283 research outputs found
Variability of Be Stars in Southern Open Clusters
We recently discovered a large number of highly active Be stars in the open
cluster NGC 3766, making it an excellent location to study the formation
mechanism of Be star disks. To explore whether similar disk appearances and/or
disappearances are common among the Be stars in other open clusters, we present
here multiple epochs of H-alpha spectroscopy for 296 stars in eight open
clusters. We identify 12 new transient Be stars and confirm 17 additional Be
stars with relatively stable disks. By comparing the H-alpha equivalent widths
to the photometric y - H-alpha colors, we present a method to estimate the
strength of the H-alpha emission when spectroscopy is not available. For a
subset of 128 stars in four open clusters, we also use blue optical
spectroscopy and available Stromgren photometry to measure their projected
rotational velocities, effective temperatures, and polar surface gravities. We
combine our Be star detections from these four clusters to investigate physical
differences between the transient Be stars, stable Be stars, and normal B-type
stars with no line emission. Both types of Be stars are faster rotating
populations than normal B-type stars, and we find no significant physical
differences between the transient and stable Be stars in our sample.Comment: Accepted to ApJ; small corrections to Table 5 and associated figure
Understanding Mediation Support
Recent decades have witnessed increasing institutionalization of mediation support through the establishment of mediation support structures (MSS) within foreign ministries and secretariats of multilateral organizations. This study sheds light on this trend and aims to better understand the emergence, design and development of different MSS. This study analyzes six MSS, namely those established in the United Nations (UN), the Organization for Security and Co-operation in Europe (OSCE), the European Union (EU), the Intergovernmental Authority on Development, (IGAD), Switzerland and Germany. It provides five main findings. First, the emergence of dedicated MSS is the result of an interplay of three factors. These refer to the political interests of states that have made mediation a priority of their foreign policies; a normative evolution emphasizing that effective peace mediation requires technical knowledge and resources provided by specialized units; and the operational needs of mediators confronted with the growing complexity of peace processes. Second, looking at their design, MSS in highly different institutional contexts nonetheless exhibit a striking similarity. All of them provide a mixture of training, knowledge management and operational support, ranging from political advice to logistical help. This similarity can be understood by the influence of professional networks of mediation support practitioners that have proven to be influential when new structures are set up. Third, despite the similarity, there are differences between MSS in terms of the balance between different lines of activity, topics they specialize in, whether or not they support external mediation operations, and the degree to which they involve civil society actors. These differences are due to mandates, political environment and organizational culture, which are specific to each organization. Fourth, as for development over time, some MSS examined in this report, i.e. the UN and Switzerland, are fully embedded and deeply involved in mediation processes pursued by their respective institutions. Other MSS, in particular IGAD and Germany, are in the early stages of development with still limited direct involvement in mediation processes. The EU and the OSCE cover the middle ground of this spectrum. The study finds that institutional entrenchment is fostered by demands for support by envoys, a conducive political environment, availability of human and financial resources, and, for multilateral organizations, the strength of secretariats vis-à-vis member states. Fifth, the study showed that the prevalent model of mediation support puts a premium on technical knowledge and generalist expertise that is transferable from one context to the next. Given that today there are fewer comprehensive peace processes and more decentralized dialogue engagements focusing on one issue or one actor in a particular context for a limited period of time, there is a need for existing MSS to adapt, and for practitioners and policymakers to consider a broad range of mediation support approaches
"Broken Heart" and "Broken Brain": Which Connection?
The interconnections between brain and heart are increasingly recognized. Takotsubo cardiomyopathy, also known as "broken heart syndrome", is characterized by a cardiovascular dysfunction provoked by an emotional or stressful situation. Similar events can trigger a neurological pathology called transient global amnesia. These conditions can occur simultaneously, although their precise connection is not well understood. We aim to present the case of a patient who experienced them and to review the relevant literature
Modelling of the ultraviolet and visual SED variability in the hot magnetic Ap star CU Vir
The spectral energy distribution (SED) in chemically peculiar stars may be
significantly affected by their abundance anomalies. The observed SED
variations are usually assumed to be a result of inhomogeneous surface
distribution of chemical elements, flux redistribution and stellar rotation.
