642 research outputs found
Freshly ionized matter around the final Helium shell flash object V4334 Sgr (Sakurai's object)
We report on the discovery of recently ionized hydrogen-deficient gas in the
immediate circumstellar environment of the final helium shell flash star V4334
Sgr (Sakurai's object). On spectra obtained with FORS2 multi-object
spectroscopy we have found spatially extended (about 2") emission from [N II],
[O I], [O II] and very faint Halpha and [S II]. In the [N II] (ll6548,83) lines
we have identified two components located at velocities -350 +/-50 and +200
+/-50 km/s, relative to V4334 Sgr itself. The full width of the [N II] l6583
feature at zero intensity corresponds to a velocity spread of about 1500 km/s.
Based on the available data it is not possible to conclusively determine the
mechanism of ionization. Both photo-ionization, from a rapidly evolving central
star, and shock excitation, as the result of the collision of the fast ouflows
with slower circumstellar matter, could account for the observed lines. The
central star is still hidden behind strong dust absorption, since only a faint
highly reddened continuum is apparent in the spectra. Theory states that it
will become hotter and will retrace its post-asymptotic giant branch evolution
towards the planetary nebula domain. Our detection of the ionized ejecta from
the very late helium shell flash marks the beginning of a new phase in this
star's amazingly rapid evolution.Comment: 11 pages, 2 figures. Accepted by ApJ
Realistic atomistic structure of amorphous silicon from machine-learning-driven molecular dynamics
Amorphous silicon (a-Si) is a widely studied noncrystalline material, and yet the subtle details of its atomistic structure are still unclear. Here, we show that accurate structural models of a-Si can be obtained using a machine-learning-based interatomic potential. Our best a-Si network is obtained by simulated cooling from the melt at a rate of 1011 K/s (that is, on the 10 ns time scale), contains less than 2% defects, and agrees with experiments regarding excess energies, diffraction data, and 29Si NMR chemical shifts. We show that this level of quality is impossible to achieve with faster quench simulations. We then generate a 4096-atom system that correctly reproduces the magnitude of the first sharp diffraction peak (FSDP) in the structure factor, achieving the closest agreement with experiments to date. Our study demonstrates the broader impact of machine-learning potentials for elucidating structures and properties of technologically important amorphous materials
V605 Aql: The Older Twin of Sakurai's Object
New optical spectra have been obtained with VLT/FORS2 of the final helium
shell flash (FF) star, V605 Aql, which peaked in brightness in 1919. New models
suggest that this star is experiencing a very late thermal pulse. The evolution
to a cool luminous giant and then back to a compact hot star takes place in
only a few years. V605 Aql, the central star of the Planetary Nebula (PN), A58,
has evolved from T5000 K in 1921 to 95,000 K today. There are
indications that the new FF star, Sakurai's Object (V4334 Sgr), which appeared
in 1996, is evolving along a similar path. The abundances of Sakurai's Object
today and V605 Aql 80 years ago mimic the hydrogen deficient R Coronae Borealis
(RCB) stars with 98% He and 1% C. The new spectra show that V605 Aql has
stellar abundances similar to those seen in Wolf-Rayet [WC] central stars of
PNe with ~55% He, and ~40% C. The stellar spectrum of V605 Aql can be seen even
though the star is not directly detected. Therefore, we may be seeing the
spectrum in light scattered around the edge of a thick torus of dust seen
edge-on. In the present state of evolution of V605 Aql, we may be seeing the
not too distant future of Sakurai's Object.Comment: 12 pages, 1 figure, ApJ Letters in pres
Analytical Investigation of Innovation Dynamics Considering Stochasticity in the Evaluation of Fitness
We investigate a selection-mutation model for the dynamics of technological
innovation,a special case of reaction-diffusion equations. Although mutations
are assumed to increase the variety of technologies, not their average success
("fitness"), they are an essential prerequisite for innovation. Together with a
selection of above-average technologies due to imitation behavior, they are the
"driving force" for the continuous increase in fitness. We will give analytical
solutions for the probability distribution of technologies for special cases
and in the limit of large times.
