2,068 research outputs found
Orbital Evolution of Compact White Dwarf Binaries
The new-found prevalence of extremely low mass (ELM, Mhe<0.2 Msun) helium
white dwarfs (WDs) in tight binaries with more massive WDs has raised our
interest in understanding the nature of their mass transfer. Possessing small
(Menv~1e-3 Msun) but thick hydrogen envelopes, these objects have larger radii
than cold WDs and so initiate mass transfer of H-rich material at orbital
periods of 6-10 minutes. Building on the original work of D'Antona et al., we
confirm the 1e6 yr period of continued inspiral with mass transfer of H-rich
matter and highlight that the inspiraling direct-impact double WD binary HM
Cancri likely has an ELM WD donor. The ELM WDs have less of a radius expansion
under mass loss, thus enabling a larger range of donor masses that can stably
transfer matter and become a He mass transferring AM CVn binary. Even once in
the long-lived AM CVn mass transferring stage, these He WDs have larger radii
due to their higher entropy from the prolonged H burning stage.Comment: 9 pages, 8 figures. Accepted for publication in the Astrophysical
Journa
LHC Charge Asymmetry as Constraint on Models for the Tevatron Top Anomaly
The forward-backward asymmetry in top quark production at
the Tevatron has been observed to be anomalously large by both CDF and D0. It
has been suggested that a model with a coupling to and might
explain this anomaly, and other anomalies in mesons. Single-top-quark
production in this model is large, and arguably in conflict with Tevatron
measurements. However the model might still be viable if is
somewhat smaller than its current measured central value. We show that even
with smaller couplings, the model can be discovered (or strongly excluded) at
the LHC using the 2010 data sets. We find that a suitable charge-asymmetry
measurement is a powerful tool that can be used to constrain this and other
sources of anomalous single-top production, and perhaps other new high-energy
charge-asymmetric processes.Comment: 25 pages, 4 figures, note adde
First Results from Pan-STARRS1: Faint, High Proper Motion White Dwarfs in the Medium-Deep Fields
The Pan-STARRS1 survey has obtained multi-epoch imaging in five bands
(Pan-STARRS1 gps, rps, ips, zps, and yps) on twelve "Medium Deep Fields", each
of which spans a 3.3 degree circle. For the period between Apr 2009 and Apr
2011 these fields were observed 50-200 times. Using a reduced proper motion
diagram, we have extracted a list of 47 white dwarf (WD) candidates whose
Pan-STARRS1 astrometry indicates a non-zero proper motion at the 6-sigma level,
with a typical 1-sigma proper motion uncertainty of 10 mas/yr. We also used
astrometry from SDSS (when available) and USNO-B to assess our proper motion
fits. None of the WD candidates exhibits evidence of statistically significant
parallaxes, with a typical 1-sigma uncertainty of 8 mas. Twelve of these
candidates are known WDs, including the high proper motion (1.7"/yr) WD LHS
291. We confirm three more objects as WDs through optical spectroscopy. Based
on the Pan-STARRS1 colors, ten of the stars are likely to be cool WDs with 4170
K Teff 5000 K and cooling ages <9 Gyr. We classify these objects as likely
thick disk WDs based on their kinematics. Our current sample represents only a
small fraction of the Pan-STARRS1 data. With continued coverage from the Medium
Deep Field Survey and the 3pi survey, Pan-STARRS1 should find many more high
proper motion WDs that are part of the old thick disk and halo.Comment: 33 pages, 8 figures, submitted to Ap
Echoes of a decaying planetary system: the gaseous and dusty disks surrounding three white dwarfs
We have performed a comprehensive ground-based observational program aimed at
characterizing the circumstellar material orbiting three single white dwarf
stars previously known to possess gaseous disks. Near-infrared imaging
unambiguously detects excess infrared emission towards Ton 345 and allows us to
refine models for the circumstellar dust around all three white dwarf stars. We
find that each white dwarf hosts gaseous and dusty disks that are roughly
spatially coincident, a result that is consistent with a scenario in which
dusty and gaseous material has its origin in remnant parent bodies of the white
dwarfs' planetary systems. We briefly describe a new model for the gas disk
heating mechanism in which the gaseous material behaves like a "Z II" region.
