New intensity and visibility aspects of a double loop neutron interferometer

Various phase shifters and absorbers can be put into the arms of a double loop neutron interferometer. The mean intensity levels of the forward and diffracted beams behind an empty four plate interferometer of this type have been calculated. It is shown that the intensities in the forward and diffracted direction can be made equal using certain absorbers. In this case the interferometer can be regarded as a 50/50 beam splitter. Furthermore the visibilities of single and double loop interferometers are compared to each other by varying the transmission in the first loop using different absorbers. It can be shown that the visibility becomes exactly 1 using a phase shifter in the second loop. In this case the phase shifter in the second loop must be strongly correlated to the transmission coefficient of the absorber in the first loop. Using such a device homodyne-like measurements of very weak signals should become possible.Comment: 12 pages, 9 figures, accepted for publication in the Journal of Optics B - Quantum and Semiclassical Optic

3D Photoionisation Modelling of NGC 6302

We present a three-dimensional photoionisation and dust radiative transfer model of NGC 6302, an extreme, high-excitation planetary nebula. We use the 3D photoionisation code Mocassin} to model the emission from the gas and dust. We have produced a good fit to the optical emission-line spectrum, from which we derived a density distribution for the nebula. A fit to the infrared coronal lines places strong constraints on the properties of the unseen ionising source. We find the best fit comes from using a 220,000 K hydrogen-deficient central star model atmosphere, indicating that the central star of this PN may have undergone a late thermal pulse. We have also fitted the overall shape of the ISO spectrum of NGC 6302 using a dust model with a shallow power-law size distribution and grains up to 1.0 micron in size. To obtain a good fit to the infrared SED the dust must be sufficiently recessed within the circumstellar disk to prevent large amounts of hot dust at short wavelengths, a region where the ISO spectrum is particularly lacking. These and other discoveries are helping to unveil many properties of this extreme object and trace it's evolutionary history.Comment: 8 pages, 4 figures; for the proceedings of "Asymmetric Planetary Nebuale IV," R. L. M. Corradi, A. Manchado, N. Soker ed

Regulation of Human Hsp70 by its Nucleotide Exchange Factors (NEFs).

Heat shock protein 70 (Hsp70) is an abundant and ubiquitous molecular chaperone that is responsible for maintenance of the human proteome. Accordingly, Hsp70 has become an attractive drug target for neurodegenerative and hyperproliferative disorders; however it is difficult to imagine strategies for inhibiting its pathobiology without impacting its essential roles. Fortunately, Hsp70 does not work alone, and instead employs a large network of co-chaperone proteins, which can tune Hsp70 activity and influence disease state. These co-chaperone proteins provide potential handles for targeting Hsp70 without disrupting overall proteostasis. One such class of co-chaperones proteins known as the Nucleotide Exchange Factors (NEFs), are a particular appealing target. NEFs bind Hsp70 and help to facilitate the exchange of ADP for ATP. The biochemistry of the NEF family of co-chaperones has classically been investigated using the prokaryotic NEF, GrpE, as a model. However, the eukaryotic cytosol does not contain a GrpE homolog. Rather, there are three main sub-classes of human NEFs: Hsp110, HspBP1, and the BAG proteins, all of which are structurally distinct with little sequence homology. Consistent with their diverse structures, they also differ in their mode of binding to Hsp70 and their roles in guiding Hsp70 biology. For example, BAG2 is associated with proteasomal degradation of the Hsp70 substrate, tau, while BAG1-Hsp70 is linked to increased tau stability. These observations suggest that the formation of specific NEF-Hsp70 complexes may help decide the fate of Hsp70-bound substrates. Additionally, these findings illustrate that differential disruption of specific Hsp70-NEF contacts might be beneficial in disease. In this thesis work I have systematically characterized the human Hsp70 NEFs, including how they interact with Hsp70, how the influence Hsp70 biochemistry and how they can bridge Hsp70 with other classes of chaperone proteins. I have used high throughput screening methods to search for chemical matter that can modulate Hsp70-NEF interactions, and we have shown that inhibitors of Hsp70-NEF interactions can be beneficial for treating disease. This thesis work has significantly advanced our knowledge of human Hsp70 regulation, and has provided groundwork for future studies on other Hsp70 co-chaperones and proteostasis components.PhDBiological ChemistryUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/111611/1/rauchjn_1.pd

