281 research outputs found
MOA-II Galactic Microlensing Constraints: The Inner Milky Way has a Low Dark Matter Fraction and a Near Maximal Disk
Microlensing provides a unique tool to break the stellar to dark matter
degeneracy in the inner Milky Way. We combine N-body dynamical models fitted to
the Milky Way's Boxy/Peanut bulge with exponential disk models outside this,
and compute the microlensing properties. Considering the range of models
consistent with the revised MOA-II data, we find low dark matter fractions in
the inner Galaxy: at the peak of their stellar rotation curve a fraction
of the circular velocity is baryonic (at , at ). These results are in agreement with constraints from the
EROS-II microlensing survey of brighter resolved stars, where we find
at . Our fiducial model of a disk with scale length
2.6kpc, and a bulge with a low dark matter fraction of 12%, agrees with both
the revised MOA-II and EROS-II microlensing data. The required baryonic
fractions, and the resultant low contribution from dark matter, are consistent
with the NFW profiles produced by dissipationless cosmological simulations in
Milky Way mass galaxies. They are also consistent with recent prescriptions for
the mild adiabatic contraction of Milky Way mass haloes without the need for
strong feedback, but there is some tension with recent measurements of the
local dark matter density. Microlensing optical depths from the larger OGLE-III
sample could improve these constraints further when available.Comment: 14 pages, 13 figures, submitted to MNRA
Corrosion inhibition of 2024 aluminium alloy by sodium decanoate
The present study concerns the corrosion protection of the aluminium alloy (AA) 2024 by sodium decanoate (a long-carbon-chain carboxylate). This compound-type is known to form hydrophobic films on the metal surface. The characterization of the inhibition mechanisms was studied for different experimental conditions (pH, NaCl concentrations) by using electrochemical techniques. Special attention was paid to the action of the carboxylate on the intermetallic particles by performing local electrochemical impedance measurements on a model system (Al/Cu couple). The decanoate afforded high protection to the AA2024 both by preventing chloride ion attack of the oxide layer and by limiting galvanic coupling between the intermetallic particles and the surrounding matrix. A passivation effect of the compound was also shown
The Structure of the Milky Way's Bar Outside the Bulge
While it is incontrovertible that the inner Galaxy contains a bar, its
structure near the Galactic plane has remained uncertain, where extinction from
intervening dust is greatest. We investigate here the Galactic bar outside the
bulge, the long bar, using red clump giant (RCG) stars from UKIDSS, 2MASS, VVV,
and GLIMPSE. We match and combine these surveys to investigate a wide area in
latitude and longitude, |b|<9deg and |l|<40deg. We find: (1) The bar extends to
l~25deg at |b|~5deg from the Galactic plane, and to l~30deg at lower latitudes.
(2) The long bar has an angle to the line-of-sight in the range (28-33)deg,
consistent with studies of the bulge at |l|<10deg. (3) The scale-height of RCG
stars smoothly transitions from the bulge to the thinner long bar. (4) There is
evidence for two scale heights in the long bar. We find a ~180pc thin bar
component reminiscent of the old thin disk near the sun, and a ~45pc super-thin
bar component which exists predominantly towards the bar end. (5) Constructing
parametric models for the RC magnitude distributions, we find a bar half length
of 5.0+-0.2kpc for the 2-component bar, and 4.6+-0.3kpc for the thin bar
component alone. We conclude that the Milky Way contains a central box/peanut
bulge which is the vertical extension of a longer, flatter bar, similar as seen
in both external galaxies and N-body models.Comment: Accepted for publication by MNRA
The Initial Mass Function of the Inner Galaxy Measured From OGLE-III Microlensing Timescales
We use the timescale distribution of ~3000 microlensing events measured by
the OGLE-III survey, together with accurate new made-to-measure dynamical
models of the Galactic bulge/bar region, to measure the IMF in the inner Milky
Way. The timescale of each event depends on the mass of the lensing object,
together with the relative distances and velocities of the lens and source. The
dynamical model provides statistically these distances and velocities allowing
us to constrain the lens mass function, and from this to infer the IMF.
