345 research outputs found
Symmetry implies independence
Given a quantum system consisting of many parts, we show that symmetry of the
system's state, i.e., invariance under swappings of the subsystems, implies
that almost all of its parts are virtually identical and independent of each
other. This result generalises de Finetti's classical representation theorem
for infinitely exchangeable sequences of random variables as well as its
quantum-mechanical analogue. It has applications in various areas of physics as
well as information theory and cryptography. For example, in experimental
physics, one typically collects data by running a certain experiment many
times, assuming that the individual runs are mutually independent. Our result
can be used to justify this assumption.Comment: LaTeX, contains 4 figure
The connection between missing AGB stars and extended horizontal branches
Recent surveys confirm early results about a deficiency or even absence of
CN-strong stars on the asymptotic giant branch (AGB) of globular clusters
(GCs), although with quite large cluster-to-cluster variations. In general,
this is at odds with the distribution of CN band strengths among first ascent
red giant branch (RGB) stars. Norris et al. proposed that the lack of CN-strong
stars in some clusters is a consequence of a smaller mass of these stars that
cannot evolve through the full AGB phase. In this short paper we found that the
relative frequency of AGB stars can change by a factor of two between different
clusters. We also find a very good correlation between the minimum mass of
stars along the horizontal branch (Gratton et al. 2010) and the relative
frequency of AGB stars, with a further dependence on metallicity. We conclude
that indeed the stars with the smallest mass on the HB cannot evolve through
the full AGB phase, being AGB-manque'. These stars likely had large He and N
content, and large O-depletion. We then argue that there should not be AGB
stars with extreme O depletion, and few of them with a moderate one.Comment: 5 Pages, 2 figures, A&A Accepte
NGC 2419: a large and extreme second generation in a currently undisturbed cluster
We analyse complementary HST and SUBARU data for the globular cluster NGC
2419. We make a detailed analysis of the horizontal branch (HB), that appears
composed by two main groups of stars: the luminous blue HB stars ---that extend
by evolution into the RR Lyrae and red HB region--- and a fainter, extremely
blue population. We examine the possible models for this latter group and
conclude that a plausible explanation is that they correspond to a significant
(~30 %) extreme second generation with a strong helium enhancement (Y~0.4). We
also show that the color dispersion of the red giant branch is consistent with
this hypothesis, while the main sequence data are compatible with it, although
the large observational error blurs the possible underlying splitting. While it
is common to find an even larger (50 -- 80) percentage of second generation in
a globular cluster, the presence of a substantial and extreme fraction of these
stars in NGC 2419 might be surprising, as the cluster is at present well inside
the radius beyond which the galactic tidal field would be dominant. If a
similar situation had been present in the first stages of the cluster life, the
cluster would have retained its initial mass, and the percentage of second
generation stars should have been quite small (up to ~10 %). Such a large
fraction of extreme second generation stars implies that the system must have
been initially much more massive and in different dynamical conditions than
today. We discuss this issue in the light of existing models of the formation
of multiple populations in globular clusters.Comment: 14 pages, 14 figures (5 in low resolution format), 3 tables, accepted
for publication in MNRA
Fortnightly Fluctuations in the O-C Diagram of CS 1246
Dominated by a single, large-amplitude pulsation mode, the rapidly-pulsating
hot subdwarf B star CS 1246 is a prime candidate for a long-term O-C diagram
study. We collected nearly 400 hours of photometry with the PROMPT telescopes
over a time span of 14 months to begin looking for secular variations in the
pulse timings. Interestingly, the O-C diagram is dominated by a strong
sinusoidal pattern with a period of 14.1 days and an amplitude of 10.7
light-seconds. Underneath this sine wave is a secular trend implying a decrease
in the 371.7-s pulsational period of Pdot = -1.9 x 10^-11, which we attribute
to the evolution of the star through the H-R diagram. The sinusoidal variation
could be produced by the presence of a low-mass companion, with m sin i ~ 0.12
Msun, orbiting the subdwarf B star at a distance of 20 Rsun. An analysis of the
combined light curve reveals the presence of a low-amplitude first harmonic to
the main pulsation mode.Comment: Accepted for publication in MNRAS. 11 pages, 8 figures, 5 table
Action research and democracy
