The orbit of the nearby low-mass binary Gliese 600     

A. A. Tokovinin  Yu. Yu. Balega  K.-H. Hofmann  G. Weigelt 《Astronomy Letters》2000,26(10):668-671

We have computed a combined spectroscopic-interferometric orbit for the nearby binary Gliese 600 discovered by us. The orbital period is 2.78 years, and the semimajor axis is 100 mas (0.1″). Its M0 V components are almost identical and have a mass of 0.5M _⊙. The mass ratio is uncertain because of the low radialvelocity semiamplitude (7 km s^?1) associated with the low orbital inclination (37°). The orbital parallax of the binary (52±11 mas) matches its dynamical and photometric parallaxes but differs significantly from the Hipparcos parallax (44.3±1.6) mas; the latter was probably distorted by the orbital motion that was not taken into account.  相似文献   

Geostrophic Wind, Gradient Wind, Thermal Wind And The Vertical Wind Profile – A Sample Analysis Within A Planetary Boundary Layer Over Arctic Sea-Ice     

R. Roth  M. Hofmann  C. Wode 《Boundary-Layer Meteorology》1999,92(2):327-339

For the interpretation of many boundary-layer field experiments the geostrophic wind is needed as an external parameter. However, quite often and especially in remote areas this wind is not known at all or difficult to determine because there are not enough measurements of the surface pressure.Here it is shown how measurements carried out with the HELIPOD system, a helicopter-borne meteorological turbulence measuring system, may be used to evaluate the geostrophic wind. This is done by the analysis of the pressure field at different heights. An additional analysis of the temperature field in the same heights allows for the discussion of the quality of the derived geostrophic wind. An intercomparison with the vertical wind profile enables us to discuss the influence of the curvature of the isobars. From this, finally, also the gradient wind can be estimated.  相似文献   

Facies architecture of an isolated carbonate platform: tracing the cycles of the Latemàr (Middle Triassic, northern Italy)     

Sven O. Egenhoff  Arndt Peterhänsel  Thilo Bechstädt  Rainer Zühlke  & Jürgen Grötsch 《Sedimentology》1999,46(5):893-912

The 720-m-thick succession of the Middle Triassic Latemàr Massif (Dolomites, Italy) was used to reconstruct the lagoonal facies architecture of a small atoll-like carbonate platform. Facies analysis of the lagoonal sediments yields a bathymetric interpretation of the lateral facies variations, which reflect a syndepositional palaeorelief. Based on tracing of lagoonal flooding surfaces, the metre-scale shallowing-upward cycles are interpreted to be of allocyclic origin. Short-term sea-level changes led to subaerial exposure of wide parts of the marginal zone, resulting in the development of a tepee belt of varying width. Occasional emergence of the entire lagoon produced lagoon-wide decimetre-thick red exposure horizons. The supratidal tepee belt in the backreef area represented the zone of maximum elevation, which circumscribed the sub- to peritidal lagoonal interior during most of the platform's development. This tepee rim, the subtidal reef and a sub- to peritidal transition zone in between stabilized the platform margin. The asymmetric width of facies belts within individual metre-scale cycles was caused by redistribution processes that reflect palaeowinds and storm paths from the present-day south and west. The overall succession shows stratigraphic changes on a scale of tens of metres from a basal subtidal unit, overlain by three tepee-rich intervals, separated by tepee-poor units composed of subtidal to peritidal facies. This stacking pattern reflects two third-order sequences during the late Anisian to early middle Ladinian.  相似文献   

Flares in Sigmoidal Coronal Structures – a Case Study     

Aurass  H.  Vršnak  B.  Hofmann  A.  Rudžjak  V. 《Solar physics》1999,190(1-2):267-293

