42,050 research outputs found
Cortical cells should fire regularly, but do not
When a typical nerve cell is injected with enough current, it fires a regular stream of action potentials. But cortical cells in vivo usually fire irregularly, reflecting synaptic input from presynaptic cells as well as intrinsic biophysical properties. We have applied the theory of stochastic
processes to spike trains recorded from cortical neurons (Tuckwell 1989) and find a fundamental contradiction between the large interspike variability observed and the much lower values predicted by well-accepted biophysical models of single cells
A statistical analysis of the late-type stellar content in the Andromeda halo
We present a statistical characterization of the carbon-star to M-giant (C/M)
ratio in the halo of M31. Based on application of pseudo-filter band passes to
our Keck/DEIMOS spectra we measure the 81-77-color index of 1288 stars in the
giant stellar stream and in halo fields out to large distances. From this
well-established narrow-band system, supplemented by V-I colors, we find only a
low number (five in total) of C-star candidates. The resulting low C/M ratio of
10% is consistent with the values in the M31 disk and inner halo from the
literature. Although our analysis is challenged by small number statistics and
our sample selection, there is an indication that the oxygen-rich M-giants
occur in similar number throughout the entire halo. We also find no difference
in the C-star population of the halo fields compared to the giant stream. The
very low C/M ratio is at odds with the observed low metallicities and the
presence of intermediate-age stars at large radii. Our observed absence of a
substantial carbon star population in the these regions indicates that the
(outer) M31 halo cannot be dominated by the debris of disk-like or SMC-type
galaxies, but rather resemble the dwarf elliptical NGC 147.Comment: 11 pages, 10 figures, accepted for publication in the A
A silicon implementation of the fly's optomotor control system
Flies are capable of stabilizing their body during free flight by using visual motion information to estimate self-rotation. We have built a hardware model of this optomotor control system in a standard CMOS VLSI process. The result is a small, low-power chip that receives input directly from the real world through on-board photoreceptors and generates motor commands in real time. The chip was tested under closed-loop conditions typically used for insect studies. The silicon system exhibited stable control sufficiently analogous to the biological system to allow for quantitative comparisons
Carleman estimates and absence of embedded eigenvalues
Let L be a Schroedinger operator with potential W in L^{(n+1)/2}. We prove
that there is no embedded eigenvalue. The main tool is an Lp Carleman type
estimate, which builds on delicate dispersive estimates established in a
previous paper. The arguments extend to variable coefficient operators with
long range potentials and with gradient potentials.Comment: 26 page
A chemical confirmation of the faint Bootes II dwarf Spheroidal Galaxy
We present a chemical abundance study of the brightest confirmed member star
of the ultrafaint dwarf galaxy Bootes II from Keck/HIRES high-resolution
spectroscopy at moderate signal-to-noise ratios. At [Fe/H] = -2.93 +/- 0.03
(stat.) +/- 0.17 (sys.) this star chemically resembles metal-poor halo field
stars and the signatures of other faint dwarf spheroidal galaxies at the same
metallicities in that it shows enhanced [alpha/Fe] ratios, Solar Fe-peak
element abundances, and low upper limits on the neutron-capture element Ba.
Moreover, this star shows no chemical peculiarities in any of the eight
elements we were able to measure. This implies that the chemical outliers found
in other systems remain outliers pertaining to the unusual enrichment histories
of the respective environments, while Bootes II appears to have experienced an
enrichment history typical of its very low mass. We also re-calibrated previous
measurements of the galaxy's metallicity from the calcium triplet (CaT) and
find a much lower value than reported before. The resulting broad metallicity
spread, in excess of one dex, the very metal poor mean, and the chemical
abundance patterns of the present star imply that Bootes II is a low-mass, old,
metal poor dwarf galaxy and not an overdensity associated with the Sagittarius
Stream as has been previously suggested based on its sky position and
kinematics. The low, mean CaT metallicity of -2.7 dex falls right on the
luminosity-metallicity relation delineated over four orders of magnitude from
the more luminous to the faintest galaxies. Thus Bootes II's chemical
enrichment appears representative of the galaxy's original mass, while tidal
stripping and other mass loss mechanisms were probably not significant as for
other low-mass satellites.Comment: 10 pages, 5 figures, accepted for publication in the Astrophysical
Journa
Computer aided processing using laser measurements
The challenge exists of processing the STS and its cargo through KSC facilities in the most timely and cost effective manner possible. To do this a 3-D computer graphics data base was established into which was entered the STS, payloads, and KSC facilities. The facility drawing data are enhanced by laser theodolite measurements into an as-built configuration. Elements of the data base were combined to study orbiter/facility interfaces payload/facility access problems and design/arrangement of various GSE to support processing requirements. With timely analysis/design utilizing the 3-D computer graphics system, costly delays can be avoided. Better methodology can be analyzed to determine procedures for cost avoidance
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