1,826 research outputs found
Convective instability of 3-D fluid-saturated geological fault zones heated from below
We conduct a theoretical analysis to investigate the convective instability of 3-D fluid-saturated geological fault zones when they are heated uniformly from below. In particular, we have derived exact analytical solutions for the critical Rayleigh numbers of different convective flow structures. Using these critical Rayleigh numbers, three interesting convective flow structures have been identified in a geological fault zone system. It has been recognized that the critical Rayleigh numbers of the system have a minimum value only for the fault zone of infinite length, in which the corresponding convective flow structure is a 2-D slender-circle flow. However, if the length of the fault zone is finite, the convective flow in the system must be 3-D. Even if the length of the fault zone is infinite, since the minimum critical Rayleigh number for the 2-D slender-circle flow structure is so close to that for the 3-D convective flow structure, the system may have almost the same chance to pick up the 3-D convective flow structures. Also, because the convection modes are so close for the 3-D convective flow structures, the convective flow may evolve into the 3-D finger-like structures, especially for the case of the fault thickness to height ratio approaching zero. This understanding demonstrates the beautiful aspects of the present analytical solution for the convective instability of 3-D geological fault zones, because the present analytical solution is valid for any value of the ratio of the fault height to thickness. Using the present analytical solution, the conditions, under which different convective flow structures may take place, can be easily determined
Alpha-particle-induced complex chromosome exchanges transmitted through extra-thymic lymphopoiesis in vitro show evidence of emerging genomic instability
Human exposure to high-linear energy transfer α-particles includes environmental (e.g. radon gas and its decay progeny), medical (e.g. radiopharmaceuticals) and occupational (nuclear industry) sources. The associated health risks of α-particle exposure for lung cancer are well documented however the risk estimates for leukaemia remain uncertain. To further our understanding of α-particle effects in target cells for leukaemogenesis and also to seek general markers of individual exposure to α-particles, this study assessed the transmission of chromosomal damage initially-induced in human haemopoietic stem and progenitor cells after exposure to high-LET α-particles. Cells surviving exposure were differentiated into mature T-cells by extra-thymic T-cell differentiation in vitro. Multiplex fluorescence in situ hybridisation (M-FISH) analysis of naïve T-cell populations showed the occurrence of stable (clonal) complex chromosome aberrations consistent with those that are characteristically induced in spherical cells by the traversal of a single α-particle track. Additionally, complex chromosome exchanges were observed in the progeny of irradiated mature T-cell populations. In addition to this, newly arising de novo chromosome aberrations were detected in cells which possessed clonal markers of α-particle exposure and also in cells which did not show any evidence of previous exposure, suggesting ongoing genomic instability in these populations. Our findings support the usefulness and reliability of employing complex chromosome exchanges as indicators of past or ongoing exposure to high-LET radiation and demonstrate the potential applicability to evaluate health risks associated with α-particle exposure.This work was supported by the Department of Health, UK. Contract RRX95 (RMA NSDTG)
Frequent burning promotes invasions of alien plants into a mesic African savanna