This contribution explores the relationship between research and learning democracy. Action research is seen as being compatible with the orientation of educational and social work research towards social justice and democracy. Nevertheless, the history of action research is characterized by a tension between democracy and social engineering. In the social-engineering approach, action research is conceptualized as a process of innovation aimed at a specific Bildungsideal. In a democratic approach action research is seen as research based on cooperation between research and practice. However, the notion of democratic action research as opposed to social engineering action research needs to be theorized. So called democratic action research involving the implementation by the researcher of democracy as a model and as a preset goal, reduces cooperation and participation into instruments to reach this goal, and becomes a type of social engineering in itself. We argue that the relationship between action research and democracy is in the acknowledgment of the political dimension of participation: âa democratic relationship in which both sides exercise power and shared control over decision-making as well as interpretationâ. This implies an open research design and methodology able to understand democracy as a learning process and an ongoing experiment
Virtual Valcamonica: collaborative exploration of prehistoric petroglyphs and their surrounding environment in multi-user virtual reality
In this paper, we present a novel, multi-user, virtual reality environment for the interactive, collaborative 3D analysis of large 3D scans and the technical advancements that were necessary to build it: a multi-view rendering system for large 3D point clouds, a suitable display infrastructure and a suite of collaborative 3D interaction techniques. The cultural heritage site of Valcamonica in Italy with its large collection of prehistoric rock-art served as an exemplary use case for evaluation. The results show that our output-sensitive level-of-detail rendering system is capable of visualizing a 3D dataset with an aggregate size of more than 14 billion points at interactive frame rates. The system design in this exemplar application results from close exchange with a small group of potential users: archaeologists with expertise in rock-art and allows them to explore the prehistoric art and its spatial context with highly realistic appearance. A set of dedicated interaction techniques was developed to facilitate collaborative visual analysis. A multi-display workspace supports the immediate comparison of geographically distributed artifacts. An expert review of the final demonstrator confirmed the potential for added value in rock-art research and the usability of our collaborative interaction techniques
Hydrogen-like nitrogen radio line from hot interstellar and warm-hot intergalactic gas
Hyperfine structure lines of highly-charged ions may open a new window in
observations of hot rarefied astrophysical plasmas. In this paper we discuss
spectral lines of isotopes and ions abundant at temperatures 10^5-10^7 K,
characteristic for warm-hot intergalactic medium, hot interstellar medium,
starburst galaxies, their superwinds and young supernova remnants. Observations
of these lines will allow to study bulk and turbulent motions of the observed
target and will broaden the information about the gas ionization state,
chemical and isotopic composition.
The most prospective is the line of the major nitrogen isotope having
wavelength 5.65 mm (Sunyaev and Churazov 1084). Wavelength of this line is
well-suited for observation of objects at z=0.15-0.6 when it is redshifted to
6.5-9 mm spectral band widely-used in ground-based radio observations, and, for
example, for z>=1.3, when the line can be observed in 1.3 cm band and at lower
frequencies. Modern and future radio telescopes and interferometers are able to
observe the absorption by 14-N VII in the warm-hot intergalactic medium at
redshifts above z=0.15 in spectra of brightest mm-band sources. Sub-millimeter
emission lines of several most abundant isotopes having hyperfine splitting
might also be detected in spectra of young supernova remnants.Comment: 12 pages, 5 figures, accepted by Astronomy Letters; v3: details
added; error fixe
Analysis and applications of respiratory surface EMG:report of a round table meeting
Surface electromyography (sEMG) can be used to measure the electrical activity of the respiratory muscles. The possible applications of sEMG span from patients suffering from acute respiratory failure to patients receiving chronic home mechanical ventilation, to evaluate muscle function, titrate ventilatory support and guide treatment. However, sEMG is mainly used as a monitoring tool for research and its use in clinical practice is still limitedâin part due to a lack of standardization and transparent reporting. During this round table meeting, recommendations on data acquisition, processing, interpretation, and potential clinical applications of respiratory sEMG were discussed. This paper informs the clinical researcher interested in respiratory muscle monitoring about the current state of the art on sEMG, knowledge gaps and potential future applications for patients with respiratory failure.</p
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