排序方式： 共有35条查询结果，搜索用时 15 毫秒
1.
M.S. Hanner E. Tedesco A.T. Tokunaga G.J. Veeder D.F. Lester F.C. Witteborn J.D. Bregman J. Gradie L. Lebofsky 《Icarus》1985,64(1):11-19
The dust coma of Comet P/Churyumov-Gerasimenko was monitored in the infrared (1–20 μm) from September 1982 to March 1983. Maximum dust production rate of ～2 × 105 g/sec occured in December, 1 month postperihelion. The ratio of dust/gas production was higher than that in other short-period comets. No silicate feature was visible in the 8- to 13-μm spectrum on 23 October. The mean geometric albedo of the grains was ～0.04 at 1.25 μm and ～0.05 at 2.2 μm. 相似文献
2.
Dr. Enno Schubert 《Contributions to Mineralogy and Petrology》1959,6(6):388-398
Zusammenfassung
Allen, Crenshaw, Johnston andLarsen (1915) untersuchten den Existenzbereich des Markasits gegenüber thermischer Beanspruchung. Sie konnten eine Modifikationsänderung mit meßbarer Geschwindigkeit his hinunter zur Temperatur von 450° C nachweisen. Umwandlungsversuche bei geringeren Temperaturen waren ergebnislos.Durch Erhitzungsversuche an Erzen von den telethermalen Verdrängungslagerstätten um Aachen wurde gezeigt, daß bei Normaldruck his herab zur Temperatur von etwa 350° C eine Umwandlung mit meßbarer Geschwindigkeit stattfindet. Wahrscheinlich ist also Markasit bei Normaldruck bis 350° C existent gegenüber thermischer Beanspruchung.Die Umwandlungserscheinungen in den verschiedenen Stadien wurden durch Mikrophotographien dargestellt.Als geologisches Thermometer ist das Auftreten von Markasit in natürlichen Paragenesen anwendbar mit folgenden Aussagen:Der mögliche Bildungsbereich der betreffenden Paragenese reicht bei normalem Druck bis zur Temperatur von etwa 350° C.Das Existenzfeld des Markasits gegeniiber rein thermischer Beanspruchung reicht bei normalem Druck bis 350° C, also kann die betreffende Paragenese niemals auf höhere Temperaturen erhitzt gewesen sein.Für die Anregung and fur ihr stets Interesse an meiner Arbeit möchte ich Frau Professor Dr.Schachner and Herrn Doz. Dr.Lindley aufrichtig danken.Auszug aus der Dissertation des Verfassers, TH Aachen 1959. 相似文献
3.
The connection between avalanche dynamics and space physics has been studied for several years. In that context we recently suggested an avalanche model which explains the phenomena of reconnection. In this work the model is generalized to include the influence of an extremely strong perturbation, reflecting the effect of plasma storms originating from the sun. In addition, we allow for diffusion processes and show that the behavior changes with the onset of diffusion processes, rendering it quasi-periodic, along with the supression of small-size avalanches. 相似文献
4.
Witteborn FC Sandford SA Bregman JD Allamandola LJ Cohen M Wooden DH Graps AL 《The Astrophysical journal》1989,341(1):270-277
If the "11.3 microns" emission feature seen in the spectra of many planetary nebulae, H II regions, and reflection nebulae is due to polycyclic aromatic hydrocarbons (PAHs), then additional features should be present between 11.3 and 13.0 microns (885 and 770 cm-1). Moderate-resolution spectra of NGC 7027, HD 44179, IRAS 21282+5050, and BD + 30 degrees 3639 are presented which show that the "11.3 microns" feature actually peaks at 11.22 microns (891 cm-1). The spectra also show evidence of new emission features near 11.9 and 12.7 microns (840 and 787 cm-1). These are consistent with an origin from PAHs and can be used to constrain the molecular structure of the family of PAHs responsible for the infrared features. The observed asymmetry of the "11.3 microns" band is consistent with the slight anharmonicity expected in the C--H out-of-plane bending mode in PAHs. Laboratory experiments show that the intensity of this mode relative to the higher frequency modes depends on the extent of molecular "clustering." The observed strengths of the "11.3 microns" interstellar bands relative to the higher frequency bands are most consistent with the features originating from free molecular PAHs. The intensity and profile of the underlying broad structure, however, may well arise from PAH clusters and amorphous carbon particles. Analysis of the 11-13 microns (910-770 cm-1) emission suggests that the molecular structures of the most intensity emitting free PAHs vary somewhat between the high-excitation environment in NGC 7027 and the low-excitation but high-flux environment close to HD 44179. Finally, a previously undetected series of regularly spaced features between 10 and 11 microns (1000 and 910 cm-1) in the spectrum of HD 44179 suggests that a simple polyatomic hydride is present in the gas phase in this object. 相似文献
5.
