Accurate age determinations of several nearby open clusters containing magnetic Ap stars

Aims: Our aim is to obtain ages that are as accurate as possible for the seven nearby open clusters alpha Per, Coma Ber, IC 2602, NGC 2232, NGC 2451A, NGC 2516, and NGC 6475, each of which contains at least one magnetic Ap or Bp star. Simultaneously, we test the current calibrations of Te and luminosity for the Ap/Bp star members, and identify clearly blue stragglers in the clusters studied. Methods: We explore the possibility that isochrone fitting in the theoretical Hertzsprung-Russell diagram (i.e. log (L/L&sun;) vs. log Te), rather than in the conventional colour-magnitude diagram, can provide more precise and accurate cluster ages, with well-defined uncertainties. Results: Well-defined ages are found for all the clusters studied. For the nearby clusters studied, the derived ages are not very sensitive to the small uncertainties in distance, reddening, membership, metallicity, or choice of isochrones. Our age determinations are all within the range of previously determined values, but the associated uncertainties are considerably smaller than the spread in recent age determinations from the literature. Furthermore, examination of proper motions and HR diagrams confirms that the Ap stars identified in these clusters are members, and that the presently accepted temperature scale and bolometric corrections for Ap stars are approximately correct. We show that in these theoretical HR diagrams blue stragglers are particularly easy to identify. Conclusions: Constructing the theoretical HR diagram of a nearby open cluster makes possible an accurate age determination, with well defined uncertainty. This diagnostic of a cluster also provides a useful tool for studying unusual stars such as Ap stars and blue stragglers

The Hα Spectroscopy of Classical B-emission Stars

Classical B-emission (Be) stars are rapidly-rotating, massive stars that possess a dense, equatorial, gaseous disk. The presence of a disk was first inferred from the Balmer series emission that these stars exhibit, and Hα emission lines remain both a hallmark observational feature and one of the key diagnostics in determining the physical conditions within the disk. In the first chapter of this thesis, we investigate the possible role of line-driven winds in disk formation. To test if line-driven winds could supply enough material to account for the equatorial disk, we check for the presence of Hα emission in the models that result from different combinations of line-force parameters. We find that certain combinations of line-force parameters can indeed produce significant Hα emission, and that line-driven winds may be an important mechanism in removing material from the central star. The next chapter of this thesis employs the code Beray to perform a detailed study of the Hα emission profiles of Be shell stars. Modelling the peak height and separation of these profiles gives an indication of the average density structure of the disk, although in some cases we found that more than one density structure could adequately reproduce these features. This finding indicates that, ideally, other observables should be simultaneously considered in order to constrain the models. It was found, however, that the absorption depth of the synthetic Hα profiles was relatively insensitive to the choice of density structure, depending instead largely on the inclination angle of the model. Thus, the inclination angles of these stars are determined to within a few degrees in our models, and we find that Be shell stars may not be oriented as close to 90° as originally thought. The final chapter of this thesis investigates the density structure of an asymmetric disk surrounding the well-known Be shell star 48 Librae. We begin our study with a thorough investigation of the central star’s spectral type (as it is not well-determined and has a significant impact on all other aspects of the model) by modelling the star’s spectral energy distribution (SED). Next, the SED and polarization levels are used to determine the inclination angle of the system. We then employ the HDUST code to model an observed Hα emission profile. We consider each peak separately, and then average the initial densities required to reproduce each peak to obtain the average initial disk density. We find that 48 Librae is best represented by a B3V central star surrounded by a very dense disk whose average initial density is 1.1 × 10−10 g cm−3 , and that the system is oriented at 85°

Searching for links between magnetic fields and stellar evolution. I. A survey of magnetic fields in open cluster A- and B-type stars with FORS1

About 5% of upper main sequence stars are permeated by a strong magnetic field, the origin of which is still matter of debate. With this work we provide observational material to study how magnetic fields change with the evolution of stars on the main sequence, and to constrain theory explaining the presence of magnetic fields in A and B-type stars. Using FORS1 in spectropolarimetric mode at the ESO VLT, we have carried out a survey of magnetic fields in early-type stars belonging to open clusters and associations of various ages. We have measured the magnetic field of 235 early-type stars with a typical uncertainty of about 100 G. In our sample, 97 stars are Ap or Bp stars. For these targets, the median error bar of our field measurements was about 80 G. A field has been detected in about 41 of these stars, 37 of which were not previously known as magnetic stars. For the 138 normal A and B-type stars, the median error bar was 136 G, and no field was detected in any of them.Comment: Accepted by A&A; 15 pages (article)+15 pages (tables), 8 figure

