1,484 research outputs found
Standing Swells Surveyed Showing Surprisingly Stable Solutions for the Lorenz '96 Model
The Lorenz '96 model is an adjustable dimension system of ODEs exhibiting
chaotic behavior representative of dynamics observed in the Earth's atmosphere.
In the present study, we characterize statistical properties of the chaotic
dynamics while varying the degrees of freedom and the forcing. Tuning the
dimensionality of the system, we find regions of parameter space with
surprising stability in the form of standing waves traveling amongst the slow
oscillators. The boundaries of these stable regions fluctuate regularly with
the number of slow oscillators. These results demonstrate hidden order in the
Lorenz '96 system, strengthening the evidence for its role as a hallmark
representative of nonlinear dynamical behavior.Comment: 10 pages, 8 figure
A Detection of Gas Associated with the M 31 Stellar Stream
Detailed studies of stellar populations in the halos of the Milky Way and the
Andromeda (M 31) galaxies have shown increasing numbers of tidal streams and
dwarf galaxies, attesting to a complicated and on-going process of hierarchical
structure formation. The most prominent feature in the halo of M 31 is the
Giant Stellar Stream, a structure ~4.5 degrees in extent along the sky, which
is close to, but not coincident with the galaxy's minor axis. The stars that
make up this stream are kinematically and chemically distinct from the other
stars in the halo. Here, we present HST/COS high-resolution ultraviolet
absorption spectra of three Active Galactic Nuclei sight lines which probe the
M 31 halo, including one that samples gas in the main southwestern portion of
the Giant Stream. We see two clear absorption components in many metal species
at velocities typical of the M 31 halo and a third, blue-shifted component
which arises in the stream. Photoionization modeling of the column density
ratios in the different components shows gas in an ionization state typical of
that seen in other galaxy halo environments and suggests solar to slightly
super-solar metallicity, consistent with previous findings from stellar
spectroscopy.Comment: 10 pages, 5 figures, accepted for publication in the Astrophysical
Journa
An X-ray WHIM metal absorber from a Mpc-scale empty region of space
We report a detection of an absorption line at ~44.8 {\AA} in a > 500 ks
Chandra HRC-S/LETG X-ray grating spectrum of the blazar H 2356-309. This line
can be identified as intervening CV-K{\alpha} absorption, at z\approx0.112,
produced by a warm (log T = 5.1 K) intergalactic absorber. The feature is
significant at a 2.9{\sigma} level (accounting for the number of independent
redshift trials). We estimate an equivalent hydrogen column density of log
N_H=19.05 (Z/Zsun)^-1 cm^-2. Unlike other previously reported FUV/X-ray metal
detections of warm-hot intergalactic medium (WHIM), this CV absorber lies in a
region with locally low galaxy density, at ~2.2 Mpc from the closest galaxy at
that redshift, and therefore is unlikely to be associated with an extended
galactic halo. We instead tentatively identify this absorber with an
intervening Warm-Hot Intergalactic Medium filament possibly permeating a
large-scale, 30 Mpc extended, structure of galaxies whose redshift centroid,
within a cylinder of 7.5 Mpc radius centered on the line of sight to H
2356-309, is marginally consistent (at a 1.8{\sigma} level) with the redshift
of the absorber.Comment: ApJ accepted, 6 pages, 3 figure
Attitudes of College Students towards Agriculture, Food and the Role of Government
In 2002 and 2007 we surveyed Agribusiness students’ attitudes about agriculture, farming, food and agricultural policies. Responses were analyzed by year and student characteristics including farm background, citizenship and gender. Citizenship was a significant variable explaining differences in agreement with statements. Year and interactions with year were not significant.agricultural policy, farming, logistic regression, student attitudes, Agricultural and Food Policy, Teaching/Communication/Extension/Profession, A13, A22, C42, Q18,
A Detailed Analysis of a Cygnus Loop Shock-Cloud Interaction
The XA region of the Cygnus Loop is a complex zone of radiative and
