arXiv subjects

K. Werner

Publications and source records attributed to K. Werner.

At least 235 records · Page 13Linked to original sources

arXiv2002

Spectral Types of Planetary Host Star Candidates: Two New Transiting Planets?

Recently, 46 low-luminosity object transits were reported from the Optical Gravitational Lensing Experiment. Our follow-up spectroscopy of the 16 most promising candidates provides a spectral classification of the primary. Together with the radius ratio from the transit measurements, we derived the radii of the low-luminosity companions. This allows to examine the possible sub-stellar nature of these objects. Fourteen of them can be clearly identified as low-mass stars. Two objects, OGLE-TR-03 and OGLE-TR-10 have companions with radii of 0.15 R_sun which is very similar to the radius of the transiting planet HD209458B. The planetary nature of these two objects should therefore be confirmed by dynamical mass determinations.

astro-ph↗

arXiv2002

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.

hep-ph↗

arXiv2002

A Novel Mechanism of H^0 Di-baryon Production in Proton-Proton Interactions from Parton Based Gribov-Regge Theory

A novel mechanism of H^0 and strangelet production in hadronic interactions within the Gribov-Regge approach is presented. In contrast to traditional distillation approaches, here the production of multiple (strange) quark bags does not require large baryon densities or a QGP. The production cross section increases with center of mass energy. Rapidity and transverse momentum distributions of the H^0 are predicted for pp collisions at E_lab = 160 AGeV (SPS) and \sqrt s = 200 AGeV (RHIC). The predicted total H^0 multiplicities are of order of the Omega-baryon yield and can be accessed by the NA49 and the STAR experiments.

hep-ph↗

arXiv2002

Screening Corrections In Simulating Heavy Ion Collisions

One year ago, we presented a new approach to treat hadronic interactions for the initial stage of nuclear collisions. It is an effective theory based on the Gribov-Regge formalism, where the internal structure of the Pomerons at high energies is governed by perturbative parton evolution, therefore the name "Parton-Based Gribov-Regge Theory". The main improvement compared to models used so-far is the appropriate treatment of the energy sharing between the different elementary interactions in case of multiple scattering. It is clear that the above formalism is not yet complete. At high energies (RHIC, LHC), the multiple elementary interactions (Pomerons) can not be purely parallel, they interact. So we introduce multiple Pomeron vertices into the theory.

hep-ph↗

arXiv2002

H1504+65 - The Naked Stellar C/O Core of a Former Red Giant Observed with FUSE and Chandra

H1504+65 is an extremely hot hydrogen-deficient white dwarf with an effective temperature close to 200,000 K. We present new FUV and soft X-ray spectra obtained with FUSE and Chandra, which confirm that H1504+65 has an atmosphere primarily composed of carbon and oxygen. The Chandra spectra show a wealth of absorption lines from highly ionized oxygen, neon and magnesium and suggest relatively high Ne and Mg abundances. This corroborates an earlier suggestion that H1504+65 represents a naked C/O stellar core or even the C/O envelope of a O-Ne-Mg white dwarf.

astro-ph↗

arXiv2002

Iron abundance in hot hydrogen-deficient central stars and white dwarfs from FUSE, HST, and IUE spectroscopy

We present a first systematic investigation of the iron abundance in very hot (Teff>50,000K) hydrogen-deficient post-AGB stars. Our sample comprises 16 PG1159 stars and four DO white dwarfs. We use recent FUSE observations as well as HST and IUE archival data to perform spectral analyses with line blanketed NLTE model atmospheres. Iron is not detected in any PG1159 star. In most cases this is compatible with a solar iron abundance due to limited quality of HST and IUE data, although the tendency to an iron underabundance may be recognized. However, the absence of iron lines in excellent FUSE spectra suggests an underabundance by at least 1 dex in two objects (K1-16 NGC 7094). A similar result has been reported recently in the [WC]-PG1159 transition object Abell 78 (Werner et al. 2002). We discuss dust fractionation and s-process neutron-captures as possible origins. We also announce the first identification of sulfur in PG1159 stars.

astro-ph↗

arXiv2002

Probing Hadronization with Strangeness

The Omega-bar/Omega ratio originating from string decays is predicted to be larger than unity in proton-proton interaction at SPS energies. The anti-omega dominance increases with decreasing beam energy. This surprising behavior is caused by the combinatorics of quark-antiquark production in small and low-mass strings. Since this behavior is not found in a statistical description of hadron production in proton-proton collisions, it may serve as a key observable to probe the hadronization mechanism in such collisions.

nucl-th↗

arXiv2002

Is RX J185635-375 a Quark Star?

