5,651 research outputs found
Continuous monitoring of the lunar or Martian subsurface using on-board pattern recognition and neural processing of Rover geophysical data
The ultimate goal is to create an extraterrestrial unmanned system for subsurface mapping and exploration. Neural networks are to be used to recognize anomalies in the profiles that correspond to potentially exploitable subsurface features. The ground penetrating radar (GPR) techniques are likewise identical. Hence, the preliminary research focus on GPR systems will be directly applicable to seismic systems once such systems can be designed for continuous operation. The original GPR profile may be very complex due to electrical behavior of the background, targets, and antennas, much as the seismic record is made complex by multiple reflections, ghosting, and ringing. Because the format of the GPR data is similar to the format of seismic data, seismic processing software may be applied to GPR data to help enhance the data. A neural network may then be trained to more accurately identify anomalies from the processed record than from the original record
Radiation and String Atmosphere for Relativistic Stars
We extend the Vaidya radiating metric to include both a radiation field and a
string fluid. Assuming diffusive transport for the string fluid, we find new
analytic solutions of Einstein's field equations. Our new solutions represent
an extention of Xanthopoulos superposition.Comment: To appear in Phys. Rev. D, Rapid Communicatio
Collapsing Layers on Schwarzschild-Lemaitre Geodesics
We discuss Israel layers collapsing inward from rest at infinity along
Schwarzschild-Lemaitre geodesics. The dynamics of the collapsing layer and its
equation of state are developed. There is a general equation of state which is
approximately polytropic in the limit of very low pressure. The equation of
state establishes a new limit on the stress-density ratio.Comment: To appear in Phys. Rev. D 1
Deep ALTAIR + NIRI Imaging of the Disk and Bulge of M31
Deep J, H, and K' images, recorded with the ALTAIR adaptive optics system and
NIRI imager on Gemini North, are used to probe the stellar content of the disk
and bulge of the Local Group galaxy M31. With FWHM near 0.08 arcsec in K, these
are the highest angular resolution near-infrared images yet obtained of this
galaxy. Four fields that sample M31 at galactocentric radii of 62, 9, 4, and 2
arcmin were observed. The RGB-tip occurs between K = 17.0 and 17.2, and the
color of the RGB in the field closest to the center of M31 is consistent with
that of NGC 6528. After accounting for random photometric errors, the upper RGB
in each field has a width on the (K, J-K) CMD that is consistent with a +/- 0.5
dex dispersion in [Fe/H], in rough agreement with what is seen in other disk
and spheroid fields in M31. A population of very bright red stars, which we
identify as C stars, are seen in the three fields that are closest to the
center of M31. The spatial distribution of these objects suggests that they are
well mixed throughout this part of M31, and so likely did not form in a compact
region near the galactic nucleus, but more probably formed in the inner disk.
We speculate that these C stars may be the most luminous members of the
intermediate age population that has been detected previously in studies of the
integrated spectrum of the central regions of M31.Comment: 36 pages of text + 16 eps figures; Astronomical Journal in pres
High Spatial Resolution Imaging of NGC 1068 in the Mid Infrared
Mid-infrared observations of the central source of NGC 1068 have been
obtained with a spatial resolution in the deconvolved image of 0.1" (~ 7pc).
The central source is extended by ~1" in the north-south direction but appears
unresolved in the east-west direction over most of its length. About two-thirds
of its flux can be ascribed to a core structure which is itself elongated
north-south and does not show a distinct unresolved compact source. The source
is strongly asymmetric, extending significantly further to the north than to
the south. The morphology of the mid-infrared emission appears similar to that
of the radio jet, and has features which correlate with the images in [O III].
Its 12.5-24.5 micron color temperature ranges from 215 to 260 K and does not
decrease smoothly with distance from the core. Silicate absorption is strongest
in the core and to the south and is small in the north.
