5,518 research outputs found
To grate a liquid into tiny droplets by its impact on a hydrophobic micro-grid
We report on experiments of drop impacting a hydrophobic micro-grid, of
typical spacing a few tens of m. Above a threshold in impact speed, liquid
emerges to the other side, forming micro-droplets of size about that of the
grid holes. We propose a method to produce either a mono-disperse spray or a
single tiny droplet of volume as small as a few picoliters corresponding to a
volume division of the liquid drop by a factor of up to 10. We also discuss
the discrepancy of the measured thresholds with that predicted by a balance
between inertia and capillarity.Comment: 3 pages, 5 figures, Accepted for publication in Applied Physics
Letter
Biases in the determination of dynamical parameters of star clusters: today and in the Gaia era
The structural and dynamical properties of star clusters are generally
derived by means of the comparison between steady-state analytic models and the
available observables. With the aim of studying the biases of this approach, we
fitted different analytic models to simulated observations obtained from a
suite of direct N-body simulations of star clusters in different stages of
their evolution and under different levels of tidal stress to derive mass, mass
function and degree of anisotropy. We find that masses can be
under/over-estimated up to 50% depending on the degree of relaxation reached by
the cluster, the available range of observed masses and distances of radial
velocity measures from the cluster center and the strength of the tidal field.
The mass function slope appears to be better constrainable and less sensitive
to model inadequacies unless strongly dynamically evolved clusters and a
non-optimal location of the measured luminosity function are considered. The
degree and the characteristics of the anisotropy developed in the N-body
simulations are not adequately reproduced by popular analytic models and can be
detected only if accurate proper motions are available. We show how to reduce
the uncertainties in the mass, mass-function and anisotropy estimation and
provide predictions for the improvements expected when Gaia proper motions will
be available in the near future.Comment: 14 pages, 8 figures, accepted for publication by MNRA
Antioxidant properties and heat damage of water biscuits enriched with sprouted wheat and barley
The nutritional value of cereal kernels is markedly improved by the germination process. Aim of this study was to evaluate protein, ash, sugars, heat damage (furosine, hydroxymethylfurfural, glucosylisomaltol), carotenoids, tocols, phenolics and antioxidant capacity (FRAP, ABTS, DPPH, reducing power, superoxide anion, beta-carotene bleaching tests) of water biscuits enriched with increasing quantities (0, 5, 10 and 20%) of sprouted bread wheat or barley. The wholemeals from sprouted wheat and barley showed high concentrations of total carotenoids (82.6 and 119.7 mg/kg, respectively), tocols (53.4 and 88.2 mg/kg), conjugated (368.0 and 564.2 mg/kg) and bound (1811.6 and 3022.0 mg/kg) phenolics. Therefore, the enriched water biscuits had higher carotenoids, tocols and phenolics content, heat damage and antioxidant capacity than the controls. The greatest increase was recorded in barley-enriched samples. The addition of 15%-20% sprouted wheat or 5%-10% sprouted barley improved the nutritional quality of water biscuits while limiting heat damage
A rare early-type star revealed in the Wing of the Small Magellanic Cloud
Sk 183 is the visually-brightest star in the N90 nebula, a young star-forming
region in the Wing of the Small Magellanic Cloud (SMC). We present new optical
spectroscopy from the Very Large Telescope which reveals Sk 183 to be one of
the most massive O-type stars in the SMC. Classified as an O3-type dwarf on the
basis of its nitrogen spectrum, the star also displays broadened He I
absorption which suggests a later type. We propose that Sk 183 has a composite
spectrum and that it is similar to another star in the SMC, MPG 324. This
brings the number of rare O2- and O3-type stars known in the whole of the SMC
to a mere four. We estimate physical parameters for Sk 183 from analysis of its
spectrum. For a single-star model, we estimate an effective temperature of
46+/-2 kK, a low mass-loss rate of ~10^-7 Msun yr^-1, and a spectroscopic mass
of 46^+9_-8 Msun (for an adopted distance modulus of 18.7 mag to the young
population in the SMC Wing). An illustrative binary model requires a slightly
