1,470 research outputs found
Technetium and the third dredge up in AGB stars. I. Field stars
We searched for Tc in a sample of long period variables selected by stellar
luminosity derived from Hipparcos parallaxes. Tc, as an unstable s-process
element, is a good indicator for the evolutionary status of stars on the
asymptotic giant branch (AGB). In this paper we study the occurrence of Tc as a
function of luminosity to provide constraints on the minimum luminosity for the
third dredge up as estimated from recent stellar evolution models.
A large number of AGB stars above the estimated theoretical limit for the
third dredge up are found not to show Tc. We confirm previous findings that
only a small fraction of the semiregular variables show Tc lines in their
spectra. Contrary to earlier results by Little et al. (1987) we find also a
significant number of Miras without Tc.
The presence and absence of Tc is discussed in relation to the mass
distribution of AGB stars. We find that a large fraction of the stars of our
sample must have current masses of less than 1.5 M_{\sun}. Combining our
findings with stellar evolution scenarios we conclude that the fraction of time
a star is observed as a SRV or a Mira is dependent on its mass.Comment: 10 pages, 4 figures, accepted for publication in A&
Concrete Dropout
Dropout is used as a practical tool to obtain uncertainty estimates in large
vision models and reinforcement learning (RL) tasks. But to obtain
well-calibrated uncertainty estimates, a grid-search over the dropout
probabilities is necessary - a prohibitive operation with large models, and an
impossible one with RL. We propose a new dropout variant which gives improved
performance and better calibrated uncertainties. Relying on recent developments
in Bayesian deep learning, we use a continuous relaxation of dropout's discrete
masks. Together with a principled optimisation objective, this allows for
automatic tuning of the dropout probability in large models, and as a result
faster experimentation cycles. In RL this allows the agent to adapt its
uncertainty dynamically as more data is observed. We analyse the proposed
variant extensively on a range of tasks, and give insights into common practice
in the field where larger dropout probabilities are often used in deeper model
layers
Nonlinear Analysis of Composite Cross-Section With Respect To Effects of Shrinkage, Creep, Ageing and Temperature
Pokročilé výpočetní metody pro řešení stavebních konstrukcí se v projekční praxi používají stále častěji. Jejich použití je vynuceno nejen nároky na estetiku, funkčnost a vysokou hospodárnost výstavby, ale často přímo normovými předpoklady a požadavky. Je proto nezbytné poskytnout projektantovi robustní, ale transparentní nástroj, pomocí kterého provede efektivní návrh konstrukce a její posouzení v průběhu celé životnosti. Práce se zabývá vytvořením výpočetního systému pro časovou analýzu betonových a spřažených rámových konstrukcí v prostoru. Předpokládá se jednoduchý nebo spřažený betonový průřez s možnou aplikací předem a/nebo dodatečně předpjatých kabelů. Jednotlivé fáze postupně budovaného spřaženého průřezu mají obecnou geometrii. Použité algoritmy dávají možnost detailní analýzy konstrukce v jednotlivých fázích výstavby, poskytují projektantovi náhled na chování konstrukce v čase s vlivem stárnutí, smršťování a dotvarování betonu, relaxace předpínací výztuže a poskytují přehled o redistribuci vnitřních sil v konstrukci a v jednotlivých částech průřezu. Takto vypočtená napjatost konstrukce se dále uplatňuje v posouzení mezních stavů únosnosti a použitelnosti. Výsledky časové analýzy jsou verifikovány ručními výpočty a srovnáním s výsledky dosaženými zjednodušenými metodami.Advanced computational methods for the analysis of building structures are used more often in engineering practice. Their use is enforced not only by the demands for aesthetics, functionality and high economy of the construction, but often directly by code provisions and requirements. Therefore, it is necessary to provide for an engineer the robust, but transparent tool, which can be used for an efficient design of structure all over it's design working life. This work deals with the creation of a computational system for time dependent analysis of concrete and composite structures. It is assumed a solid or composite concrete section with possible application of pre-tensioned and/or post-tensioned tendons. Each phase of step-by-step build composite cross-section has a general geometry. The used algorithms give us the possibility of a detailed analysis of the structure in individual construction stages, provide the designer better view on the behavior of structures with respect to concrete aging, shrinkage and creep, relaxation of prestressed tendons and provide an information on the redistribution of internal forces in the structure and in different phases of the cross-section. Stress state of the structure calculated in this way come in useful further in the design and checking of ultimate and serviceability limit states. Results of time dependent analysis are verified by manual calculations and by comparing with the results obtained by simplified methods.
Field #3 of the Palomar-Groningen Survey II. Near-infrared photometry of semiregular variables
Near-infrared photometry (JHKL'M) was obtained for 78 semiregular variables
(SRVs) in field #3 of the Palomar-Groningen survey (PG3, l=0, b=-10). Together
with a sample of Miras in this field a comparison is made with a sample of
field SRVs and Miras. The PG3 SRVs form a sequence (period-luminosity
& period-colour) with the PG3 Miras, in which the SRVs are the short period
extension to the Miras. The field and PG3 Miras follow the same P/(J--K)o
relation, while this is not the case for the field and PG3 SRVs. Both the PG3
SRVs and Miras follow the SgrI period-luminosity relation adopted from Glass et
al. (1995, MNRAS 273, 383). They are likely pulsating in the fundamental mode
and have metallicities spanning the range from intermediate to approximately
solar.Comment: 14 pages LaTeX (2 tables, 8 figures), to appear in A&A 338 (1998);
minor modifications in tex
Inversion of partitioned matrices in investigation of the twoepoch linear model with the nuisance parameters
summary:The estimation procedures in the multiepoch (and specially twoepoch) linear regression models with the nuisance parameters that were described in [2], Chapter 9, frequently need finding the inverse of a partitioned matrix. We use different kinds of such inversion in dependence on simplicity of the result, similarly as in well known Rohde formula for partitioned matrix. We will show some of these formulas, also methods how to get the other formulas, and then we applicate the formulas in estimation of the mean value parameters in the twoepoch linear regression model with the nuisance parameters
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