However, the direct evidence for this is still only scarce. We aim to identify
the processes that determine the SED and its variability in the UV and visual
spectral domains of the helium-weak star CU Vir. We used the model atmospheres
to obtain the emergent flux and predict the rotationally modulated flux
variability of the star. We show that most of the light variations in the vby
filters of the Stromgren photometric system are a result of the uneven surface
distribution of silicon, chromium, and iron. Our models are only able to
explain a part of the variability in the u filter, however. The observed UV
flux distribution is very well reproduced, and the models are able to explain
most of the observed features in the UV light curve. The variability observed
in the visible is merely a faint gleam of that in the UV. While the amplitude
of the light curves reaches only several hundredths of magnitude in the visual
domain, it reaches about 1 mag in the UV. The visual and UV light variability
of CU Vir is caused by the flux redistribution from the far UV to near UV and
visible regions, inhomogeneous distribution of the elements and stellar
rotation. Bound-free transitions of silicon and bound-bound transitions of iron
and chromium contribute the most to the flux redistribution. This mechanism can
explain most of the rotationally modulated light variations in the filters
centred on the Paschen continuum and on the UV continuum of the star CU Vir.
However, another mechanism(s) has to be invoked to fully explain the observed
light variations in the u filter and in the region 2000-2500 A.Comment: 14 pages, 13 figures, accepted for publication in Astronomy and
Astrophysic
Las Vegas Flute Club Teacher\u27s Exposition Recital
Program listing performers and works performe
Atmospheric parameters and rotational velocities for a sample of Galactic B-type supergiants
High resolution optical spectra of 57 Galactic B-type supergiant stars have
been analyzed to determine their rotational and macroturbulent velocities. In
addition, their atmospheric parameters (effective temperature, surface gravity
and microturbulent velocity) and surface nitrogen abundances have been
estimated using a non-LTE grid of model atmospheres. Comparisons of the
projected rotational velocities have been made with the predictions of stellar
evolutionary models and in general good agreement was found. However for a
small number of targets, their observed rotational velocities were
significantly larger than predicted, although their nitrogen abundances were
consistent with the rest of the sample. We conclude that binarity may have
played a role in generating their large rotational velocities. No correlation
was found between nitrogen abundances and the current projected rotational
velocities. However a correlation was found with the inferred projected
rotational velocities of the main sequence precursors of our supergiant sample.
This correlation is again in agreement with the predictions of single star
evolutionary models that incorporate rotational mixing. The origin of the
macroturbulent and microturbulent velocity fields is discussed and our results
support previous theoretical studies that link the former to sub-photospheric
convection and the latter to non-radial gravity mode oscillations. In addition,
we have attempted to identify differential rotation in our most rapidly
rotating targets.Comment: Submitted to MNRAS, 16 page
Adding cetuximab to capecitabine plus oxaliplatin (XELOX) in first-line treatment of metastatic colorectal cancer: a randomized phase II trial of the Swiss Group for Clinical Cancer Research SAKK
Background: To determine the activity and tolerability of adding cetuximab to the oxaliplatin and capecitabine (XELOX) combination in first-line treatment of metastatic colorectal cancer (MCC). Patients and methods: In a multicenter two-arm phase II trial, patients were randomized to receive oxaliplatin 130 mg/m2 on day 1 and capecitabine 1000 mg/m2 twice daily on days 1-14 every 3 weeks alone or in combination with standard dose cetuximab. Treatment was limited to a maximum of six cycles. Results: Seventy-four patients with good performance status entered the trial. Objective partial response rates after external review and radiological confirmation were 14% and 41% in the XELOX and in the XELOX + Cetuximab arm, respectively. Stable disease has been observed in 62% and 35% of the patients, with 76% disease control in both arms. Cetuximab led to skin rash in 65% of the patients. The median overall survival was 16.5 months for arm A and 20.5 months for arm B. The median time to progression was 5.8 months for arm A and 7.2 months for arm B. Conclusion: Differences in response rates between the treatment arms indicate that cetuximab may improve outcome with XELOX. The correct place of the cetuximab, oxaliplatin and fluoropyrimidine combinations in first-line treatment of MCC has to be assessed in phase III trial
Stellar Rotation in Young Clusters. II. Evolution of Stellar Rotation and Surface Helium Abundance
We derive the effective temperatures and gravities of 461 OB stars in 19
young clusters by fitting the H-gamma profile in their spectra. We use
synthetic model profiles for rotating stars to develop a method to estimate the
polar gravity for these stars, which we argue is a useful indicator of their
evolutionary status. We combine these results with projected rotational
velocity measurements obtained in a previous paper on these same open clusters.
