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DSpace at MIT (74,934 recursos)
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MIT Open Access Articles

Mostrando recursos 41 - 60 de 12,613

41. Understanding microbial cooperation - Damore, James A.; Gore, Jeff
The field of microbial cooperation has grown enormously over the last decade, leading to improved experimental techniques and a growing awareness of collective behavior in microbes. Unfortunately, many of our theoretical tools and concepts for understanding cooperation fail to take into account the peculiarities of the microbial world, namely strong selection strengths, unique population structure, and non-linear dynamics. Worse yet, common verbal arguments are often far removed from the math involved, leading to confusion and mistakes. Here, we review the general mathematical forms of Price's equation, Hamilton's rule, and multilevel selection as they are applied to microbes and provide some...

42. DERIVING STELLAR EFFECTIVE TEMPERATURES OF METAL-POOR STARS WITH THE EXCITATION POTENTIAL METHOD - Frebel, Anna L.; Casey, Andrew R.; Jacobson, Heather; Yu, Qinsi
It is well established that stellar effective temperatures determined from photometry and spectroscopy yield systematically different results. We describe a new, simple method to correct spectroscopically derived temperatures ("excitation temperatures") of metal-poor stars based on a literature sample with –3.3 < [Fe/H] < –2.5. Excitation temperatures were determined from Fe I line abundances in high-resolution optical spectra in the wavelength range of ~3700-~7000 Å, although shorter wavelength ranges, up to 4750-6800 Å, can also be employed, and compared with photometric literature temperatures. Our adjustment scheme increases the temperatures up to several hundred degrees for cool red giants, while leaving the...

43. A CHANDRA VIEW OF NON-THERMAL EMISSION IN THE NORTHWESTERN REGION OF SUPERNOVA REMNANT RCW 86: PARTICLE ACCELERATION AND MAGNETIC FIELDS - Castro, Daniel; Lopez, Laura A.; Slane, Patrick O.; Yamaguchi, Hiroya; Ramirez-Ruiz, Enrico; Figueroa-Feliciano, Enectali
The shocks of supernova remnants are believed to accelerate particles to cosmic ray (CR) energies. The amplification of the magnetic field due to CRs propagating in the shock region is expected to have an impact on both the emission from the accelerated particle population as well as the acceleration process itself. Using a 95 ks observation with the Advanced CCD Imaging Spectrometer on board the Chandra X-Ray Observatory, we map and characterize the synchrotron emitting material in the northwest region of RCW 86. We model spectra from several different regions, both filamentary and diffuse, where emission appears to be dominated...

44. Toward directed energy planetary defense - Lubin, Philip; Hughes, Gary B.; Bible, Johanna; Bublitz, Jesse; Arriola, Josh; Motta, Caio; Suen, Jon; Johansson, Isabella; Riley, Jordan; Sarvian, Nilou; Clayton-Warwick, Deborah; Wu, Jane; Milich, Andrew; Oleson, Mitch; Pryor, Mark; Krogen, Peter Ra; Kangas, Miikka; O’Neill, Hugh
Asteroids and comets that cross Earth’s orbit pose a credible risk of impact, with potentially severe disturbances to Earth and society. We propose an orbital planetary defense system capable of heating the surface of potentially hazardous objects to the vaporization point as a feasible approach to impact risk mitigation. We call the system DE-STAR, for Directed Energy System for Targeting of Asteroids and exploRation. The DE-STAR is a modular-phased array of kilowatt class lasers powered by photovoltaic’s. Modular design allows for incremental development, minimizing risk, and allowing for technological codevelopment. An orbiting structure would be developed in stages. The main...

45. CONSTRAINING EXPLOSION TYPE OF YOUNG SUPERNOVA REMNANTS USING 24 μm EMISSION MORPHOLOGY - Peters, Charee L.; Lopez, Laura A.; Ramirez-Ruiz, Enrico; Stassun, Keivan G.; Figueroa-Feliciano, Enectali
Determination of the explosion type of supernova remnants (SNRs) can be challenging, as SNRs are hundreds to thousands of years old and supernovae are classified based on spectral properties days after explosion. Previous studies of thermal X-ray emission from Milky Way and Large Magellanic Cloud SNRs have shown that Type Ia and core-collapse (CC) SNRs have statistically different symmetries, and thus these sources can be typed based on their X-ray morphologies. In this Letter, we extend the same technique, a multipole expansion technique using power ratios, to infrared (IR) images of SNRs to test whether they can be typed using...

