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  • Astrophysics  (6)
  • Chemistry
  • Length-weight relationships
  • Mice
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  • 2015-2019  (8)
  • 2017  (8)
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  • 2015-2019  (8)
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  • 1
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    Mechanochemical unzipping of insulating polyladderene to semiconducting polyacetylene (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-08-04
    Description: Biological systems sense and respond to mechanical stimuli in a complex manner. In an effort to develop synthetic materials that transduce mechanical force into multifold changes in their intrinsic properties, we report on a mechanochemically responsive nonconjugated polymer that converts to a conjugated polymer via an extensive rearrangement of the macromolecular structure in response to force. Our design is based on the facile mechanochemical unzipping of polyladderene, a polymer inspired by a lipid natural product structure and prepared via direct metathesis polymerization. The resultant polyacetylene block copolymers exhibit long conjugation length and uniform trans-configuration and self-assemble into semiconducting nanowires. Calculations support a tandem unzipping mechanism of the ladderene units.
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 2
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    Femtosecond laser reshaping yields gold nanorods with ultranarrow surface plasmon resonances (2017)
    American Association for the Advancement of Science (AAAS)
    In: Science
    Details
    Publication Date: 2017-11-03
    Description: The irradiation of gold nanorod colloids with a femtosecond laser can be tuned to induce controlled nanorod reshaping, yielding colloids with exceptionally narrow localized surface plasmon resonance bands. The process relies on a regime characterized by a gentle multishot reduction of the aspect ratio, whereas the rod shape and volume are barely affected. Successful reshaping can only occur within a narrow window of the heat dissipation rate: Low cooling rates lead to drastic morphological changes, and fast cooling has nearly no effect. Hence, a delicate balance must be achieved between irradiation fluence and surface density of the surfactant on the nanorods. This perfection process is appealing because it provides a simple, fast, reproducible, and scalable route toward gold nanorods with an optical response of exceptional quality, near the theoretical limit.
    Keywords: Chemistry
    Print ISSN: 0036-8075
    Electronic ISSN: 1095-9203
    Topics: Biology , Chemistry and Pharmacology , Geosciences , Computer Science , Medicine , Natural Sciences in General , Physics
    Published by American Association for the Advancement of Science (AAAS)
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  • 3
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    Stellar and Planetary Parameters for K2's Late-Type Dwarf Systems from C1 to C5 (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: The NASA K2 (Kepler-2) mission uses photometry to find planets transiting stars of various types. M dwarfs are of high interest since they host more short-period planets than any other type of main-sequence star and transiting planets around M dwarfs have deeper transits compared to other main-sequence stars. In this paper, we present stellar parameters from K and M dwarfs hosting transiting planet candidates discovered by our team. Using the SOFI (Son OF Isaac - ESA's earlier, similar instrument) spectrograph on the European Southern Observatory's New Technology Telescope, we obtained R approximately equal to 1000 J-, H-, and K-band (0.95-2.52 micron) spectra of 34 late-type K2 planet and candidate planet host systems and 12 bright K4-M5 dwarfs with interferometrically measured radii and effective temperatures. Out of our 34 late-type K2 targets, we identify 27 of these stars as M dwarfs. We measure equivalent widths of spectral features, derive calibration relations using stars with interferometric measurements, and estimate stellar radii, effective temperatures, masses, and luminosities for the K2 planet hosts. Our calibrations provide radii and temperatures with median uncertainties of 0.059 solar radii (16.09 percent) and 160 degrees Kelvin (4.33 percent), respectively. We then reassess the radii and equilibrium temperatures of known and candidate planets based on our spectroscopically derived stellar parameters. Since a planet's radius and equilibrium temperature depend on the parameters of its host star, our study provides more precise planetary parameters for planets and candidates orbiting late-type stars observed with K2. We find a median planet radius and an equilibrium temperature of approximately 3 solar radii and 500 degrees Kelvin, respectively, with several systems (K2-18b and K2-72e) receiving near-Earth-like levels of incident irradiation.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN56781 , The Astrophysical Journal (ISSN 2041-8205) (e-ISSN 2041-8213); 837; 1; 72
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  • 4
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    The Origin of the Iron-Rich Knot in Tycho's Supernova Remnant (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: X-ray observations of supernova remnants (SNRs) allow us to investigate the chemical inhomogeneity of ejecta, offering unique insight into the nucleosynthesis in supernova explosions. Here we present detailed imaging and spectroscopic studies of the Fe knot located along the eastern rim of the Type Ia SNR Tycho ( SN 1572) using Suzaku and Chandra long-exposure data. Surprisingly, the Suzaku spectrum of this knot shows no emission from Cr, Mn, or Ni, which is unusual for the Fe-rich regions in this SNR. Within the framework of the canonical delayed-detonation models for SN Ia, the observed mass ratios M(sub Cr)/M(sub Fe) is less than 0.023, M(sub Mn)/M(sub Fe) is less than 0.012, and M(sub Ni)/M(sub Fe) is less than 0.029 (at 90% confidence) can only be achieved for a peak temperature of (5.3 - 5.7) x 10(exp. 9) K and a neutron excess of approximately less than 2.0 x 10(exp. -3). These constraints rule out the deep, dense core of a Chandrasekhar-mass white dwarf as the origin of the Fe knot and favor either incomplete Si burning or an Alpha-rich freeze-out regime, probably close to the boundary. An explosive He burning regime is a possible alternative, although this hypothesis is in conflict with the main properties of this SNR.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN45948 , The Astrophysical Journal (ISSN 0004-637X) (e-ISSN 1538-4357); 834; 2; 124
