1,737 research outputs found
First constraints on the magnetic field strength in extra-Galactic stars: FORS2 observations of Of?p stars in the Magellanic Clouds
Massive O-type stars play a dominant role in our Universe, but many of their
properties remain poorly constrained. In the last decade magnetic fields have
been detected in all Galactic members of the distinctive Of?p class, opening
the door to a better knowledge of all O-type stars. With the aim of extending
the study of magnetic massive stars to nearby galaxies, to better understand
the role of metallicity in the formation of their magnetic fields and
magnetospheres, and to broaden our knowledge of the role of magnetic fields in
massive star evolution, we have carried out spectropolarimetry of five
extra-Galactic Of?p stars, as well as a couple of dozen neighbouring stars. We
have been able to measure magnetic fields with typical error bars from 0.2 to
1.0 kG, depending on the apparent magnitude and on weather conditions. No
magnetic field has been firmly detected in any of our measurements, but we have
been able to estimate upper limits to the field values of our target stars. One
of our targets, 2dFS 936, exhibited an unexpected strengthening of emission
lines. We confirm the unusual behaviour of BI 57, which exhibits a 787 d period
with two photometric peaks and one spectroscopic maximum. The observed
strengthening of the emission lines of 2dFS 936, and the lack of detection of a
strong magnetic field in a star with such strong emission lines is at odd with
expectations. Together with the unusual periodic behaviour of BI 57, it
represents a challenge for the current models of Of?p stars. The limited
precision that we obtained in our field measurements (in most cases as a
consequence of poor weather) has led to field-strength upper limits that are
substantially larger than those typically measured in Galactic magnetic O
stars. Further higher precision observations and monitoring are clearly
required.Comment: Accepted by A&
Optical Spectroscopy of X-Mega targets in the Carina Nebula - VI. FO 15: a new O-Type double-lined eclipsing binary
We report the discovery of a new O-type double-lined spectroscopic binary
with a short orbital period of 1.4 days. We find the primary component of this
binary, FO 15, to have an approximate spectral type O5.5Vz, i.e. a
Zero-Age-Main-Sequence star. The secondary appears to be of spectral type
O9.5V. We have performed a numerical model fit to the public ASAS photometry,
which shows that FO 15 is also an eclipsing binary. We find an orbital
inclination of ~ 80 deg. From a simultaneous light-curve and radial velocity
solution we find the masses and radii of the two components to be 30 +/- 1 and
16 +/- 1 solar masses and 7.5 +/- 0.5 and 5.3 +/- 0.5 solar radii. These radii,
and hence also the luminosities, are smaller than those of normal O-type stars,
but similar to recently born ZAMS O-type stars. The absolute magnitudes derived
from our analysis locate FO 15 at the same distance as Eta Carinae. From
Chandra and XMM X-ray images we also find that there are two close X-ray
sources, one coincident with FO 15 and another one without optical counterpart.
This latter seems to be a highly variable source, presumably due to a
pre-main-sequence stellar neighbour of FO 15.Comment: 11 pages, 9 figures, 3 tables. Accepted for publication in MNRAS.
Higher resolution version available at
http://lilen.fcaglp.unlp.edu.ar/papers2006.htm
Surface abundances of ON stars
Massive stars burn hydrogen through the CNO cycle during most of their
evolution. When mixing is efficient, or when mass transfer in binary systems
happens, chemically processed material is observed at the surface of O and B
stars. ON stars show stronger lines of nitrogen than morphologically normal
counterparts. Whether this corresponds to the presence of material processed
through the CNO cycle or not is not known. Our goal is to answer this question.
We perform a spectroscopic analysis of a sample of ON stars with atmosphere
models. We determine the fundamental parameters as well as the He, C, N, and O
surface abundances. We also measure the projected rotational velocities. We
compare the properties of the ON stars to those of normal O stars. We show that
ON stars are usually helium-rich. Their CNO surface abundances are fully
consistent with predictions of nucleosynthesis. ON stars are more chemically
evolved and rotate - on average - faster than normal O stars. Evolutionary
models including rotation cannot account for the extreme enrichment observed
among ON main sequence stars. Some ON stars are members of binary systems, but
others are single stars as indicated by stable radial velocities. Hence, mass
transfer is not a simple explanation for the observed chemical properties. We
conclude that ON stars show extreme chemical enrichment at their surface,
consistent with nucleosynthesis through the CNO cycle. Its origin is not clear
at present.Comment: 18 pages, 10 figures (+ appendix). A&A accepte
The HD5980 multiple system: Masses and evolutionary status
New spectroscopic observations of the LBV/WR multiple system HD5980 in the
Small Magellanic Cloud are used to address the question of the masses and
evolutionary status of the two very luminous stars in the 19.3d eclipsing
binary system. Two distinct components of the N V 4944 A line are detected in
emission and their radial velocity variations are used to derive masses of 61
and 66 Mo, under the assumption that binary interaction effects on this atomic
transition are negligible. We propose that this binary system is the product of
quasi-chemically homogeneous evolution with little or no mass transfer. Thus,
both of these binary stars may be candidates for gamma-ray burst progenitors or
even pair instability supernovae. Analysis of the photospheric absorption lines
belonging to the third-light object in the system confirm that it consists of
an O-type star in a 96.56d eccentric orbit (e=0.82) around an unseen companion.