The core, apparently containing two-thirds of the bolometric luminosity of
the inner 4" diameter area, may be explained by a thick, dusty torus near the
central AGN viewed at an angle of ~65 deg to its plane. There are, however,
detailed difficulties with existing models, especially the narrow east-west
width of the thin extended mid-infrared "tongue" to the north of the core. We
interpret the tongue as re-processed visual and ultraviolet radiation that is
strongly beamed and that originates in the AGN.Comment: 42 pages, 2 tables, 9 figures; Accepted for publication in A
Deep Mid-Infrared Silicate Absorption as a Diagnostic of Obscuring Geometry Toward Galactic Nuclei
The silicate cross section peak near 10um produces emission and absorption
features in the spectra of dusty galactic nuclei observed with the Spitzer
Space Telescope. Especially in ultraluminous infrared galaxies, the observed
absorption feature can be extremely deep, as IRAS 08572+3915 illustrates. A
foreground screen of obscuration cannot reproduce this observed feature, even
at large optical depth. Instead, the deep absorption requires a nuclear source
to be deeply embedded in a smooth distribution of material that is both
geometrically and optically thick. In contrast, a clumpy medium can produce
only shallow absorption or emission, which are characteristic of
optically-identified active galactic nuclei. In general, the geometry of the
dusty region and the total optical depth, rather than the grain composition or
heating spectrum, determine the silicate feature's observable properties. The
apparent optical depth calculated from the ratio of line to continuum emission
generally fails to accurately measure the true optical depth. The obscuring
geometry, not the nature of the embedded source, also determines the far-IR
spectral shape.Comment: To appear in ApJ
Morphology of the 12-micron Seyfert Galaxies: II. Optical and Near-Infrared Image Atlas
We present 263 optical and near-infrared (NIR) images for 42 Seyfert 1s and
48 Seyfert 2s, selected from the Extended 12-micron Galaxy Sample.
Elliptically-averaged profiles are derived from the images, and isophotal radii
and magnitudes are calculated from these. We also report virtual aperture
photometry, that judging from comparison with previous work, is accurate to
roughly 0.05mag in the optical, and 0.07mag in the NIR. Our B-band isophotal
magnitude and radii, obtained from ellipse fitting, are in good agreement with
those of RC3. When compared with the B band, V, I, J, and K isophotal diameters
show that the colors in the outer regions of Seyferts are consistent with the
colors of normal spirals. Differences in the integrated isophotal colors and
comparison with a simple model show that the active nucleus+bulge is stronger
and redder in the NIR than in the optical. Finally, roughly estimated Seyfert
disk surface brightnesses are significantly brighter in B and K than those in
normal spirals of similar morphological type.Comment: 17 pgs including figures; Table 2 is a separate file. Complete Figure
1 is available by contacting the authors. Accepted for publication in ApJ
Debris disks around Sun-like stars
We have observed nearly 200 FGK stars at 24 and 70 microns with the Spitzer
Space Telescope. We identify excess infrared emission, including a number of
cases where the observed flux is more than 10 times brighter than the predicted
photospheric flux, and interpret these signatures as evidence of debris disks
in those systems. We combine this sample of FGK stars with similar published
results to produce a sample of more than 350 main sequence AFGKM stars. The
incidence of debris disks is 4.2% (+2.0/-1.1) at 24 microns for a sample of 213
Sun-like (FG) stars and 16.4% (+2.8/-2.9) at 70 microns for 225 Sun-like (FG)
stars. We find that the excess rates for A, F, G, and K stars are statistically
indistinguishable, but with a suggestion of decreasing excess rate toward the
later spectral types; this may be an age effect. The lack of strong trend among
FGK stars of comparable ages is surprising, given the factor of 50 change in
stellar luminosity across this spectral range. We also find that the incidence
of debris disks declines very slowly beyond ages of 1 billion years.Comment: ApJ, in pres
Collective dynamics of two-mode stochastic oscillators
We study a system of two-mode stochastic oscillators coupled through their
collective output. As a function of a relevant parameter four qualitatively
distinct regimes of collective behavior are observed. In an extended region of
the parameter space the periodicity of the collective output is enhanced by the
considered coupling. This system can be used as a new model to describe
synchronization-like phenomena in systems of units with two or more oscillation
modes. The model can also explain how periodic dynamics can be generated by
coupling largely stochastic units. Similar systems could be responsible for the
emergence of rhythmic behavior in complex biological or sociological systems.Comment: 4 pages, RevTex, 5 figure
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