Deep Chandra LETG+HRC-S observations of the isolated neutron star candidate RX J1856.5-3754 have been analysed to search for metallic and resonance cyclotron spectral features and for pulsation behaviour. As found from earlier observations, the X-ray spectrum is well-represented by a ~ 60 eV (7e5 K) blackbody. No unequivocal evidence of spectral line or edge features has been found, arguing against metal-dominated models. The data contain no evidence for pulsation and we place a 99% confidence upper limit of 2.7% on the unaccelerated pulse fraction over a wide frequency range from 1e-4 to 100 Hz. We argue that the derived interstellar medium neutral hydrogen column density of 8e19 <= N_H <= 1.1e20 per sq. cm favours the larger distance from two recent HST parallax analyses, placing RX J1856.5-3754 at ~ 140 pc instead of ~ 60 pc, and in the outskirts of the R CrA dark molecular cloud. That such a comparatively rare region of high ISM density is precisely where an isolated neutron star re-heated by accretion of interstellar matter would be expected is either entirely coincidental, or current theoretical arguments excluding this scenario for RX J1856.5-3754 are premature. Taken at face value, the combined observational evidence -- a lack of spectral and temporal features and an implied radius at infinity of 3.8-8.2 km that is too small for current neutron star models -- points to a more compact object, such as allowed for quark matter equations of state.

astro-ph↗

arXiv2002

Temperature Scale and Iron Abundances of Very Hot Central Stars of Planetary Nebulae

The determination of effective temperatures of very hot central stars (Teff>70000K) by model atmosphere analyses of optical H and He line profiles is afflicted with considerable uncertainty, primarily due to the lack of neutral helium lines. Ionization balances of metals, accessible only with UV lines, allow more precise temperature estimates. The potential of iron lines is pointed out. At the same time iron and other metal abundances, hardly investigated until today, may be derived from UV spectra. We describe recent HST spectroscopy performed for this purpose. A search for iron lines in FUV spectra of the hottest H-deficient central stars (PG1159-type, Teff>100000K) taken with FUSE was unsuccessful. The derived deficiency is interpreted in terms of iron depletion due to n-capture nucleosynthesis in intershell matter, which is now exposed at the stellar surface as a consequence of a late He shell flash.

astro-ph↗

arXiv2002

Initial Condition for QGP Evolution from NEXUS

We recently proposed a new approach to high energy nuclear scattering, which treats the initial stage of heavy ion collisions in a sophisticated way. We are able to calculate macroscopic quantities like energy density and velocity flow at the end of this initial stage, after the two nuclei having penetrated each other. In other words, we provide the initial conditions for a macroscopic treatment of the second stage of the collision. We address in particular the question of how to incorporate the soft component properly. We find almost perfect "Bjorken scaling": the rapidity coincides with the space-time rapidity, whereas the transverse flow is practically zero. The distribution of the energy density in the transverse plane shows typically a very "bumpy" structure.

hep-ph↗

arXiv2002

FUSE observations of the central star of Abell 78

FUSE high resolution spectra of two PG1159 type central stars (K1-16 and NGC 7094) have revealed an unexpected iron deficiency of at least 1 or 2 dex (Miksa et al. 2002). Here we present early results of FUSE spectroscopy of the CSPN Abell 78. It is shown that iron is strongly deficient in this star, too.

astro-ph↗

arXiv2002

Spectral analysis of the sdO K 648, the exciting star of the planetary nebula Ps 1 in the globular cluster M 15 (NGC 7078)

We present a spectral analysis of the sdO central star K 648 based on medium-resolution optical and high-resolution UV spectra. The photospheric parameters are determined by means of state-of-the-art NLTE model atmosphere techniques. We found Teff = 39 +/- 2 kK and log g = 3.9 +/- 0.2. The helium (He/H=0.08) and oxygen (O/H=0.001) abundances are about solar while carbon is enriched by a factor of 2.5 (C/H=0.001). Nitrogen (N/H = 10**(-6), [N/H] = -2.0) appears at a sub-solar value. However, these metal abundances are much higher than the cluster's metallicity M 15: [Fe/H] = -2.25). The surface composition appears to be a mixture of the original hydrogen-rich material and products of helium burning (3 alpha process) which have been mixed up to the surface. The abundances of He, C, and N are consistent with the nebular abundance, while O is considerably more abundant in the photosphere than in the nebula. From a comparison of its position in the log Teff - log g plane with evolutionary calculations a mass of 0.57 (+0.02, -0.01) Msun and a luminosity of 3810 +/- 1200 Lsun are deduced. Our spectroscopic distance d = 11.1 (+2.4, -2.9) kpc is in agreement with the distance of M 15 as determined by Alves et al. (2000). From the GHRS spectra we measure a radial velocity of vrad = -130 km/sec.

