193,023 research outputs found
A Near-Infrared Spectroscopic Study of Young Field Ultracool Dwarfs
We present a near-infrared (0.9-2.4 microns) spectroscopic study of 73 field
ultracool dwarfs having spectroscopic and/or kinematic evidence of youth
(~10-300 Myr). Our sample is composed of 48 low-resolution (R~100) spectra and
41 moderate-resolution spectra (R>~750-2000). First, we establish a method for
spectral typing M5-L7 dwarfs at near-IR wavelengths that is independent of
gravity. We find that both visual and index-based classification in the near-IR
provide consistent spectral types with optical spectral types, though with a
small systematic offset in the case of visual classification at J and K band.
Second, we examine features in the spectra of ~10 Myr ultracool dwarfs to
define a set of gravity-sensitive indices based on FeH, VO, K, Na and H-band
continuum shape. We then create an index-based method for classifying the
gravities of M6-L5 dwarfs that provides consistent results with gravity
classifications from optical spectroscopy. Our index-based classification can
distinguish between young and dusty objects. Guided by the resulting
classifications, we propose a set of low-gravity spectral standards for the
near-IR. Finally, we estimate the ages corresponding to our gravity
classifications.Comment: Published in ApJ. IDL program for calculating indices
(allers13_index.pro) included in the source gzipped ta
Kelu-1 is a Binary L Dwarf: First Brown Dwarf Science from Laser Guide Star Adaptive Optics
(Abridged) We present near-IR imaging of the nearby L dwarf Kelu-1 obtained
with the Keck sodium laser guide star adaptive optics (LGS AO) system as part
of a high angular resolution survey for substellar binaries. Kelu-1 was one of
the first free-floating L dwarfs identified, and the origin of its
overluminosity compared to other similar objects has been a long-standing
question. Our images clearly resolve Kelu-1 into a 0.29'' (5.4 AU) binary, and
a previous non-detection by HST demonstrates that the system is a true physical
pair. Binarity explains the properties of Kelu-1 that were previously noted to
be anomalous compared to other early-L dwarfs. We estimate spectral types of
L1.5-L3 and L3-L4.5 for the two components, giving model-derived masses of
0.05-0.07 Msun and 0.045-0.065 Msun for an estimated age of 0.3-0.8 Gyr. More
distant companions are not detected to a limit of 5-9 Mjup. The presence of
lithium absorption indicates that both components are substellar, but the
weakness of this feature relative to other L dwarfs can be explained if only
Kelu-1B is Li-bearing. Determining whether both or only one of the components
possesses lithium could constrain the age of Kelu-1 (and other Li-bearing L
binaries) with higher precision than is possible for most ultracool field
objects. These results are the first LGS AO observations of brown dwarfs and
demonstrate the potential of this new instrumental capability for substellar
astronomy.Comment: 24 pages, Astrophysical Journal, in press (Nov 20, 2005 issue). Note
that Figure 1 of the PDF version is degraded by arxiv.org, but the Postscript
version is fine. Version 2 includes very minor changes to match the published
versio
The Micro-Bubble Distribution in the Wake of a Cavitating Circular Cylinder
Bubble nuclei populations in the wake of a circular cylinder under cavitating and noncavitating conditions were measured using a Phase Doppler Anemometry (PDA) system. In addition, the mean velocity defect and the turbulent fluctuations were monitored in order to try to understand the nuclei population dynamics within the flow. At the Reynolds numbers of these experiments (20000->33000) the laminar near-wake is fairly steady and under very limited cavitation conditions nuclei accumulate in this wake so that the population there is several orders of magnitude larger than in the upstream flow. Further downstream the population declines again as nuclei are entrained into the wake. However at fifteen diameters downstream the population is still much larger than in the upstream flow
Superior removal of arsenic from water with zirconium metal-organic framework UiO-66
10.1038/srep16613Scientific Reports51661
The role of fracture mechanics in the design of fuel tanks in space vehicles
With special reference to design of fuel tanks in space vehicles, the principles of fracture mechanics are reviewed. An approximate but extremely simple relationship is derived among the operating stress level, the length of crack, and the number of cycles of failure. Any one of the variables can be computed approximately from the knowledge of the other two, if the loading schedule (mission of the tank) is not greatly altered. Two sample examples illustrating the procedures of determining the allowable safe operating stress corresponding to a set of assumed loading schedule are included. The selection of sample examples is limited by the relatively meager available data on the candidate material for various stress ratios in the cycling
Energy dependence of Normal Branch Oscillation in Scorpius X-1
We report the energy dependence of normal branch oscillations (NBOs) in
Scorpius X-1, a low-mass X-ray binary Z-source. Three characteristic quantities
(centroid frequency, quality factor, and fractional root-mean-squared (rms)
amplitude) of a quasi-periodic oscillation signal as functions of photon energy
are investigated. We found that, although it is not yet statistically well
established, there is a signature indicating that the NBO centroid frequency
decreases with increasing photon energy when it is below 6-8 keV, which turns
out to be positively correlated with the photon energy at the higher energy
side. In addition, the rms amplitude increases significantly with the photon
energy below 13 keV and then decreases in the energy band of 13-20 keV. There
is no clear dependence on photon energy for the quality factor. Based on these
results, we suggest that the NBO originates mainly in the transition layer.Comment: 6 pages, 4 figure
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