8,621 research outputs found
Exciton trapping in magnetic wire structures
The lateral magnetic confinement of quasi two-dimensional excitons into wire
like structures is studied. Spin effects are take into account and two
different magnetic field profiles are considered, which experimentally can be
created by the deposition of a ferromagnetic stripe on a semiconductor quantum
well with magnetization parallel or perpendicular to the grown direction of the
well. We find that it is possible to confine excitons into one-dimensional (1D)
traps. We show that the dependence of the confinement energy on the exciton
wave vector, which is related to its free direction of motion along the wire
direction, is very small. Through the application of a background magnetic
field it is possible to move the position of the trapping region towards the
edge of the ferromagnetic stripe or even underneath the stripe. The exact
position of this 1D exciton channel depends on the strength of the background
magnetic field and on the magnetic polarisation direction of the ferromagnetic
film.Comment: 10 pages, 7 figures, to be published in J. Phys: Condens. Matte
Constraining the relative inclinations of the planets B and C of the millisecond pulsar PSR B1257+12
We investigate on the relative inclination of the planets B and C orbiting
the pulsar PSR B1257+12 in connection with potential violations of the
equivalence principle (Abridged).Comment: LaTex2e, 10 pages, 1 table, 3 figures, 17 references. Small stylistic
changes. Version to appear in Journal of Astrophysics and Astronomy (JAA
Optical Identification of He White Dwarfs Orbiting Four Millisecond Pulsars in the Globular Cluster 47 Tucanae
We used ultra-deep UV observations obtained with the Hubble Space Telescope
to search for optical companions to binary millisecond pulsars (MSPs) in the
globular cluster 47 Tucanae. We identified four new counterparts (to MSPs
47TucQ, 47TucS, 47TucT and 47TucY) and confirmed those already known (to MSPs
47TucU and 47TucW). In the color magnitude diagram, the detected companions are
located in a region between the main sequence and the CO white dwarf cooling
sequences, consistent with the cooling tracks of He white dwarfs of mass
between 0.15 Msun and 0.20 Msun. For each identified companion, mass, cooling
age, temperature and pulsar mass (as a function of the inclination angle) have
been derived and discussed. For 47TucU we also found that the past accretion
history likely proceeded in a sub-Eddington rate. The companion to the redback
47TucW is confirmed to be a non degenerate star, with properties particularly
similar to those observed for black widow systems. Two stars have been
identified within the 2-sigma astrometric uncertainty from the radio positions
of 47TucH and 47TucI, but the available data prevent us from firmly assessing
whether they are the true companions of these two MSPs.Comment: 27 pages, 7 figures, Accepted for publication by Ap
Can we see pulsars around Sgr A*? - The latest searches with the Effelsberg telescope
Radio pulsars in relativistic binary systems are unique tools to study the
curved space-time around massive compact objects. The discovery of a pulsar
closely orbiting the super-massive black hole at the centre of our Galaxy, Sgr
A*, would provide a superb test-bed for gravitational physics. To date, the
absence of any radio pulsar discoveries within a few arc minutes of Sgr A* has
been explained by one principal factor: extreme scattering of radio waves
caused by inhomogeneities in the ionized component of the interstellar medium
in the central 100 pc around Sgr A*. Scattering, which causes temporal
broadening of pulses, can only be mitigated by observing at higher frequencies.
Here we describe recent searches of the Galactic centre region performed at a
frequency of 18.95 GHz with the Effelsberg radio telescope.Comment: 3 pages, 2 figures, Proceedings of IAUS 291 "Neutron Stars and
Pulsars: Challenges and Opportunities after 80 years", 201
Prognostic scores in a gastroenterology intensive care unit
BACKGROUND: Several prognostic systems have been developed and validated in general Intensive Care Units (ICUs). No assessment of these scores was performed in specialized Gastroenterology Intensive Care Units (GICUs).
AIM: To assess the prognostic accuracy of Acute Physiology and Chronic Health Evaluation (APACHE) II, Simplified Acute Physiology Score (SAPS) II and Sequential Organ Failure Assessment (SOFA) scores systems to predict mortality in a GICU.
METHODS: Retrospective study of 300 consecutively admissions in a GICU. Demographics, indication for admission, APACHE II, SAPS II and SOFA scores and survival at GICU discharge were recorded. Discrimination was evaluated using receiver operations characteristic (ROC) curves and area under a ROC curve (AUC). Calibration was estimated using the Hosmer-Lemeshow goodness-of-fit test.
RESULTS: Overall GICU mortality was 5.3%. APACHE II, SAPS II and SOFA mean scores of nonsurvivors (21.9, 46.2 and 9.3, respectively) were found to be significantly higher than those of survivors (11.9, 26.7 and 2.2, respectively) (p < 0.001). Discrimination was excellent for all the prognostic systems, with AUC = 0.900, 0.903 and 0.965 for APACHE II, SAPS II and SOFA, respectively. Similarly, APACHE II, SAPS II and SOFA scores achieved good calibration, with p = 0.671, 0.928 and 0.775, respectively. Among the three scores, SOFA showed the best performance, with overall correctness of prediction of 94.0%, while it was 86.2% for APACHE II and 82.7% for SAPS II.
CONCLUSIONS: in GICU, APACHE II, SAPS II and SOFA scores have excellent prognostic accuracy and, among the three scores, SOFA has the greatest overall correctness of prediction
Determination of the orbital parameters of binary pulsars
We present a simple method for determination of the orbital parameters of
binary pulsars, using data on the pulsar period at multiple observing epochs.
This method uses the circular nature of the velocity space orbit of Keplerian
motion and produces preliminary values based on two one dimensional searches.
Preliminary orbital parameter values are then refined using a computationally
efficient linear least square fit. This method works for random and sparse
sampling of the binary orbit. We demonstrate the technique on (a) the highly
eccentric binary pulsar PSR J0514-4002 (the first known pulsar in the globular
cluster NGC 1851) and (b) 47 Tuc T, a binary pulsar with a nearly circular
orbit.Comment: 7 pages, 7 figures, 2 tables, Accepted by MNRAS, minor clarifications
in references adde
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