3,619 research outputs found
Degenerate mixing of plasma waves on cold, magnetized single-species plasmas
In the cold-fluid dispersion relation ω = ω_p/[1+(k_⊥/k_z)^(2]1/2) for Trivelpiece-Gould waves on an infinitely long magnetized plasma cylinder, the transverse and axial wavenumbers appear only in the combination k_⊥/k_z. As a result, for any frequency ω<ω_p, there are infinitely many degenerate waves, all having the same value of k_⊥/k_z. On a cold finite-length plasma column, these degenerate waves reflect into one another at the ends; thus, each standing-wave normal mode of the bounded plasma is a mixture of many degenerate waves, not a single standing wave as is often assumed. A striking feature of the many-wave modes is that the short-wavelength waves often add constructively along resonance cones given by dz/dr = ±(ω_p^2/ω^2-1)^(1/2). Also, the presence of short wavelengths in the admixture for a predominantly long-wavelength mode enhances the viscous damping beyond what the single-wave approximation would predict. Here, numerical solutions are obtained for modes of a cylindrical plasma column with rounded ends. Exploiting the fact that the modes of a spheroidal plasma are known analytically (the Dubin modes), a perturbation analysis is used to investigate the mixing of low-order, nearly degenerate Dubin modes caused by small deformations of a plasma spheroid
Where are the Baryons?
New, high resolution, large-scale, cosmological hydrodynamic galaxy formation
simulations of a standard cold dark matter model (with a cosmological constant)
are utilized to predict the distribution of baryons at the present and at
moderate redshift. It is found that the average temperature of baryons is an
increasing function of time, with most of the baryons at the present time
having a temperature in the range 10^{5-7} K. Thus, not only is the universe
dominated by dark matter, but more than one half of the normal matter is yet to
be detected. Detection of this warm/hot gas poses an observational challenge,
requiring sensitive EUV and X-ray satellites. Signatures include a soft, cosmic
X-ray background, apparent warm components in hot clusters due to both
intrinsic warm intra-cluster gas and warm inter-cluster gas projected onto
clusters along the line of sight, absorption lines in X-ray and UV quasar
spectra [e.g., O VI (1032,1038)A lines, OVII 574 eV line], strong emission
lines (e.g., O VIII 653 eV line) and low redshift, broad, low column density
\lya absorption lines. We estimate that approximately 1/4 of the
extragalactic soft X-ray background (SXRB) (at 0.7 keV) arises from the
warm/hot gas, half of it coming from and three-quarters from ,
so the source regions should be identifiable on deep optical images.Comment: ApJ in press, revised (fig 3 is in jpg). Whole paper including
fig3.ps can be obtained at
"http://astro.princeton.edu/~cen/PAPERS_TO_APPEAR/64
Phase transition in the collisionless regime for wave-particle interaction
Gibbs statistical mechanics is derived for the Hamiltonian system coupling
self-consistently a wave to N particles. This identifies Landau damping with a
regime where a second order phase transition occurs. For nonequilibrium initial
data with warm particles, a critical initial wave intensity is found: above it,
thermodynamics predicts a finite wave amplitude in the limit of infinite N;
below it, the equilibrium amplitude vanishes. Simulations support these
predictions providing new insight on the long-time nonlinear fate of the wave
due to Landau damping in plasmas.Comment: 12 pages (RevTeX), 2 figures (PostScript
Trends in Depressed Mood and Suicidal Behaviors Among Female High School Students Who Engaged in Physical Fighting
The objective of this article is to identify prevalence changes in depressed mood/suicidal behaviors among female high school students reporting physical fighting. This research analyzed the national combined data set of the Youth Risk Behavior Surveillance (YRBS) cross-sectional surveys from 2001 to 2015. Logistic regression analyzed the time trends. Two multiple logistic regression models were built. A quadratic trend was present with an initial decrease followed by an increase 2009 to 2015 (p < .001). The odds and severity of depressed mood/suicidal behaviors were greater among female youths with four or more fights and for other violent events, which were even greater when accounting for electronic bullying. The odds of depressed mood/suicidal behaviors among female adolescents engaged in physical fighting has been increasing with electronic bullying contributing to polyvictimization
Star Cluster Formation and Evolution in Nearby Starburst Galaxies: II. Initial Conditions
We use the ages, masses and metallicities of the rich young star cluster
systems in the nearby starburst galaxies NGC 3310 and NGC 6745 to derive their
cluster formation histories and subsequent evolution. We further expand our
analysis of the systematic uncertainties involved in the use of broad-band
observations to derive these parameters by examining the effects of a priori
assumptions on the individual cluster metallicities. The age (and metallicity)
distributions of both the clusters in the circumnuclear ring in NGC 3310 and of
those outside the ring are statistically indistinguishable, but there is a
clear and significant excess of higher-mass clusters IN the ring compared to
the non-ring cluster sample; it is likely that the physical conditions in the
starburst ring may be conducive for the formation of higher-mass star clusters,
on average, than in the relatively more quiescent environment of the main
galactic disc. For the NGC 6745 cluster system we derive a median age of ~10
Myr. NGC 6745 contains a significant population of high-mass "super star
clusters", with masses in the range 6.5 <= log(M_cl/M_sun) <= 8.0. This
detection supports the scenario that such objects form preferentially in the
extreme environments of interacting galaxies. The age of the cluster
populations in both NGC 3310 and NGC 6745 is significantly lower than their
respective characteristic cluster disruption time-scales. This allows us to
obtain an independent estimate of the INITIAL cluster mass function slope,
alpha = 2.04(+- 0.23)(+0.13)(-0.43) for NGC 3310, and 1.96(+- 0.15)(+- 0.19)
for NGC 6745, respectively, for masses M_cl >= 10^5 M_sun and M_cl >= 4 x 10^5
M_sun. These mass function slopes are consistent with those of other young star
cluster systems in interacting and starburst galaxies.Comment: 17 pages LaTeX, incl. 11 postscript figures, accepted for publication
in MNRA
The Effects of Starburst Activity on Low Surface Brightness Disk Galaxies
Although numerous simulations have been done to understand the effects of
intense bursts of star formation on high surface brightness galaxies, few
attempts have been made to understand how localized starbursts would affect
both the color and surface brightness of low surface brightness (LSB) galaxies.