On helium-dominated stellar evolution: the mysterious role of the O(He)-type stars

About a quarter of all post-asymptotic giant branch (AGB) stars are hydrogen-deficient. Stellar evolutionary models explain the carbon-dominated H-deficient stars by a (very) late thermal pulse scenario where the hydrogen-rich envelope is mixed with the helium-rich intershell layer. Depending on the particular time at which the final flash occurs, the entire hydrogen envelope may be burned. In contrast, helium-dominated post-AGB stars and their evolution are yet not understood. A small group of very hot, helium-dominated stars is formed by O(He)-type stars. We performed a detailed spectral analysis of ultraviolet and optical spectra of four O(He) stars by means of state-of-the-art non-LTE model-atmosphere techniques. We determined effective temperatures, surface gravities, and the abundances of H, He, C, N, O, F, Ne, Si, P, S, Ar, and Fe. By deriving upper limits for the mass-loss rates of the O(He) stars, we found that they do not exhibit enhanced mass-loss. The comparison with evolutionary models shows that the status of the O(He) stars remains uncertain. Their abundances match predictions of a double helium white dwarf merger scenario, suggesting that they might be the progeny of the compact and of the luminous helium-rich sdO-type stars. The existence of planetary nebulae that do not show helium enrichment around every other O(He) star, precludes a merger origin for these stars. These stars must have formed in a different way, for instance via enhanced mass-loss during their post-AGB evolution or a merger within a common-envelope (CE) of a CO-WD and a red giant or AGB star. A helium-dominated stellar evolutionary sequence exists, that may be fed by different types of mergers or CE scenarios. It appears likely, that all these pass through the O(He) phase just before they become white dwarfs.Comment: 29 pages, 27 figures, accepted for publication in A&

Analysis of cool DO-type white dwarfs from the Sloan Digital Sky Survey Data Release 10

We report on the identification of 22 new cool DO-type white dwarfs (WD) detected in Data Release 10 (DR10) of the Sloan Digital Sky Survey (SDSS). Among them, we found one more member of the so-called hot-wind DO WDs, which show ultrahigh excitation absorption lines. Our non-LTE model atmosphere analyses of these objects and two not previously analyzed hot-wind DO WDs, revealed effective temperatures and gravities in the ranges Teff=45-80kK and log g= 7.50-8.75. In eight of the spectra we found traces of C (0.001-0.01, by mass). Two of these are the coolest DO WDs ever discovered that still show a considerable amount of C in their atmospheres. This is in strong contradiction with diffusion calculations, and probably, similar to what is proposed for DB WDs, a weak mass-loss is present in DO WDs. One object is the most massive DO WD discovered so far with a mass of 1.07 M_sun if it is an ONe-WD or 1.09 M_sun if it is a CO-WD. We furthermore present the mass distribution of all known hot non-DA (pre-) WDs and derive the hot DA to non-DA ratio for the SDSS DR7 spectroscopic sample. The mass distribution of DO WDs beyond the wind limit strongly deviates from the mass distribution of the objects before the wind limit. We address this phenomenon by applying different evolutionary input channels. We argue that the DO WD channel may be fed by about 13% by post-extreme-horizontal branch stars and that PG1159 stars and O(He) stars may contribute in a similar extent to the non-DA WD channel.Comment: 13 pages, accepted for publication in A&

Dynamics of the Tippe Top -- properties of numerical solutions versus the dynamical equations

We study the relationship between numerical solutions for inverting Tippe Top and the structure of the dynamical equations. The numerical solutions confirm oscillatory behaviour of the inclination angle $\theta(t)$ for the symmetry axis of the Tippe Top. They also reveal further fine features of the dynamics of inverting solutions defining the time of inversion. These features are partially understood on the basis of the underlying dynamical equations

