Regular and quasi black hole solutions for spherically symmetric charged dust distributions in the Einstein-Maxwell theory

Static spherically symmetric distributions of electrically counterpoised dust (ECD) are used to construct solutions to Einstein-Maxwell equations in Majumdar--Papapetrou formalism. Unexpected bifurcating behaviour of solutions with regard to source strength is found for localized, as well as for the delta-function ECD distributions. Unified treatment of general ECD distributions is accomplished and it is shown that for certain source strengths one class of regular solutions approaches Minkowski spacetime, while the other comes arbitrarily close to black hole solutions.Comment: LaTeX (IOP style) 17 pages, 10 figure

Gravastar Solutions with Continuous Pressures and Equation of State

We study the gravitational vacuum star (gravastar) configuration as proposed by other authors in a model where the interior de Sitter spacetime segment is continuously extended to the exterior Schwarzschild spacetime. The multilayered structure in previous papers is replaced by a continuous stress-energy tensor at the price of introducing anisotropy in the (fluid) model of the gravastar. Either with an ansatz for the equation of state connecting the radial $p_r$ and tangential $p_t$ pressure or with a calculated equation of state with non-homogeneous energy/fluid density, solutions are obtained which in all aspects satisfy the conditions expected for an anisotropic gravastar. Certain energy conditions have been shown to be obeyed and a polytropic equation of state has been derived. Stability of the solution with respect to possible axial perturbation is shown to hold.Comment: 19 pages, 9 figures. Latest version contains new and updated references along with some clarifying remarks in the stability analysi

Gravastar energy conditions revisited

We consider the gravastar model where the vacuum phase transition between the de Sitter interior and the Schwarzschild or Schwarzschild-de Sitter exterior geometries takes place at a single spherical delta-shell. We derive sharp analytic bounds on the surface compactness (2m/r) that follow from the requirement that the dominant energy condition (DEC) holds at the shell. In the case of Schwarzschild exterior, the highest surface compactness is achieved with the stiff shell in the limit of vanishing (dark) energy density in the interior. In the case of Schwarzschild-de Sitter exterior, in addition to the gravastar configurations with the shell under surface pressure, gravastar configurations with vanishing shell pressure (dust shells), as well as configurations with the shell under surface tension, are allowed by the DEC. Respective bounds on the surface compactness are derived for all cases. We also consider the speed of sound on the shell as derived from the requirement that the shell is stable against the radial perturbations. The causality requirement (sound speed not exceeding that of light) further restricts the space of allowed gravastar configurations.Comment: LaTeX/IOP-style, 16 pages, 2 figures, changes wrt v1: motivation for eq. (6) clarified, several referecnes added (to appear in Class. Quantum Grav.

Detection of gravity modes in the massive binary V380 Cyg from Kepler spacebased photometry and high-resolution spectroscopy

We report the discovery of low-amplitude gravity-mode oscillations in the massive binary star V380 Cyg, from 180 d of Kepler custom-aperture space photometry and 5 months of high-resolution high signal-to-noise spectroscopy. The new data are of unprecedented quality and allowed to improve the orbital and fundamental parameters for this binary. The orbital solution was subtracted from the photometric data and led to the detection of periodic intrinsic variability with frequencies of which some are multiples of the orbital frequency and others are not. Spectral disentangling allowed the detection of line-profile variability in the primary. With our discovery of intrinsic variability interpreted as gravity mode oscillations, V380 Cyg becomes an important laboratory for future seismic tuning of the near-core physics in massive B-type stars.Comment: 5 pages, 4 figures, 2 tables. Accepted for publication in MNRAS Letter

