Element Abundance Determination in Hot Evolved Stars

The hydrogen-deficiency in extremely hot post-AGB stars of spectral class PG1159 is probably caused by a (very) late helium-shell flash or a AGB final thermal pulse that consumes the hydrogen envelope, exposing the usually-hidden intershell region. Thus, the photospheric element abundances of these stars allow us to draw conclusions about details of nuclear burning and mixing processes in the precursor AGB stars. We compare predicted element abundances to those determined by quantitative spectral analyses performed with advanced non-LTE model atmospheres. A good qualitative and quantitative agreement is found for many species (He, C, N, O, Ne, F, Si, Ar) but discrepancies for others (P, S, Fe) point at shortcomings in stellar evolution models for AGB stars. Almost all of the chemical trace elements in these hot stars can only be identified in the UV spectral range. The Far Ultraviolet Spectroscopic Explorer and the Hubble Space Telescope played a crucial role for this research.Comment: To appear in: Recent Advances in Spectroscopy: Theoretical, Astrophysical, and Experimental Perspectives, Proceedings, Jan 28 - 31, 2009, Kodaikanal, India (Springer

Is the centrality dependence of the elliptic flow v2v_2 and of the average <pT><p_T> in RHIC experiments more than a Core-Corona Effect?

Recently we have shown that the centrality dependence of the multiplicity of different hadron species observed in RHIC and SPS experiments can be well understood in a simple model, dubbed core-corona model. There it is assumed that those incoming nucleons which scatter only once produce hadrons as in pp collisions whereas those which scatter more often form an equilibrated source which decays according to phase space. In this article we show that also kinematical variables like $v_2/\epsilon_{part} (N_{part})$ as well as $v_2^i/\epsilon_{part} (N_{part})$ and $$of identified particles are well described in this model. The correlation of$$ between peripheral heavy ion collisions and pp collisions for different hadrons, reproduced in this model, questions whether hydrodynamical calculations are the proper tool to describe non-central heavy ion collision. The model explains as well the centrality dependence of $v_2/\epsilon_{part}$ of charged particles, considered up to now as an observable which allows to determine the viscosity of the quark gluon plasma. The observed dependence of $v_2^i/\epsilon_{part}(N_{part})$ on the particle species is a simple consequence of the different ratios of core to corona particles.Comment: Figure added, text partially rewritten, interpretation of v2 of identified particle

Quark-Gluon-Plasma Formation at SPS Energies?

By colliding ultrarelativistic ions, one achieves presently energy densities close to the critical value, concerning the formation of a quark-gluon-plasma. This indicates the importance of fluctuations and the necessity to go beyond the investigation of average events. Therefore, we introduce a percolation approach to model the final stage ($\tau > 1$ fm/c) of ion-ion collisions, the initial stage being treated by well-established methods, based on strings and Pomerons. The percolation approach amounts to finding high density domains, and treating them as quark-matter droplets. In this way, we have a {\bf realistic, microscopic, and Monte--Carlo based model which allows for the formation of quark matter.} We find that even at SPS energies large quark-matter droplets are formed -- at a low rate though. In other words: large quark-matter droplets are formed due to geometrical fluctuation, but not in the average event.Comment: 7 Pages, HD-TVP-94-6 (1 uuencoded figure

Deposing the Cool Corona of KPD 0005+5106

The ROSAT PSPC pulse height spectrum of the peculiar He-rich hot white dwarf KPD 0005+5106 provided a great surprise when first analysed by Fleming, Werner & Barstow (1993). It defied the best non-LTE modelling attempts in terms of photospheric emission from He-dominated atmospheres including C, N and O and was instead interpreted as the first evidence for a coronal plasma around a white dwarf. We show here that a recent high resolution Chandra LETGS spectrum has more structure than expected from a thermal bremsstrahlung continuum and lacks the narrow lines of H-like and He-like C expected from a coronal plasma. Moreover, a coronal model requires a total luminosity more than two orders of magnitude larger than that of the star itself. Instead, the observed 20-80 AA flux is consistent with photospheric models containing trace amounts of heavier elements such as Fe. The soft X-ray flux is highly sensitive to the adopted metal abundance and provides a metal abundance diagnostic. The weak X-ray emission at 1 keV announced by O'Dwyer et al (2003) instead cannot arise from the photosphere and requires alternative explanations. We echo earlier speculation that such emission arises in a shocked wind. Despite the presence of UV-optical O VIII lines from transitions between levels n=7-10, no X-ray O VIII Ly alpha flux is detected. We show that O VIII Lyman photons can be trapped by resonant scattering within the emitting plasma and destroyed by photoelectric absorption.Comment: 15 Pages, 4 figures. Accepted for the Astrophysical Journa

Physics of Event Generators

An event generator for nuclear collisions is a microscopic model, obtained from extrapolating elementary interactions -- as electron-positron annihilation, deep inelastic scattering, and proton-proton interactions -- towards proton-nucleus and nucleus-nucleus scattering, by using Monte Carlo techniques. In this paper, we will discuss the physical concepts behind such event generators. We first present some qualitative discussion of nuclear scattering, before discussing particle production and strings. We then discuss the parton model, and finally multiple scattering theory.Comment: Invited lecture, given at the Pan-American Advanced Study Institute "New States of Matter in Hadronic Interactions" Campos de Jordao, Brazil, January 7-18, 200

Discovery of an OVI Emitting Nebula around the Hot White Dwarf KPD 0005+5106

A survey of diffuse interstellar sight lines observed with the Far Ultraviolet Spectroscopic Explorer has led to the serendipitous discovery of a high-ionization nebula around the hot white dwarf KPD 0005+5106. The nebula has an OVI 1032A surface brightness of up to 25,000 photons/s/cm^2/sr, making it the brightest region of extended OVI emission in our survey. Photoionization models using the incident white dwarf continuum successfully reproduce the observed OVI intensity. The OVI emission arises in the highly ionized inner region of a planetary nebula around KPD 0005+5106. This newly discovered nebula may be one member of a class of high-ionization planetary nebulae that are difficult to detect in the optical, but which can be easily identified in the ultraviolet.Comment: accepted for publication in ApJL, 11 pages including 2 figure

Bose-Einstein Correlations in a Fluid Dynamical Scenario for Proton-Proton Scattering at 7 TeV

Using a fluid dynamical scenario for $pp$ scattering at 7 TeV, we compute correlation functions for $\pi^+\pi^+$ pairs. Femtoscopic radii are extracted based on three-dimensional parametrizations of the correlation functions. We study the radii as a function of the transverse momenta of the pairs, for different multiplicity classes, corresponding to recent experimental results from ALICE. We find the same decrease of the radii with $k_T$, more and more pronounced with increasing multiplicity, but absent for the lowest multiplicities. In the model we understand this as transition from string expansion (low multiplicity) towards a three-dimensional hydrodynamical expansion (high multiplicity)

Drawer drive for space shuttle vacuum canister

A sliding drawer type canister was designed to contain long duration exposure facility experiments which require vacuum storage before and after space exposure. The elastomeric seals require high closing loads which are generated through camming levers and transmitted through a spring loaded pressure plate. Lubrication was provided by various dry surface coatings. Higher than expected friction required some redesign after which the assembly functioned well and provided good sealing