Krishna-Swamy KS Sandford SA Allamandola LJ Witteborn FC Bregman JD 《The Astrophysical journal》1989,340(1):527-549
A brief discussion of the infrared observations from 4 to 20 micrometers of seven comets is presented. The observed infrared emission from comets depends primarily on their heliocentric distance. A model based on grain populations composed of a mixture of silicate and amorphous carbon particles in the mass ratio of about 40 to 1, with a power-law size distribution similar to that inferred for comet Halley, is applied to the observations. The model provides a good match to the observed heliocentric variation of both the 10 micrometers feature and the overall thermal emission from comets West and Halley. Matches to the observations of comet IRAS-Araki-Alcock and the antitail of comet Kohoutek require slightly larger grains. While the model does not match the exact profile and position of the 3.4 micrometers feature discovered in comet Halley, it does produce a qualitative fit to the observed variation of the feature's strength as a function of heliocentric distance. The calculations predict that the continuum under the 3.4 micrometers feature is due primarily to thermal emission from the comet dust when the comet is close to the Sun and to scattered solar radiation at large heliocentric distances, as is observed. A brief discussion of the determination of cometary grain temperatures from the observed infrared emission is presented. It is found that the observed shape of the emission curve from about 4 to 8 micrometers provides the best spectral region for estimating the cometary grain temperature distribution. 相似文献
6.
Joel N. Bregman 《Astrophysics and Space Science》2004,289(3-4):181-192
The interstellar medium of the Milky Way possesses a remarkable rangeof components, which have individual lifetimes considerably less than aHubble time. Consequently, there has been a major effort inunderstanding the properties of these components, their heating andcooling mechanisms, and the process by which one component istransformed into another. We review several of the issues in an effort tohighlight controversies that are under study presently. One of the centraldebates involves star formation, where there are two general andconflicting theoretical models. Another issue deals with the efforts tounderstand the fractional amounts of gas in the various phases, whichbrings together star formation with global gas dynamical reactions. Thisinteraction is responsible for heating of the gas by supernovae, causingsuperbubbles, and subsequently, either galactic winds or galacticfountains. The relative amounts of material in the hot and cold phasesnot only depends upon the supernova rate, it also depends upon details ofinterstellar models, such as whether thermal conduction is an importantprocess. We review the progress that people have made onself-consistent modeling of the ISM. The constraints on such models comefrom a wide variety of observations of molecular, neutral atomic, warmionized, and X-ray gas. Both observations and theory are represented atthis meeting and we highlight areas in which contributions are likely toresolve the existing controversies. 相似文献
7.
Stefan J. Wijnholds A. Ger De Bruyn Jaap D. Bregman Jan Geralt Bij De Vaate 《Experimental Astronomy》2004,17(1-3):59-64
The thousand element array (THEA) system is a phased array system consisting of 1 m2 tiles having 64 Vivaldi elements each, arranged on a regular 8-by-8 grid, which has been developed as a demonstrator of technology and applicability for SKA. In this paper we present imaging results of Galactic neutral hydrogen with THEA. Measurements have been taken using a dense 2-by-2 array of four tiles as a four tile adder. The results are compared with results from the Leiden-Dwingeloo Survey, showing qualitative agreement, but also indicating that further studies are needed on the instrumental characteristics. 相似文献
8.
Short dipoles are a key element in new low frequency array antennas as proposed for LOFAR and other astronomical applications. Unfortunately standard texts on short dipole antennas are based on the effective area and do not lead to an astronomically useful sensitivity formulation in a straightforward manner. The concept of maximum effective area is applied to arrays of short dipoles and allows expressing the sensitivity as the ratio of this area over the effective sky brightness temperature as long as the output noise power is dominated by the antenna input radiation. For both quantities we only need to know the array directivity pattern that includes the mutual coupling effects when the actual loading conditions of the array elements are taken into account. Short dipole elements have a constant directivity pattern for frequencies below resonance, but they exhibit strong complex impedance variations that provide only narrow band performance when power matching is applied as required in transmit applications. However, in receive applications voltage or current sensing can be realized, for example with an active balun. Assisted by the steep increase of the sky brightness with wavelength for frequencies below 300 MHz, this can provide sky noise dominated performance over at least a three to one frequency range. Still the low frequency limit is determined by the amplifier noise contribution and the losses in the antenna and in the dielectric ground surrounding the elements. We show that for a sparse array with the elements non-uniformly distributed according to an exponential shell model, a constant sensitivity can be obtained over a frequency range of at least two octaves. In addition, such a configuration has a factor of six greater sensitivity than a rectangular array for a large part of the frequency band. 相似文献
9.
