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Max Pettini Samantha A. Rix Chuck C. Steidel Matthew P. Hunt Alice E. Shapley Kurt L. Adelberger 《Astrophysics and Space Science》2002,281(1-2):461-466
Recent advances in instrumentation and observing techniques have made it possible to begin to study in detail the stellar
populations and the interstellar media of galaxies at redshift z = 3, when the universe was still in its ‘teen years’. I illustrate recent progress in this field with the latest observations
of the gravitationally lensed galaxy MS 1512- cB58.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
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Moritz Liebl Jörg Robl David Lundbek Egholm Günther Prasicek Kurt Stüwe Gerit Gradwohl Stefan Hergarten 《地球表面变化过程与地形》2021,46(10):1964-1980
The Pleistocene glaciations left a distinct topographic footprint in mountain ranges worldwide. The geometric signature of glacial topography has been quantified in various ways, but the temporal development of landscape metrics has not been traced in a landscape evolution model so far. However, such information is needed to interpret the degree of glacial imprint in terms of the integrated signal of temporal and spatial variations in erosion as a function of glacial occupation time. We apply a surface process model for cold-climate conditions to an initially fluvial mountain range. By exploring evolving topographic patterns in model time series, we determine locations where topographic changes reach a maximum and where the initial landscape persists. The signal of glacial erosion, expressed by the overdeepening of valleys and the steepening of valley flanks, develops first at the glacier front and migrates upstream with ongoing glacial erosion. This leads to an increase of mean channel slope and its variance. Above steep flanks and head-walls, however, the observed mean channel slope remains similar to the mean channel slope of the initial fluvial topography. This leads to a characteristic turning point in the channel slope–elevation distribution above the equilibrium line altitude, where a transition from increasing to decreasing channel slope with elevation occurs. We identify this turning point and a high channel slope variance as diagnostic features to quantify glacial imprint. Such features are abundant in glacially imprinted mid-latitude mountain ranges such as the Eastern Alps. By analysing differently glaciated parts of the mountain range, we observe a decreasing clarity of this diagnostic morphometric property with decreasing glacial occupation. However, catchments of the unglaciated eastern fringe of the Alps also feature turning points in their channel slope–elevation distributions, but in contrast to the glaciated domain, the variance of channel slope is small at all elevation levels. 相似文献
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Maria LUGARO Carolyn L. DOHERTY Amanda I. KARAKAS Sarah T. MADDISON Kurt LIFFMAN D. A. GARCÍA‐HERNÁNDEZ Lionel SIESS John C. LATTANZIO 《Meteoritics & planetary science》2012,47(12):1998-2012
Abstract– The composition of the most primitive solar system condensates, such as calcium‐aluminum‐rich inclusions (CAIs) and micron‐sized corundum grains, show that short‐lived radionuclides (SLR), e.g., 26Al, were present in the early solar system. Their abundances require a local or stellar origin, which, however, is far from being understood. We present for the first time the abundances of several SLR up to 60Fe predicted from stars with initial mass in the range approximately 7–11 M⊙. These stars evolve through core H, He, and C burning. After core C burning they go through a “Super”‐asymptotic giant branch (Super‐AGB) phase, with the H and He shells activated alternately, episodic thermal pulses in the He shell, a very hot temperature at the base of the convective envelope (approximately 108 K), and strong stellar winds driving the H‐rich envelope into the surrounding interstellar medium. The final remnants of the evolution of Super‐AGB stars are mostly O–Ne white dwarfs. Our Super‐AGB models produce 26Al/27Al yield ratios approximately 0.02–0.26. These models can account for the canonical value of the 26Al/27Al ratio using dilutions with the solar nebula of the order of 1 part of Super‐AGB mass per several 102 to several 103 of solar nebula mass, resulting in associated changes in the O‐isotope composition in the range Δ17O from 3 to 20‰. This is in agreement with observations of the O isotopic ratios in primitive solar system condensates, which do not carry the signature of a stellar polluter. The radionuclides 41Ca and 60Fe are produced by neutron captures in Super‐AGB stars and their meteoritic abundances are also matched by some of our models, depending on the nuclear and stellar physics uncertainties as well as the meteoritic experimental data. We also expect and are currently investigating Super‐AGB production of SLR heavier than iron, such as 107Pd. 相似文献
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As gas flowed from the solar accretion disk or Solar Nebula onto the proto-Sun, magnetic pressure gradients in the solar magnetosphere and the inner Solar Nebula provided an environment where some of this infalling flow was diverted to produce a low pressure, high temperature, gaseous, “infall” atmosphere around the inner Solar Nebula. The pressure in this inner disk atmosphere was mainly dependant on the accretion flow rate onto the star. High flow rates implied relatively high pressures, which decreased over time as the accretion rate decreased.In the first hundred thousand years after the formation of the Solar Nebula, accretional flow gas pressures were high enough to create submicron-sized Refractory Metal Nuggets (RMNs) – the precursors to Calcium Aluminum Inclusions (CAIs). Optimal temperatures and pressures for RMN formation may have occurred between 20,000 and 100,000 years after the formation of the Solar Nebula. It is possible that conditions were conducive to RMN/CAI formation over an 80,000 year timescale. The “infall” atmosphere and the condensation of refractory particles within this atmosphere may be observable around the inner disks of other protostellar systems.The interaction of forces from magnetic fields with the radiation pressure from the proto-Sun and the inner solar accretion disk potentially produced an optical-magnetic trap above and below the inner Solar Nebula, which provided a relatively stable environment in which the RMNs/proto-CAIs could form and grow. These RMN formation sites only existed during accretion events from the proto-solar disk onto the proto-Sun. As such, the formation and growth time of a particular RMN was dependent on the timescale of its nascent accretion event.Observational evidence suggests that RMNs were the nucleation particles for CAIs. As a consequence, the observed bimodal distribution of 26Al in CAIs, where some CAIs have 26Al while others do not, is probably due to the injection 26Al during the short CAI formation period, where 26Al was not present when the first CAIs were formed. 相似文献