hotter temperature (~47.5 kK) for the primary component. In either scenario, Sk
183 is the earliest-type star known in N90 and will therefore be the dominant
source of hydrogen-ionising photons. This suggests Sk 183 is the primary
influence on the star formation along the inner edge of the nebula.Comment: Accepted by ApJ, 10 pages, 7 figures, v2 after proof
Free-energy distribution of the directed polymer at high temperature
We study the directed polymer of length in a random potential with fixed
endpoints in dimension 1+1 in the continuum and on the square lattice, by
analytical and numerical methods. The universal regime of high temperature
is described, upon scaling 'time' and space (with for the discrete model) by a continuum model with
-function disorder correlation. Using the Bethe Ansatz solution for the
attractive boson problem, we obtain all positive integer moments of the
partition function. The lowest cumulants of the free energy are predicted at
small time and found in agreement with numerics. We then obtain the exact
expression at any time for the generating function of the free energy
distribution, in terms of a Fredholm determinant. At large time we find that it
crosses over to the Tracy Widom distribution (TW) which describes the fixed
infinite limit. The exact free energy distribution is obtained for any time
and compared with very recent results on growth and exclusion models.Comment: 6 pages, 3 figures large time limit corrected and convergence to
Tracy Widom established, 1 figure changed
The VLT-FLAMES Tarantula Survey. VII. A low velocity dispersion for the young massive cluster R136
Detailed studies of resolved young massive star clusters are necessary to
determine their dynamical state and evaluate the importance of gas expulsion
and early cluster evolution. In an effort to gain insight into the dynamical
state of the young massive cluster R136 and obtain the first measurement of its
velocity dispersion, we analyse multi-epoch spectroscopic data of the inner
regions of 30 Doradus in the Large Magellanic Cloud (LMC) obtained as part of
the VLT-FLAMES Tarantula Survey. Following a quantitative assessment of the
variability, we use the radial velocities of non-variable sources to place an
upper limit of 6 km/s on the line-of-sight velocity dispersion of stars within
a projected distance of 5 pc from the centre of the cluster. After accounting
for the contributions of undetected binaries and measurement errors through
Monte Carlo simulations, we conclude that the true velocity dispersion is
likely between 4 and 5 km/s given a range of standard assumptions about the
binary distribution. This result is consistent with what is expected if the
cluster is in virial equilibrium, suggesting that gas expulsion has not altered
its dynamics. We find that the velocity dispersion would be ~25 km/s if
binaries were not identified and rejected, confirming the importance of the
multi-epoch strategy and the risk of interpreting velocity dispersion
measurements of unresolved extragalactic young massive clusters.Comment: 18 pages, 7 figures, accepted by A&
The VLT-FLAMES Tarantula Survey XVII. Physical and wind properties of massive stars at the top of the main sequence
The evolution and fate of very massive stars (VMS) is tightly connected to
their mass-loss properties. Their initial and final masses differ significantly
as a result of mass loss. VMS have strong stellar winds and extremely high
ionising fluxes, which are thought to be critical sources of both mechanical
and radiative feedback in giant Hii regions. However, how VMS mass-loss
properties change during stellar evolution is poorly understood. In the
framework of the VLT-Flames Tarantula Survey (VFTS), we explore the mass-loss
transition region from optically thin O to denser WNh star winds, thereby
testing theoretical predictions. To this purpose we select 62 O, Of, Of/WN, and
WNh stars, an unprecedented sample of stars with the highest masses and
luminosities known. We perform a spectral analysis of optical VFTS as well as
near-infrared VLT/SINFONI data using the non-LTE radiative transfer code CMFGEN
to obtain stellar and wind parameters. For the first time, we observationally
resolve the transition between optically thin O and optically thick WNh star
winds. Our results suggest the existence of a kink between both mass-loss
regimes, in agreement with recent MC simulations. For the optically thick
regime, we confirm the steep dependence on the Eddington factor from previous
theoretical and observational studies. The transition occurs on the MS near a