Prikaz novog skraćenog izdanja Univerzalne decimalne klasifikacije za područje religije te usporedbe s prvim hrvatskim srednjim izdanjem Univerzalne decimalne klasifikacije

U radu je ukratko prikazana nova shema Univerzalne decimalne klasifikacije za religiju koja se u knjižničnoj praksi Nacionalne i sveučilišne knjižnice u Zagrebu primjenjuje od siječnja 2015. godine. U klasificiranju religije prema Univerzalnoj decimalnoj klasifikaciji sada i prije nove prakse, koja se radi prema skraćenom izdanju UDK, način klasificiranja je u velikoj mjeri različit. U prvom hrvatskom srednjem izdanju UDK 80 posto oznaka pripadalo je kršćanstvu i svim temama koje se vežu uz tu religiju, a otprilike 20 posto drugim nekršćanskim religijama. U novom izdanju postotak je proporcionalno različit, otprilike 20 posto UDK pripada kršćanstvu, a proširena je klasifikacija drugih nekršćanskih religija, posebno judaizma, islama, budizma i drugih religija i vjera čiji opseg sada zauzima gotovo 80 posto. Novi UDK knjižničarima je pored izazova donio i puno klasifikacijskih prednosti. Prednosti se odnose na raspon različitih tema koje pojedine publikacije obrađuju u praksi, a koje su raznovrsne. Prilagođavanjem tema religijama lakše je odrediti sadržaje i to je jednostavnije i jasnije nego u starijem izdanju UDK koji je najširim izborom tema pokrivao kršćanstvo. Tako nove tablice otvaraju više prostora za druge religije i njihove specifičnosti

Protection from floods

Poplave su u cijelom svijetu, a tako i kod nas, jedna od najučestalijih elementarnih nepogoda. Rješavanju takvih problema uglavnom se pristupalo uređivanjem vodenih tokova i gradnjom nasipa kao preventivnih mjera, te poduzimanjem različitih operativnih mjera kao što su postavljanje vodenih pregrada u hitnim situacijama. Danas vidimo da te mjere nisu dovoljne jer su poplave sve učestalije, a samim time dovode se u opasnost i ljudski životi, životinje i imovina. Kako napreduje tehnologija u zaštiti i spašavanju, tako dolazi i do promjena u obrani od poplava. Postoji niz takvih obrambenih mjera koje kod nas još uvijek nisu poznate, npr. uzmimo samo za primjer koliko ljudi i tehnike treba da se postavi vodena brana sagrađena od vreća s pijeskom, dok postoji niz drugih jednostavnijih i učinkovitijih vodenih pregrada, ali sve one nose sa sobom svoje mane i prednosti. Kako bi se što efikasnije i brže mogle provoditi mjere obrane od poplava, odnosno izgradnja vodenih pregrada u izvanrednim situacijama, došlo se do izuma nekih novih vodenih pregrada koje bi trebale olakšati i ubrzati kriznu situaciju nastalu na određenom području. Više o tim novim metodama obrane od poplava bit će prikazano u daljnjem radu.Floods in the world, but also in our country, are one of the most common natural disasters. Preventive measures in solving problems caused by floods are regulating water flows and building of dams. In emergencies various operative measures such as setting up water barriers are used. Nowadays we realise that these measures are not sufficient because floods occur more and more often, which puts into danger human lives, animals and properties. As the technology of protection and saving is improving, some shifts in the flood defense have been done. There is a whole variety of defense measures that we are still not familiar with. For example, a lot of people and technics is needed to set up a water barrier built of sandbags, whereas there are a lot of other simpler and more efficient water barriers, but all of them have their advantages and disadvantages. Building of regulating and protective water structures is conducted according to the Programme of building of regulating and protective water structures, which is enacted by the Government of the Republic of Croatia and proposed by the sectoral minister. Technical and economical maintenance of watercourses, water resources and water structures is based on the Programme of arranging of rivers and other watercourses. This programme is part of the annual plan of water management and is enacted by Croatian Waters in accordance with the Ministry of the regional development, forestry sector and water economics. Policies and measures to protect nature and the environment will be incorporated In the above plans of water management. The idea to be able to carry out flood protection measures and the construction of water barriers in emergency situations more efficiently and more quickly led to the invention of some new water barriers which would facilitate and accelerate the crisis situation caused on the specific area. In the further work more about these new methods of flood defense will be revealed