Far-UV Emission from Elliptical Galaxies at z=0.55

The restframe UV-to-optical flux ratio, characterizing the ``UV upturn'' phenomenon, is potentially the most sensitive tracer of age in elliptical galaxies; models predict that it may change by orders of magnitude over the course of a few Gyr. In order to trace the evolution of the UV upturn as a function of redshift, we have used the far-UV camera on the Space Telescope Imaging Spectrograph to image the galaxy cluster CL0016+16 at z=0.55. Our 25''x25'' field includes four bright elliptical galaxies, spectroscopically confirmed to be passively evolving cluster members. The weak UV emission from the galaxies in our image demonstrates that the UV upturn is weaker at a lookback time 5.6 Gyr earlier than our own, as compared to measurements of the UV upturn in cluster E and S0 galaxies at z=0 and z=0.375. These images are the first with sufficient depth to demonstrate the fading of the UV upturn expected at moderate redshifts. We discuss these observations and the implications for the formation history of galaxies.Comment: 4 pages, Latex. 2 figures. Uses corrected version of emulateapj.sty and apjfonts.sty (included). Accepted for publication in ApJ Letter

Efficient multiple time scale molecular dynamics: using colored noise thermostats to stabilize resonances

Multiple time scale molecular dynamics enhances computational efficiency by updating slow motions less frequently than fast motions. However, in practice the largest outer time step possible is limited not by the physical forces but by resonances between the fast and slow modes. In this paper we show that this problem can be alleviated by using a simple colored noise thermostatting scheme which selectively targets the high frequency modes in the system. For two sample problems, flexible water and solvated alanine dipeptide, we demonstrate that this allows the use of large outer time steps while still obtaining accurate sampling and minimizing the perturbation of the dynamics. Furthermore, this approach is shown to be comparable to constraining fast motions, thus providing an alternative to molecular dynamics with constraints.Comment: accepted for publication by the Journal of Chemical Physic

Modeling, simulation, and control of a polypyrrole-based conducting polymer actuator

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2004.Includes bibliographical references.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.A detailed model was developed for an ionic electro-active polymer (EAP) actuator. The electrical and chemical domains of the system were modeled using a simple electrical circuit. Ionic charge storage within the polymer was described using a linear reticulated model. This model improves upon the continuum diffusive model introduced in prior work by providing a low order model of diffusion that can be analyzed in the context of modern and classical control methods. Additionally, the reticulated diffusion model describes the dynamics of ionic charge distribution within the polymer, which enables a more precise calculation of electromechanical coupling. An interesting observation of ionic electro-active polymers is that they exhibit enormous asymmetry in coupling from electrical to mechanical domains. While electrical potentials produce large linear displacements (5% strain or greater), uniaxially-applied mechanical loads result in a negligible electrical back effect. This is surprising, suggesting that there are huge entropic losses when applying mechanical loads. After examining the mechanics of the system it was theorized that the apparent lack of coupling is actually the result of the Poisson Effect, which causes changes in the volume of an object when uniaxial loads are applied. A derivation of the stored electrical energy and strain energy led directly to a set of constitutive equations that are able to account for the asymmetric coupling observed in EAP. The solution to the uniaxial loading boundary condition was developed fully and compared to prior work. Experimental results from an EAP actuator composed of polypyrrole, a widely-used conducting polymer, validate the electro-mechanical coupling model. MATLAB was used to simulate the(cont.) response of the actuator and the results compared to the experimental data. Results verify that the model accurately describes the electrical, mechanical, and coupled behavior of the system. The correlation between the model and experimental data is very good for electrically-induced strains up to 3% and applied potentials up to 1 Volt above the potential of zero charge (PZC); these are within the typical operating range of polypyrrole. The model is sufficiently simple to allow real-time control while also exceeding prior models in its ability to predict polymer behavior in normal operating ranges.by Thomas A. Bowers.S.M

Far-Ultraviolet Emission from Elliptical Galaxies at z=0.33

We present far-ultraviolet (far-UV) images of the rich galaxy cluster ZwCl1358.1+6245, taken with the Space Telescope Imaging Spectrograph on board the Hubble Space Telescope (HST). When combined with archival HST observations, our data provide a measurement of the UV-to-optical flux ratio in 8 early-type galaxies at z=0.33. Because the UV flux originates in a population of evolved, hot, horizontal branch (HB) stars, this ratio is potentially one of the most sensitive tracers of age in old populations -- it is expected to fade rapidly with lookback time. We find that the UV emission in these galaxies, at a lookback time of 3.9 Gyr, is significantly weaker than it is in the current epoch, yet similar to that in galaxies at a lookback time of 5.6 Gyr. Taken at face value, these measurements imply different formation epochs for the massive ellipticals in these clusters, but an alternative explanation is a "floor" in the UV emission due to a dispersion in the parameters that govern HB morphology.Comment: 4 pages, Latex. 2 figures. Uses corrected version of emulateapj.sty and apjfonts.sty (included). Accepted for publication in ApJ Letter

A common cardiac sodium channel variant associated with sudden infant death in African Americans, SCN5A S1103Y.

