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361.
Constraints on the formation environment of two chondrule‐like igneous particles from comet 81P/Wild 2
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Zack Gainsforth Anna L. Butterworth Julien Stodolna Andrew J. Westphal Gary R. Huss Kazu Nagashima Ryan Ogliore Donald E. Brownlee David Joswiak Tolek Tyliszczak Alexandre S. Simionovici 《Meteoritics & planetary science》2015,50(5):976-1004
Using chemical and petrologic evidence and modeling, we deduce that two chondrule‐like particles named Iris and Callie, from Stardust cometary track C2052,12,74, formed in an environment very similar to that seen for type II chondrules in meteorites. Iris was heated near liquidus, equilibrated, and cooled at ≤100 °C h‐1 and within ≈2 log units of the IW buffer with a high partial pressure of Na such as would be present with dust enrichments of ≈103. There was no detectable metamorphic, nebular, or aqueous alteration. In previous work, Ogliore et al. (2012) reported that Iris formed late, >3 Myr after CAIs, assuming 26Al was homogenously distributed, and was rich in heavy oxygen. Iris may be similar to assemblages found only in interplanetary dust particles and Stardust cometary samples called Kool particles. Callie is chemically and isotopically very similar, but not identical to Iris. 相似文献
362.
Multiple-point statistics are widely used for the simulation of categorical variables because the method allows for integrating a conceptual model via a training image and then simulating complex heterogeneous fields. The multiple-point statistics inferred from the training image can be stored in several ways. The tree structure used in classical implementations has the advantage of being efficient in terms of CPU time, but is very RAM demanding and then implies limitations on the size of the template, which serves to make a proper reproduction of complex structures difficult. Another technique consists in storing the multiple-point statistics in lists. This alternative requires much less memory and allows for a straightforward parallel algorithm. Nevertheless, the list structure does not benefit from the shortcuts given by the branches of the tree for retrieving the multiple-point statistics. Hence, a serial algorithm based on list structure is generally slower than a tree-based algorithm. In this paper, a new approach using both list and tree structures is proposed. The idea is to index the lists by trees of reduced size: the leaves of the tree correspond to distinct sublists that constitute a partition of the entire list. The size of the indexing tree can be controlled, and then the resulting algorithm keeps memory requirements low while efficiency in terms of CPU time is significantly improved. Moreover, this new method benefits from the parallelization of the list approach. 相似文献
363.
Julien Grunfelder Thomas Sick Nielsen 《Geografisk tidskrift / udgivet af Bestyrelsen for Det Kongelige danske geografiske selskab》2013,113(1):2-14
This paper is an empirical investigation of the relation between urban form and commuting behaviour in a polycentric urban region. It explores to what extent urban form and location variables help to explain commuting time, distance and mode based on an empirical case, East Jutland, which is a polycentric urban region in Denmark. Data from the National Transport Survey of Denmark were used for this quantitative analysis and two time periods were selected to highlight any potential changes over time. Empirical findings indicate that urban form and location variables help to explain the three selected aspects of commuting. However, urban form variables have greater explanatory power in explaining commuting modes than commuting time and commuting distance. No general trends in commuting were detectable from the data. Finally, the empirical findings revealed specificities of the case study. For instance, two of the important findings are that the distance to the closest urban centre is an important factor which affects the three selected aspects of commuting, while the distance from the residence to the closest large city (i.e. second-ranked city) is associated with a relatively short commuting distance in East Jutland. 相似文献
364.
Adrianne D. Slyz Julien E. G. Devriendt Joseph Silk reas Burkert 《Monthly notices of the Royal Astronomical Society》2002,333(4):894-910
We argue for implementing star formation on a viscous time-scale in hydrodynamical simulations of disc galaxy formation and evolution. Modelling two-dimensional isolated disc galaxies with the Bhatnagar–Gross–Krook (BGK) hydrocode, we verify the analytic claim of various authors that if the characteristic time-scale for star formation is equal to the viscous time-scale in discs, the resulting stellar profile is exponential on several scalelengths whatever the initial gas and dark matter profile. This casts new light on both numerical and semi-analytical disc formation simulations that either (a) commence star formation in an already exponential gaseous disc, (b) begin a disc simulation with conditions known to lead to an exponential, i.e. the collapse of a spherically symmetric nearly uniform sphere of gas in solid-body rotation under the assumption of specific angular momentum conservation, or (c) in simulations performed in a hierarchical context, tune their feedback processes to delay disc formation until the dark matter haloes are slowly evolving and without much substructure so that the gas has the chance to collapse under conditions known to give exponentials. In such models, star formation follows a Schmidt-like law, which for lack of a suitable time-scale, resorts to an efficiency parameter. With star formation prescribed on a viscous time-scale, however, we find gas and star fractions after ∼12 Gyr that are consistent with observations without having to invoke a 'fudge factor' for star formation. Our results strongly suggest that despite our gap in understanding the exact link between star formation and viscosity, the viscous time-scale is indeed the natural time-scale for star formation. 相似文献
365.