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  • 5
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    The Clumpy Absorber in the High-Mass X-Ray Binary Vela X-1 (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: Bright and eclipsing, the high-mass X-ray binary Vela X-1 offers a unique opportunity to study accretion onto a neutron star from clumpy winds of O/B stars and to disentangle the complex accretion geometry of these systems. In Chandra-HETGS spectroscopy at orbital phase approximately 0.25, when our line of sight towards the source does not pass through the large-scale accretion structure such as the accretion wake, we observe changes in overall spectral shape on timescales of a few kiloseconds. This spectral variability is, at least in part, caused by changes in overall absorption and we show that such strongly variable absorption cannot be caused by unperturbed clumpy winds of O/B stars. We detect line features from high and low ionization species of silicon, magnesium, and neon whose strengths and presence depend on the overall level of absorption. These features imply a co-existence of cool and hot gas phases in the system, which we interpret as a highly variable, structured accretion flow close to the compact object such as has been recently seen in simulations of wind accretion in high-mass X-ray binaries.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN59935 , Astronomy & Astrophysics (ISSN 0004-6361) (e-ISSN 2197-3504); 608; A143
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  • 6
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    A Precessing Be Disc as a Possible Model for Occultation Events in GX 304-1 (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We report on the RXTE detection of a sudden increase in the absorption column density, NH, during the 2011 May outburst of GX 3041. The NH increased up to 16 10(exp 22) atoms cm(exp 2), which is a factor of 34 larger than what is usually measured during the outbursts of GX 3041 as covered by RXTE. Additionally, an increase in the variability of the hardness ratio as calculated from the energy resolved RXTE-Proportional Counter Array light curves is measured during this time range. We interpret these facts as an occultation event of the neutron star by material in the line of sight. Using a simple 3D model of an inclined and precessing Be disc around the Be-type companion, we are able to qualitatively explain the NH evolution over time. We are able to constrain the Be disc density to be of the order of 10(exp 11)g cm(exp 3). Our model strengthens the idea of inclined Be discs as origin of double-peaked outbursts as the derived geometry allows accretion twice per orbit under certain conditions.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN60528 , Monthly Notices of the Royal Astronomical Society (ISSN 0035-8711) (e-ISSN 1365-2966); 471; 2; 1553–1564
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  • 7
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    POEMMA: Probe of Extreme Multi-Messenger Astrophysics (2017)
    In:  CASI
    Details
    Publication Date: 2019-08-14
    Description: The Probe Of Extreme Multi-Messenger Astrophysics (POEMMA) mission is being designed to establish charged-particle astronomy with ultra-high energy cosmic rays (UHECRs) and to observe cosmogenic tau neutrinos (CTNs). The study of UHECRs and CTNs from space will yield orders-of-magnitude increase in statistics of observed UHECRs at the highest energies, and the observation of the cosmogenic flux of neutrinos for a range of UHECR models. These observations should solve the long-standing puzzle of the origin of the highest energy particles ever observed, providing a new window onto the most energetic environments and events in the Universe, while studying particle interactions well beyond accelerator energies. The discovery of CTNs will help solve the puzzle of the origin of UHECRs and begin a new field of Astroparticle Physics with the study of neutrino properties at ultra-high energies.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN70876 , Proceedings of Science (e-ISSN 1824-8039); 301; 542|International Cosmic Ray Conference; Jul 12, 2017 - Jul 20, 2017; Bexco, Busan; Korea, Republic of
    Format: application/pdf
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  • 8
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    High Count-Rate Study of Two TES X-Ray Microcalorimeters With Different Transition Temperatures (2017)
    In:  Other Sources
    Details
    Publication Date: 2019-07-13
    Description: We have developed transition-edge sensor (TES) microcalorimeter arrays with high count-rate capability and high energy resolution to carry out x-ray imaging spectroscopy observations of various astronomical sources and the Sun. We have studied the dependence of the energy resolution and throughput (fraction of processed pulses) on the count rate for such microcalorimeters with two different transition temperatures T(sub c). Devices with both transition temperatures were fabricated within a single microcalorimeter array directly on top of a solid substrate where the thermal conductance of the microcalorimeter is dependent upon the thermal boundary resistance between the TES sensor and the dielectric substrate beneath. Because the thermal boundary resistance is highly temperature dependent, the two types of device with different T(sub c)(sup s) had very different thermal decay times, approximately one order of magnitude different. In our earlier report, we achieved energy resolutions of 1.6 and 2.eV at 6 keV from lower and higher T(sub c) devices, respectively, using a standard analysis method based on optimal filtering in the low flux limit. We have now measured the same devices at elevated x-ray fluxes ranging from 50 Hz to 1000 Hz per pixel. In the high flux limit, however, the standard optimal filtering scheme nearly breaks down because of x-ray pile-up. To achieve the highest possible energy resolution for a fixed throughput, we have developed an analysis scheme based on the socalled event grade method. Using the new analysis scheme, we achieved 5.0 eV FWHM with 96 Percent throughput for 6 keV x-rays of 1025 Hz per pixel with the higher T(sub c) (faster) device, and 5.8 eV FWHM with 97 Percent throughput with the lower T(sub c) (slower) device at 722 Hz.
    Keywords: Astrophysics
    Type: GSFC-E-DAA-TN50847 , Superconducter Science & Technology (ISSN 0953-2048) (e-ISSN 1361-6668); 30; 10; 104005
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