The selection dynamics is modelled by a "proportional imitation" of better
technologies. However, the assessment of a technology's fitness may be
imperfect and, therefore, vary stochastically. We will derive conditions, under
which wrong assessment of fitness can accelerate the innovation dynamics, as it
has been found in some surprising numerical investigations.Comment: For related work see http://www.helbing.or
Concept and optical design of the cross-disperser module for CRIRES
This is the peer reviewed version of the following article: Oliva, Ernesto, A. Tozzi, D. Ferruzzi, L. Origlia, A. Hatzes, R. Follert, T. Loewinger et al. "Concept and optical design of the cross-disperser module for CRIRES+." In SPIE Astronomical Telescopes+ Instrumentation, pp. 91477R-91477R. International Society for Optics and Photonics, 2014, which has been published in final form at 10.1117/12.2054381
Reliability of old and new ventricular fibrillation detection algorithms for automated external defibrillators
BACKGROUND: A pivotal component in automated external defibrillators (AEDs) is the detection of ventricular fibrillation by means of appropriate detection algorithms. In scientific literature there exists a wide variety of methods and ideas for handling this task. These algorithms should have a high detection quality, be easily implementable, and work in real time in an AED. Testing of these algorithms should be done by using a large amount of annotated data under equal conditions. METHODS: For our investigation we simulated a continuous analysis by selecting the data in steps of one second without any preselection. We used the complete BIH-MIT arrhythmia database, the CU database, and the files 7001 – 8210 of the AHA database. All algorithms were tested under equal conditions. RESULTS: For 5 well-known standard and 5 new ventricular fibrillation detection algorithms we calculated the sensitivity, specificity, and the area under their receiver operating characteristic. In addition, two QRS detection algorithms were included. These results are based on approximately 330 000 decisions (per algorithm). CONCLUSION: Our values for sensitivity and specificity differ from earlier investigations since we used no preselection. The best algorithm is a new one, presented here for the first time
From counting to construction of BPS states in N=4 SYM
We describe a universal element in the group algebra of symmetric groups,
whose characters provides the counting of quarter and eighth BPS states at weak
coupling in N=4 SYM, refined according to representations of the global
symmetry group. A related projector acting on the Hilbert space of the free
theory is used to construct the matrix of two-point functions of the states
annihilated by the one-loop dilatation operator, at finite N or in the large N
limit. The matrix is given simply in terms of Clebsch-Gordan coefficients of
symmetric groups and dimensions of U(N) representations. It is expected, by
non-renormalization theorems, to contain observables at strong coupling. Using
the stringy exclusion principle, we interpret a class of its eigenvalues and
eigenvectors in terms of giant gravitons. We also give a formula for the action
of the one-loop dilatation operator on the orthogonal basis of the free theory,
which is manifestly covariant under the global symmetry.Comment: 41 pages + Appendices, 4 figures; v2 - refs and acknowledgments adde
Winds in R Coronae Borealis Stars
We present new spectroscopic observations of the He I 10830 line in
R Coronae Borealis (RCB) stars which provide the first strong evidence that
most, if not all, RCB stars have winds. It has long been suggested that when
dust forms around an RCB star, radiation pressure accelerates the dust away
from the star, dragging the gas along with it. The new spectra show that nine
of the ten stars observed have P-Cygni or asymmetric blue-shifted profiles in
the He I 10830 line. In all cases, the He I line indicates a mass
outflow - with a range of intensity and velocity. Around the RCB stars, it is
likely that this state is populated by collisional excitation rather than
photoionization/recombination. The line profiles have been modeled with an SEI
code to derive the optical depth and the velocity field of the helium gas. The
results show that the typical RCB wind has a steep acceleration with a terminal
velocity of \Vinf = 200-350 \kms and a column density of N
cm in the He I 10830 line. There is a possible relationship
between the lightcurve of an RCB star and its He I 10830 profile.
Stars which have gone hundreds of days with no dust-formation episodes tend to
have weaker He I features. The unusual RCB star, V854 Cen, does not follow this
trend, showing little or no He I absorption despite high mass-loss activity.
The He I 10830 line in R CrB itself, which has been observed at four
epochs between 1978 and 2001, seems to show a P-Cygni or asymmetric
blue-shifted profile at all times whether it is in decline or at maximum light.Comment: 14 pages, 4 figures, ApJ in pres
ESPRESSO: The next European exoplanet hunter
The acronym ESPRESSO stems for Echelle SPectrograph for Rocky Exoplanets and
Stable Spectroscopic Observations; this instrument will be the next VLT high
resolution spectrograph. The spectrograph will be installed at the
Combined-Coud\'e Laboratory of the VLT and linked to the four 8.2 m Unit
Telescopes (UT) through four optical Coud\'e trains. ESPRESSO will combine
efficiency and extreme spectroscopic precision. ESPRESSO is foreseen to achieve
a gain of two magnitudes with respect to its predecessor HARPS, and to improve
the instrumental radial-velocity precision to reach the 10 cm/s level. It can
be operated either with a single UT or with up to four UTs, enabling an
additional gain in the latter mode. The incoherent combination of four
telescopes and the extreme precision requirements called for many innovative
design solutions while ensuring the technical heritage of the successful HARPS
experience. ESPRESSO will allow to explore new frontiers in most domains of
astrophysics that require precision and sensitivity. The main scientific
drivers are the search and characterization of rocky exoplanets in the
habitable zone of quiet, nearby G to M-dwarfs and the analysis of the
variability of fundamental physical constants. The project passed the final
design review in May 2013 and entered the manufacturing phase. ESPRESSO will be
installed at the Paranal Observatory in 2016 and its operation is planned to
start by the end of the same year.Comment: 12 pages, figures included, accepted for publication in Astron. Nach
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