Bacteriohopanepolyols (BHPs) are lipid constituents of many bacterial groups. Geohopanoids, the diagenetic products, are therefore ubiquitous in organic matter of the geosphere. To examine the potential of BHPs as environmental markers in marine sediments, we investigated a Holocene sediment core from the Black Sea. The concentrations of BHPs mirror the environmental shift from a well-mixed lake to a stratified marine environment by a strong and gradual increase from low values (∼30 μg g−1 TOC) in the oldest sediments to ∼170 μg g−1 TOC in sediments representing the onset of a permanently anoxic water body at about 7500 years before present (BP). This increase in BHP concentrations was most likely caused by a strong increase in bacterioplanktonic paleoproductivity brought about by several ingressions of Mediterranean Sea waters at the end of the lacustrine stage (∼9500 years BP). δ15N values coevally decreasing with increasing BHP concentrations may indicate a shift from a phosphorus- to a nitrogen-limited setting supporting growth of N2-fixing, BHP-producing bacteria. In sediments of the last ∼3000 years BHP concentrations have remained relatively stable at about 50 μg g−1 TOC.The distributions of major BHPs did not change significantly during the shift from lacustrine (or oligohaline) to marine conditions. Tetrafunctionalized BHPs prevailed throughout the entire sediment core, with the common bacteriohopanetetrol and 35-aminobacteriohopanetriol and the rare 35-aminobacteriohopenetriol, so far only known from a purple non-sulfur α-proteobacterium, being the main components. Other BHPs specific to cyanobacteria and pelagic methanotrophic bacteria were also found but only in much smaller amounts.Our results demonstrate that BHPs from microorganisms living in deeper biogeochemical zones of marine water columns are underrepresented or even absent in the sediment compared to the BHPs of bacteria present in the euphotic zone. Obviously, the assemblage of molecular fossils in the sediments does not represent an integrated image of the entire community living in the water column. Remnants of organisms living in zones where effective transport mechanisms - such as the fecal pellet express - exist are accumulated while those of others are underrepresented. Our work shows a high stability of BHPs over geological time scales. Largely uniform distributions and only minor changes in structures like an increasing prevalence of saturated over unsaturated BHPs with time were observed. Consequently, sedimentary BHP distributions are less suitable as markers for in situ living bacteria but are useful for paleoreconstructions of bacterioplanktonic communities and productivity changes. 相似文献
10.
Sebastiaan W. Rampen Stefan Schouten Enno Schefuß Jaap S. Sinninghe Damst 《Organic Geochemistry》2009,40(11):1124-1131
Long chain 1,14-diols and 12-hydroxy methyl alkanoates are biomarker lipids for Proboscia diatoms and occur widely in Quaternary sediments. To determine the effect of temperature on the lipid composition of these algae, a new Proboscia sp. culture grown at 8 °C and Proboscia indica cultures grown at 18, 21, 24 and 27 °C were examined. The results were combined with lipid data from a P. indica culture and a Proboscia alata culture, grown at 20 and 2 °C, respectively, from previous studies. The data showed a strong relationship between long chain diol and 12-hydroxy methyl alkanoate composition and growth temperature, i.e. the chain length increases and the degree of unsaturation of long chain 1,14-diols decreases with increasing growth temperature. To determine the effect of temperature on Proboscia lipid compositions in natural environments, we also analyzed fossil long chain 1,14-diols and 12-hydroxy methyl alkanoates in surface sediments derived from Proboscia diatoms living in the water column of the eastern South Atlantic. The results indicate a significant relationship between sea surface temperature and chain length distribution of saturated long chain diols, but also suggest that the relative abundances of unsaturated long chain diols and 12-hydroxy methyl alkanoates in sediments are predominantly determined by factors other than temperature. 相似文献