We find that the more massive B-stars experience a spin down as predicted by
the theories for the evolution of rotating stars. Furthermore, we find that the
members of binary stars also experience a marked spin down with advanced
evolutionary state due to tidal interactions. We also derive non-LTE-corrected
helium abundances for most of the sample by fitting the He I 4026, 4387, 4471
lines. A large number of helium peculiar stars are found among cooler stars
with Teff < 23000 K. The analysis of the high mass stars (8.5 solar masses < M
< 16 solar masses) shows that the helium enrichment process progresses through
the main sequence (MS) phase and is greater among the faster rotators. This
discovery supports the theoretical claim that rotationally induced internal
mixing is the main cause of surface chemical anomalies that appear during the
MS phase. The lower mass stars appear to have slower rotation rates among the
low gravity objects, and they have a large proportion of helium peculiar stars.
We suggest that both properties are due to their youth. The low gravity stars
are probably pre-main sequence objects that will spin up as they contract.
These young objects very likely host a remnant magnetic field from their natal
cloud, and these strong fields sculpt out surface regions with unusual chemical
abundances.Comment: 50 pages 18 figures, accepted by Ap
The evolution stage and massive disc of the interacting binary V393 Scorpii
V393 Scorpii is a bright Galactic Double Periodic Variable showing a long
photometric cycle of approx. 253 days. We present new VIJK photometric time
series for V393 Scorpii along with the analysis of ASAS V-band photometry. We
disentangled all light curves into the orbital and long cycle components. The
ASAS V-band orbital light curve was modeled with two stellar components plus a
circumprimary optically thick disc assuming a semidetached configuration. We
present the results of this calculation, giving physical parameters for the
stars and the disc, along with general system dimensions. Our results are in
close agreement with those previously found by Mennickent et al. (2010) from IR
spectroscopy and the modeling of the spectral energy distribution. The
stability of the orbital light curve suggests that the stellar + disc
configuration remains stable during the long cycle. Therefore, the long cycle
should be produced by an additional variable and not-eclipsed emitting
structure. We discuss the evolutionary stage of the system finding the best
match with one of the evolutionary models of van Rensbergen et al. (2008).
According to these models, the system is found to be after an episode of fast
mass exchange that transferred 4 M_sun from the donor to the gainer in a period
of 400.000 years. We argue that a significant fraction of this mass has not
been accreted by the gainer but remains in an optically thick massive (about 2
M_sun) disc-like surrounding pseudo-photosphere whose luminosity is not driven
by viscosity but probably by reprocessed stellar radiation. Finally, we provide
the result of our search for Galactic Double Periodic Variables and briefly
discuss the outliers beta Lyr and RX Cas.Comment: 10 pages, 5 tables, 6 figures, accepted for publication in MNRAS main
journa
Rotational velocities of A-type stars IV. Evolution of rotational velocities
In previous works of this series, we have shown that late B- and early A-type
stars have genuine bimodal distributions of rotational velocities and that late
A-type stars lack slow rotators. The distributions of the surface angular
velocity ratio \Omega/\Omega_crit (\Omega_crit is the critical angular
velocity) have peculiar shapes according to spectral type groups, which can be
caused by evolutionary properties. We aim to review the properties of these
rotational velocity distributions in some detail as a function of stellar mass
and age. We have gathered v sin i for a sample of 2014 B6- to F2-type stars. We
have determined the masses and ages for these objects with stellar evolution
models. The (Teff, log L/Lsun)-parameters were determined from the uvby-\beta
photometry and the HIPPARCOS parallaxes. The velocity distributions show two
regimes that depend on the stellar mass. Stars less massive than 2.5 Msun have
a unimodal equatorial velocity distribution and show a monotonical acceleration
with age on the main sequence (MS). Stars more massive have a bimodal
equatorial velocity distribution. Contrarily to theoretical predictions, the
equatorial velocities of stars from about 1.7 Msun to 3.2 Msun undergo a strong
acceleration in the first third of the MS evolutionary phase, while in the last
third of the MS they evolve roughly as if there were no angular momentum
redistribution in the external stellar layers. The studied stars might start in
the ZAMS not necessarily as rigid rotators, but with a total angular momentum
lower than the critical one of rigid rotators. The stars seem to evolve as
differential rotators all the way of their MS life span and the variation of
the observed rotational velocities proceeds with characteristic time scales
\delta(t)\sim 0.2 t_MS, where t_MS is the time spent by a star in the MS.Comment: 22 pages, 18 figures, A&A in pres
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