In this Z II region, gas primarily composed of metals is photoionized by
ultraviolet light and cools through optically thick allowed Ca II-line
emission.Comment: 43 pages, 9 tables, 9 figures. Accepted to Ap
Searching for Multijet Resonances at the LHC
Recently it was shown that there is a class of models in which colored vector
and scalar resonances can be copiously produced at the Tevatron with decays to
multijet final states, consistent with all experimental constraints and having
strong discovery potential. We investigate the collider phenomenology of TeV
scale colored resonances at the LHC and demonstrate a strong discovery
potential for the scalars with early data as well as the vectors with
additional statistics. We argue that the signal can be self-calibrating and
using this fact we propose a search strategy which we show to be robust to
systematic errors typically expected from Monte Carlo background estimates. We
model the resonances with a phenomenological Lagrangian that describes them as
bound states of colored vectorlike fermions due to new confining gauge
interactions. However, the phenomenological Lagrangian treatment is quite
general and can represent other scenarios of microscopic physics as well.Comment: 28 pages, 13 figures, pdflatex. Discussion of background expanded,
minor modifications made. Version to appear in JHE
VEGF overexpression induces post-ischaemic neuroprotection, but facilitates haemodynamic steal phenomena
Therapeutic angiogenesis with vascular endothelial growth factor (VEGF) is a clinically promising strategy in ischaemic disease. The pathophysiological consequences of enhanced vessel formation, however, are poorly understood. We established mice overexpressing human VEGF165 under a neuron-specific promoter, which exhibited an increased density of brain vessels under physiological conditions and enhanced angiogenesis after brain ischaemia. Following transient intraluminal middle cerebral artery (MCA) occlusions, VEGF overexpression significantly alleviated neurological deficits and infarct volume, and reduced disseminated neuronal injury and caspase-3 activity, confirming earlier observations that VEGF has neuroprotective properties. Brain swelling was not influenced in VEGF-overexpressing animals, while sodium fluorescein extravasation was moderately increased, suggesting that VEGF induces a mild blood-brain barrier leakage. To elucidate whether enhanced angiogenesis improves regional cerebral blood flow in the ischaemic brain, [14C]iodoantipyrine autoradiography was performed. Autoradiographies revealed that VEGF induces haemodynamic steal phenomena with reduced blood flow in ischaemic areas and increased flow values only outside the MCA territory. Our data demonstrate that VEGF protects neurons from ischaemic cell death by a direct action rather than by promoting angiogenesis, and suggest that strategies aiming at increasing vascular density in the whole brain, e.g. by VEGF overexpression, may worsen rather than improve cerebral haemodynamics after strok
Testing Gluino Spin with Three-Body Decays
We examine the possibility of distinguishing a supersymmetric gluino from a
Kaluza-Klein gluon of universal extra dimensions (UED) at the Large Hadron
Collider (LHC). We focus on the case when all kinematically allowed tree-level
decays of this particle are 3-body decays into two jets and a massive daughter
(typically weak gaugino or Kaluza-Klein weak gauge boson). We show that the
shapes of the dijet invariant mass distributions differ significantly in the
two models, as long as the mass of the decaying particle mA is substantially
larger than the mass of the massive daughter mB. We present a simple analysis
estimating the number of events needed to distinguish between the two models
under idealized conditions. For example, for mA/mB=10, we find the required
number of events to be of order several thousand, which should be available at
the LHC within a few years. This conclusion is confirmed by a parton level
Monte Carlo study which includes the effects of experimental cuts and the
combinatoric background.Comment: 19 pages, 10 figure
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