To remedy this, we have run 53 simulations involving bursts of star formation
activity on LSB galaxies, varying both the underlying galaxy properties and the
parameters describing the starbursts. We discovered that although changing the
total color of a galaxy was fairly straightforward, it was virtually impossible
to alter a galaxy's central surface brightness and thereby remove it from the
LSB galaxy classification without placing a high (and fairly artificial)
threshold for the underlying gas density. The primary effect of large amounts
of induced star formation was to produce a centralized core (bulge) component
which is generally not observed in LSB galaxies. The noisy morphological
appearance of LSB galaxies as well as their noisy surface brightness profiles
can be reproduced by considering small bursts of star formation that are
localized within the disk. The trigger mechanism for such bursts is likely
distant/weak tidal encounters. The stability of disk central surface brightness
to these periods of star formation argues that the large space density of LSB
galaxies at z = 0 should hold to substantially higher redshifts.Comment: 38 pages, 5 figures, 4 tables, tarred and compressed Also available
on http://guernsey.uoregon.edu/~kare
Family Caregiving Challenges in Advanced Colorectal Cancer: Patient and Caregiver Perspectives
Purpose
Family caregivers of advanced colorectal cancer patients may be at increased risk for psychological distress. Yet their key challenges in coping with the patient’s illness are not well understood. Soliciting both patient and caregiver perspectives on these challenges would broaden our understanding of the caregiving experience. Thus, the purpose of this research was to identify caregivers’ key challenges in coping with their family member’s advanced colorectal cancer from the perspective of patients and caregivers.
Methods
Individual, semi-structured qualitative interviews were conducted with 23 advanced colorectal cancer patients and 23 primary family caregivers. Interview data were analyzed via thematic analysis.
Results
In nearly all cases, patient and caregiver reports of the caregiver’s key challenge were discrepant. Across patient and caregiver reports, caregivers’ key challenges included processing emotions surrounding the patient’s initial diagnosis or recurrence and addressing the patient’s practical and emotional needs. Other challenges included coping with continual uncertainty regarding the patient’s potential functional decline and prognosis and observing the patient suffer from various physical symptoms.
Conclusions
Findings suggest that eliciting the perspectives of both patients and caregivers regarding caregivers’ challenges provides a more comprehensive understanding of their experience. Results also point to the need to assist caregivers with the emotional and practical aspects of caregiving
Positive changes among patients with advanced colorectal cancer and their family caregivers: a qualitative analysis
OBJECTIVE:
This study assessed positive changes in patients with advanced colorectal cancer and their family caregivers following diagnosis. We compared self-reported positive changes within patient-caregiver dyads as well as self-reports and patient reports of positive changes in caregivers.
DESIGN:
Individual, semi-structured qualitative interviews were conducted with 23 patients with advanced colorectal cancer and 23 caregivers. A theoretical thematic analysis of interview transcripts was framed by posttraumatic growth theory.
RESULTS:
Patients and caregivers described five positive changes: closer relationships with others, greater appreciation of life, clarifying life priorities, increased faith, and more empathy for others. Additionally, only caregivers reported better health habits following the cancer diagnosis, and a minority of patients and caregivers reported no positive changes. In about half of cases, patients reported at least one positive change that was identical to that of their caregiver. However, in most cases, patient and caregiver reports of the caregiver's positive change were discrepant.
CONCLUSION:
Findings suggest that positive changes are a shared experience for many patient-caregiver dyads and obtaining both patient and caregiver reports of caregiver positive changes provides a more comprehensive understanding of their experience. Interventions may capitalise on positive changes to promote meaningful living in the context of advanced cancer
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