1,841 research outputs found
Measuring stellar oscillations using equivalent widths of absorption lines
Kjeldsen et al. (1995, AJ 109, 1313; astro-ph/9411016) have developed a new
technique for measuring stellar oscillations and claimed a detection in the G
subgiant eta Boo. The technique involves monitoring temperature fluctuations in
a star via their effect on the equivalent width of Balmer lines. In this paper
we use synthetic stellar spectra to investigate the temperature dependence of
the Balmer lines, Ca II, Fe I, the Mg b feature and the G~band. We present a
list of target stars likely to show solar-like oscillations and estimate their
expected amplitudes. We also show that centre-to-limb variations in Balmer-line
profiles allow one to detect oscillation modes with l<=4, which accounts for
the detection by Kjeldsen et al. of modes with degree l=3 in integrated
sunlight.Comment: MNRAS (accepted); 7 pages, LaTeX with necessary style file and
PostScript figures in a single uuencoded Z-compressed .tar fil
Finding the First Stars: The Hamburg/ESO Objective Prism Survey
We report on a search for extremely metal-poor ([Fe/H]<-3.0) turnoff stars in
the Hamburg/ESO objective prism survey (HES). Metal-poor stars are selected by
automatic spectral classification. Extensive simulations show that the
selection efficiency for turnoff stars of [Fe/H]25% at B<16.5. Since
the HES is more than 1 mag deeper than the HK survey of Beers et al. (1992),
the HES offers the possibility to efficiently increase the total number of
metal-poor stars by at least a factor of 4.Comment: To appear in: Proceedings of ESO/MPA conference "The First Stars". 2
pages, 1 figur
Mechanical effect of van der Waals interactions observed in real time in an ultracold Rydberg gas
We present time-resolved spectroscopic measurements of Rydberg-Rydberg
interactions in an ultracold gas, revealing the pair dynamics induced by
long-range van der Waals interactions between the atoms. By detuning the
excitation laser, a specific pair distribution is prepared. Penning ionization
on a microsecond timescale serves as a probe for the pair dynamics under the
influence of the attractive long-range forces. Comparison with a Monte Carlo
model not only explains all spectroscopic features but also gives quantitative
information about the interaction potentials. The results imply that the
interaction-induced ionization rate can be influenced by the excitation laser.
Surprisingly, interaction-induced ionization is also observed for Rydberg
states with purely repulsive interactions
Modeling many-particle mechanical effects of an interacting Rydberg gas
In a recent work [Phys. Rev. Lett. 98, 023004 (2007)] we have investigated
the influence of attractive van der Waals interaction on the pair distribution
and Penning ionization dynamics of ultracold Rydberg gases. Here we extend this
description to atoms initially prepared in Rydberg states exhibiting repulsive
interaction. We present calculations based on a Monte Carlo algorithm to
simulate the dynamics of many atoms under the influence of both repulsive and
attractive longrange interatomic forces. Redistribution to nearby states
induced by black body radiation is taken into account, changing the effective
interaction potentials. The model agrees with experimental observations, where
the ionization rate is found to increase when the excitation laser is
blue-detuned from the atomic resonance
Lipase-Catalyzed Dynamic Kinetic Resolution of Chiral Amines: Use of Palladium as the Racemization Catalyst
The simultaneous use of a biocatalyst (lipase Candida antarctica) and a transition-metal catalyst (palladium) makes the dynamic kinetic resolution of racemic phenylethylamine possible, conversion to the enantiomerically pure N-acylated form being 75–77% (ee = 99%)
Highly Efficient Lipase-Catalyzed Kinetic Resolution of Chiral Amines
The lipase Candida antarctica catalyzes the enantioselective acetylation of chiral primary amines, kinetic resolution leading to an enantiomeric excess (ee) of 90–98%
Suppression of Excitation and Spectral Broadening Induced by Interactions in a Cold Gas of Rydberg Atoms
We report on the observation of ultralong range interactions in a gas of cold
Rubidium Rydberg atoms. The van-der-Waals interaction between a pair of Rydberg
atoms separated as far as 100,000 Bohr radii features two important effects:
Spectral broadening of the resonance lines and suppression of excitation with
increasing density. The density dependence of these effects is investigated in
detail for the S- and P- Rydberg states with main quantum numbers n ~ 60 and n
~ 80 excited by narrow-band continuous-wave laser light. The density-dependent
suppression of excitation can be interpreted as the onset of an
interaction-induced local blockade
Accounting Facilities in the European Supercomputing Grid DEISA
Account management and resource usage monitoring are essential services for production Grids. The scope of a production Grid infrastructure, the heterogeneity of resources and services, the typical community usage profiles, and the depth of integration of the resource providers regarding operational procedures and policies imply specific requirements for accounting facilities. We present the accounting facilities currently used in production in the Distributed European Infra-structure for the Supercomputing Applications (DEISA). DEISA is a consortium of leading national supercomputing centres currently deploying and operating a persistent, production quality, distributed su-percomputing environment with continental scope. The DEISA accounting facilities gather information from the site-local batch systems and the distributed DEISA user administration system, and generate XML usage records conforming to the OGF usage record specification which are then stored locally in a XML data base at each DEISA site. The distributed accounting information can be fetched by clients such as users, project supervisors, site accounting managers and DEISA supervisors. The information is made available by site-local WSRF-compliant accounting information services that allow for a fine-grained setting of access rights. Each authorized client gets a specific view on the accounting information according to one of the following roles: a) a site accounting manager imports usage records of related home-site users from all DEISA sites for longterm archiving, b) a project supervisor retrieves information to assess the resource usage by his project partners, c) a DEISA supervisor (e.g. someone overlooking the usage on behalf of the DEISA executive committee) gets a report on the global usage of DEISA resources, and d) the user who can retrieve all the accounting information related to his own jobs. The privacy and integrity of the data provided and transferred from the accounting information service running at each site is guaranteed using X.509 certificates for mutual authentication and secure communication channels
Preparation and Catalytic Activity of Boron-Substituted Zirconocenes
Borylated zirconocenes of the type (L2B–C5H4)2ZrCl2 and (L2B–C5H4)(C5H5)ZrCl2 are readily available in two steps. In the polymerization of ethylene and propylene in the presence of AlEt3 or methylalumoxan (MAO), these zirconocenes are more active than the unsubstituted reagent Cp2ZrCl2
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