3,310 research outputs found
Nonlinear dynamics of phase separation in thin films
We present a long-wavelength approximation to the Navier-Stokes Cahn-Hilliard
equations to describe phase separation in thin films. The equations we derive
underscore the coupled behaviour of free-surface variations and phase
separation. We introduce a repulsive substrate-film interaction potential and
analyse the resulting fourth-order equations by constructing a Lyapunov
functional, which, combined with the regularizing repulsive potential, gives
rise to a positive lower bound for the free-surface height. The value of this
lower bound depends on the parameters of the problem, a result which we compare
with numerical simulations. While the theoretical lower bound is an obstacle to
the rupture of a film that initially is everywhere of finite height, it is not
sufficiently sharp to represent accurately the parametric dependence of the
observed dips or `valleys' in free-surface height. We observe these valleys
across zones where the concentration of the binary mixture changes sharply,
indicating the formation of bubbles. Finally, we carry out numerical
simulations without the repulsive interaction, and find that the film ruptures
in finite time, while the gradient of the Cahn--Hilliard concentration develops
a singularity.Comment: 26 pages, 20 figures, PDFLaTeX with RevTeX4 macros. A thorough
analysis of the equations is presented in arXiv:0805.103
The VLT-FLAMES Survey of Massive Stars: Observations centered on the Magellanic Cloud clusters NGC 330, NGC 346, NGC 2004, and the N11 region
We present new observations of 470 stars using the Fibre Large Array
Multi-Element Spectrograph (FLAMES) instrument in fields centered on the
clusters NGC 330 and NGC 346 in the Small Magellanic Cloud (SMC), and NGC 2004
and the N11 region in the Large Magellanic Cloud (LMC). A further 14 stars were
observed in the N11 and NGC 330 fields using the Ultraviolet and Visual Echelle
Spectrograph (UVES) for a separate programme. Spectral classifications and
stellar radial velocities are given for each target, with careful attention to
checks for binarity. In particular we have investigated previously unexplored
regions around the central LH9/LH10 complex of N11, finding ~25 new O-type
stars from our spectroscopy. We have observed a relatively large number of
Be-type stars that display permitted Fe II emission lines. These are primarily
not in the cluster cores and appear to be associated with classical Be-type
stars, rather than pre main-sequence objects. The presence of the Fe II
emission, as compared to the equivalent width of H, is not obviously
dependent on metallicity. We have also explored the relative fraction of Be- to
normal B-type stars in the field-regions near to NGC 330 and NGC 2004, finding
no strong evidence of a trend with metallicity when compared to Galactic
results. A consequence of service observations is that we have reasonable
time-sampling in three of our FLAMES fields. We find lower limits to the binary
fraction of O- and early B-type stars of 23 to 36%. One of our targets
(NGC346-013) is especially interesting with a massive, apparently hotter, less
luminous secondary component.Comment: 35 pages, 17 figures (some reduced in size). Replacement copy,
includes an erratum on the final page. A copy with full res. & embedded
figures is at http://www.roe.ac.uk/~cje/flamesMC.ps.g
Associations between fibrin D-dimer, markers of inflammation, incident self-reported mobility limitation, and all-cause mortality in older men
Objectives<p></p>
To examine the independent relationships between fibrin D-dimer, interleukin 6 (IL-6), C-reactive protein (CRP), and fibrinogen and incident mobility limitation and mortality.<p></p>
Design<p></p>
Prospective.<p></p>
Setting<p></p>
General practice in 24 British towns.<p></p>
Participants<p></p>
Men aged 60 to 79 without prevalent heart failure followed up for an average of 11.5 years (N = 3,925).<p></p>
Measurements<p></p>
All-cause mortality (n = 1,286) and self-reported mobility disability obtained at examination in 1998 to 2000 and in a postal questionnaire 3 to 5 years later in 2003.<p></p>
Results<p></p>
High D-dimer (top vs lowest tertile: adjusted odds ratio (aOR) = 1.46, 95% confidence interval = 1.02–2.05) and IL-6 (aOR = 1.43, 95% CI = 1.01–2.02) levels (but not CRP or fibrinogen) were associated with greater incident mobility limitation after adjustment for confounders and prevalent disease status. IL-6, CRP, fibrinogen, and D-dimer were significantly associated with total mortality after adjustment for confounders. Only D-dimer and IL-6 predicted total mortality independent of each other and the other biomarkers. The adjusted hazard ratio (aHR) was 1.16 (95% CI = 1.10–1.22) for a standard deviation increase in log D-dimer and 1.10 (95% CI = 1.04–1.18) for a standard deviation increase in log IL-6. D-dimer was independently related to vascular and nonvascular mortality, and IL-6 was independently related to vascular mortality. Risks of mobility limitation and mortality were greatest in those with a combination of high D-dimer and IL-6 levels.<p></p>
