3,113 research outputs found
Morphological Changes Accompanying the Transition from Juvenile (Atmospheric) to Adult (Tank) Forms in the Mexican Epiphyte Tillandsia Deppeana (Bromeliaceae)
This is the publisher's official version, also available at: http://www.jstor.org/stable/10.2307/2443800.Two distinct morphological forms characterize the ontogenetic development of the epiphytic
bromcliad Tillandsia deppeana. Juveniles are characterized by a non-impounding rosette of
small, linear leaves covered with elaborate trichomes possessing a 4 + 8 + 16 + 64 shield cell
pattern. The broader transitional leaves, which form an impounding rosette prior to the initiation
of true adult leaves, also possess trichomes with the 4 + 8 + 16 + 64 cell pattern. Adult
individuals have large, broad leaves with overlapping, sheathing bases which impound water
and debris. These leaves have trichomes with shields exhibiting a 4 + 8 + 32 cell pattern.
Trichome density is fairly uniform in the juvenile leaves with trichomes covering 100% of leaf
surfaces, whereas in the adult leaves density is high at the base and diminishes significantly
toward the apex. Stomatal density of both juvenile and adult leaves increases from the base to
the apex, although this is most pronounced in the adults. Stomata in the adults are also arranged
in longitudinal series parallel and abaxial to parallel rows of mesophyll tissue. The results of
this study indicate that juveniles of 7". deppeana are more similar morphologically to adult
atmospheric-type tillandsioid species than to the tank-forming adults into which they eventually
develop
Broadband X-ray Imaging and Spectroscopy of the Crab Nebula and Pulsar with NuSTAR
We present broadband (3 -- 78 keV) NuSTAR X-ray imaging and spectroscopy of
the Crab nebula and pulsar. We show that while the phase-averaged and spatially
integrated nebula + pulsar spectrum is a power-law in this energy band,
spatially resolved spectroscopy of the nebula finds a break at 9 keV in
the spectral photon index of the torus structure with a steepening
characterized by . We also confirm a previously reported
steepening in the pulsed spectrum, and quantify it with a broken power-law with
break energy at 12 keV and . We present spectral
maps of the inner 100\as\ of the remnant and measure the size of the nebula as
a function of energy in seven bands. These results find that the rate of
shrinkage with energy of the torus size can be fitted by a power-law with an
index of , consistent with the predictions of Kennel
and Coroniti (1984). The change in size is more rapid in the NW direction,
coinciding with the counter-jet where we find the index to be a factor of two
larger. NuSTAR observed the Crab during the latter part of a -ray
flare, but found no increase in flux in the 3 - 78 keV energy band
Observational Artifacts of NuSTAR: Ghost Rays and Stray Light
The Nuclear Spectroscopic Telescope Array (NuSTAR), launched in June 2012,
flies two conical approximation Wolter-I mirrors at the end of a 10.15m mast.
The optics are coated with multilayers of Pt/C and W/Si that operate from 3--80
keV. Since the optical path is not shrouded, aperture stops are used to limit
the field of view from background and sources outside the field of view.
However, there is still a sliver of sky (~1.0--4.0 degrees) where photons may
bypass the optics altogether and fall directly on the detector array. We term
these photons Stray-light. Additionally, there are also photons that do not
undergo the focused double reflections in the optics and we term these Ghost
Rays. We present detailed analysis and characterization of these two components
and discuss how they impact observations. Finally, we discuss how they could
have been prevented and should be in future observatories.Comment: Published in Journal of Astronomical Telescopes, Instruments, and
Systems. Open Access. http://dx.doi.org/10.1117/1.JATIS.3.4.04400
Classifying the mechanisms of electrochemical shock in ion-intercalation materials
“Electrochemical shock” – the electrochemical cycling-induced fracture of materials – contributes to impedance growth and performance degradation in ion-intercalation batteries, such as lithium-ion. Using a combination of micromechanical models and acoustic emission experiments, the mechanisms of electrochemical shock are identified, classified, and modeled in targeted model systems with different composition and microstructure. A particular emphasis is placed on mechanical degradation occurring in the first electrochemical cycle. Three distinct mechanisms of electrochemical shock are identified, and a fracture mechanics failure criterion is derived for each mechanism. In a given material system, crystal symmetry and phase-behavior determine the active mechanisms. A surprising result is that electrochemical shock in commercial lithium-storage materials occurs by mechanisms that are insensitive to the electrochemical cycling rate. This fundamental understanding of electrochemical shock leads naturally to practical design criteria for battery materials and microstructures that improve performance and energy storage efficiency. These microstructure and crystal chemical design criteria are demonstrated experimentally for spinel materials such as LiMn2O4 and LiMn1.5Ni0.5O4. A case study of LiMn1.5Ni0.5O4 is presented, in which small changes in composition that have negligible impact on electrochemical properties induce a significant change in phase behavior that allow electrochemical shock at relevant electrochemical cycling rates to be avoided. Although lithium-storage materials are used as model systems for experimental study, the physical phenomena are common to other ion-intercalation systems, including sodium- and magnesium-storage compounds
Government-Industry Cooperative Fisheries Research in the North Pacific under the MSFCMA
The National Marine Fisheries Service’s Alaska Fisheries Science Center (AFSC) has a long and successful history of conducting research in cooperation with the fishing industry. Many of the AFSC’s annual resource assessment surveys are carried out aboard chartered commercial vessels and the skill and experience of captains and crew are integral to the success of this work. Fishing companies have been contracted to provide vessels and expertise for many different types of research, including testing and evaluation of survey and commercial fishing gear and development of improved methods for estimating commercial catch quantity and composition. AFSC scientists have also participated in a number of industry-initiated research projects including development of selective fishing gears for bycatch reduction and evaluating and improving observer catch composition sampling. In this paper, we describe the legal and regulatory provisions for these types of cooperative work and present examples to illustrate the process and identify the requirements for successful cooperative research
