734 research outputs found
Evolution of the IBDM Structural Latch Development into a Generic Simplified Design
This paper presents the evolution in the development of the structural latch for the International Berthing Docking Mechanism (IBDM, see Figure 1). It reports on the lessons learned since completion of the test program on the engineering development unit of the first generation latching system in 2007. The initial latch design has been through a second generation concept in 2008, and now evolved into a third generation of this mechanism. Functional and structural testing on the latest latch hardware has recently been completed with good results. The IBDM latching system will provide the structural connection between two mated space vehicles after berthing or docking. The mechanism guarantees that the interface seals become compressed to form a leak-tight pressure system that creates a passageway for the astronauts
The Projected Rotational Velocity Distribution of a Sample of OB stars from a Calibration based on Synthetic He I lines
We derive projected rotational velocities (vsini) for a sample of 156
Galactic OB star members of 35 clusters, HII regions, and associations. The HeI
lines at 4026, 4388, and 4471A were analyzed in order to define
a calibration of the synthetic HeI full-widths at half maximum versus stellar
vsini. A grid of synthetic spectra of HeI line profiles was calculated in
non-LTE using an extensive helium model atom and updated atomic data. The
vsini's for all stars were derived using the He I FWHM calibrations but also,
for those target stars with relatively sharp lines, vsini values were obtained
from best fit synthetic spectra of up to 40 lines of CII, NII, OII, AlIII,
MgII, SiIII, and SIII. This calibration is a useful and efficient tool for
estimating the projected rotational velocities of O9-B5 main-sequence stars.
The distribution of vsini for an unbiased sample of early B stars in the
unbound association Cep OB2 is consistent with the distribution reported
elsewhere for other unbound associations.Comment: Accepted for publication in The Astronomical Journa
Discovery of magnetic fields in the very young, massive stars W601 (NGC 6611) and OI 201 (NGC 2244)
Context: Recent spectropolarimetric observations of Herbig Ae/Be stars have
yielded new arguments in favour of a fossil origin for the magnetic fields of
intermediate mass stars. Aims: To study the evolution of these magnetic fields,
and their impact on the evolution of the angular momentum of these stars during
the pre-main sequence phase, we observed Herbig Ae/Be members of young open
clusters of various ages. Methods: We obtained high-resolution
spectropolarimetric observations of Herbig Ae/Be stars belonging to the young
open clusters NGC 6611 (< 6 Myr), NGC 2244 (~1.9 Myr), and NGC 2264 (~8 Myr),
using ESPaDOnS at theCanada-France-Hawaii Telescope. Results: Here we report
the discovery of strong magnetic fields in two massive pre-main sequence
cluster stars. We detected, for the first time, a magnetic field in a pre-main
sequence rapid rotator: the 10.2 Msun Herbig B1.5e star W601 (NGC 6611; v sin i
~ 190 km/s). Our spectropolarimetric observations yield a longitudinal magnetic
field larger than 1 kG, and imply a rotational period shorter than 1.7 days.
The spectrum of this very young object (age ~ 0.017 Myr) shows strong and
variable lines of He and Si. We also detected a magnetic field in the 12.1 Msun
B1 star OI 201 (NGC 2244; v sin i = 23.5 km/s). The Stokes V profile of this
star does not vary over 5 days, suggesting a long rotational period, a pole-on
orientation, or aligned magnetic and rotation axes. OI 201 is situtated near
the Zero-Age Main Sequence on the HR diagram, and exhibits normal chemical
abundances and no spectrum variability.Comment: Accepted for publication as a letter in A&
Expressiveness modulo Bisimilarity of Regular Expressions with Parallel Composition (Extended Abstract)
The languages accepted by finite automata are precisely the languages denoted
by regular expressions. In contrast, finite automata may exhibit behaviours
that cannot be described by regular expressions up to bisimilarity. In this
paper, we consider extensions of the theory of regular expressions with various
forms of parallel composition and study the effect on expressiveness. First we
prove that adding pure interleaving to the theory of regular expressions
strictly increases its expressiveness up to bisimilarity. Then, we prove that
replacing the operation for pure interleaving by ACP-style parallel composition
gives a further increase in expressiveness. Finally, we prove that the theory
of regular expressions with ACP-style parallel composition and encapsulation is
expressive enough to express all finite automata up to bisimilarity. Our
results extend the expressiveness results obtained by Bergstra, Bethke and
Ponse for process algebras with (the binary variant of) Kleene's star
operation.Comment: In Proceedings EXPRESS'10, arXiv:1011.601
Boron Abundances in Main Sequence B-type Stars: A Test of Rotational Depletion during Main Sequence Evolution
Boron abundances have been derived for seven main sequence B-type stars from
HST STIS spectra around the B III 2066 A line. In two stars, boron appears to
be undepleted with respect to the presumed initial abundance. In one star,
boron is detectable but it is clearly depleted. In the other four stars, boron
is undetectable implying depletions of 1 to 2 dex. Three of these four stars
are nitrogen enriched, but the fourth shows no enrichment of nitrogen. Only
rotationally induced mixing predicts that boron depletions are unaccompanied by
nitrogen enrichments. The inferred rate of boron depletion from our
observations is in good agreement with these predictions. Other boron-depleted
nitrogen-normal stars are identified from the literature. Also, several
boron-depleted nitrogen-rich stars are identified, and while all fall on the
boron-nitrogen trend predicted by rotationally-induced mixing, a majority have
nitrogen enrichments that are not uniquely explained by rotation.
