experiments detecting collective emission. Probably this is because present theory is inadequate, and should not be used to rule out collective radiation processes. 相似文献