Anomalous optical surface absorption in nominally pure silicon samples at 1550 nm
The announcement of the direct detection of Gravitational Waves (GW) by the LIGO and Virgo collaboration in February 2016 has removed any uncertainty around the possibility of GW astronomy. It has demonstrated that future detectors with sensitivities ten times greater than the Advanced LIGO detectors would see thousands of events per year. Many proposals for such future interferometric GW detectors assume the use of silicon test masses. Silicon has low mechanical loss at low temperatures, which leads to low displacement noise for a suspended interferometer mirror. In addition to the low mechanical loss, it is a requirement that the test masses have a low optical loss. Measurements at 1550 nm have indicated that material with a low enough bulk absorption is available; however there have been suggestions that this low absorption material has a surface absorption of > 100 ppm which could preclude its use in future cryogenic detectors. We show in this paper that this surface loss is not intrinsic but is likely to be a result of particular polishing techniques and can be removed or avoided by the correct polishing procedure. This is an important step towards high gravitational wave detection rates in silicon based instruments
NuSTAR discovery of a cyclotron line in the accreting X-ray pulsar IGR J16393-4643
The high-mass X-ray binary and accreting X-ray pulsar IGR J16393-4643 was
observed by NuSTAR in the 3-79 keV energy band for a net exposure time of 50
ks. We present the results of this observation which enabled the discovery of a
cyclotron resonant scattering feature with a centroid energy of 29.3(+1.1/-1.3)
keV. This allowed us to measure the magnetic field strength of the neutron star
for the first time: B = (2.5+/-0.1)e12 G. The known pulsation period is now
observed at 904.0+/-0.1 s. Since 2006, the neutron star has undergone a
long-term spin-up trend at a rate of P' = -2e-8 s/s (-0.6 s per year, or a
frequency derivative of nu' = 3e-14 Hz/s ). In the power density spectrum, a
break appears at the pulse frequency which separates the zero slope at low
frequency from the steeper slope at high frequency. This addition of angular
momentum to the neutron star could be due to the accretion of a quasi-spherical
wind, or it could be caused by the transient appearance of a prograde accretion
disk that is nearly in corotation with the neutron star whose magnetospheric
radius is around 2e8 cm.Comment: Accepted for publication in the Astrophysical Journal, 7 pages, 8
figures, 2 table
High-Energy X-ray Imaging of the Pulsar Wind Nebula MSH~15-52: Constraints on Particle Acceleration and Transport
We present the first images of the pulsar wind nebula (PWN) MSH 15-52 in the
hard X-ray band (>8 keV), as measured with the Nuclear Spectroscopic Telescope
Array (NuSTAR). Overall, the morphology of the PWN as measured by NuSTAR in the
3-7 keV band is similar to that seen in Chandra high-resolution imaging.
However, the spatial extent decreases with energy, which we attribute to
synchrotron energy losses as the particles move away from the shock. The
hard-band maps show a relative deficit of counts in the northern region towards
the RCW 89 thermal remnant, with significant asymmetry. We find that the
integrated PWN spectra measured with NuSTAR and Chandra suggest that there is a
spectral break at 6 keV which may be explained by a break in the
synchrotron-emitting electron distribution at ~200 TeV and/or imperfect cross
calibration. We also measure spatially resolved spectra, showing that the
spectrum of the PWN softens away from the central pulsar B1509-58, and that
there exists a roughly sinusoidal variation of spectral hardness in the
azimuthal direction. We discuss the results using particle flow models. We find
non-monotonic structure in the variation with distance of spectral hardness
within 50" of the pulsar moving in the jet direction, which may imply particle
and magnetic-field compression by magnetic hoop stress as previously suggested
for this source. We also present 2-D maps of spectral parameters and find an
interesting shell-like structure in the NH map. We discuss possible origins of
the shell-like structure and their implications.Comment: 15 pages, 9 figures, accepted for publication in Ap
The Hard X-Ray View of the Young Supernova Remnant G1.9+0.3
NuSTAR observed G1.9+0.3, the youngest known supernova remnant in the Milky
Way, for 350 ks and detected emission up to 30 keV. The remnant's X-ray
morphology does not change significantly across the energy range from 3 to 20
keV. A combined fit between NuSTAR and CHANDRA shows that the spectrum steepens
with energy. The spectral shape can be well fitted with synchrotron emission
from a power-law electron energy distribution with an exponential cutoff with
no additional features. It can also be described by a purely phenomenological
model such as a broken power-law or a power-law with an exponential cutoff,
though these descriptions lack physical motivation. Using a fixed radio flux at
1 GHz of 1.17 Jy for the synchrotron model, we get a column density of N = cm, a spectral index of
, and a roll-off frequency of Hz. This can be explained by particle
acceleration, to a maximum energy set by the finite remnant age, in a magnetic
field of about 10 G, for which our roll-off implies a maximum energy of
about 100 TeV for both electrons and ions. Much higher magnetic-field strengths
would produce an electron spectrum that was cut off by radiative losses, giving
a much higher roll-off frequency that is independent of magnetic-field
strength. In this case, ions could be accelerated to much higher energies. A
search for Ti emission in the 67.9 keV line results in an upper limit of
assuming a line width of 4.0 keV (1 sigma).Comment: 9 pages, 6 figures, accepted Ap
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