24,891 research outputs found
BONNSAI: a Bayesian tool for comparing stars with stellar evolution models
Powerful telescopes equipped with multi-fibre or integral field spectrographs
combined with detailed models of stellar atmospheres and automated fitting
techniques allow for the analysis of large number of stars. These datasets
contain a wealth of information that require new analysis techniques to bridge
the gap between observations and stellar evolution models. To that end, we
develop BONNSAI (BONN Stellar Astrophysics Interface), a Bayesian statistical
method, that is capable of comparing all available observables simultaneously
to stellar models while taking observed uncertainties and prior knowledge such
as initial mass functions and distributions of stellar rotational velocities
into account. BONNSAI can be used to (1) determine probability distributions of
fundamental stellar parameters such as initial masses and stellar ages from
complex datasets, (2) predict stellar parameters that were not yet
observationally determined and (3) test stellar models to further advance our
understanding of stellar evolution. An important aspect of BONNSAI is that it
singles out stars that cannot be reproduced by stellar models through
hypothesis tests and posterior predictive checks. BONNSAI can be
used with any set of stellar models and currently supports massive
main-sequence single star models of Milky Way and Large and Small Magellanic
Cloud composition. We apply our new method to mock stars to demonstrate its
functionality and capabilities. In a first application, we use BONNSAI to test
the stellar models of Brott et al. (2011a) by comparing the stellar ages
inferred for the primary and secondary stars of eclipsing Milky Way binaries.
Ages are determined from dynamical masses and radii that are known to better
than 3%. We find that the stellar models reproduce the Milky Way binaries well.
BONNSAI is available through a web-interface at
http://www.astro.uni-bonn.de/stars/bonnsai.Comment: Accepted for publication in A&A; 15 pages, 10 figures, 4 tables;
BONNSAI is available through a web-interface at
http://www.astro.uni-bonn.de/stars/bonnsa
High-precision astrometry on the VLT/FORS1 at time scales of few days
We investigate the accuracy of astrometric measurements with the VLT/FORS1
camera and consider potential applications. The study is based on two-epoch
(2000 and 2002/2003) frame series of observations of a selected Galactic Bulge
sky region that were obtained with FORS1 during four consecutive nights each.
Reductions were carried out with a novel technique that eliminates atmospheric
image motion and does not require a distinction between targets and reference
objects. The positional astrometric precision was found to be limited only by
the accuracy of the determination of the star photocentre, which is typically
200-300 microarcsec per single measurement for bright unsaturated stars
B=18-19. Several statistical tests have shown that at time-scales of 1-4 nights
the residual noise in measured positions is essentially a white noise with no
systematic instrumental signature and no significant deviation from a Gaussian
distribution. Some evidence of a good astrometric quality of the VLT for frames
separated by two years has also been found. Our data show that the VLT with
FORS1/2 cameras can be effectively used for astrometric observations of
planetary microlensing events and other applications where a high accuracy is
required, that is expected to reach 30-40 microarcsec for a series of 50 frames
(one hours with R filter).Comment: 11 pages, 9 figures, accepted for publication in A&
Space biology initiative program definition review. Trade study 4: Design modularity and commonality
The relative cost impacts (up or down) of developing Space Biology hardware using design modularity and commonality is studied. Recommendations for how the hardware development should be accomplished to meet optimum design modularity requirements for Life Science investigation hardware will be provided. In addition, the relative cost impacts of implementing commonality of hardware for all Space Biology hardware are defined. Cost analysis and supporting recommendations for levels of modularity and commonality are presented. A mathematical or statistical cost analysis method with the capability to support development of production design modularity and commonality impacts to parametric cost analysis is provided
On exponential cosmological type solutions in the model with Gauss-Bonnet term and variation of gravitational constant
A D-dimensional gravitational model with Gauss-Bonnet term is considered.
When ansatz with diagonal cosmological type metrics is adopted, we find
solutions with exponential dependence of scale factors (with respect to
"synchronous-like" variable) which describe an exponential expansion of "our"
3-dimensional factor-space and obey the observational constraints on the
temporal variation of effective gravitational constant G. Among them there are
two exact solutions in dimensions D = 22, 28 with constant G and also an
infinite series of solutions in dimensions D \ge 2690 with the variation of G
obeying the observational data.Comment: 21 pages, 12 figures, LaTex; eq. (2.1) is modified, several sentences
are added, a typo in eq. (3.13) is eliminate
Space biology initiative program definition review. Trade study 3: Hardware miniaturization versus cost
The optimum hardware miniaturization level with the lowest cost impact for space biology hardware was determined. Space biology hardware and/or components/subassemblies/assemblies which are the most likely candidates for application of miniaturization are to be defined and relative cost impacts of such miniaturization are to be analyzed. A mathematical or statistical analysis method with the capability to support development of parametric cost analysis impacts for levels of production design miniaturization are provided
Locating Encrypted Data Hidden Among Non-Encrypted Data using Statistical Tools
This research tests the security of software protection techniques that use encryption to protect code segments containing critical algorithm implementation to prevent reverse engineering. Using the National Institute of Standards and Technology (NIST) Tests for Randomness encrypted regions hidden among non-encrypted bits of a binary executable file are located. The location of ciphertext from four encryption algorithms (AES, DES, RSA, and TEA) and three block sizes (10, 100, and 500 32-bit words) were tested during the development of the techniques described in this research. The test files were generated from the Win32 binary executable file of Adobe\u27s Acrobat Reader version 7.0.9. The culmination of this effort developed a technique capable of locating 100% of the encryption regions with no false negative error and minimal false positive error with a 95% confidence. The encrypted region must be encrypted with a strong encryption algorithm whose ciphertext appears statistically random to the NIST Tests for Randomness, and the size of the encrypted region must be at least 100 32-bit words (3,200 bits)
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