A Model for the Strings of Eta Carinae

We propose a model based on ionization shadows to explain the formation of the long and narrow strings of Eta Carinae. Five strings are known, all located along the symmetry axis outside the Homunculus. The model assumes that each string is formed in a shadow behind a dense clump near the symmetry axis. The surrounding gas is ionized first, becomes much hotter, and compresses the gas in the shadow. This leads to the formation of a radial, dense, long, and narrow region, i.e., a string. Later the neutral material in the strings is ionized, and becomes brighter. Still later it re-expands, and we predict that in about 200 years the strings will fade. The condition for the model to work is that the ionization front, due to the diffuse ionizing recombination radiation of the surrounding gas, proceeds into the shadow at a velocity slower than the compression speed. From that we get a condition on the mass loss rate of the mass loss episode that formed the strings, which should be less than 10^{-4} Mo/year. The model can also explain the strings in the planetary nebula NGC 6543.Comment: 8 pages; Submitted to A&

Analytical solution of the optimal laser control problem in two-level systems

The optimal control of two-level systems by time-dependent laser fields is studied using a variational theory. We obtain, for the first time, general analytical expressions for the optimal pulse shapes leading to global maximization or minimization of different physical quantities. We present solutions which reproduce and improve previous numerical results.Comment: 12 pages, 2 figure

Resistive jet simulations extending radially self-similar magnetohydrodynamic models

Numerical simulations with self-similar initial and boundary conditions provide a link between theoretical and numerical investigations of jet dynamics. We perform axisymmetric resistive magnetohydrodynamic (MHD) simulations for a generalised solution of the Blandford & Payne type, and compare them with the corresponding analytical and numerical ideal-MHD solutions. We disentangle the effects of the numerical and physical diffusivity. The latter could occur in outflows above an accretion disk, being transferred from the underlying disk into the disk corona by MHD turbulence (anomalous turbulent diffusivity), or as a result of ambipolar diffusion in partially ionized flows. We conclude that while the classical magnetic Reynolds number $R_{\rm m}$ measures the importance of resistive effects in the induction equation, a new introduced number, \rbeta=(\beta/2)R_{\rm m} with $\beta$ the plasma beta, measures the importance of the resistive effects in the energy equation. Thus, in magnetised jets with $\beta<2$, when \rbeta \la 1 resistive effects are non-negligible and affect mostly the energy equation. The presented simulations indeed show that for a range of magnetic diffusivities corresponding to \rbeta \ga 1 the flow remains close to the ideal-MHD self-similar solution.Comment: Accepted for publication in MNRA

Comparison of fouling between aerobic and anaerobic MBR treating municipal wastewater

The key driver for anaerobic membrane bioreactors (AnMBR) for municipal wastewater treatment is enabling the transition to energy neutral wastewater treatment. However, municipal wastewater delivers a comparatively constrained methane yield, which means energy conservation must be prioritised to achieve the proposed energy neutral ambition. A critical focus on membrane fouling is therefore warranted, as membrane operation represents the primary energy demand in MBRs. This review seeks to quantify the characteristics of the prevailing AnMBR biological suspension and to ascertain whether knowledge transfer exists between fouling characteristics in aerobic and anaerobic MBRs for municipal applications. Analysis of literature data revealed that the level of extractable extracellular polymeric substrate is slightly higher in aerobic MBRs than in anaerobic MBRs. However, AnMBR comprises considerably higher soluble microbial product concentrations, which have been widely reported to increase fouling propensity in aerobic systems. More distinct is the difference in the colloidal and fine solids fraction (between 1 and 10–15 μm), which is likely to dominate fouling in anaerobic systems and limit knowledge transfer from aerobic MBRs. Literature data on energy production was compared to that employed for membrane operation, and evidences that despite the challenging character of the particle matrix, energy neutral operation is achievable for AnMBR applied to municipal wastewater treatment

Efficient construction of maximally localized photonic Wannier functions: locality criterion and initial conditions

