241,644 research outputs found
Pipelined digital SAR azimuth correlator using hybrid FFT-transversal filter
A synthetic aperture radar system (SAR) having a range correlator is provided with a hybrid azimuth correlator which utilizes a block-pipe-lined fast Fourier transform (FFT). The correlator has a predetermined FFT transform size with delay elements for delaying SAR range correlated data so as to embed in the Fourier transform operation a corner-turning function as the range correlated SAR data is converted from the time domain to a frequency domain. The azimuth correlator is comprised of a transversal filter to receive the SAR data in the frequency domain, a generator for range migration compensation and azimuth reference functions, and an azimuth reference multiplier for correlation of the SAR data. Following the transversal filter is a block-pipelined inverse FFT used to restore azimuth correlated data in the frequency domain to the time domain for imaging
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
Deletion mutants in COP9/Signalosome subunits in fission yeast Schizosaccharomyces pombe display distinct phenotypes
The COP9/signalosome complex is highly conserved in evolution and possesses significant structural similarity to the 19S regulatory lid complex of the proteasome. It also shares limited similarity to the translation initiation factor eIF3. The signalosome interacts with multiple cullins in mammalian cells. In the fission yeast Schizosaccharomyces pombe, the Csn1 subunit is required for the removal of covalently attached Nedd8 from Pcu1, one of three S. pombe cullins. It remains unclear whether this activity is required for all the functions ascribed to the signalosome. We previously identified Csn1 and Csn2 as signalosome subunits in S. pombe. csn1 and csn2 null mutants are DNA damage sensitive and exhibit slow DNA replication. Two further putative subunits, Csn4 and Csn5, were identified from the S. pombe genome database. Herein, we characterize null mutations of csn4 and csn5 and demonstrate that both genes are required for removal of Nedd8 from the S. pombe cullin Pcu1 and that their protein products associate with Csn1 and Csn2. However, neither csn4 nor csn5 null mutants share the csn1 and csn2 mutant phenotypes. Our data suggest that the subunits of the signalosome cannot be considered as a distinct functional unit and imply that different subunits of the signalosome mediate distinct functions
- ā¦