292 research outputs found
A Multi-Epoch HST Study of the Herbig-Haro Flow from XZ Tauri
We present nine epochs of Hubble Space Telescope optical imaging of the
bipolar outflow from the pre-main sequence binary XZ Tauri. Our data monitors
the system from 1995-2005 and includes emission line images of the flow. The
northern lobe appears to be a succession of bubbles, the outermost of which
expanded ballistically from 1995-1999 but in 2000 began to deform and
decelerate along its forward edge. It reached an extent of 6" from the binary
in 2005. A larger and fainter southern counterbubble was detected for the first
time in deep ACS images from 2004. Traces of shocked emission are seen as far
as 20" south of the binary. The bubble emission nebulosity has a low excitation
overall, as traced by the [S II]/H-alpha line ratio, requiring a nearly
comoving surrounding medium that has been accelerated by previous ejections or
stellar winds.
Within the broad bubbles there are compact emission knots whose alignments
and proper motions indicate that collimated jets are ejected from each binary
component. The jet from the southern component, XZ Tau A, is aligned with the
outflow axis of the bubbles and has tangential knot velocities of 70-200 km/s.
Knots in the northern flow are seen to slow and brighten as they approach the
forward edge of the outermost bubble. The knots in the jet from the other star,
XZ Tau B, have lower velocities of ~100 km/s
Comprehensive aerodynamic and dynamic study of independence of ukraine monument
Розглянуто результати комплексних досліджень вирішення інженерної проблеми створення висотного монумента колонного типу в Києві, динамічних досліджень моделі в масштабі 1:25 та аеродинамічних досліджень великомасштабної моделі (1:8) в аеродинамічній трубі ТАД-2 Національного авіаційного університету. Наведено процедуру визначення на натурному об’єкті реальних динамічних характеристик та забезпечення ефективності демпфіруванняРассмотрены результаты комплексных исследований решения инженерной задачи создания высотного монумента колонного типа в Киеве, динамических исследований модели в масштабе 1:25 и аэродинамических испытаний крупномасштабной (1:8) модели в аэродинамической трубе ТАД-2 Национального авиационного университета. Приведена процедура определения на натурном объекте реальных динамических характеристик и обеспечения эффективности демпфированияComprehensive approach for solution the engineering problems o f creation the high-rise extended pillartype Monument in Kiyv is described. The results o f dynamic tests o f 1:25 scale model and aerodynamictests o f 1:8 scale model o f the Monument in TAD-2 wind tunnel are given. The procedures fordetermination o f actual dynamic characteristics and assurance the efficiency o f damping on the siteare describe
The Catalytic Machinery of a Key Enzyme in Amino Acid Biosynthesis
The aspartate pathway of amino acid biosynthesis is essential for all microbial life but is absent in mammals. Characterizing the enzyme-catalyzed reactions in this pathway can identify new protein targets for the development of antibiotics with unique modes of action. The enzyme aspartate β-semialdehyde dehydrogenase (ASADH) catalyzes an early branch point reaction in the aspartate pathway. Kinetic, mutagenic, and structural studies of ASADH from various microbial species have been used to elucidate mechanistic details and to identify essential amino acids involved in substrate binding, catalysis, and enzyme regulation. Important structural and functional differences have been found between ASADHs isolated from these bacterial and fungal organisms, opening the possibility for developing species-specific antimicrobial agents that target this family of enzymes
Constraining the optical emission from the double pulsar system J0737-3039
We present the first optical observations of the unique system J0737-3039
(composed of two pulsars, hereafter PSR-A and PSR-B). Ultra-deep optical
observations, performed with the High Resolution Camera of the Advanced Camera
for Surveys on board the Hubble Space Telescope could not detect any optical
emission from the system down to m_F435W=27.0 and m_F606W=28.3. The estimated
optical flux limits are used to constrain the three-component (two thermal and
one non-thermal) model recently proposed to reproduce the XMM-Newton X-ray
spectrum. They suggest the presence of a break at low energies in the
non-thermal power law component of PSR-A and are compatible with the expected
black-body emission from the PSR-B surface. The corresponding efficiency of the
optical emission from PSR-A's magnetosphere would be comparable to that of
other Myr-old pulsars, thus suggesting that this parameter may not dramatically
evolve over a time-scale of a few Myr.Comment: 16 pages, 3 figures, ApJ accepte
3D Gamma-ray and Neutron Mapping in Real-Time with the Localization and Mapping Platform from Unmanned Aerial Systems and Man-Portable Configurations
Nuclear Scene Data Fusion (SDF), implemented in the Localization and Mapping
Platform (LAMP) fuses three-dimensional (3D), real-time volumetric
reconstructions of radiation sources with contextual information (e.g. LIDAR,
camera, etc.) derived from the environment around the detector system. This
information, particularly when obtained in real time, may be transformative for
applications, including directed search for lost or stolen sources, consequence
management after the release of radioactive materials, or contamination
avoidance in security-related or emergency response scenarios. 3D
reconstructions enabled by SDF localize contamination or hotspots to specific
areas or objects, providing higher resolution over larger areas than
conventional 2D approaches, and enabling more efficient planning and response,
particularly in complex 3D environments.