46. Effect of Microstimulation of the Superior Colliculus on Visual Space Attention - Gattass, Ricardo; Desimone, Robert
It is well established that stellar effective temperatures determined from photometry and spectroscopy yield systematically different results. We describe a new, simple method to correct spectroscopically derived temperatures ("excitation temperatures") of metal-poor stars based on a literature sample with –3.3 < [Fe/H] < –2.5. Excitation temperatures were determined from Fe I line abundances in high-resolution optical spectra in the wavelength range of ~3700-~7000 Å, although shorter wavelength ranges, up to 4750-6800 Å, can also be employed, and compared with photometric literature temperatures. Our adjustment scheme increases the temperatures up to several hundred degrees for cool red giants, while leaving the...

47. Visual Apollo: A Graphical Exploration of Computer-Human Relationships - Loukissas, Yanni Alexander; Mindell, David A.
Introduction As NASA struggles with an uncertain national policy environment, it is seeking new combinations of human and robotic modes of exploration. During the Apollo era, establishing human presence in space was an integral part of NASA’s work. More recently, the Mars Exploration Rover missions have demonstrated how rich and successful remote exploration of a planetary surface can be. Space exploration is one of many examples of technical operations conducted in extreme environments that are raising new questions about the relative importance of human and remote presence. What does it mean to “be there?”

48. Frame-Semantic Parsing - Das, Dipanjan; Chen, Desai; Martins, André F. T.; Schneider, Nathan; Smith, Noah A.
Frame semantics is a linguistic theory that has been instantiated for English in the FrameNet lexicon. We solve the problem of frame-semantic parsing using a two-stage statistical model that takes lexical targets (i.e., content words and phrases) in their sentential contexts and predicts frame-semantic structures. Given a target in context, the first stage disambiguates it to a semantic frame. This model uses latent variables and semi-supervised learning to improve frame disambiguation for targets unseen at training time. The second stage finds the target's locally expressed semantic arguments. At inference time, a fast exact dual decomposition algorithm collectively predicts all the...

49. Simulations of Noise in Phase-Separated Transition-Edge Sensors for SuperCDMS - Anderson, Adam Jonathan; Leman, Steven W.; Pyle, M.; Figueroa-Feliciano, Enectalí; McCarthy, K.; Doughty, T.; Cherry, M.; Young, B.
We briefly review a simple model of superconducting-normal phase-separation in transition-edge sensors (TESs) in the SuperCDMS experiment. After discussing some design considerations relevant to the TESs in the experiment, we study noise sources in both the phase-separated and phase-uniform cases. Such simulations will be valuable for optimizing the critical temperature and TES length of future SuperCDMS detectors.

50. The Genome Analysis Toolkit: A MapReduce framework for analyzing next-generation DNA sequencing data - McKenna, Aaron; Hanna, Matthew; Banks, Eric; Sivachenko, Andrey; Cibulskis, Kristian; Kernytsky, Andrew; Garimella, Kiran; Altshuler, David; Gabriel, Stacey; Daly, Mark; DePristo, Mark A.
Next-generation DNA sequencing (NGS) projects, such as the 1000 Genomes Project, are already revolutionizing our understanding of genetic variation among individuals. However, the massive data sets generated by NGS—the 1000 Genome pilot alone includes nearly five terabases—make writing feature-rich, efficient, and robust analysis tools difficult for even computationally sophisticated individuals. Indeed, many professionals are limited in the scope and the ease with which they can answer scientific questions by the complexity of accessing and manipulating the data produced by these machines. Here, we discuss our Genome Analysis Toolkit (GATK), a structured programming framework designed to ease the development of efficient...