The spectra have also been used to determine iron-group (Cr, Mn, Fe, and Ni)
abundances. The seven B-type stars have near solar iron-group abundances, as
expected for young stars in the solar neighborhood. We have also analysed the
halo B-type star, PG0832+676. We find [Fe/H] = -0.88 +/- 0.10, and the absence
of the B III line gives the upper limit [B/H]<2.5. These and other published
abundances are used to infer the star's evolutionary status as a post-AGB star.Comment: 31 pages, 14 figures. accepted to Ap
Short-term mangrove browsing by feral water buffalo: conflict between natural resources, wildlife and subsistence interests?
The VLT-FLAMES survey of massive stars: Nitrogen abundances for Be-type stars in the Magellanic Clouds
Aims. We compare the predictions of evolutionary models for early-type stars
with atmospheric parameters, projected rotational velocities and nitrogen
abundances estimated for a sample of Be-type stars. Our targets are located in
4 fields centred on the Large Magellanic Cloud cluster: NGC 2004 and the N 11
region as well as the Small Magellanic Cloud clusters: NGC 330 and NGC 346.
Methods. Atmospheric parameters and photospheric abundances have been
determined using the non-LTE atmosphere code tlusty. Effective temperature
estimates were deduced using three different methodologies depending on the
spectral features observed; in general they were found to yield consistent
estimates. Gravities were deduced from Balmer line profiles and
microturbulences from the Si iii spectrum. Additionally the contributions of
continuum emission from circumstellar discs were estimated. Given its
importance in constraining stellar evolutionary models, nitrogen abundances (or
upper limits) were deduced for all the stars analysed. Results. Our nitrogen
abundances are inconsistent with those predicted for targets spending most of
their main sequence life rotating near to the critical velocity. This is
consistent with the results we obtain from modelling the inferred rotational
velocity distribution of our sample and of other investigators. We consider a
number of possibilities to explain the nitrogen abundances and rotational
velocities of our Be-type sample.Comment: 14 pages, 9 figures, submitted to A&
The added value of immediate breast reconstruction to health-related quality of life of breast cancer patients
Background: Postmastectomy immediate breast reconstruction (IBR) may improve the quality of life (QoL) of breast cancer patients. Guidelines recommend to discuss the option IBR with all patients undergoing mastectomy. However, substantial hospital variation in IBR-rates was previously observed in the Netherlands, influenced by patient, tumour and hospital factors and clinicians’ believes. Information provision about IBR may have a positive effect on receiving IBR and therefore QoL. This study investigated patient-reported QoL of patients treated with mastectomy with and without IBR. Methods: An online survey, encompassing the validated BREAST-Q questionnaire, was distributed to a representative sample of 1218 breast cancer patients treated with mastectomy. BREAST-Q scores were compared between patients who had undergone mastectomy either with or without IBR. Results: A total of 445 patients were included for analyses: 281 patients with and 164 without IBR. Patients who had received IBR showed significantly higher BREAST-Q scores on “psychosocial well-being” (75 versus 67, p < 0.001), “sexual well-being” (62 versus 52, p < 0.001) and “physical well-being” (77 versus 74, p = 0.021) compared to patients without IBR. No statistically significant difference was found for “satisfaction with breasts” (64 versus 62, p = 0.21). Similar results were found after multivariate regression analyses, revealing IBR to be an independent factor for a better patient-reported QoL. Conclusions: Patients diagnosed with breast cancer with IBR following mastectomy report a better QoL on important psychosocial, sexual and physical well-being domains. This further supports the recommendation to discuss the option of IBR with all patients with an indication for mastectomy and to enable shared decision-making
The VLT-FLAMES survey of massive stars: constraints on stellar evolution from the chemical compositions of rapidly rotating Galactic and Magellanic Cloud B-type stars
We have previously analysed the spectra of 135 early B-type stars in the LMC
and found several groups of stars that have chemical compositions that conflict
with the theory of rotational mixing. Here we extend this study to Galactic and
SMC metallicities with the analysis of ~50 Galactic and ~100 SMC early B-type
stars with rotational velocities up to ~300km/s. The surface nitrogen
abundances are utilised as a probe of the mixing process.
In the SMC, we find a population of slowly rotating nitrogen-rich stars
amongst the early B type core-hydrogen burning stars, similar to the LMC. In
the Galactic sample we find no significant enrichment amongst the core
hydrogen-burning stars, which appears to be in contrast with the expectation
from both rotating single-star and close binary evolution models. However, only
a small number of the rapidly rotating stars have evolved enough to produce a
significant nitrogen enrichment, and these may be analogous to the non-enriched
rapid rotators previously found in the LMC sample. Finally, in each metallicity
regime, a population of highly enriched supergiants is observed, which cannot
be the immediate descendants of core-hydrogen burning stars. Their abundances
are, however, compatible with them having gone through a previous red
supergiant phase. Together, these observations paint a complex picture of the
nitrogen enrichment in massive main sequence and supergiant stellar
atmospheres, where age and binarity cause crucial effects. Whether rotational
mixing is required to understand our results remains an open question at this
time, but could be answered by identifying the true binary fraction in those
groups of stars that do not agree with single-star evolutionary models
(abridged).Comment: Accepted paper - 86 pages with tables and figure
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