Wannier function expansions are well suited for the description of photonic- crystal-based defect structures, but constructing maximally localized Wannier functions by optimizing the phase degree of freedom of the Bloch modes is crucial for the efficiency of the approach. We systematically analyze different locality criteria for maximally localized Wannier functions in two- dimensional square and triangular lattice photonic crystals, employing (local) conjugate-gradient as well as (global) genetic-algorithm-based, stochastic methods. Besides the commonly used second moment (SM) locality measure, we introduce a new locality measure, namely the integrated modulus (IM) of the Wannier function. We show numerically that, in contrast to the SM criterion, the IM criterion leads to an optimization problem with a single extremum, thus allowing for fast and efficient construction of maximally localized Wannier functions using local optimization techniques. We also present an analytical formula for the initial choice of Bloch phases, which under certain conditions represents the global maximum of the IM criterion and, thus, further increases the optimization efficiency in the general case

Accretion-Induced Lithium Line Enhancements in Classical T Tauri Stars: RW Aur

It is widely accepted that much of the stochastic variability of T Tauri stars is due to accretion by a circumstellar disk. The emission line spectrum as well as the excess continuum emission are common probes of this process. In this communication, we present additional probes of the circumstellar environment in the form of resonance lines of low ionization potential elements. Using a set of 14 high resolution echelle observations of the classical T Tauri star (CTTS), RW Aur, taken between 1986 and 1996, we carefully measure the continuum veiling at each epoch by comparing more than 500 absorption lines with those of an appropriate template. This allows us to accurately subtract out the continuum emission and to recover the underlying photospheric spectrum. In doing so, we find that selected photospheric lines are enhanced by the accretion process, namely the resonance lines of LiI and KI. A resonance line of TiI and a low excitation potential line of CaI also show weak enhancements. Simple slab models and computed line bisectors lead us to propose that these line enhancements are markers of cool gas at the beginning of the accretion flow which provides an additional source of line opacity. These results suggest that published values of surface lithium abundances of classical T Tauri stars are likely to be overestimated. This would account for the various reports of surface lithium abundances in excess of meteoritic values among the extreme CTTS. Computing LTE lithium abundances of RW Aur in a low and then high accretion state yields abundances which vary by one order of magnitude. The low accretion state lithium abundance is consistent with theoretical predictions for a star of this age and mass while the high accretion state spectrum yields a super-meteoritic lithium abundance.Comment: 28 pages, 8 figures, accepted by Ap

On carbon and oxygen isotope ratios in starburst galaxies: New data from NGC253 and Mrk231 and their implications

Using the IRAM 30-m telescope, CN and CO isotopologues have been measured toward the central regions of the nearby starburst galaxy NGC253 and the prototypical ultraluminous infrared galaxy Mrk231. In NGC253, the 12C/13C ratio is 40+-10. Assuming that the ratio also holds for the CO emitting gas, this yields 16O/18O = 145+-36 and 16O/17O = 1290+-365 and a 32S/34S ratio close to that measured for the local interstellar medium (20-25). No indication for vibrationally excited CN is found. Peak line intensity ratios between NGC253 and Mrk231 are ~100 for 12C16O and 12C18O J=1-0, while the ratio for 13C16O J=1-0 is ~250. This and similar 13CO and C18O line intensities in the J=1-0 and 2-1 transitions of Mrk231 suggest 12C/13C ~ 100 and 16O/18O ~ 100, in agreement with values obtained for the less evolved ultraluminous merger Arp220. Also accounting for other extragalactic data, 12C/13C ratios appear to vary over a full order of magnitude, from >100 in ultraluminous high redshift galaxies to ~100 in more local such galaxies to ~40 in weaker starbursts not undergoing a large scale merger to 25 in the Central Molecular Zone of the Milky Way. With 12C being predominantly synthesized in massive stars, while 13C is mostly ejected by longer lived lower mass stars at later times, this is qualitatively consistent with our results of decreasing carbon isotope ratios with time and rising metallicity. It is emphasized, however, that both infall of poorly processed material, initiating a nuclear starburst, as well as the ejecta from newly formed massive stars (in particular in case of a top-heavy stellar initial mass function) can raise the carbon isotope ratio for a limited amount of time.Comment: Accepted by Astronomy & Astrophysics, 6 figures, 4 table