235 research outputs found
Distinguishing Solar Flare Types by Differences in Reconnection Regions
Observations show that magnetic reconnection and its slow shocks occur in
solar flares. The basic magnetic structures are similar for long duration event
(LDE) flares and faster compact impulsive (CI) flares, but the former require
less non-thermal electrons than the latter. Slow shocks can produce the
required non-thermal electron spectrum for CI flares by Fermi acceleration if
electrons are injected with large enough energies to resonate with scattering
waves. The dissipation region may provide the injection electrons, so the
overall number of non-thermal electrons reaching the footpoints would depend on
the size of the dissipation region and its distance from the chromosphere. In
this picture, the LDE flares have converging inflows toward a dissipation
region that spans a smaller overall length fraction than for CI flares. Bright
loop-top X-ray spots in some CI flares can be attributed to particle trapping
at fast shocks in the downstream flow, the presence of which is determined by
the angle of the inflow field and velocity to the slow shocks.Comment: 15 pages TeX and 2 .eps figures, accepted to Ap.J.Let
Indeterminacy and instability in Petschek reconnection
We explain two puzzling aspects of Petschek's model for fast reconnection. One is its failure to occur in plasma simulations with uniform resistivity. The other is its inability to provide anything more than an upper limit for the reconnection rate. We have found that previously published analytical solutions based on Petschek's model are structurally unstable if the electrical resistivity is uniform. The structural instability is associated with the presence of an essential singularity at the X-line that is unphysical. By requiring that such a singularity does not exist, we obtain a formula that predicts a specific rate of reconnection. For uniform resistivity, reconnection can only occur at the slow, Sweet-Parker rate. For nonuniform resistivity, reconnection can occur at a much faster rate provided that the resistivity profile is not too flat near the X-line. If this condition is satisfied, then the scale length of the nonuniformity determines the reconnection rate
Use of Track Plates to Quantify Predation Risk at Small Spatial Scales
Spatial heterogeneity in risk is a critical component of predator-prey interactions. However, at small spatial scales, it is difficult to quantify predation risk without altering it. We used track plates to measure local predation risk created by white-footed mouse (Peromyscus leucopus) foraging activity on oak-forest plots in Millbrook, New York. Live gypsy moth pupae (Lymantria dispar) were placed at 2 heights on trees and monitored for predation. Pupae deployed on trees visited by mice were more likely to be eaten than those on trees not visited. Logistic regression indicated that predation rates on gypsy moth pupae were positively correlated with track activity, indicating that areas of concentrated mouse activity were areas of heightened risk for gypsy moths. Survival of individual oat grains placed on and 50 cm from track plates were not statistically different, indicating that mice exhibited no detectable behavioral reaction toward track plates. We conclude that track plates offer an economical and reliable means of quantifying local risk of attack by terrestrial mammals without substantially altering the spatial distribution of risk
The Kinematics and Dynamics of the Globular Clusters and the Planetary Nebulae of NGC 5128
A new kinematic and dynamic study of the halo of the giant elliptical galaxy,
NGC 5128, is presented. From a spectroscopically confirmed sample of 340
globular clusters and 780 planetary nebulae, the rotation amplitude, rotation
axis, velocity dispersion, and the total dynamical mass are determined for the
halo of NGC 5128. The globular cluster kinematics were searched for both radial
dependence and metallicity dependence by subdividing the globular cluster
sample into 158 metal-rich ([Fe/H] > -1.0) and 178 metal-poor ([Fe/H] < -1.0)
globular clusters. Our results show the kinematics of the metal-rich and
metal-poor subpopulations are quite similar. The kinematics are compared to the
planetary nebula population where differences are apparent in the outer regions
of the halo. The total mass of NGC 5128 is found using the Tracer Mass
estimator (Evans et al. 2003), to determine the mass supported by internal
random motions, and the spherical component of the Jeans equation to determine
the mass supported by rotation. We find a total mass of (1.0+/-0.2) x 10^(12)
Msun from the planetary nebulae data out to a projected radius of 90 kpc and
(1.3+/-0.5) x 10^(12) Msun from the globular clusters out to a projected radius
of 50 kpc. Lastly, we present a new and homogeneous catalog of known globular
clusters in NGC 5128. This catalog combines all previous definitive cluster
identifications from radial velocity studies and HST imaging studies, as well
as 80 new globular clusters from a study of M.A. Beasley et al. (2007, in
preparation).Comment: Accepted in the Astronomical Journal,52 pages, 13 figures, 6 tables -
Changes made to Table 1 from originally submitted 0704.118
High Repetition-Rate Pulse Shaping of a Spectrally Broadened Yb Femtosecond Laser
We demonstrate compression and shaping of few cycle pulses from a high
average power Ytterbium laser system. The pulses from a commercial 20 W, 100
kHz Yb laser system are spectrally broadened in two-stages using gas-filled,
stretched hollow-core fibers and then compressed and shaped in an acousto-optic
modulator-based pulse-shaper. The pulse-shaper allows for compression,
characterization, and shaping all in one system, producing ~10 fs pulses with
50 uJ of energ
