6,169 research outputs found
Metal-Ion Absorption in Conductively Evaporating Clouds
We present computations of the ionization structure and metal-absorption
properties of thermally conductive interface layers that surround evaporating
warm spherical clouds, embedded in a hot medium. We rely on the analytical
formalism of Dalton & Balbus to calculate the temperature profile in the
evaporating gas, and explicitly solve the time-dependent ionization equations
for H, He, C, N, O, Si, and S in the interface. We include photoionization by
an external field. We estimate how departures from equilibrium ionization
affect the resonance-line cooling efficiencies in the evaporating gas, and
determine the conditions for which radiative losses may be neglected in the
solution for the evaporation dynamics and temperature profile. Our results
indicate that non-equilibrium cooling significantly increases the value of the
saturation parameter at which radiative losses begin to affect the flow
dynamics. As applications we calculate the ion fractions and projected column
densities arising in the evaporating layers surrounding dwarf-galaxy-scale
objects that are also photoionized by metagalactic radiation. We compare our
results to the UV metal-absorption column densities observed in local
highly-ionized metal-absorbers, located in the Galactic corona or intergalactic
medium. Conductive interfaces significantly enhance the formation of high-ions
such as C^3+, N^4+, and O^5+ relative to purely photoionized clouds, especially
for clouds embedded in a high-pressure corona. However, the enhanced columns
are still too low to account for the O VI columns (~1e14 cm^-2) observed in the
local high-velocity absorbers. We find that O VI columns larger than ~1e13
cm^-2 cannot be produced in evaporating clouds. Our results do support the
conclusion of Savage & Lehner, that absorption due to evaporating O VI likely
occurs in the local interstellar medium, with characteristic columns of ~1e13
cm^-2.Comment: Accepted for Publication in Ap
MIDAS prototype Multispectral Interactive Digital Analysis System for large area earth resources surveys. Volume 2: Charge coupled device investigation
MIDAS is a third-generation, fast, low cost, multispectral recognition system able to keep pace with the large quantity and high rates of data acquisition from large regions with present and projected sensors. MIDAS, for example, can process a complete ERTS frame in forty seconds and provide a color map of sixteen constituent categories in a few minutes. A principal objective of the MIDAS Program is to provide a system well interfaced with the human operator and thus to obtain large overall reductions in turn-around time and significant gains in throughput. The need for advanced onboard spacecraft processing of remotely sensed data is stated and approaches to this problem are described which are feasible through the use of charge coupled devices. Tentative mechanizations for the required processing operations are given in large block form. These initial designs can serve as a guide to circuit/system designers
Autonomous proximity operations using machine vision for trajectory control and pose estimation
A machine vision algorithm was developed which permits guidance control to be maintained during autonomous proximity operations. At present this algorithm exists as a simulation, running upon an 80386 based personal computer, using a ModelMATE CAD package to render the target vehicle. However, the algorithm is sufficiently simple, so that following off-line training on a known target vehicle, it should run in real time with existing vision hardware. The basis of the algorithm is a sequence of single camera images of the target vehicle, upon which radial transforms were performed. Selected points of the resulting radial signatures are fed through a decision tree, to determine whether the signature matches that of the known reference signatures for a particular view of the target. Based upon recognized scenes, the position of the maneuvering vehicle with respect to the target vehicles can be calculated, and adjustments made in the former's trajectory. In addition, the pose and spin rates of the target satellite can be estimated using this method
Distinct roles of transcription factors EGL-46 and DAF-19 in specifying the functionality of a polycystin-expressing sensory neuron necessary for C. elegans male vulva location behavior
Caenorhabditis elegans polycystins LOV-1 and PKD-2 are expressed in the male-specific HOB neuron, and are necessary for sensation of the hermaphrodite vulva during mating. We demonstrate that male vulva location behavior and expression of lov-1 and pkd-2 in the ciliated sensory neuron HOB require the activities of transcription factor EGL-46 and to some extent also EGL-44. This EGL-46- regulated program is specific to HOB and is distinct from a general ciliogenic pathway functioning in all ciliated neurons. The ciliogenic pathway regulator DAF-19 affects downstream components of the HOB-specific program indirectly and is independent of EGL-46 activity. The sensory function of HOB requires the combined action of these two distinct regulatory pathways
Eddington limited starbursts in the central 10pc of AGN, and the Torus in NGC1068
We present results from a survey of nearby AGN using the near infrared
adaptive optics integral field spectrograph SINFONI. These data enable us to
probe the distribution and kinematics of the gas and stars at spatial
resolutions as small as 0.085arcsec. We find strong evidence for recent but
short lived starbursts residing in very dense nuclear disks. On scales of less
than 10pc these would have reached Eddington-limited luminosities when active,
perhaps accounting for their short duration. In addition, for NGC1068 at a
resolution of 6pc, we present direct observations of molecular gas close around
the AGN which we identify with the obscuring torus.Comment: Conference proceedings to appear in "The Central Engine of Active
Galactic Nuclei", ed. L. C. Ho and J.-M. Wang (San Francisco: ASP
An Archaeomagnetic Paleointensity Study of Some Hohokam Potsherds from Snaketown, Arizona
A paleointensity study on nine potsherds from the Hohokam Indian site of Snaketown, Arizona is described. The sherds range in age from A.D. 200-1400. Examination of different temperature subintervals from the Thellier-Thellier double heating experiment reveals that conventional statistical measures sometimes can unambiguously determine the best data subset for paleointensity calculations. However, it is often necessary to visually inspect the data and utilize physical insight in determining this data subset. Results suggest that the paleointensity was about 0.94 FO (FO, present intensity ā 0.506 oe) at A.D. 200, 0.72 FO at A.D. 600, and 1.2 FO at A.D. 1400. The shape of our curve of paleointensity vs. age is congruent with a curve previously derived from other Snaketown artifacts, but our paleointensities are systematically lower by about 0.15 oe
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