Fire is both inevitable and necessary for maintaining the structure and functioning of mesic savannas. Without disturbances such as fire and herbivory, tree cover can increase at the expense of grass cover and over time dominate mesic savannas. Consequently, repeated burning is widely used to suppress tree recruitment and control bush encroachment. However, the effect of regular burning on invasion by alien plant species is little understood. Here, vegetation data from a long-term fire experiment, which began in 1953 in a mesic Zimbabwean savanna, were used to test whether the frequency of burning promoted alien plant invasion. The fire treatments consisted of late season fires, lit at 1-, 2-, 3-, and 4-year intervals, and these regularly burnt plots were compared with unburnt plots. Results show that over half a century of frequent burning promoted the invasion by alien plants relative to areas where fire was excluded. More alien plant species became established in plots that had a higher frequency of burning. The proportion of alien species in the species assemblage was highest in the annually burnt plots followed by plots burnt biennially. Alien plant invasion was lowest in plots protected from fire but did not differ significantly between plots burnt triennially and quadrennially. Further, the abundance of five alien forbs increased significantly as the interval (in years) between fires became shorter. On average, the density of these alien forbs in annually burnt plots was at least ten times as high as the density of unburnt plots. Plant diversity was also altered by long-term burning. Total plant species richness was significantly lower in the unburnt plots compared to regularly burnt plots. These findings suggest that frequent burning of mesic savannas enhances invasion by alien plants, with short intervals between fires favouring alien forbs. Therefore, reducing the frequency of burning may be a key to minimising the risk of alien plant spread into mesic savannas, which is important because invasive plants pose a threat to native biodiversity and may alter savanna functioning
Deep Synoptic Array Science: Polarimetry of 25 New Fast Radio Bursts Provides Insights into their Origins
We report on a full-polarization analysis of the first 25 as yet
non-repeating FRBs detected at 1.4 GHz by the 110-antenna Deep Synoptic Array
(DSA-110) during commissioning observations. We present details of the data
reduction, calibration, and analysis procedures developed for this novel
instrument. The data have 32 s time resolution and sensitivity to Faraday
rotation measures (RMs) between rad m. RMs are detected for
20 FRBs with magnitudes ranging from rad m. FRBs are
found to have high () linear-polarization fractions. The remaining
FRBs exhibit significant circular polarization (), or are either
partially depolarized () or unpolarized (). We investigate the
mechanism of depolarization, disfavoring stochastic RM variations within a
scattering screen as a dominant cause. Polarization-state and possible RM
variations are observed in the four FRBs with multiple sub-components, but only
one other FRB shows a change in polarization state. We combine the DSA-110
sample with polarimetry of previously published FRBs, and compare the
polarization properties of FRB sub-populations and FRBs with Galactic pulsars.
Although FRBs are typically far more polarized than the average profiles of
Galactic pulsars, and exhibit greater spread in polarization fractions than
pulsar single pulses, we find a remarkable similarity between FRB polarization
fractions and the youngest (characteristic ages yr) pulsars. Our
results support a scenario wherein FRB emission is intrinsically highly
linearly polarized, and where propagation effects within progenitor
magnetospheres can result in conversion to circular polarization and
depolarization. Young pulsar emission and magnetospheric-propagation geometries
may form a useful analogy for the origin of FRB polarization.Comment: 43 pages, 17 figure
Deep Synoptic Array Science: Implications of Faraday Rotation Measures of Localized Fast Radio Bursts
Faraday rotation measures (RMs) of fast radio bursts (FRBs) offer the
prospect of directly measuring extragalactic magnetic fields. We present an
analysis of the RMs of ten as yet non-repeating FRBs detected and localized to
host galaxies by the 110-antenna Deep Synoptic Array (DSA-110). We combine this
sample with published RMs of 15 localized FRBs, nine of which are repeating
sources. For each FRB in the combined sample, we estimate the host-galaxy
dispersion measure (DM) contributions and extragalactic RM. We find compelling
evidence that the extragalactic components of FRB RMs are often dominated by
contributions from the host-galaxy interstellar medium (ISM). Specifically, we
find that both repeating and as yet non-repeating FRBs show a correlation
between the host-DM and host-RM in the rest frame, and we find an
anti-correlation between extragalactic RM (in the observer frame) and redshift
for non-repeaters, as expected if the magnetized plasma is in the host galaxy.
