Stellar abundances of beryllium and CUBES

Stellar abundances of beryllium are useful in different areas of astrophysics, including studies of the Galactic chemical evolution, of stellar evolution, and of the formation of globular clusters. Determining Be abundances in stars is, however, a challenging endeavor. The two Be II resonance lines useful for abundance analyses are in the near UV, a region strongly affected by atmospheric extinction. CUBES is a new spectrograph planned for the VLT that will be more sensitive than current instruments in the near UV spectral region. It will allow the observation of fainter stars, expanding the number of targets where Be abundances can be determined. Here, a brief review of stellar abundances of Be is presented together with a discussion of science cases for CUBES. In particular, preliminary simulations of CUBES spectra are presented, highlighting its possible impact in investigations of Be abundances of extremely metal-poor stars and of stars in globular clusters.Comment: 11 pages, 8 figures. To appear in Astrophysics and Space Science "Special Issue: UV Astronomy 3", Proceedings of the ESO/NUVA/IAG Workshop on Challenges in UV Astronomy, ESO Garching, 7-11 October 201

Beryllium abundances along the evolutionary sequence of the open cluster IC 4651 - New test for hydrodynamical stellar models

[abridged] Previous analyses of lithium abundances in main sequence and red giant stars have revealed the action of mixing mechanisms other than convection in stellar interiors. Beryllium abundances in stars with lithium abundance determinations can offer valuable complementary information on the nature of these mechanisms. Our aim is to derive beryllium abundances along the whole evolutionary sequence of an open cluster, IC 4651. These Be abundances are used together with previously determined Li abundances, in the same sample stars, to investigate the mixing mechanisms in a range of stellar masses and evolutionary stages. New beryllium abundances are determined from high-resolution, high signal-to-noise UVES spectra using spectrum synthesis and model atmospheres. The careful synthetic modelling of the Be lines region is used to calculate reliable abundances in rapidly rotating stars. The observed behavior of Be and Li is compared to theoretical predictions from stellar models including rotation-induced mixing, internal gravity waves, atomic diffusion, and thermohaline mixing. Beryllium is detected in all the main sequence and turn-off sample stars, both slow- and fast-rotating stars, including the Li-dip stars, but was not detected in the red giants. Confirming previous results, we find that the Li dip is also a Be dip, although the depletion of Be is more modest than that of Li in the corresponding effective temperature range. For post-main-sequence stars, the Be dilution starts earlier within the Hertzsprung gap than expected from classical predictions as does the Li dilution. A clear dispersion in the Be abundances is also observed. Theoretical stellar models including the hydrodynamical transport processes mentioned above are able to reproduce well all the observed features.Comment: 12 pages, accepted for publication in A&A, revised final versio

Using Element Clustering to Increase the Efficiency of XML Schema Matching

Schema matching attempts to discover semantic mappings between elements of two schemas. Elements are cross compared using various heuristics (e.g., name, data-type, and structure similarity). Seen from a broader perspective, the schema matching problem is a combinatorial problem with an exponential complexity. This makes the naive matching algorithms for large schemas prohibitively inefficient. In this paper we propose a clustering based technique for improving the efficiency of large scale schema matching. The technique inserts clustering as an intermediate step into existing schema matching algorithms. Clustering partitions schemas and reduces the overall matching load, and creates a possibility to trade between the efficiency and effectiveness. The technique can be used in addition to other optimization techniques. In the paper we describe the technique, validate the performance of one implementation of the technique, and open directions for future research

Beryllium abundances and the formation of the halo and the thick disk

The single stable isotope of beryllium is a pure product of cosmic-ray spallation in the ISM. Assuming that the cosmic-rays are globally transported across the Galaxy, the beryllium production should be a widespread process and its abundance should be roughly homogeneous in the early-Galaxy at a given time. Thus, it could be useful as a tracer of time. In an investigation of the use of Be as a cosmochronometer and of its evolution in the Galaxy, we found evidence that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined. We also found evidence that the star formation rate was lower in the outer regions of the thick disk, pointing towards an inside-out formation.Comment: 6 pages, 5 figures, To appear in the Proceedings of IAU Symp. 268 - Light Elements in the Universe (C. Charbonnel, M. Tosi, F. Primas, C. Chiappini, eds

A view of the Galactic halo using beryllium as a time scale

Beryllium stellar abundances were suggested to be a good tracer of time in the early Galaxy. In an investigation of its use as a cosmochronometer, using a large sample of local halo and thick-disk dwarfs, evidence was found that in a log(Be/H) vs. [alpha/Fe] diagram the halo stars separate into two components. One is consistent with predictions of evolutionary models while the other is chemically indistinguishable from the thick-disk stars. This is interpreted as a difference in the star formation history of the two components and suggests that the local halo is not a single uniform population where a clear age-metallicity relation can be defined.Comment: To appear in Proceedings of the International Astronomical Union, IAU Symposium, Volume 265, Chemical abundances in the Universe: connecting first stars to planets, K. Cunha, M. Spite and B. Barbuy, eds. 2 Pages, 2 figure

Some studies of artificial enzyme systems.

This thesis describes a novel approach to the rational design of artificial esterases and aldolases. The Introduction provides a literature summary of the previous approaches that have been employed towards the design and synthesis of artificial enzyme systems. Chapter 2 describes the preparation and reactivity of a number of polymer based artificial enzymes, which are capable of catalysing ester hydrolysis. The study has involved the incorporation of a histidine catalytic group together with specifically designed peptide binding groups within a polymeric backbone. The binding groups were specifically selected according to their binding affinity towards an appropriate transition state analogue. The synthesis of peptide binding sites and thus incorporation of these, together with the histidine catalytic group into a polymer backbone, using standard peptide chemistry has been outlined. The results to an investigation of the influence of different pH, solvent and substrate concentration on the activity of artificial esterases are presented. Chapter 3 describes preliminary work undertaken towards the design and synthesis of artificial aldol catalysts. The aldolases, which feature a proline residue attached to a polymer backbone are shown to selectively catalyse aldol reactions using aromatic aldehydes as electrophilic partner. Chapter 4 describes the detailed experimental procedures used

XML schema matching : balancing efficiency and effectiveness by means of clustering

In this thesis we place our research in the scope of a tool which looks for information within XML data on the Internet. We envision a personal schema querying system which enables a user to express his information need by specifying a personal XML schema. The user can also ask queries over his personal schema. The first task the personal schema querying system must perform is to match the personal schema against the real schemas of the XML data on the Internet. In this way, the system establishes a meaningful link between the user¿s information need and the data available on the Internet. The second task is the evaluation of the personal query, which is performed by means of distributed query processing