The 5:1 period ratio and high eccentricities of the two binaries suggest that
they may constitute a hierarchical quadruple system.Comment: 27 pages, 8 tables, 15 figures; accepted A
Nanoscale phase separation in the iron chalcogenide superconductor K0.8Fe1.6Se2 as seen via scanning nanofocused x-ray diffraction
Advanced synchrotron radiation focusing down to a size of 300 nm has been
used to visualize nanoscale phase separation in the K0.8Fe1.6Se2
superconducting system using scanning nanofocus single-crystal X-ray
diffraction. The results show an intrinsic phase separation in K0.8Fe1.6Se2
single crystals at T< 520 K, revealing coexistence of i) a magnetic phase
characterized by an expanded lattice with superstructures due to Fe vacancy
ordering and ii) a non-magnetic phase with an in-plane compressed lattice. The
spatial distribution of the two phases at 300 K shows a frustrated or arrested
nature of the phase separation. The space-resolved imaging of the phase
separation permitted us to provide a direct evidence of nanophase domains
smaller than 300 nm and different micrometer-sized regions with percolating
magnetic or nonmagnetic domains forming a multiscale complex network of the two
phases.Comment: 5 pages, 4 figure
Spanning Properties of Theta-Theta Graphs
We study the spanning properties of Theta-Theta graphs. Similar in spirit
with the Yao-Yao graphs, Theta-Theta graphs partition the space around each
vertex into a set of k cones, for some fixed integer k > 1, and select at most
one edge per cone. The difference is in the way edges are selected. Yao-Yao
graphs select an edge of minimum length, whereas Theta-Theta graphs select an
edge of minimum orthogonal projection onto the cone bisector. It has been
established that the Yao-Yao graphs with parameter k = 6k' have spanning ratio
11.67, for k' >= 6. In this paper we establish a first spanning ratio of
for Theta-Theta graphs, for the same values of . We also extend the class of
Theta-Theta spanners with parameter 6k', and establish a spanning ratio of
for k' >= 5. We surmise that these stronger results are mainly due to a
tighter analysis in this paper, rather than Theta-Theta being superior to
Yao-Yao as a spanner. We also show that the spanning ratio of Theta-Theta
graphs decreases to 4.64 as k' increases to 8. These are the first results on
the spanning properties of Theta-Theta graphs.Comment: 20 pages, 6 figures, 3 table
IGR J08408--4503: a new recurrent Supergiant Fast X-ray Transient
The supergiant fast X-ray transient IGR J08408-4503 was discovered by
INTEGRAL on May 15, 2006, during a bright flare. The source shows sporadic
recurrent short bright flares, reaching a peak luminosity of 10^36 erg s^-1
within less than one hour. The companion star is HD 74194, an Ob5Ib(f)
supergiant star located at 3 kpc in the Vela region. We report the light curves
and broad-band spectra (0.1-200 keV) of all the three flares of IGR J08408-4503
detected up to now based on INTEGRAL and Swift data. The flare spectra are well
described by a power-law model with a high energy cut-off at ~15 keV. The
absorption column density during the flares was found to be ~10^21 cm^-2,
indicating a very low matter density around the compact object. Using the
supergiant donor star parameters, the wind accretion conditions imply an
orbital period of the order of one year, a spin period of the order of hours
and a magnetic field of the order of 10^13 G.Comment: 5 pages, 2 figures, accepted for publication in Astrophysical Journal
Letter
Clinical Processes - The Killer Application for Constraint-Based Process Interactions?
For more than a decade, the interest in aligning information
systems in a process-oriented way has been increasing. To enable operational
support for business processes, the latter are usually specified in
an imperative way. The resulting process models, however, tend to be too
rigid to meet the flexibility demands of the actors involved. Declarative
process modeling languages, in turn, provide a promising alternative in
scenarios in which a high level of flexibility is demanded. In the scientific
literature, declarative languages have been used for modeling rather simple
processes or synthetic examples. However, to the best of our knowledge,
they have not been used to model complex, real-world scenarios
that comprise constraints going beyond control-flow. In this paper, we
propose the use of a declarative language for modeling a sophisticated
healthcare process scenario from the real world. The scenario is subject to
complex temporal constraints and entails the need for coordinating the
constraint-based interactions among the processes related to a patient
treatment process. As demonstrated in this work, the selected real process
scenario can be suitably modeled through a declarative approach.Ministerio de Economía y Competitividad TIN2016-76956-C3-2-RMinisterio de Economía y Competitividad TIN2015-71938-RED
Contribución a la caracterización productiva del cerdo manchado de Jabugo en el período predestete: resultados preliminares
En este trabajo se estudia el comportamiento de la raza Manchado de Jabugo en el periodo predestete con vistas a la caracterización productiva de la raza en este estadio fisiológico. Para ello se analiza una muestra de 140 animales entre machos y hembras. De los resultados obtenidos se desprende un peso al nacimiento medio en torno a 1,6 kg, un peso medio al destete alrededor de 12,5 kg y una ganancia media diaria en el periodo 0-60 días de 206 gramos/día. Se concluye que el Manchado de Jabugo presenta un peso al nacimiento mayor al descrito en el resto de las razas porcinas autóctonas españolas (diferentes variedades de cerdo Ibérico, Chato Murciano y Negro Canario) y una menor capacidad de crecimiento en las primeras fases de vida en comparación a dichas razas. Las variables estudiadas nos informan de un escaso dimorfismo sexual en la raza
- …