Radial stability analysis of the continuous pressure gravastar

Radial stability of the continuous pressure gravastar is studied using the conventional Chandrasekhar method. The equation of state for the static gravastar solutions is derived and Einstein equations for small perturbations around the equilibrium are solved as an eigenvalue problem for radial pulsations. Within the model there exist a set of parameters leading to a stable fundamental mode, thus proving radial stability of the continuous pressure gravastar. It is also shown that the central energy density possesses an extremum in rho_c(R) curve which represents a splitting point between stable and unstable gravastar configurations. As such the rho_c(R) curve for the gravastar mimics the famous M(R) curve for a polytrope. Together with the former axial stability calculations this work completes the stability problem of the continuous pressure gravastar.Comment: 17 pages, 5 figures, References corrected, minor changes wrt v1, matches published versio

The Rest-frame Optical Colors of 99,000 SDSS Galaxies

We synthesize the rest-frame Stroemgren colors using SDSS spectra for 99,088 galaxies selected from Data Release 1. This narrow-band ~200 AA photometric system (uz, vz, bz, yz), first designed for the determination of effective temperature, metallicity and gravity of stars, measures the continuum spectral slope of galaxies in the rest-frame 3200-5800 AA wavelength range. Galaxies form a remarkably narrow locus (~0.03 mag) in the resulting color-color diagram. The Bruzual & Charlot population synthesis models suggest that the position of a galaxy along this locus is controlled by a degenerate combination of metallicity and age of the dominant stellar population. Galaxy distribution along the locus is bimodal, with the local minimum corresponding to an ~1 Gyr old single stellar population. The position perpendicular to the locus is independent of metallicity and age, and reflects the galaxy's dust content, as implied by both the models and the statistics of IRAS detections. A comparison of this locus with the galaxy locus in the H_delta-D_n(4000) diagram, utilized by Kauffmann et al. (2003) to estimate stellar masses, reveals a tight correlation, although the two analyzed spectral ranges barely overlap. Overall, the galaxy spectral energy distribution in the entire UV to near-IR range can be described as a single-parameter family with an accuracy of 0.1 mag, or better. This nearly one-dimensional distribution of galaxies in the multi-dimensional space of measured parameters strongly supports the conclusion of Yip et al. (2004), based on a principal component analysis, that SDSS galaxy spectra can be described by a small number of eigenspectra. Apparently, the contributions of stellar populations that dominate the optical emission from galaxies are combined in a simple and well-defined way.Comment: Accepted for publication in MNRAS; 19 pages, 28 color figure

