1,742 research outputs found
The Evolution of Carbon and Oxygen in the Bulge and Disk of the Milky Way
The evolution of C and O abundances in the Milky Way can impose strong
constraints on stellar nucleosynthesis and help understanding the formation and
evolution of our Galaxy. The aim is to review the measured C and O abundances
in the disk and bulge of the Galaxy and compare them with model predictions. We
adopt two successful chemical evolution models for the bulge and the disk,
which assume the same nucleosynthesis prescriptions but different histories of
star formation. The data show a clear distinction between the trend of [C/O] in
the thick and thin Galactic disks, while the thick disk and bulge trends are
indistinguishable with a large (>0.5 dex) increase in the C/O ratio in the
range from -0.1 to +0.4 dex for [O/H]. In our models we consider yields from
massive stars with and without the inclusion of metallicity-dependent stellar
winds. The observed increase in the [C/O] ratio with metallicity in the bulge
and thick disk lies between the predictions utilizing the mass-loss rates of
Maeder (1992) and those of Meynet & Maeder (2002). A model without
metallicity-dependent yields completely fails to match the observations. Thus,
the relative increase in carbon abundance at high metallicity appears to be due
to metallicity-dependent stellar winds in massive stars. These results also
explain the steep decline of the [O/Fe] ratio with [Fe/H] in the Galactic
bulge, while the [Mg/Fe] ratio is enhanced at all [Fe/H]. (abridged)Comment: 18 pages, 6 figures, submitted to Astronomy & Astrophysic
The impact of stellar rotation on the CNO abundance patterns in the Milky Way at low metallicities
We investigate the effect of new stellar models, which take rotation into
account, computed for very low metallicities on the chemical evolution of the
earliest phases of the Milky Way. We check the impact of these new stellar
yields on a model for the halo of the Milky Way that can reproduce the observed
halo metallicity distribution. In this way we try to better constrain the ISM
enrichment timescale, which was not done in our previous work. The stellar
models adopted in this work were computed under the assumption that the ratio
of the initial rotation velocity to the critical velocity of stars is roughly
constant with metallicity. This naturally leads to faster rotation at lower
metallicity, as metal poor stars are more compact than metal rich ones. We find
that the new Z = 10-8 stellar yields computed for large rotational velocities
have a tremendous impact on the interstellar medium nitrogen enrichment for
log(O/H)+12 < 7 (or [Fe/H]< -3). We show that upon the inclusion of the new
stellar calculations in a chemical evolution model for the galactic halo with
infall and outflow, both high N/O and C/O ratios are obtained in the very-metal
poor metallicity range in agreement with observations. Our results give further
support to the idea that stars at very low metallicities could have initial
rotational velocities of the order of 600-800kms-1. An important contribution
to N from AGB stars is still needed in order to explain the observations at
intermediate metallicities. One possibility is that AGB stars at very low
metallicities also rotate fast. This could be tested in the future, once
stellar evolution models for fast rotating AGB stars will be available.Comment: Contribution to Nuclei in the Cosmos IX (Proceedings of Science - 9
pages, 4 figs., accepted) - Version 2: one reference added in the caption of
Fig.
SPINSTARS at low metallicities
The main effect of axial rotation on the evolution of massive PopIII stars is
to trigger internal mixing processes which allow stars to produce significant
amounts of primary nitrogen 14 and carbon 13. Very metal poor massive stars
produce much more primary nitrogen than PopIII stars for a given initial mass
and rotation velocity. The very metal poor stars undergo strong mass loss
induced by rotation. One can distinguish two types of rotationnaly enhanced
stellar winds: 1) Rotationally mechanical winds occurs when the surface
velocity reaches the critical velocity at the equator, {\it i.e.} the velocity
at which the centrifugal acceleration is equal to the gravity; 2) Rotationally
radiatively line driven winds are a consequence of strong internal mixing which
brings large amounts of CNO elements at the surface. This enhances the opacity
and may trigger strong line driven winds. These effects are important for an
initial value of of 0.54 for a 60 M at
, {\it i.e.} for initial values of
higher than the one (0.4) corresponding to observations at solar .