We analyze radio observations, magnetograms and extrapolated field line maps, Hα filtergrams, and X-ray observations of two flare events (6 February 1992 in AR 7042 and 25 October 1994 in AR 7792) and study properties, evolution and energy release signatures of sigmoidal loop systems. During both events, the loop configuration seen in soft X-ray (SXR) images changes from a preflare sigmoidal shape to a relaxed post-flare loop system. The underlying magnetic field system consists of a quadrupolar configuration formed by a sheared arcade core and a remote field concentration. We demonstrate two possibilities: a sigmoidal SXR pattern can be due to a single continuous flux tube (the 1992 event). Alternatively, it can be due to a set of independent loops appearing like a sigmoid (the 1994 event). In both cases, the preflare and post-flare loops can be well reproduced by a linear force-free field and potential field, respectively, computed using preflare magnetograms. We find that thermal and non-thermal flare energy release indicators of both events become remarkably similar after applying spatial and temporal scale transformations. Using the spatial scaling between both events we estimated that the non-thermal energy release in the second event liberated about 1.7 times more energy per unit volume. A two-and-a-half times faster evolution indicates that the rate of the energy release per unit volume is more than four times higher in this event. A coronal type II burst reveals ignition and propagation of a coronal shock wave. In contrast, the first event, which was larger and released about a 10 times more energy during the non-thermal phase, was associated with a CME, but no type II burst was recorded. During both events, in addition to the two-ribbon flare process an interaction was observed between the flaring arcade and an emerging magnetic flux region of opposite polarity next to the dominant leading sunspot. The arcade flare seems to stimulate the reconnection process in an `emerging flux-type' configuration, which significantly contributes to the energy release. This regime is characterized by the quasiperiodic injection of electron beams into the surrounding extended field line systems. The repeated beam injections excite pulsating broadband radio emission in the decimetric-metric wavelength range. Each radio pulse is due to a new electron beam injection. The pulsation period (seconds) reflects the spatial scale of the emerging flux-type field configuration. Since broadband decimetric-metric radio pulsations are a frequent radio flare phenomenon, we speculate that opposite-polarity small-scale flux intrusions located in the vicinity of strong field regions may be an essential component of the energy release process in dynamic flares.  相似文献   

Design study of the PEPSI polarimeter for the LBT     

A. Hofmann  K.G. Strassmeier  M. Woche 《Astronomische Nachrichten》2002,323(5):510-519

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Accuracy of tretyakov precipitation gauge: Result of wmo intercomparison     

Daqing Yang  Barry E. Goodison  John R. Metcalfe  Valentin S. Golubev  Esko Elomaa  Thilo Gunther  Roy Bates  Timothy Pangburn  Clayton L. Hanson  Douglas Emerson  Voilete Copaciu  Janja Milkovic 《水文研究》1995,9(8):877-895

The Tretyakov non-recording precipitation gauge has been used historically as the official precipitation measurement instrument in the Russian (formerly the USSR) climatic and hydrological station network and in a number of other European countries. From 1986 to 1993, the accuracy and performance of this gauge were evaluated during the WMO Solid Precipitation Measurement Intercomparison at 11 stations in Canada, the USA, Russia, Germany, Finland, Romania and Croatia. The double fence intercomparison reference (DFIR) was the reference standard used at all the Intercomparison stations in the Intercomparison. The Intercomparison data collected at the different sites are compatible with respect to the catch ratio (measured/DFIR) for the same gauge, when compared using mean wind speed at the height of the gauge orifice during the observation period. The Intercomparison data for the Tretyakov gauge were compiled from measurements made at these WMO intercomparison sites. These data represent a variety of climates, terrains and exposures. The effects of environmental factors, such as wind speed, wind direction, type of precipitation and temperature, on gauge catch ratios were investigated. Wind speed was found to be the most important factor determining the gauge catch and air temperature had a secondary effect when precipitation was classified into snow, mixed and rain. The results of the analysis of gauge catch ratio versus wind speed and temperature on a daily time step are presented for various types of precipitation. Independent checks of the correction equations against the DFIR have been conducted at those Intercomparison stations and a good agreement (difference less than 10%) has been obtained. The use of such adjustment procedures should significantly improve the accuracy and homogeneity of gauge-measured precipitation data over large regions of the former USSR and central Europe.  相似文献   

Microbial remains in some earliest Earth rocks: Comparison with a potential modern analogue   总被引：1，自引：1，他引：0  

Miryam Glikson  Lawrence J. Duck  Suzanne D. Golding  Axel Hofmann  Robert Bolhar  Robyn Webb  Justice C.F. Baiano  Lindsay I. Sly 《Precambrian Research》2008,164(3-4):187-200