luminosity of 10^6.1Lsun, or a mass of 80...90Msun. Above this limit, we find
that - even when accounting for moderate wind clumping (with f = 0.1) - wind
mass-loss rates are enhanced with respect to standard prescriptions currently
adopted in stellar evolution calculations. We also show that this results in
substantial helium surface enrichment. Based on our spectroscopic analyses, we
are able to provide the most accurate ionising fluxes for VMS known to date,
confirming the pivotal role of VMS in ionising and shaping their environments.Comment: Accepted for publication in A&A, 19 pages, 14 figures, 6 tables, (74
pages appendix, 68 figures, 4 tables
Mass modelling globular clusters in the Gaia era: a method comparison using mock data from an N-body simulation of M 4
As we enter a golden age for studies of internal kinematics and dynamics of Galactic globular clusters (GCs), it is timely to assess the performance of modelling techniques in recovering the mass, mass profile, and other dynamical properties of GCs. Here, we compare different mass-modelling techniques (distribution function (DF)-based models, Jeans models, and a grid of N-body models) by applying them to mock observations from a star-by-star N-body simulation of the GC M 4 by Heggie. The mocks mimic existing and anticipated data for GCs: surface brightness or number density profiles, local stellar mass functions, line-of-sight velocities, and Hubble Space Telescope-and Gaia-like proper motions. We discuss the successes and limitations of the methods. We find that multimass DF-based models, Jeans, and N-body models provide more accurate mass profiles compared to single-mass DF-based models. We highlight complications in fitting the kinematics in the outskirts due to energetically unbound stars associated with the cluster ('potential escapers', captured neither by truncated DF models nor by N-body models of clusters in isolation), which can be avoided with DF-based models including potential escapers, or with Jeans models. We discuss ways to account for mass segregation. For example, three-component DF-based models with freedom in their mass function are a simple alternative to avoid the biases of single-mass models (which systematically underestimate the total mass, half-mass radius, and central density), while more realistic multimass DF-based models with freedom in the remnant content represent a promising avenue to infer the total mass and the mass function of remnants
The VLT-FLAMES Tarantula Survey XXI. Stellar spin rates of O-type spectroscopic binaries
The initial distribution of spin rates of massive stars is a fingerprint of
their elusive formation process. It also sets a key initial condition for
stellar evolution and is thus an important ingredient in stellar population
synthesis. So far, most studies have focused on single stars. Most O stars are
however found in multiple systems. By establishing the spin-rate distribution
of a sizeable sample of O-type spectroscopic binaries and by comparing the
distributions of binary sub-populations with one another as well as with that
of presumed single stars in the same region, we aim to constrain the initial
spin distribution of O stars in binaries, and to identify signatures of the
physical mechanisms that affect the evolution of the massive stars spin rates.
We use ground-based optical spectroscopy obtained in the framework of the
VLT-FLAMES Tarantula Survey (VFTS) to establish the projected equatorial
rotational velocities (\vrot) for components of 114 spectroscopic binaries in
30 Doradus. The \vrot\ values are derived from the full-width at half-maximum
(FWHM) of a set of spectral lines, using a FWHM vs. \vrot\ calibration that we
derive based on previous line analysis methods applied to single O-type stars
in the VFTS sample. The overall \vrot\ distribution of the primary stars
resembles that of single O-type stars in the VFTS, featuring a low-velocity
peak (at \vrot < 200 kms) and a shoulder at intermediate velocities (200 <
\vrot < 300 kms). The distributions of binaries and single stars however
differ in two ways. First, the main peak at \vrot \sim100 kms is broader and
slightly shifted toward higher spin rates in the binary distribution compared
to that of the presumed-single stars. Second, the \vrot distribution of
primaries lacks a significant population of stars spinning faster than 300 kms
while such a population is clearly present in the single star sample.Comment: 16 pages, 16 figures, paper accepted in Astronomy & Astrophysic
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