These two effects, strong internal mixing leading to the synthesis of large
amounts of primary nitrogen and important mass losses induced by rotation,
occur for between about 10 and 0.001. For metallicities above 0.001
and for reasonable choice of the rotation velocities, internal mixing is no
longer efficient enough to trigger these effects.Comment: 5 pages, 4 figures, to be published in the conference proceedings of
First Stars III, Santa Fe, 200
Nanomedicine Approaches to Negotiate Local Biobarriers for Topical Drug Delivery
AbstractTopical treatments have been widely adopted to address a broad range of conditions across multiple sites thanks to their convenience, versatility, and effectiveness. While bypassing systemic biobarriers and avoiding systemic side effects by delivering directly to the target tissue, topical treatments still face significant local biobarriers that limit their efficacy. The toolset available for nanodelivery systems and their inherent multifunctionality can contribute to simultaneously address otherwise intractable challenges related to barrier function evasion, drug solubility, bioavailability, pharmacokinetics, smart and sustained release, quantitative coâdelivery, and local targeting which are key to successful topical treatments. This review summarizes the outstanding challenges associated with the topical treatments of key diseases of the skin, mucosae, eyes, and ears, and highlights how nanodelivery systems are being developed to address them effectively
Lifting of the Landau level degeneracy in graphene devices in a tilted magnetic field
We report on transport and capacitance measurements of graphene devices in
magnetic fields up to 30 T. In both techniques, we observe the full splitting
of Landau levels and we employ tilted field experiments to address the origin
of the observed broken symmetry states. In the lowest energy level, the spin
degeneracy is removed at filling factors and we observe an enhanced
energy gap. In the higher levels, the valley degeneracy is removed at odd
filling factors while spin polarized states are formed at even . Although
the observation of odd filling factors in the higher levels points towards the
spontaneous origin of the splitting, we find that the main contribution to the
gap at , and is due to the Zeeman energy.Comment: 5 pages, 4 figure
Abundance patterns in early-type galaxies: is there a 'knee' in the [Fe/H] vs. [alpha/Fe] relation?
Early-type galaxies (ETGs) are known to be enhanced in alpha elements, in
accordance with their old ages and short formation timescales. In this
contribution we aim to resolve the enrichment histories of ETGs. This means we
study the abundance of Fe ([Fe/H]) and the alpha-element groups ([alpha/Fe])
separately for stars older than 9.5 Gyr ([Fe/H]o, [alpha/Fe]o) and for stars
between 1.5 and 9.5 Gyr ([Fe/H]i, [alpha/Fe]i). Through extensive simulation we
show that we can indeed recover the enrichment history per galaxy. We then
analyze a spectroscopic sample of 2286 early-type galaxies from the SDSS
selected to be ETGs. We separate out those galaxies for which the abundance of
iron in stars grows throughout the lifetime of the galaxy, i.e. in which
[Fe/H]o < [Fe/H]i. We confirm earlier work where the [Fe/H] and [alpha/Fe]
parameters are correlated with the mass and velocity dispersion of ETGs. We
emphasize that the strongest relation is between [alpha/Fe] and age. This
relation falls into two regimes, one with a steep slope for old galaxies and
one with a shallow slope for younger ETGs. The vast majority of ETGs in our
sample do not show the 'knee' in the plot of [Fe/H] vs. [alpha/Fe] commonly
observed in local group galaxies. This implies that for the vast majority of
ETGs, the stars younger than 9.5 Gyrs are likely to have been accreted or
formed from accreted gas. The properties of the intermediate-age stars in
accretion-dominated ETGs indicate that mass growth through late (minor) mergers
in ETGs is dominated by galaxies with low [Fe/H] and low [alpha/Fe]. The method
of reconstructing the stellar enrichment histories of ETGs introduced in this
paper promises to constrain the star formation and mass assembly histories of
large samples of galaxies in a unique way.Comment: 22 pages, 25 figures, accepted for publication by A&
An alternative 3D inversion method for magnetic anomalies with depth resolution