astro-ph↗

arXiv2001

Overpopulation of $\bar Ω$ in pp collisions: a way to distinguish statistical hadronization from string dynamics

The $\barΩ/Ω$ ratio originating from string decays is predicted to be larger than unity in proton proton interactions at SPS energies ($E_{\rm lab}$=160 GeV). The anti-omega dominance increases with decreasing beam energy. This surprising behavior is caused by the combinatorics of quark-antiquark production in small and low-mass strings. Since this behavior is not found in a statistical description of hadron production in proton proton collisions, it may serve as a key observable to probe the hadronization mechanism in such collisions.

hep-ph↗

arXiv2001

Consistent Treatment of Soft and Hard Processes in Hadronic Interactions

The QCD improved parton model is a very successful concept to treat processes in hadronic interactions, whenever large partonic transverse momenta are involved. However, cross sections diverge in the limit p_T -> 0, and the usual treatment is the definition of a lower cutoff p_T_min, such that processes with a smaller p_T -- so-called soft processes -- are simply ignored, which is certainly not correct for example at RHIC energies. A more consistent procedure amounts to introduce a technical parameter Q_0^2, referred to as soft virtuality scale, which is nothing but an artificial borderline between soft and hard physics. We will discuss such a formalism, which coincides with the improved parton model for high p_T processes and with the phenomenological treatment of soft scattering, when only small virtualities are involved. The most important aspect of our approach is that it allows to obtain a smooth transition between soft and hard scattering, and therefore no artificial dependence on a cutoff parameter should appear.

hep-ph↗

arXiv2001

Inconsistencies in Models for RHIC and LHC

The interpretation of experimental results at RHIC and in the future also at LHC requires very reliable and realistic models. Considerable effort has been devoted to the development of such models during the past decade, many of them being heavily used in order to analyze data. It is the purpose of this paper to point out serious inconsistencies in the above-mentioned approaches. We will demonstrate that requiring theoretical self-consistency reduces the freedom in modeling high energy nuclear scattering enormously. We will introduce a fully self-consistent formulation of the multiple-scattering scheme in the framework of a Gribov-Regge type effective theory. In addition, we develop new computational techniques which allow for the first time a satisfactory solution of the problem in the sense that calculations of observable quantities can be done strictly within a self-consistent formalism.

hep-ph↗

arXiv2001

Models for RHIC and LHC: New Developments

We outline inconsistencies in presently used models for high energy nuclear scattering, which make their application quite unreliable. Many "successes" are essentially based on an artificial freedom of parameters, which does not exist when the models are constructed properly. The problem is the fact that any multiple scattering theory requires an appropriate treatment of the energy sharing between the individual interactions, which is technically very difficult to implement. Lacking a satisfying solution to this problem, it has been simply ignored. We introduce a fully self-consistent formulation of the multiple-scattering scheme. Inclusion of soft and hard components - very crucial at high energies - appears in a "natural way", providing a smooth transition from soft to hard physics. We can show that the effect of appropriately considering energy conservation has a big influence on the results, and MUST therefore be included in any serious calculation.

hep-ph↗

arXiv2001

Nuclear Scattering at Very High Energies

We discuss the current understanding of nuclear scattering at very high energies. We point out several serious inconsistencies in nowadays models, which provide big problems for any interpretation of data at high energy nuclear collisions. We outline how to develop a fully self-consistent formalism, which in addition uses all the knowledge available from studying electron-positron annihilation and deep inelastic scattering, providing a solid basis for further developments concerning secondary interactions.

hep-ph↗

arXiv2001

Self-Consistency Requirement in High-Energy Nuclear Scattering

Practically all serious calculations of exclusive particle production in ultra-relativistic nuclear or hadronic interactions are performed in the framework of Gribov-Regge theory or the eikonalized parton model scheme. It is the purpose of this paper to point out serious inconsistencies in the above-mentioned approaches. We will demonstrate that requiring theoretical self-consistency reduces the freedom in modeling high energy nuclear scattering enormously. We will introduce a fully self-consistent formulation of the multiple-scattering scheme in the framework of a Gribov-Regge type effective theory. In addition, we develop new computational techniques which allow for the first time a satisfactory solution of the problem in the sense that calculation s of observable quantities can be done strictly within a self-consistent formalism.

hep-ph↗