Thousands die each year from sudden infant death syndrome (SIDS). Neither the cause nor basis for varied prevalence in different populations is understood. While 2 cases have been associated with mutations in type Valpha, cardiac voltage-gated sodium channels (SCN5A), the "Back to Sleep" campaign has decreased SIDS prevalence, consistent with a role for environmental influences in disease pathogenesis. Here we studied SCN5A in African Americans. Three of 133 SIDS cases were homozygous for the variant S1103Y. Among controls, 120 of 1,056 were carriers of the heterozygous genotype, which was previously associated with increased risk for arrhythmia in adults. This suggests that infants with 2 copies of S1103Y have a 24-fold increased risk for SIDS. Variant Y1103 channels were found to operate normally under baseline conditions in vitro. As risk factors for SIDS include apnea and respiratory acidosis, Y1103 and wild-type channels were subjected to lowered intracellular pH. Only Y1103 channels gained abnormal function, demonstrating late reopenings suppressible by the drug mexiletine. The variant appeared to confer susceptibility to acidosis-induced arrhythmia, a gene-environment interaction. Overall, homozygous and rare heterozygous SCN5A missense variants were found in approximately 5% of cases. If our findings are replicated, prospective genetic testing of SIDS cases and screening with counseling for at-risk families warrant consideration

The Dearth of UV-Bright Stars in M32: Implications for Stellar Evolution Theory

Using the Space Telescope Imaging Spectrograph on the Hubble Space Telescope, we have obtained deep far-ultraviolet images of the compact elliptical galaxy M32. When combined with earlier near-ultraviolet images of the same field, these data enable the construction of an ultraviolet color-magnitude diagram of the hot horizontal branch (HB) population and other hot stars in late phases of stellar evolution. We find few post-asymptotic giant branch (PAGB) stars in the galaxy, implying that these stars either cross the HR diagram more rapidly than expected, and/or that they spend a significant fraction of their time enshrouded in circumstellar material. The predicted luminosity gap between the hot HB and its AGB-Manque (AGBM) progeny is less pronounced than expected, especially when compared to evolutionary tracks with enhanced helium abundances, implying that the presence of hot HB stars in this metal-rich population is not due to (Delta Y)/(Delta Z) > 4. Only a small fraction (~2%) of the HB population is hot enough to produce significant UV emission, yet most of the UV emission in this galaxy comes from the hot HB and AGBM stars, implying that PAGB stars are not a significant source of UV emission even in those elliptical galaxies with a weak UV excess.Comment: Accepted for publication in The Astrophysical Journal. Latex, 18 pages, 18 black & white figures, in emulate-ApJ format. Figures 11 & 16 have been degraded due to size constraints; the high-quality version of the paper is at http://www.stsci.edu/~tbrown/research/m32fuv.pd

Simulations of electromagnetic effects in high frequency capacitively coupled discharges using the Darwin approximation

The Darwin approximation is investigated for its possible use in simulation of electromagnetic effects in large size, high frequency capacitively coupled discharges. The approximation is utilized within the framework of two different fluid models which are applied to typical cases showing pronounced standing wave and skin effects. With the first model it is demonstrated that Darwin approximation is valid for treatment of such effects in the range of parameters under consideration. The second approach, a reduced nonlinear Darwin approximation-based model, shows that the electromagnetic phenomena persist in a more realistic setting. The Darwin approximation offers a simple and efficient way of carrying out electromagnetic simulations as it removes the Courant condition plaguing explicit electromagnetic algorithms and can be implemented as a straightforward modification of electrostatic algorithms. The algorithm described here avoids iterative schemes needed for the divergence cleaning and represents a fast and efficient solver, which can be used in fluid and kinetic models for self-consistent description of technical plasmas exhibiting certain electromagnetic activity

Polymyxin B in Combination with Rifampin and Meropenem against Polymyxin B-Resistant KPC-Producing Klebsiella pneumoniae

Safe and effective therapies are urgently needed to treat polymyxin-resistant KPC-producing K. pneumoniae and suppress the emergence of resistance. We investigated the pharmacodynamics of polymyxin B, rifampin, and meropenem alone and as polymyxin B-based double and triple combinations against KPC-producing K. pneumoniae . The rate and extent of killing with polymyxin B (1-128mg/L), rifampin (2-16mg/L), and meropenem (10-120mg/L) were evaluated against polymyxin B-susceptible (PB S ) and -resistant (PB R ) clinical isolates using 48h static time-kills. Additionally, humanized triple drug regimens of polymyxin B (C ss : 0.5, 1, 2mg/L), rifampin 600mg every 12 or 8h, and meropenem 1 or 2g every 8h dosed as an extended 3h infusion were simulated over 48h using a one-compartment in vitro dynamic infection model. Serial bacterial counts were measured to quantify pharmacodynamic effect. Population analysis profiles (PAPs) were used to assess the emergence of polymyxin B resistance. Monotherapy was ineffective against both isolates. Polymyxin B with rifampin demonstrated early bactericidal activity against the PB S isolate followed by regrowth by 48h. Bactericidal activity was sustained at all polymyxin B concentrations ≥2 mg/L in combination with meropenem. No two-drug combinations were effective against the PB R isolate, but all simulated triple-drug regimens showed early bactericidal activity by 8h that was sustained over 48h against both strains. PAPs did not reveal the emergence of resistant subpopulations. The triple drug combination of polymyxin B, rifampin, and meropenem may be a viable consideration for the treatment of PB R KPC-producing K. pneumoniae . Further investigation is warranted to optimize triple combination therapy