5,274 research outputs found
The oxidation and sublimation of graphite in simulated re-entry environments
Graphite oxidation and sublimation in simulated reentry environment
The time evolution of marginally trapped surfaces
In previous work we have shown the existence of a dynamical horizon or
marginally trapped tube (MOTT) containing a given strictly stable marginally
outer trapped surface (MOTS). In this paper we show some results on the global
behavior of MOTTs assuming the null energy condition. In particular we show
that MOTSs persist in the sense that every Cauchy surface in the future of a
given Cauchy surface containing a MOTS also must contain a MOTS. We describe a
situation where the evolving outermost MOTS must jump during the coalescence of
two seperate MOTSs. We furthermore characterize the behavior of MOTSs in the
case that the principal eigenvalue vanishes under a genericity assumption. This
leads to a regularity result for the tube of outermost MOTSs under the
genericity assumption. This tube is then smooth up to finitely many jump times.
Finally we discuss the relation of MOTSs to singularities of a space-time.Comment: 21 pages. This revision corrects some typos and contains more
detailed proofs than the original versio
Is Small Perfect? Size Limit to Defect Formation in Pyramidal Pt Nanocontacts
We report high resolution transmission electron microscopy and ab initio
calculation results for the defect formation in Pt nanocontacts (NCs). Our
results show that there is a size limit to the existence of twins (extended
structural defects). Defects are always present but blocked away from the tip
axes. The twins may act as scattering plane, influencing contact electron
transmission for Pt NC at room temperature and Ag/Au NC at low temperature.Comment: 4 pages, 3 figure
A Search for Distant Galactic Cepheids Toward l=60
We present results of a survey of a 6-square-degree region near l=60, b=0 to
search for distant Milky Way Cepheids. Few MW Cepheids are known at distances
>~ R_0, limiting large-scale MW disk models derived from Cepheid kinematics;
this work was designed to find a sample of distant Cepheids for use in such
models. The survey was conducted in the V and I bands over 8 epochs, to a
limiting I~=18, with a total of ~ 5 million photometric observations of ~ 1
million stars. We present a catalog of 578 high-amplitude variables discovered
in this field. Cepheid candidates were selected from this catalog on the basis
of variability and color change, and observed again the following season. We
confirm 10 of these candidates as Cepheids with periods from 4 to 8 days, most
at distances > 3 kpc. Many of the Cepheids are heavily reddened by intervening
dust, some with implied extinction A_V > 10 mag. With a future addition of
infrared photometry and radial velocities, these stars alone can provide a
constraint on R_0 to 8%, and in conjunction with other known Cepheids should
provide good estimates of the global disk potential ellipticity.Comment: 18 pages, 4 tables, 13 figures (LaTeX / AASTeX
Interpreting BEC in e+e- annihilation
The usual interpretation of Bose-Einstein correlations (BEC) of identical
boson pairs relates the width of the peak in the correlation function at small
relative four-momentum to the spatial extent of the source of the bosons.
However, in the tau-model, which successfully describes BEC in hadronic Z
decay, the width of the peak is related to the temporal extent of boson
emission. Some new checks on the validity of both the tau-model and the usual
descriptions are presented.Comment: Talk given at XLVIII International Symposium on Multiparticle
Dynamics, Singapore, 3-7 Sep. 2018. To appear in the proceeding
The genealogy of judgement: towards a deep history of academic freedom
The classical conception of academic freedom associated with Wilhelm von Humboldt and the rise of the modern university has a quite specific cultural foundation that centres on the controversial mental faculty of 'judgement'. This article traces the roots of 'judgement' back to the Protestant Reformation, through its heyday as the signature feature of German idealism, and to its gradual loss of salience as both a philosophical and a psychological concept. This trajectory has been accompanied by a general shrinking in the scope of academic freedom from the promulgation of world-views to the offering of expert opinion
A "kilonova" associated with short-duration gamma-ray burst 130603B
Short-duration gamma-ray bursts (SGRBs) are intense flashes of cosmic
gamma-rays, lasting less than ~2 s, whose origin is one of the great unsolved
questions of astrophysics today. While the favoured hypothesis for their
production, a relativistic jet created by the merger of two compact stellar
objects (specifically, two neutron stars, NS-NS, or a neutron star and a black
hole, NS-BH), is supported by indirect evidence such as their host galaxy
properties, unambiguous confirmation of the model is still lacking. Mergers of
this kind are also expected to create significant quantities of neutron-rich
radioactive species, whose decay should result in a faint transient in the days
following the burst, a so-called "kilonova". Indeed, it is speculated that this
mechanism may be the predominant source of stable r-process elements in the
Universe. Recent calculations suggest much of the kilonova energy should appear
in the near-infrared (nIR) due to the high optical opacity created by these
heavy r-process elements. Here we report strong evidence for such an event
accompanying SGRB 130603B. If this simplest interpretation of the data is
correct, it provides (i) support for the compact object merger hypothesis of
SGRBs, (ii) confirmation that such mergers are likely sites of significant
r-process production and (iii) quite possibly an alternative, un-beamed
electromagnetic signature of the most promising sources for direct detection of
gravitational waves.Comment: preprint of paper appearing in Nature (3 Aug 2013
Magnetism, X-rays, and Accretion Rates in WD 1145+017 and other Polluted White Dwarf Systems
This paper reports circular spectropolarimetry and X-ray observations of
several polluted white dwarfs including WD 1145+017, with the aim to constrain
the behavior of disk material and instantaneous accretion rates in these
evolved planetary systems. Two stars with previously observed Zeeman splitting,
WD 0322-019 and WD 2105-820, are detected above 5 sigma and > 1 kG, while
WD 1145+017, WD 1929+011, and WD 2326+049 yield (null) detections below this
minimum level of confidence. For these latter three stars, high-resolution
spectra and atmospheric modeling are used to obtain limits on magnetic field
strengths via the absence of Zeeman splitting, finding B* < 20 kG based on data
with resolving power R near 40 000. An analytical framework is presented for
bulk Earth composition material falling onto the magnetic polar regions of
white dwarfs, where X-rays and cyclotron radiation may contribute to accretion
luminosity. This analysis is applied to X-ray data for WD 1145+017, WD
1729+371, and WD 2326+049, and the upper bound count rates are modeled with
spectra for a range of plasma kT = 1 - 10 keV in both the magnetic and
non-magnetic accretion regimes. The results for all three stars are consistent
with a typical dusty white dwarf in a steady-state at 1e8 - 1e9 g/s. In
particular, the non-magnetic limits for WD 1145+017 are found to be well below
previous estimates of up to 1e12 g/s, and likely below 1e10 g/s, thus
suggesting the star-disk system may be average in its evolutionary state, and
only special in viewing geometry.Comment: 14 pages, 7 figures, 3 tables; accepted to MNRA
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