Parameterising the IMF as a broken power-law, we find slopes in the main
sequence and brown
dwarf region where we
use a fiducial 50% binary fraction, and the systematic uncertainty covers the
range of binary fractions 0-100%. Similarly for a log-normal IMF we conclude
and
. These values are very
similar to a Kroupa or Chabrier IMF respectively, showing that the IMF in the
bulge is indistinguishable from that measured locally, despite the lenses lying
in the inner Milky Way where the stars are mostly ~10Gyr old and formed on a
fast -element enhanced timescale. This therefore constrains models of
IMF variation that depend on the properties of the collapsing gas cloud.Comment: 6 pages, 3 figures. Accepted by ApJ
The Stellar Halo in the Inner Milky Way: Predicted Shape and Kinematics
We have used N-body simulations for the Milky Way to investigate the
kinematic and structural properties of the old metal-poor stellar halo in the
barred inner region of the Galaxy. We find that the extrapolation of the
density distribution for bulge RR Lyrae stars, , approximately
matches the number density of RR Lyrae in the nearby stellar halo. We follow
the evolution of such a tracer population through the formation and evolution
of the bar and box/peanut bulge in the N-body model. We find that its density
distribution changes from oblate to triaxial, and that it acquires slow
rotation in agreement with recent measurements. The maximum radial velocity is
km/s at , and the velocity dispersion is
km/s. Even though the simulated metal-poor halo in the bulge has a
barred shape, just of the orbits follow the bar, and it does not trace
the peanut/X structure. With these properties, the RR Lyrae population in the
Galactic bulge is consistent with being the inward extension of the Galactic
metal-poor stellar halo.Comment: 5 pages, 5 figures. Accepted for publication in MNRAS Letter
Made-to-Measure models of the Galactic Box/Peanut bulge: stellar and total mass in the bulge region
We construct dynamical models of the Milky Way's Box/Peanut (B/P) bulge,
using the recently measured 3D density of Red Clump Giants (RCGs) as well as
kinematic data from the BRAVA survey. We match these data using the NMAGIC
Made-to-Measure method, starting with N-body models for barred discs in
different dark matter haloes. We determine the total mass in the bulge volume
of the RCGs measurement (+-2.2 x +- 1.4 x +- 1.2 kpc) with unprecedented
accuracy and robustness to be 1.84 +- 0.07 x10^10 Msun. The stellar mass in
this volume varies between 1.25-1.6 x10^10 Msun, depending on the amount of
dark matter in the bulge. We evaluate the mass-to-light and mass-to-clump
ratios in the bulge and compare them to theoretical predictions from population
synthesis models. We find a mass-to-light ratio in the K-band in the range
0.8-1.1. The models are consistent with a Kroupa or Chabrier IMF, but a
Salpeter IMF is ruled out for stellar ages of 10 Gyr. To match predictions from
the Zoccali IMF derived from the bulge stellar luminosity function requires
about 40% or 0.7 x10^10 Msun dark matter in the bulge region. The BRAVA data
together with the RCGs 3D density imply a low pattern speed for the Galactic
B/P bulge of 25-30 km.s-1.kpc-1. This would place the Galaxy among the slow
rotators (R >= 1.5). Finally, we show that the Milky Way's B/P bulge has an
off-centred X structure, and that the stellar mass involved in the peanut shape
accounts for at least 20% of the stellar mass of the bulge, significantly
larger than previously thought.Comment: Accepted for publication in MNRA
Corrosion protection of AA2024 sealed anodic layers using the hydrophobic properties of carboxylic acids
The present study investigates the use of carboxylic acids as a post-treatment for sealed AA2024 anodised in tartaricâsulphuric acid electrolyte. Four monocarboxylic acids with different carbon chain lengths were tested ((CH3â(CH2)nâCOOH with n=4, 8, 12 and 16). Hydrophobic surface properties after the posttreatment were characterized by contact angle measurements. Electrochemical impedance spectroscopy (EIS) was performed to assess the ability of the four carboxylic acids to form protective films. It was shown that stearic acid (n=16) used in its pure molten state was the most efficient. The organic film formed very rapidly (under 5 min) and contributed to the enhancement of the protection in terms of corrosion resistance of the sealed anodic layers. EIS measurements showed the presence of the organic films on the specimen surface
Revisiting the Tale of Hercules: how stars orbiting the Lagrange points visit the Sun
We propose a novel explanation for the Hercules stream consistent with recent
measurements of the extent and pattern speed of the Galactic bar. We have
adapted a made-to-measure dynamical model tailored for the Milky Way to
investigate the kinematics of the solar neighborhood (SNd). The model matches
the 3D density of the red clump giant stars (RCGs) in the bulge and bar as well
as stellar kinematics in the inner Galaxy, with a pattern speed of 39 km
s kpc. Cross-matching this model with the DR1 TGAS data
combined with RAVE and LAMOST radial velocities, we find that the model
naturally predicts a bimodality in the -velocity distribution for
nearby stars which is in good agreement with the Hercules stream. In the model,
the Hercules stream is made of stars orbiting the Lagrange points of the bar
which move outward from the bar's corotation radius to visit the SNd. While the
model is not yet a quantitative fit of the velocity distribution, the new
picture naturally predicts that the Hercules stream is more prominent inward
from the Sun and nearly absent only a few pc outward of the Sun, and
plausibly explains that Hercules is prominent in old and metal-rich stars.Comment: 7 pages, 5 figures. ApJ Letters, in pres
Magnetoresistance of disordered graphene: from low to high temperatures
We present the magnetoresistance (MR) of highly doped monolayer graphene
layers grown by chemical vapor deposition on 6H-SiC. The magnetotransport
studies are performed on a large temperature range, from = 1.7 K up to room
temperature. The MR exhibits a maximum in the temperature range K.
The maximum is observed at intermediate magnetic fields ( T), in between
the weak localization and the Shubnikov-de Haas regimes. It results from the
competition of two mechanisms. First, the low field magnetoresistance increases
continuously with and has a purely classical origin. This positive MR is
induced by thermal averaging and finds its physical origin in the energy
dependence of the mobility around the Fermi energy. Second, the high field
negative MR originates from the electron-electron interaction (EEI). The
transition from the diffusive to the ballistic regime is observed. The
amplitude of the EEI correction points towards the coexistence of both long and
short range disorder in these samples
3C-SiC â From Electronic to MEMS Devices
Since decades, silicon carbide (SiC) has been avowed as an interesting material for high-power and high-temperature applications because of its significant properties including its wide bandgap energy and high temperature stability. SiC is also professed as an ideal candidate for microsystem applications due to its excellent mechanical properties and chemical inertia, making it suitable for harsh environments. Among the 250 different SiC polytypes, only 4H, 6H and 3C-SiC are commercially available. The cubic structure, 3C-SiC, is the only one that can be grown on cheap silicon substrates. Hence, 3C-SiC is more interesting than any other polytype for reducing fabrication costs and increasing wafer diameter. This huge property has been evidenced for more than 30 years using chemical vapor deposition. Despite this key achievement and the growing interest for silicon carbide, no 3C-SiC-based devices can be found on the market whereas 4H-SiC-based devices are more and more largely commercialized. Even so, important headways have been reached for electrical and microelectromechanical systems (MEMS) applications. Therefore, the purpose of this chapter is to address concerns related to electronic applications and MEMS fabrication of 3C-SiC-based devices, trying to give a broad overview on specific issues and challenging solutions
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