In this work, we present the expansion of these gamma-ray mapping concepts to
neutron source localization. Here we integrate LAMP with a custom
(CLLBC) scintillator detector sensitive to both
gamma-rays and neutrons, which we dub Neutron Gamma LAMP (NG-LAMP). NG-LAMP
enables simultaneous neutron and gamma-ray mapping with high resolution
gamma-ray spectroscopy. We demonstrate the ability to detect and localize
surrogate Special Nuclear Materials (SNM) in real-time and in 3D based on
neutron signatures alone, which is critical for the detection of heavily
shielded SNM, when gamma-ray signatures are attenuated. In this work, we show
for the first time the ability to localize, in 3D and realtime, a neutron
source in the presence of a strong gamma-ray source, simultaneous and
spectroscopic localization of three gamma-ray sources and a neutron source, and
finally the localization of a surrogate SNM source based on neutron signatures
alone, where gamma-ray data are consistent with background
Morphological Composition of z~0.4 groups: The site of S0 formation
The low redshift Universe (z<~0.5) is not a dull place. Processes leading to
the suppression of star formation and morphological transformation are
prevalent: this is particularly evident in the dramatic upturn in the fraction
of S0-type galaxies in clusters. However, until now, the process and
environment of formation has remained unidentified. We present a HST-based
morphological analysis of galaxies in the redshift-space selected group and
field environments at z~0.4. Groups contain a much higher fraction of S0s at
fixed luminosity than the lower density field, with >99.999% confidence. Indeed
the S0 fraction in groups is at least as high as in z~0.4 clusters and X-ray
selected groups, which have more luminous Intra Group Medium (IGM). An 97%
confident excess of S0s at >=0.3Mpc from the group centre at fixed luminosity,
tells us that formation is not restricted to, and possibly even avoids, the
group cores. Interactions with a bright X-ray emitting IGM cannot be important
for the formation of the majority of S0s in the Universe. In contrast to S0s,
the fraction of elliptical galaxies in groups at fixed luminosity is similar to
the field, whilst the brightest ellipticals are strongly enhanced towards the
group centres (>99.999% confidence within 0.3Mpc). We conclude that the group
and sub-group environments must be dominant for the formation of S0 galaxies,
and that minor mergers, galaxy harassment and tidal interactions are the most
likely responsible mechanisms. This has implications not only for the inferred
pre-processing of cluster galaxies, but also for the global morphological and
star formation budget of galaxies: as hierarchical clustering progresses, more
galaxies will be subject to these transformations as they enter the group
environment.Comment: 13 pages, 6 figures. Accepted for publication in Ap
The Massive Star Content of NGC 3603
We investigate the massive star content of NGC 3603, the closest known giant
H II region. We have obtained spectra of 26 stars in the central cluster using
the Baade 6.5-m telescope (Magellan I). Of these 26 stars, 16 had no previous
spectroscopy. We also obtained photometry of all of the stars with previous or
new spectroscopy, primarily using archival HST ACS/HRC images. We use these
data to derive an improved distance to the cluster, and to construct an H-R
diagram for discussing the masses and ages of the massive star content of this
cluster.Comment: Accepted by the Astronomical Journal. This revision updates the
coordinates in Table 1 by (-0.18sec, +0.2") to place them on the UCAC2 syste
Free-moving Quantitative Gamma-ray Imaging
The ability to map and estimate the activity of radiological source
distributions in unknown three-dimensional environments has applications in the
prevention and response to radiological accidents or threats as well as the
enforcement and verification of international nuclear non-proliferation
agreements. Such a capability requires well-characterized detector response
functions, accurate time-dependent detector position and orientation data, an
algorithmic understanding of the surrounding 3D environment, and appropriate
image reconstruction and uncertainty quantification methods. We have previously
demonstrated 3D mapping of gamma-ray emitters with free-moving detector systems
on a relative intensity scale using a technique called Scene Data Fusion (SDF).
Here we characterize the detector response of a multi-element gamma-ray imaging
system using experimentally benchmarked Monte Carlo simulations and perform 3D
mapping on an absolute intensity scale. We present experimental reconstruction
results from hand-carried and airborne measurements with point-like and
distributed sources in known configurations, demonstrating quantitative SDF in
complex 3D environments.Comment: 19 pages, 5 figures, 4 supplementary figures, submitted to Scientific
Reports - Natur
The Ultraviolet Spectral Energy Distributions of Quiescent Black Holes and Neutron Stars
We present HST/ACS ultraviolet photometry of three quiescent black hole X-ray
transients: X-ray Nova Muscae 1991 (GU Mus), GRO J0422+32 (V518 Per), and X-ray
Nova Vel 1993 (MM Vel), and one neutron star system, Aql X-1. These are the
first quiescent UV detections of these objects. All are detected at a much
higher level than expected from their companion stars alone and are significant
detections of the accretion flow. Three of the four UV excesses can be
characterized by a black body of temperature 5000-13,000K, hotter than expected
for the quiescent outer disk. A good fit could not be found for MM Vel. The
source of the black-body-like emission is most likely a heated region of the
inner disk. Contrary to initial indications from spectroscopy there does not
appear to be a systematic difference in the UV luminosity or spectral shape
between black holes and neutron star systems. However combining our new data
with earlier spectroscopy and published X-ray luminosities there is a
significant difference in the X-ray to UV flux ratios with the neutron stars
exhibiting Lx/Luv about 10x higher than the black hole systems. Since both
bandpasses are expected to be dominated by accretion light this suggests the
difference in X-ray luminosities cannot simply reflect differences in quiescent
accretion rates and so is a more robust discriminator between the black hole
and neutron star populations than the comparison of X-ray luminosities alone.Comment: Accepted for publication in the Astrophysical Journa
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