The Globular Cluster System of NGC 5128 II. Ages, Metallicities, Kinematics, and Formation
We present a study of the nearby post-merger giant elliptical galaxy, NGC
5128 (Centaurus A), in which we use the properties of its globular cluster (GC)
and planetary nebula (PN) systems to constrain its evolution. Using photometric
and spectroscopic data for 215 GCs presented in Paper I, we study trends in
age, metallicity, and kinematics for the GC system. We confirm that the GC
metallicity distribution is bimodal, and show that these two sub-populations
have different properties. Using spectral line index measurements of the
brightest clusters, the metal-poor GCs have old ages like the Milky Way
globular clusters, while the metal-rich GCs have H-beta line-strengths that
could be interpreted as a mean age of ~5 (+3/-2) Gyr. Both populations appear
to have [Mg/Fe] ratios consistent with that of the Galactic GC system, although
this quantity is not very well-constrained. The kinematics of the metal-rich
GCs are similar to those of the planetary nebulae, exhibiting significant
rotation about a misaligned axis, while the metal-poor GCs have a higher
velocity dispersion and show a weaker kinematic correlation with the field
stars. The total gravitating mass of NGC 5128 derived from the GCs is in
excellent agreement with the value derived from stellar (PN) kinematics. We
suggest that these and other data support a picture in which the main body of
NGC 5128 was formed 3-8 Gyr ago by the dissipational merger of two unequal-mass
disk galaxies supplemented by the continual accretion of both gas-rich and
gas-poor satellites.Comment: 15 pages, 21 figures (figures 14-20 best viewed in color), accepted
for publication in the Astrophysical Journa
The Diversity and Variability of Star Formation Histories in Models of Galaxy Evolution
Understanding the variability of galaxy star formation histories (SFHs)
across a range of timescales provides insight into the underlying physical
processes that regulate star formation within galaxies. We compile the SFHs of
galaxies at from an extensive set of models, ranging from cosmological
hydrodynamical simulations (Illustris, IllustrisTNG, Mufasa, Simba, EAGLE),
zoom simulations (FIRE-2, g14, and Marvel/Justice League), semi-analytic models
(Santa Cruz SAM) and empirical models (UniverseMachine), and quantify the
variability of these SFHs on different timescales using the power spectral
density (PSD) formalism. We find that the PSDs are well described by broken
power-laws, and variability on long timescales ( Gyr) accounts for
most of the power in galaxy SFHs. Most hydrodynamical models show increased
variability on shorter timescales ( Myr) with decreasing stellar
mass. Quenching can induce dex of additional power on timescales
Gyr. The dark matter accretion histories of galaxies have remarkably
self-similar PSDs and are coherent with the in-situ star formation on
timescales Gyr. There is considerable diversity among the different models
in their (i) power due to SFR variability at a given timescale, (ii) amount of
correlation with adjacent timescales (PSD slope), (iii) evolution of median
PSDs with stellar mass, and (iv) presence and locations of breaks in the PSDs.
The PSD framework is a useful space to study the SFHs of galaxies since model
predictions vary widely. Observational constraints in this space will help
constrain the relative strengths of the physical processes responsible for this
variability.Comment: 31 pages, 17 figures (+ appendix). Resubmitted to MNRAS after
responding to referee's comments. Comments are welcome
Economics methods in Cochrane systematic reviews of health promotion and public health related interventions.
Peer reviewedPublisher PD
A peculiar HI cloud near the distant globular cluster Pal 4
We present 21-cm observations of four Galactic globular clusters, as part of
the on-going GALFA-HI Survey at Arecibo. We discovered a peculiar HI cloud in
the vicinity of the distant (109 kpc) cluster Pal 4, and discuss its properties
and likelihood of association with the cluster. We conclude that an association
of the HI cloud and Pal 4 is possible, but that a chance coincidence between
Pal 4 and a nearby compact high-velocity cloud cannot be ruled out altogether.
New, more stringent upper limits were derived for the other three clusters: M
3, NGC 5466, and Pal 13. We briefly discuss the fate of globular cluster gas
and the interaction of compact clouds with the Galactic Halo gas.Comment: Accepted for publication in MNRA
Recommended from our members
Hematopoietic Cell Transplantation in Patients With Primary Immune Regulatory Disorders (PIRD): A Primary Immune Deficiency Treatment Consortium (PIDTC) Survey.
Primary Immune Regulatory Disorders (PIRD) are an expanding group of diseases caused by gene defects in several different immune pathways, such as regulatory T cell function. Patients with PIRD develop clinical manifestations associated with diminished and exaggerated immune responses. Management of these patients is complicated; oftentimes immunosuppressive therapies are insufficient, and patients may require hematopoietic cell transplant (HCT) for treatment. Analysis of HCT data in PIRD patients have previously focused on a single gene defect. This study surveyed transplanted patients with a phenotypic clinical picture consistent with PIRD treated in 33 Primary Immune Deficiency Treatment Consortium centers and European centers. Our data showed that PIRD patients often had immunodeficient and autoimmune features affecting multiple organ systems. Transplantation resulted in resolution of disease manifestations in more than half of the patients with an overall 5-years survival of 67%. This study, the first to encompass disorders across the PIRD spectrum, highlights the need for further research in PIRD management
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