Conclusion<p></p>
D-dimer and IL-6 are associated with risk of mobility limitation and mortality in older men without heart failure. The findings suggest that coagulation leads to functional decline and mortality s that inflammation does not explain
Disentangling the spatial substructure of Cygnus OB2 from Gaia DR2
© 2019 The Author(s) Published by Oxford University Press on behalf of the Royal Astronomical SocietyFor the first time, we have explored the spatial substructure of the Cygnus OB2 association using parallaxes from the recent second Gaia data release. We find significant line-of-sight substructure within the association, which we quantify using a parametrized model that reproduces the observed parallax distribution. This inference approach is necessary due to the non-linearity of the parallax distance transformation and the asymmetry of the resulting probability distribution. Using a Markov Chain Monte Carlo ensemble sampler and an unbinned maximum likelihood test, we identify two different stellar groups superposed on the association. We find the main Cygnus OB2 group at ∼1760 pc, further away than recent estimates have envisaged, and a foreground group at ∼1350 pc. We also calculate individual membership probabilities and identify outliers as possible non-members of the association.Peer reviewe
The nature of B supergiants: clues from a steep drop in rotation rates at 22000 K. The possibility of Bi-stability braking
The location of B supergiants in the Hertzsprung-Russell diagram (HRD)
represents a long-standing problem in massive star evolution. Here we propose
their nature may be revealed utilising their rotational properties, and we
highlight a steep drop in massive star rotation rates at an effective
temperature of 22000 K. We discuss two potential explanations for it. On the
one hand, the feature might be due to the end of the main sequence, which could
potentially constrain the core overshooting parameter. On the other hand, the
feature might be the result of enhanced mass loss at the predicted location of
the bi-stability jump. We term this effect "bi-stability breaking" and discuss
its potential consequences for the evolution of massive stars.Comment: Accepted by A&A Letters (4 pages, 5 figures); typos correcte
A Be star with a low nitrogen abundance in the SMC cluster NGC330
High-resolution UVES/VLT spectra of B12, an extreme pole-on Be star in the
SMC cluster NGC330, have been analysed using non-LTE model atmospheres to
obtain its chemical composition relative to the SMC standard star AV304. We
find a general underabundance of metals which can be understood in terms of an
extra contribution to the stellar continuum due to emission from a disk which
we estimate to be at the ~25% level. When this is corrected for, the nitrogen
abundance for B12 shows no evidence of enhancement by rotational mixing as has
been found in other non-Be B-type stars in NGC330, and is inconsistent with
evolutionary models which include the effects of rotational mixing. A second Be
star, NGC330-B17, is also shown to have no detectable nitrogen lines. Possible
explanations for the lack of rotational mixing in these rapidly rotating stars
are discussed, one promising solution being the possibility that magnetic
fields might inhibit rotational mixing.Comment: 7 pages, 4 figures. Submitted to A&
Terminal velocities of luminous, early-type SMC stars
Ultraviolet spectra from the Space Telescope Imaging Spectrograph (STIS) are
used to determine terminal velocities for 11 O and B-type giants and
supergiants in the Small Magellanic Cloud (SMC) from the Si IV and C IV
resonance lines. Using archival data from observations with the Goddard
High-Resolution Spectrograph and the International Ultraviolet Explorer
telescope, terminal velocities are obtained for a further five B-type
supergiants. We discuss the metallicity dependence of stellar terminal
velocities, finding no evidence for a significant scaling between Galactic and
SMC metallicities for Teff < 30,000 K, consistent with the predictions of
radiation driven wind theory for supergiant stars. A comparison of the
ratio between the SMC and Galactic samples, while
consistent with the above statement, emphasizes that the uncertainties in the
distances to galactic O-stars are a serious obstacle to a detailed comparison
with theory. For the SMC sample there is considerable scatter in this ratio at
a given effective temperature, perhaps indicative of uncertainties in stellar
masses.Comment: 28 pages, 8 figures, accepted by ApJ; minor revisions prior to
acceptanc
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