Carbonaceous matter (CM) from ca. 3.5 Ga hydrothermal black cherts of the Pilbara Craton of Western Australia and the Barberton Greenstone Belt of South Africa yielded transmission electron microscopy (TEM) images that are suggestive of microbial remains and possible remnants of microbial cell walls. These are compared to a potential modern analogue, the hyperthermophilic Methanocaldococcus jannaschii, derived from an active seafloor hydrothermal environment and cultured under similar conditions. A striking resemblance to the early Archaean forms was evident in wall structure and thermal degradation mode. Cell disintegration of the cultures occurred at 100 °C marking the limits of life. Complete disintegration, deformation and shrinkage occurred at 132 °C. A multidisciplinary approach to the characterisation of the CM was undertaken using organic petrology, TEM coupled with electron dispersive spectral analysis (EDS), high resolution TEM (HRTEM) to determine molecular ordering, and elemental and carbon isotope geochemistry. Reflectance measurements of the CM to determine thermal stress yielded a range of values corresponding to several populations, and pointing to different sources and processes. The δ¹³C values of Dresser Formation CM (−36.5 to −32.1‰) are negatively correlated with TOC (0.13–0.75%) and positively correlated with C/N ratio (134–569), which is interpreted to reflect the relative abundance of high R_o/oxidised/recycled CM and preferential loss of ¹²C and N during thermal maturation. TEM observations, inferred carbon isotopic heterogeneity and isotope fractionations of −27 to −32‰ are consistent with the activity of chemosynthetic microbes in a seafloor hydrothermal system where rapid silicification at relatively low temperature preserved the CM.  相似文献   

The mafic-ultramafic complex near Finero (Ivrea-Verbano Zone), II. Geochronology and isotope geochemistry   总被引：1，自引：0，他引：1  

Meihua Lu  Albrecht W. Hofmann  Maurizio Mazzucchelli  Giorgio Rivalenti 《Chemical Geology》1997,140(3-4):223-235

Whole-rock Nd and Sr isotopic compositions of the mafic-ultramafic complex near Finero demonstrate that the magma was derived from a depleted, perhaps MORB-type mantle reservoir. The Sm-Nd data for the Amphibole Peridotite unit can be interpreted as an isochron with an apparent age of 533 ± 20 Ma, which is consistent with a ²⁰⁷Pb/²⁰⁶Pb evaporation age of 549 ± 12 Ma of a single zircon grain from the Internal Gabbro unit. However, the interpretation of these apparent ages remains open to question. We therefore retain the alternative hypotheses that the intrusion occurred either about 533 or 270 Ma ago, the latter being the most likely age of emplacement of the much larger magma body near Balmuccia (Val Sesia). The implication of the older emplacement age (if correct) would be that the igneous complex may be related to the numerous amphibolite units, which are intercalated with the metapelites of the overlying Kinzigite Formation, and together with them may constitute an accretionary complex. In this case, the mafic-ultramafic complex itself might also be part of such an accretionary complex (as has been proposed for the Balmuccia peridotite).

Internal Sm-Nd isochrons involving grt, cpx, plag and amph from the Internal Gabbro unit yield concordant ages of 231 ± 23, 226 ± 7, 223 ± 10, 214 ± 17, and 203 ± 13 Ma. These results confirm published evidence for a separate, regional heating event about 215 ± 15 Ma ago.

Initial _Nd(533) values average +6.3 ± 0.4 for six samples of the Amphibole Peridotite unit and +6.0 ± 1.2 for ten samples of the External Gabbro unit. ⁸⁷Sr/⁸⁶Sr ratios require little or no age correction and range from 0.7026 to 0.7047 (with two outliers at 0.7053 and 0.7071). Strong correlations between ⁸⁷Sr/⁸⁶Sr and K₂O and weaker correlations between initial _Nd and K₂O imply a comparatively minor (≤ 10%) contamination of the External Gabbro magma by crustal material and a later alteration by a crustal or seawater-derived fluid. These results contrast sharply with the isotopic composition (negative _Nd and high ⁸⁷Sr/⁸⁶Sr values) of the associated mantle rocks, the Phlogopite Peridotite unit, which has been pervasively metasomatized by crustal fluids. This type of metasomatism and its isotopic signature are never seen in the magmatic complex. This evidence rules out any direct genetic relationship between the igneous complex and the mantle peridotite. The crust-mantle interaction is the opposite of that seen at Balmuccia, where the mantle peridotite is essentially ‘pristine’ and the magmatic body has been extensively contaminated by assimilation of crustal rocks.  相似文献   