51. The Effect of Ligand Lipophilicity on the Nanoparticle Encapsulation of Pt(IV) Prodrugs - Johnstone, Timothy; Lippard, Stephen J.
In an effort to expand the therapeutic range of platinum anticancer agents, several new approaches to platinum-based therapy, including nanodelivery, are under active investigation. To better understand the effect of ligand lipophilicity on the encapsulation of Pt(IV) prodrugs within polymer nanoparticles, the series of compounds cis,cis,trans-[Pt(NH3)2Cl2L2] was prepared, where L = acetate, propanoate, butanoate, pentanoate, hexanoate, heptanoate, octanoate, nonanoate, and decanoate. The lipophilicities of these compounds, assessed by reversed-phase HPLC, correlate with the octanol/water partition coefficients of their respective free carboxylic acid ligands, which in turn affect the degree of encapsulation of the Pt(IV) complex within the hydrophobic core of...

52. Subcortical Projections of Area V2 in the Macaque - Ungerleider, Leslie G.; Galkin, Thelma W.; Desimone, Robert; Gattass, Ricardo
To investigate the subcortical efferent connections of visual area V2, we injected tritiated amino acids under electrophysiological control into 15 V2 sites in 14 macaques. The injection sites included the fovea representation as well as representations ranging from central to far peripheral eccentricities in both the upper and lower visual fields. The results indicated that V2 projects topographically to different portions of the inferior and lateral pulvinar and to the superficial and intermediate layers of the superior colliculus. Within the pulvinar, the V2 projections terminated in fields P1, P2, and P4, with the strongest projection being in P2. Central visual...

53. Multispecies diel transcriptional oscillations in open ocean heterotrophic bacterial assemblages - Marin, R.; Schuster, S. C.; Scholin, C. A.; Ottesen, Elizabeth A.; Young, Curtis R.; Gifford, Scott Michael; Eppley, John M.; DeLong, Edward
Oscillating diurnal rhythms of gene transcription, metabolic activity, and behavior are found in all three domains of life. However, diel cycles in naturally occurring heterotrophic bacteria and archaea have rarely been observed. Here, we report time-resolved whole-genome transcriptome profiles of multiple, naturally occurring oceanic bacterial populations sampled in situ over 3 days. As anticipated, the cyanobacterial transcriptome exhibited pronounced diel periodicity. Unexpectedly, several different heterotrophic bacterioplankton groups also displayed diel cycling in many of their gene transcripts. Furthermore, diel oscillations in different heterotrophic bacterial groups suggested population-specific timing of peak transcript expression in a variety of metabolic gene suites. These...

54. Quantum Adiabatic Algorithms, Small Gaps, and Different Paths - Farhi, Edward; Goldstone, Jeffrey; Gosset, David; Gutmann, Sam; Meyer, Harvey B.; Shor, Peter W.
We construct a set of instances of 3SAT which are not solved efficiently using the simplestquantum adiabatic algorithm. These instances are obtained by picking randomclauses all consistent with two disparate planted solutions and then penalizing one ofthem with a single additional clause. We argue that by randomly modifying the beginningHamiltonian, one obtains (with substantial probability) an adiabatic path thatremoves this difficulty. This suggests that the quantum adiabatic algorithm should ingeneral be run on each instance with many different random paths leading to the problemHamiltonian. We do not know whether this trick will help for a random instance of3SAT (as opposed...

55. Angular control of optical cavities in a radiation-pressure-dominated regime: the Enhanced LIGO case - Dooley, Katherine L.; Barsotti, Lisa; Adhikari, Rana X.; Fricke, Tobin T.; Frolov, Valera; Kawabe, Keita; Evans, Matthew J.; Fritschel, Peter K.; Smith-Lefebvre, Nicolas
We describe the angular sensing and control (ASC) of 4 km detectors of the Laser Interferometer Gravitational-Wave Observatory (LIGO). Enhanced LIGO, the culmination of the first generation LIGO detectors, operated between 2009 and 2010 with about 40 kW of laser power in the arm cavities. In this regime, radiation-pressure effects are significant and induce instabilities in the angular opto-mechanical transfer functions. Here we present and motivate the ASC design in this extreme case and present the results of its implementation in Enhanced LIGO. Highlights of the ASC performance are successful control of opto-mechanical torsional modes, relative mirror motions of ≤ 1...