Important exceptions to the ISM origin include a dense, magnetized circum-burst
medium in some repeating FRBs, and the intra-cluster medium (ICM) of host or
intervening galaxy clusters. We find that the estimated ISM magnetic-field
strengths, , are characteristically larger than those inferred from
Galactic radio pulsars. This suggests either increased ISM magnetization in FRB
hosts in comparison with the Milky Way, or that FRBs preferentially reside in
regions of increased magnetic-field strength within their hosts
Deep Synoptic Array science I: discovery of the host galaxy of FRB 20220912A
We report the detection and interferometric localization of the repeating
fast radio burst (FRB) source FRB 20220912A during commissioning observations
with the Deep Synoptic Array (DSA-110). Two bursts were detected from FRB
20220912A, one each on 2022 October 18 and 2022 October 25. The best-fit
position is (R.A. J2000, decl. J2000) = (23:09:04.9, +48:42:25.4), with a 90%
confidence error ellipse of arcsec and arcsec in right ascension
and declination respectively. The two bursts have disparate polarization
properties and temporal profiles. We find a Faraday rotation measure that is
consistent with the low value of rad m reported by CHIME/FRB. The
DSA-110 localization overlaps with the galaxy PSO J347.2702+48.7066 at a
redshift , which we identify as the likely host. PSO
J347.270248.7066 has a stellar mass of approximately ,
modest internal dust extinction, and a star-formation rate likely in excess of
yr. The host-galaxy contribution to the dispersion
measure is likely pc cm. The FRB 20220912A source is
therefore likely viewed along a tenuous plasma column through the host galaxy.Comment: 10 pages, 7 figures, 2 tables, submitted to AAS Journal
The Evolution of Compact Binary Star Systems
We review the formation and evolution of compact binary stars consisting of
white dwarfs (WDs), neutron stars (NSs), and black holes (BHs). Binary NSs and
BHs are thought to be the primary astrophysical sources of gravitational waves
(GWs) within the frequency band of ground-based detectors, while compact
binaries of WDs are important sources of GWs at lower frequencies to be covered
by space interferometers (LISA). Major uncertainties in the current
understanding of properties of NSs and BHs most relevant to the GW studies are
discussed, including the treatment of the natal kicks which compact stellar
remnants acquire during the core collapse of massive stars and the common
envelope phase of binary evolution. We discuss the coalescence rates of binary
NSs and BHs and prospects for their detections, the formation and evolution of
binary WDs and their observational manifestations. Special attention is given
to AM CVn-stars -- compact binaries in which the Roche lobe is filled by
another WD or a low-mass partially degenerate helium-star, as these stars are
thought to be the best LISA verification binary GW sources.Comment: 105 pages, 18 figure
Limits on WWZ and WW\gamma couplings from p\bar{p}\to e\nu jj X events at \sqrt{s} = 1.8 TeV
We present limits on anomalous WWZ and WW-gamma couplings from a search for
WW and WZ production in p-bar p collisions at sqrt(s)=1.8 TeV. We use p-bar p
-> e-nu jjX events recorded with the D0 detector at the Fermilab Tevatron
Collider during the 1992-1995 run. The data sample corresponds to an integrated
luminosity of 96.0+-5.1 pb^(-1). Assuming identical WWZ and WW-gamma coupling
parameters, the 95% CL limits on the CP-conserving couplings are
-0.33<lambda<0.36 (Delta-kappa=0) and -0.43<Delta-kappa<0.59 (lambda=0), for a
form factor scale Lambda = 2.0 TeV. Limits based on other assumptions are also
presented.Comment: 11 pages, 2 figures, 2 table
Search For Heavy Pointlike Dirac Monopoles
We have searched for central production of a pair of photons with high
transverse energies in collisions at TeV using of data collected with the D\O detector at the Fermilab Tevatron in
1994--1996. If they exist, virtual heavy pointlike Dirac monopoles could
rescatter pairs of nearly real photons into this final state via a box diagram.
We observe no excess of events above background, and set lower 95% C.L. limits
of on the mass of a spin 0, 1/2, or 1 Dirac
monopole.Comment: 12 pages, 4 figure
The Dijet Mass Spectrum and a Search for Quark Compositeness in bar{p}p Collisions at sqrt{s} = 1.8 TeV
Using the DZero detector at the 1.8 TeV pbarp Fermilab Tevatron collider, we
have measured the inclusive dijet mass spectrum in the central pseudorapidity
region |eta_jet| < 1.0 for dijet masses greater than 200 Gev/c^2. We have also
measured the ratio of spectra sigma(|eta_jet| < 0.5)/sigma(0.5 < |eta_jet| <
1.0). The order alpha_s^3 QCD predictions are in good agreement with the data
and we rule out models of quark compositeness with a contact interaction scale
< 2.4 TeV at the 95% confidence level.Comment: 11 pages, 4 figures, 2 tables, submitted to Phys. Rev. Let
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