nonradiative shocks interacting with interstellar clouds. We combine five far
ultraviolet spectral observations from the Hopkins Ultraviolet Telescope (HUT),
a grid of 24 IUE spectra and a high-resolution longslit Halpha spectrum to
study the spatial emission line variations across the region. These spectral
data are placed in context using ground-based, optical emission line images of
the region and a far-UV image obtained by the Ultraviolet Imaging Telescope
(UIT). The presence of high-ionization ions (OVI, NV, CIV) indicates a shock
velocity near 170 km/s while other diagnostics indicate v_shock=140 km/s. It is
likely that a large range of shock velocities may exist at a spatial scale
smaller than we are able to resolve. By comparing CIV 1550, CIII 977 and CIII]
1909, we explore resonance scattering across the region. We find that a
significant column depth is present at all positions, including those not near
bright optical/UV filaments. Analysis of the OVI doublet ratio suggests an
average optical depth of about unity in that ion while flux measurements of
[SiVIII] 1443 suggest a hot component in the region at just below 10^6K. Given
the brightness of the OVI emission and the age of the interaction, we rule out
the mixing layer interpretation of the UV emission. Furthermore, we formulate a
picture of the XA region as the encounter of the blast wave with a finger of
dense gas protruding inward from the pre-SN cavity.Comment: 21 pages, 9 figures, accepted by the Astronomical Journal, July 2001
Full resolution figures available at http://fuse.pha.jhu.edu/~danforth/xa
Possible Detection of OVI from the LMC Superbubble N70
We present FUSE observations toward four stars in the LMC superbubble N70 and
compare these spectra to those of four comparison targets located in nearby
field and diffuse regions. The N70 sight lines show OVI 1032 absorption that is
consistently stronger than the comparison sight lines by ~60%. We attribute the
excess column density (logN_OVI=14.03 cm^-2) to hot gas within N70, potentially
the first detection of OVI associated with a superbubble. In a survey of 12 LMC
sight lines, Howk et al. (2002a) concluded that there was no correlation
between ISM morphology and N_OVI. We present a reanalysis of their measurements
combined with our own and find a clear difference between the superbubble and
field samples. The five superbubbles probed to date with FUSE show a
consistently higher mean N_OVI than the 12 non-superbubble sight lines, though
both samples show equivalent scatter from halo variability. Possible ionization
mechanisms for N70 are discussed, and we conclude that the observed OVI could
be the product of thermal conduction at the interface between the hot, X-ray
emitting gas inside the superbubble and the cooler, photoionized material
making up the shell seen prominently in Halpha. We calculate the total hydrogen
density n_H implied by our OVI measurements and find a value consistent with
expectations. Finally, we discuss emission-line observations of OVI from N70.Comment: 9 pages in emulateapj style. Accepted to Ap
An HST/COS Observation of Broad Ly Emission and Associated Absorption Lines of the BL Lacertae Object H 2356-309
Weak spectral features in BL Lacertae objects (BL Lac) often provide a unique
opportunity to probe the inner region of this rare type of active galactic
nucleus. We present a Hubble Space Telescope/Cosmic Origins Spectrograph
observation of the BL Lac H 2356-309. A weak Ly emission line was
detected. This is the fourth detection of a weak Ly emission feature in
the ultraviolet (UV) band in the so-called "high energy peaked BL Lacs", after
Stocke et al. Assuming the line-emitting gas is located in the broad line
region (BLR) and the ionizing source is the off-axis jet emission, we constrain
the Lorentz factor () of the relativistic jet to be with a
maximum viewing angle of 3.6. The derived is somewhat larger
than previous measurements of , implying a covering
factor of 3% of the line-emitting gas. Alternatively, the BLR clouds
could be optically thin, in which case we constrain the BLR warm gas to be
. We also detected two HI and one OVI absorption
lines that are within of the BL Lac object.