Discovery of excess O I absorption towards the z = 6.42 QSO SDSS J1148+5251

We present a search for O I in the spectra of nine 4.9 < z_qso < 6.4 QSOs taken with Keck/HIRES. We detect six systems with N(O I) > 10^13.7 cm^{-2} in the redshift intervals where O I 1302 falls redward of the Ly-alpha forest. Four of these lie towards SDSS J1148+5251 (z_qso = 6.42). This imbalance is unlikely to arise from variations in sensitivity among our data or from a statistical fluctuation. The excess O I occurs over a redshift interval that also contains transmission in Ly-alpha and Ly-beta. Therefore, if these O I systems represent pockets of neutral gas, then they must occur within or near regions of the IGM that are highly ionized. In contrast, no O I is detected towards SDSS J1030+0524 (z_qso = 6.30), whose spectrum shows complete absorption in Ly-alpha and Ly-beta over \Delta z ~ 0.2. Assuming no ionization corrections, we measure mean abundance ratios = -0.04 +/- 0.06, = -0.31 +/- 0.09, and = -0.34 +/- 0.07 (2 sigma), which are consistent with enrichment dominated by Type II supernovae. The O/Si ratio limits the fraction of silicon in these systems contributed by metal-free very massive stars to < 30%, a result which is insensitive to ionization corrections. The ionic comoving mass densities along the z_qso > 6.2 sightlines, including only the detected systems, are \Omega(O I) = (7.0 +/- 0.6) * 10^{-8}, \Omega(Si II) = (9.6 +/- 0.9) * 10^{-9}, and \Omega(C II) = (1.5 +/- 0.2) * 10^{-8}.Comment: Submitted to ApJ, with changes to reflect referee's comment

On the density-potential mapping in time-dependent density functional theory

The key questions of uniqueness and existence in time-dependent density functional theory are usually formulated only for potentials and densities that are analytic in time. Simple examples, standard in quantum mechanics, lead however to non-analyticities. We reformulate these questions in terms of a non-linear Schr\"odinger equation with a potential that depends non-locally on the wavefunction.Comment: 8 pages, 2 figure

The rapid evolution of the exciting star of the Stingray Nebula

SAO244567, the exciting star of the Stingray nebula, is rapidly evolving. Previous analyses suggested that it has heated up from an effective temperature of about 21kK in 1971 to over 50kK in the 1990s. Canonical post-asymptotic giant branch evolution suggests a relatively high mass while previous analyses indicate a low-mass star. Fitting line profiles from static and expanding non-LTE model atmospheres to the observed UV and optical spectra, taken during 1988-2013, allowed us to study the temporal change of effective temperature, surface gravity, mass-loss rate, and terminal wind velocity. In addition, we determined the chemical composition of the atmosphere. We find that the central star has steadily increased its effective temperature from 38kK in 1988 to a peak value of 60kK in 2002. During the same time, the star was contracting, as concluded from an increase in surface gravity from log g = 4.8 to 6.0 and a drop in luminosity. Simultaneously, the mass-loss rate declined from log (dM/dt/Msun/yr)=-9.0 to -11.6 and the terminal wind velocity increased from 1800km/s to 2800km/s. Since around 2002, the star stopped heating and has cooled down again to 55kK by 2006. It has a largely solar surface composition with the exception of slightly subsolar carbon, phosphorus, and sulfur. By comparison with stellar-evolution calculations, we confirm that SAO244567 must be a low-mass star (M < 0.55 Msun). However, the slow evolution of the respective stellar evolutionary models is in strong contrast to the observed fast evolution and the young planetary nebula with a kinematical age of only about 1000 years. We speculate that the star could be a late He-shell flash object. Alternatively, it could be the outcome of close-binary evolution. Then SAO244567 would be a low-mass (0.354 Msun) helium prewhite dwarf after the common-envelope phase, during which the planetary nebula was ejected.Comment: 16 pages, 13 figures, accepted for publication in A&