(RE)DEFINISANJE KONCEPTA BEZBEDNOSTI IZ ASPEKTA SAVREMENIH IMIGRACIJA

The paper analyses the contemporary security concept, with a special review of the security aspects of migrations. After the introduction, which primarily offers certain terminological concerns, and a review of the historical development of the term security, the paper points to the impact of globalization to national security, and the existence of interdependence of countries in modern age. For this reason, the paper especially emphasizes the importance of international organizations, as well as international documents, which present the basic guidelines for treatment of security risks modern society is facing. Although literature points to a series of security challenges and threats, it seems that in recent years the problem of mass migrations is a special problem, which was the reason for the other part of the paper to be dedicated to security aspects of migrations, in two basic directions: first, pointing to the principal problems appearing as the consequence of migrations, and second, presentation of the model for a response to the migration crisis. The last part of the paper is dedicated to conclusive contemplations.Autori u radu analiziraju savremeni koncept bezbednosti, uz poseban osvrt na bezbednosne aspekte migracija. Nakon uvodnih napomena, u kome su pre svega date određene terminološke odrednice, kao i osvrt na istorijski razvoj shvatanja pojma bezbednosti, ukazano je na uticaj globalizacije na nacionalnu bezbednost, te postojanje nužne međuzavisnosti država u savremenom dobu. Iz navedenog razloga, u radu je posebno istaknut značaj međunarodnih organizacija, kao i međunarodnih dokumenata koji predstavljaju osnovne smernice za postupanje sa bezbednosnim rizicima sa kojima se savremeno društvo susreće. Mada se u literaturi ukazuje na niz bezbednosnih izazova i pretnji, čini se da se poslednjih godina kao poseban problem javio problem masovnih migracija, što je bio razlog da druga velika celina u ovom radu bude posvećena upravo bezbednosnim aspektima migracija i to u dva osnovna pravca: prvo, ukazivanje na osnovne probleme koji nastaju kao posledica postojanja migracija i drugo, predstavljanje modela reagovanja na migracionu krizu. Zaključnim razmatranjima, posvećen je poslednji deo rada, pri čemu su se autori pre svega okrenuli pitanjima državne reakcije na migrantsku krizu. Kada je reč o poslednjoj krizi, primetna su dva osnova modela na osnovu kojih su države reagovale i to: humanitarni pristup sa jedne strane i socijalno-ekonomski pristup i sekuritizacija, sa druge strane, pri čemu su neke države primale migrante, druge omogućavale njihov tranzit, a neke zabranile prolaz. Smatramo da je neophodno opredeljenje za prvi, humanitarni pristup migracijama, koji bi se pre svega ogledao u integraciji, te jedinstvenoj migracionoj politici, uz obavezno poštovanje ljudskih prava. U tom smislu, ovakve ideje bi trebalo osnažiti pre svega na međunarodnom planu, uz donošenje adekvatnih Konvencija pod okriljem Ujedinjenih nacija, koje bi bile dobar putokaz i osnov za jedinstveno postupanje. Na taj način, iako migrantska kriza ne bi bila rešena u svim svojim aspektima, zaisgurno bi bio trasiran jedinstven put koji bi doprineo da se, uz niz drugih mehanizama, ostvari princip humanosti na kojima počivaju moderna društva

On the separation of component spectra in binary and higher-multiplicity stellar systems: bias progression and spurious patterns

Context. Powerful methods are available to reconstruct the spectra of stars in orbit around each other, using a time-series of observed, composite spectra. They act either on the Fourier components of the observed spectra, or directly on these spectra in a velocity grid. Aims. We discuss under which conditions spurious patterns can appear in reconstructed spectra, either as a consequence of (quasi)-degeneracy of the equations or as a consequence of bias in the observed spectra. Also we discuss the equivalence of Fourier and direct methods in practice. Methods. We show under which conditions the equations degenerate, and how to evaluate this. We pay special attention to spectra of binary stars and triple systems analysed in Fourier space. We apply the theory to real data sets and to artificial data sets with several types of data-reduction bias constructed to illustrate degeneracies and the transfer of bias to the reconstructed spectra. Results. Quasi-degeneracy of the equations depends on the lack of significant time-variability in the relative light contribution of the stars, on the length of the spectral interval in units of the involved Doppler shifts, on the presence of very faint stellar components, and on the distribution of the observations over the orbital phases. Eclipse spectra, possibly used with different weight in low- and high-frequency Fourier modes, remove quasi-degeneracies. But when the normalisation of the observed spectra is biased in a systematic way with relation to the orbital phase, then the bias amplifies strongly in the reconstructed spectra, particularly for the faintest components. Wavelength-locked bias is transferred more strongly to the spectrum of the star with the lowest velocity amplitude. Unrecognized variations in line strength lead to bias that is larger in spectral regions with high line-density. Most importantly, the bias in all reconstructed spectra is coupled in a unique, predictable way. This coupling allows us to design robust procedures for the removal of bias from the reconstructed spectra. Conclusions. When the spectral features of all stars visible in the observed spectra undergo a significant time-variable dilution effect, then the reconstructed spectra are well-defined. Otherwise, spurious patterns, mostly of low frequency, may be superimposed on the reconstructed spectra. The analysis presented in this paper allows observers to optimise their observing strategy, gives insight in the origin of spurious patterns and indicates ways either to suppress such patterns or to remove them a posteriori from the reconstructed spectra. In this way, a broader range of astrophysical analyses can be applied to the component spectra