Mineralogy and geochemistry of reduction spheroids in red beds   总被引：1，自引：0，他引：1  

Dr. B. A. Hofmann 《Mineralogy and Petrology》1991,44(1-2):107-124

Summary The mineralogy and geochemistry of reduction spheroids from continental and marine red beds of Europe, North America and Oman were investigated. Reduction spheroids are spheroidal, isolated reduction sites in hematitic rocks. They consist of a mineralized core (0.1 to 5 cm diameter) and a hematite-dissolution halo (1 to 20 cm diameter). Irrespective of origin and age of host rocks, all reduction spheroid cores show a very similar mineralogy dominated by the vanadian mica roscoelite and a similar pattern of element enrichment relative to their host rocks dominated by V and U. Element enrichments in most reduction spheroids are very similar to those of sandstone-hosted vanadium-uranium deposits except for a lack of a molybdenum enrichment. Isotopically light sulfide of in-situ, low-temperature origin is an indication for the involvement of bacterial sulfate reduction during reduction spheroid formation.

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Mineralogie und Geochemie von Reduktionshöfen in Rotsedimenten

Zusammenfassung Reduktionshöfe aus kontinentalen und marinen Rotsedimenten von Europa, Nordamerika und aus dem Oman wurden geochemisch und mineralogisch untersucht. Reduktionshöfe sind kugelsymmetrische, isolierte Reduktionszentren in hämatithaltigen Gesteinen. Sie bestehen aus einem mineralisierten Kern von 0.1 bis 5 cm Durchmesser welcher von einem infolge Hämatitlösung gebleichten Hof von 1 bis 20 cm Durchmesser umgeben ist. Reduktionshöfe zeigen unabhängig von ihrer Herkunft und vom Alter des Nebengesteins immer eine sehr ähnliche Mineralogie der mineralisierten Kerne mit dominierendem Roscoelith sowie eine ähnliche Charakteristik der Elementanreicherung relativ zum Muttergestein. Die Elementanreicherung wird von V und U dominiert und gleicht jener von Sandstein-Typ Vanadium-Uran-Lagerstätten mit Ausnahme des Fehlens einer Molybdänanreicherung. In-situ unter Tieftemperaturbedingungen produziertes, isotopisch leichtes Sulfid ist ein Hinweis darauf, dass bakterielle Sulfatreduktion bei der Bildung der Reduktionshöfe eine Rolle spielte.

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With 7 Figures  相似文献   

3D magnetic reconnection at an X-ray bright point     

C. H. Mandrini  P. Démoulin  L. Van Driel-Gesztelyi  B. Schmieder  G. Cauzzi  A. Hofmann 《Solar physics》1996,168(1):115-133

On May 1, 1993, a flaring X-ray bright point (XBP) was observed for about 16 hours in the old, disintegrating, bipolar active region (AR) NOAA 7493. During this period, a minor magnetic bipole (10²⁰ Mx) emerged in the region. We have found observational evidence showing that the XBP brightenings were due to magnetic reconnection between the new bipole and pre-existing plage fields. The aim of the present work is to substantiate with magnetic modelling what has been shown by the observations. For this purpose we extrapolate the observed photospheric magnetic fields in the linear force-free approximation and follow its evolution during the lifetime of the XBP. From the computed coronal field lines we determine the location of regions of drastic change in field-line linkage, called quasi-separatrix layers or QSLs. QSLs are open layers that behave physically like separatrices: the break down of ideal magnetohydrodynamics and the release of free magnetic energy may occur at these locations when their thickness is small enough. The extrapolated field lines, with photospheric footpoints on both sides of QSLs, match the observed chromospheric and coronal structures (arch filament system, XBP and faint X-ray loops (FXL)). We study also the evolution of the width of the QSL located over the new negative polarity pore: the calculated QSL is very thin (typically less than 100 m) during the lifetime of the XBP, but becomes much thicker ( 10⁴ m) after the XBP has faded. Furthermore we show that peaks in X-ray brightness propagate along the FXL with a velocity of 670 km s^-1, starting from the XBP location, implying that the energy is released where the emerging bipole impacts against pre-existing coronal loops. We discuss the possible mechanism of energy transport and conclude that the energy is conducted to the remote footpoints of the FXL by a thermal front. These results strongly support the supposition that the XBP brightness and flaring are due to the interaction of different flux systems, through 3D magnetic reconnection, at QSLs.Member of the Carrera del Investigador Cientifíco, CONICET.Also at Konkoly Observatory, Budapest, Pf. 67, H-1525 Hungary.  相似文献