Alexander Knebe Brett Little Ranty Islam Julien Devriendt Asim Mahmood Joe Silk 《Astrophysics and Space Science》2003,284(2):335-340
Observations on galactic scales seem to be in contradiction with recent high resolution N-body simulations. This so-called cold dark matter (CDM) crisis has been addressed in several ways, ranging from a change
in fundamental physics by introducing self-interacting cold dark matter particles to a tuning of complex astrophysical processes
such as global and/or local feedback. All these efforts attempt to soften density profiles and reduce the abundance of satellites
in simulated galaxy halos. In this contribution we are exploring the differences between a Warm Dark Matter model and a CDM
model where the power on a certain scale is reduced by introducing a narrow negative feature (`dip'). This dip is placed in
a way so as to mimic the loss of power in the WDM model: both models have the same integrated power out to the scale where
the power of the Dip model rises to the level of the unperturbed CDM spectrum again. Using N-body simulations we show that that the new Dip model appears to be a viable alternative to WDM while being based on different
physics: where WDM requires the introduction of a new particle species the Dip stems from anon-standard inflationary period.
If we are looking for an alternative to the currently challenged standard ΛCDM structure formation scenario, neither the ΛWDM
nor the new Dip model can be ruled out with respect to the analysis presented in this contribution. They both make very similar
predictions and the degeneracy between them can only be broken with observations yet to come.
This revised version was published online in August 2006 with corrections to the Cover Date. 相似文献
366.
Julien Devriendt 《Astrophysics and Space Science》2002,281(1-2):505-508
This contribution describes results obtained with the GALICS model (for Galaxies In Cosmological Simulations), which is a hybrid model for hierarchical galaxy formation studies, combining the outputs of large cosmological N-body simulations
with simple, semi-analytic recipes to describe the fate of the baryons within dark matter halos. Designed to predict the overall
statistical properties of galaxies, with special emphasis on the panchromatic spectral energy distribution emitted by galaxies
in the UV/optical and IR/submm wavelength ranges, such an approach can be used to predict the galaxy luminosity function evolution
from the ultraviolet to far infrared, along with individual galaxies star formation histories.
This revised version was published online in September 2006 with corrections to the Cover Date. 相似文献
367.
Jérémy Blaizot Yogesh Wadadekar Bruno Guiderdoni Stéphane T. Colombi Emmanuel Bertin François R. Bouchet Julien E. G. Devriendt Steve Hatton 《Monthly notices of the Royal Astronomical Society》2005,360(1):159-175
We present the Mock Map Facility, a powerful tool for converting theoretical outputs of hierarchical galaxy formation models into catalogues of virtual observations. The general principle is straightforward: mock observing cones can be generated using semi-analytically post-processed snapshots of cosmological N -body simulations. These cones can then be projected to synthesize mock sky images. To this end, the paper describes in detail an efficient technique for creating such mock cones and images from the galaxies in cosmological simulations ( galics ) semi-analytic model, providing the reader with an accurate quantification of the artefacts it introduces at every step. We show that replication effects introduce a negative bias on the clustering signal – typically peaking at less than 10 per cent around the correlation length. We also thoroughly discuss how the clustering signal is affected by finite-volume effects, and show that it vanishes at scales larger than approximately one-tenth of the simulation box size. For the purpose of analysing our method, we show that number counts and redshift distributions obtained with galics / momaf compare well with K -band observations and the two-degree field galaxy redshift survey. Given finite-volume effects, we also show that the model can reproduce the automatic plate measuring machine angular correlation function. The momaf results discussed here are made publicly available to the astronomical community through a public data base. Moreover, a user-friendly Web interface ( http://galics.iap.fr ) allows any user to recover her/his own favourite galaxy samples through simple SQL queries. The flexibility of this tool should permit a variety of uses ranging from extensive comparisons between real observations and those predicted by hierarchical models of galaxy formation, to the preparation of observing strategies for deep surveys and tests of data processing pipelines. 相似文献
368.
369.
X-ray spectra of accretion discs with dynamic coronae 总被引:1,自引:0,他引:1
Julien Malzac rei M. Beloborodov Juri Poutanen 《Monthly notices of the Royal Astronomical Society》2001,326(2):417-427
We compute the X-ray spectra produced by non-static coronae atop accretion discs around black holes and neutron stars. The hot corona is radiatively coupled to the underlying disc (the reflector) and generates an X-ray spectrum which is sensitive to the bulk velocity of the coronal plasma, β = v / c . We show that an outflowing corona reproduces the hard-state spectrum of Cyg X-1 and similar objects. The dynamic model predicts a correlation between the observed amplitude of reflection R and the X-ray spectrum slope Γ since both strongly depend on β . A similar correlation was observed and its shape was well fitted by the dynamic model. The scattering of soft radiation in an outflowing corona can also account for the observed optical–UV polarization pattern in active galactic nuclei. 相似文献
370.