The OVI and one of the HI absorbers likely coexist due to their nearly
identical velocities. We discuss several ionization models and find a
photoionization model where the ionizing photon source is the BL Lac object can
fit the observed ion column densities with reasonable physical parameters. This
absorber can either be located in the interstellar medium of the host galaxy,
or in the BLR.Comment: 7 pages, 2 figures, accepted for publication in Ap
Chandra Imaging and Spectroscopy of the Eastern XA Region of the Cygnus Loop Supernova Remnant
The XA region of the Cygnus Loop is a bright knot of X-ray emission on the
eastern edge of the supernova remnant resulting from the interaction of the
supernova blast wave with density enhancements at the edge of a precursor
formed cavity. To study the nature and origin of the X-ray emission we use high
spatial resolution images from Chandra. Our goal is to probe the density of
various spectral extraction regions to form a picture of the cavity wall and
characterize the interaction between this supernova and the local interstellar
medium. We find that a series of regions along the edge of the X-ray emission
appears to trace out the location of the cavity wall. The best fit plasma
models result in two temperature component equilibrium models for each region.
The low temperature components have densities that are an order of magnitude
higher than the high temperature components. The high density plasma may exist
in the cavity wall where it equilibrates rapidly and cools efficiently. The low
density plasma is interior to the enhancement and heated further by a reverse
shock from the wall. Calculations of shock velocities and timescales since
shock heating are consistent with this interpretation. Furthermore, we find a
bright knot of emission indicative of a discrete interaction of the blast wave
with a high density cloud in the cavity wall with a size scale ~0.1 pc. Aside
from this, other extractions made interior to the X-ray edge are confused by
line of sight projection of various components. Some of these regions show
evidence of detecting the cavity wall but their location makes the
interpretation difficult. In general, the softer plasmas are well fit at
temperatures kT~0.11 keV, with harder plasmas at temperatures of kT~0.27 keV.
All regions display consistent metal depletions most notably in N, O, and Ne at
an average of 0.54, 0.55, and 0.36 times solar
Cosmic Origins Spectrograph and FUSE Observations of T ~ 10^5 K Gas In A Nearby Galaxy Filament
We present a detection of a broad Ly-alpha absorber (BLA) with a matching O
VI line in the nearby universe. The BLA is detected at z = 0.01028 in the high
S/N spectrum of Mrk 290 obtained using the Cosmic Origins Spectrograph. The
Ly-alpha absorption has two components, with b(HI) = 55 +/- 1 km/s and b(HI) =
33 +/- 1 km/s, separated in velocity by v ~ 115 km/s. The O VI, detected by
FUSE at z = 0.01027, has a b(OVI) = 29 +/- 3 km/s and is kinematically well
aligned with the broader HI component. The different line widths of the BLA and
OVI suggest a temperature of T = 1.4 x 10^5 K in the absorber. The observed
line strength ratios and line widths favor an ionization scenario in which both
ion-electron collisions and UV photons contribute to the ionization in the gas.
Such a model requires a low-metallicity of -1.7 dex, ionization parameter of
log U ~ -1.4, a large total hydrogen column density of N(H) ~ 4 x 10^19 cm^-2,
and a path length of 400 kpc. The line of sight to Mrk 290 intercepts at the
redshift of the absorber, a megaparsec scale filamentary structure extending
over 20 deg in the sky, with several luminous galaxies distributed within 1.5
Mpc projected distance from the absorber. The collisionally ionized gas in this
absorber is likely tracing a shock-heated gaseous structure, consistent with a
few different scenarios for the origin, including an over-dense region of the
WHIM in the galaxy filament or highly ionized gas in the extended halo of one
of the galaxies in the filament. In general, BLAs with metals provide an
efficient means to study T ~ 10^5 - 10^6 K gas in galaxy halos and in the
intergalactic medium. A substantial fraction of the baryons "missing" from the
present universe is predicted to be in such environments in the form of highly
ionized plasma.Comment: Astrophysical Journal Accepte
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