56. DC readout experiment in Enhanced LIGO - Fricke, Tobin T; Abbott, Richard; Adhikari, Rana; Dooley, Katherine L; Frolov, Valery V; Kawabe, Keita; Slagmolen, Bram J J; Smith-Lefebvre, Nicolas D.; Evans, Matthew J.; Fritschel, Peter K.; Kissel, Jeffrey S.; Waldman, Sam J.
The two 4 km long gravitational wave detectors operated by the Laser Interferometer Gravitational-wave Observatory (LIGO) were modified in 2008 to read out the gravitational wave channel using the DC readout form of homodyne detection and to include an optical filter cavity at the output of the detector. As part of the upgrade to Enhanced LIGO, these modifications replaced the radio-frequency (RF) heterodyne system used previously. We describe the motivations for and the implementation of DC readout and the output mode cleaner in Enhanced LIGO. We present characterizations of the system, including measurements and models of the couplings of the...

57. Active noise cancellation in a suspended interferometer - Driggers, Jennifer C.; Pepper, Keenan; Adhikari, Rana; Evans, Matthew J.
We demonstrate feed-forward vibration isolation on a suspended Fabry-Perot interferometer using Wiener filtering and a variant of the common least mean square adaptive filter algorithm. We compare the experimental results with theoretical estimates of the cancellation efficiency. Using data from the recent Laser InterferometerGravitational Wave Observatory (LIGO) Science Run, we also estimate the impact of this technique on full scale gravitational waveinterferometers. In the future, we expect to use this technique also to remove acoustic, magnetic, and gravitational noise perturbations from the LIGOinterferometers. This noise cancellation technique is simple enough to implement in standard laboratory environments and can be used...

58. Arm-length stabilisation for interferometric gravitational-wave detectors using frequency-doubled auxiliary lasers - Mullavey, Adam J.; Slagmolen, Bram J. J.; Miller, John; Sigg, Daniel; Waldman, Sam J.; Shaddock, Daniel A.; McClelland, David E.; Evans, Matthew J.; Fritschel, Peter K.
Residual motion of the arm cavity mirrors is expected to prove one of the principal impediments to systematic lock acquisition in advanced gravitational-wave interferometers. We present a technique which overcomes this problem by employing auxiliary lasers at twice the fundamental measurement frequency to pre-stabilise the arm cavities’ lengths. Applying this approach, we reduce the apparent length noise of a 1.3 m long, independently suspended Fabry-Perot cavity to 30 pm rms and successfully transfer longitudinal control of the system from the auxiliary laser to the measurement laser.

59. Neutrino experiments and the Large Hadron Collider: friends across 14 orders of magnitude - Conrad, Janet
This paper explores some of the questions that connect the Large Hadron Collider (LHC) and neutrino experiments. What is the origin of mass? What is the meaning of flavor? Is there direct evidence of new forces or particles? The neutrino program investigating these questions is large and diverse. The strategy here, to narrow the discussion, is to focus on relatively new ideas for experiments that may be less known within the LHC community.

60. The effects of dissolved methane upon liquid argon scintillation light - Alexander, T; Back, H O; Pordes, S; Jones, Benjamin James Poyner; Collin, Gabriel Lewis; Conrad, Janet; Greene, A.; Katori, T.; Toups, Matthew
In this paper we report on measurements of the effects of dissolved methane upon argon scintillation light. We monitor the light yield from an alpha source held 20 cm from a cryogenic photomultiplier tube (PMT) assembly as methane is injected into a high-purity liquid argon volume. We observe significant suppression of the scintillation light yield by dissolved methane at the 10 part per billion (ppb) level. By examining the late scintillation light time constant, we determine that this loss is caused by an absorption process and also see some evidence of methane-induced scintillation quenching at higher concentrations (50-100 ppb). Using...

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