This paper presents a new method to invert magnetic anomaly data in a variety of non-complex contexts when
a priori information about the sources is not available. The region containing magnetic sources is discretized into
a set of homogeneously magnetized rectangular prisms, polarized along a common direction. The magnetization
distribution is calculated by solving an underdetermined linear system, and is accomplished through the simultaneous
minimization of the norm of the solution and the misfit between the observed and the calculated
field. Our algorithm makes use of a dipolar approximation to compute the magnetic field of the rectangular
blocks. We show how this approximation, in conjunction with other correction factors, presents numerous advantages
in terms of computing speed and depth resolution, and does not affect significantly the success of the
inversion. The algorithm is tested on both synthetic and real magnetic datasets
Strengthening functionally specific neural pathways with transcranial brain stimulation
Cortico-cortical paired associative stimulation (ccPAS) is a recently established offline dual-coil transcranial magnetic stimulation (TMS) protocol 1, 2, 3 based on the Hebbian principle of associative plasticity and designed to transiently enhance synaptic efficiency in neural pathways linking two interconnected (targeted) brain regions 4, 5. Here, we present a new âfunction-tuning ccPASâ paradigm in which, by pairing ccPAS with the presentation of a specific visual feature, for example a specific motion direction, we can selectively target and enhance the synaptic efficiency of functionally specific, but spatially overlapping, pathways. We report that ccPAS applied in a state-dependent manner and at a low intensity selectively enhanced detection of the specific motion direction primed during the combined visual-TMS manipulations. This paradigm significantly enhances the specificity of TMS-induced plasticity, by allowing the targeting of cortico-cortical pathways associated with specific functions
The thick disk rotation-metallicity correlation as a fossil of an "inverse chemical gradient" in the early Galaxy
The thick disk rotation--metallicity correlation, \partial
V_\phi/\partial[Fe/H] =40\div 50 km s^{-1}dex^{-1} represents an important
signature of the formation processes of the galactic disk. We use
nondissipative numerical simulations to follow the evolution of a Milky Way
(MW)-like disk to verify if secular dynamical processes can account for this
correlation in the old thick disk stellar population. We followed the evolution
of an ancient disk population represented by 10 million particles whose
chemical abundances were assigned by assuming a cosmologically plausible radial
metallicity gradient with lower metallicity in the inner regions, as expected
for the 10-Gyr-old MW. Essentially, inner disk stars move towards the outer
regions and populate layers located at higher |z|. A rotation--metallicity
correlation appears, which well resembles the behaviour observed in our Galaxy
at a galactocentric distance between 8 kpc and 10 kpc. In particular,we measure
a correlation of \partial V_\phi/\partial[Fe/H]\simeq 60 km s^{-1}dex^{-1} for
particles at 1.5 kpc < |z| < 2.0 kpc that persists up to 6 Gyr. Our pure N-body
models can account for the V_\phi vs. [Fe/H] correlation observed in the thick
disk of our Galaxy, suggesting that processes internal to the disk such as
heating and radial migration play a role in the formation of this old stellar
component. In this scenario, the positive rotation-metallicity correlation of
the old thick disk population would represent the relic signature of an ancient
"inverse" chemical (radial) gradient in the inner Galaxy, which resulted from
accretion of primordial gas.Comment: Accepted for publication on Astronomy and Astrophysic
Colloidal crystals: A suitable tool for the development of nanoscale responsive structures
In this work, we report on a colloidal composite system based on polystyrene nanoparticles embedded in an elastomeric matrix able to change its color in presence of different solvents and their mixtures. The structure presents an iridescent green color that can be attributed to the ordered assembly of the constituting nanoparticles causing the formation of a pseudo band gap. The composites realized present a colorimetric variation in their response, since a remarkable blue-shift of the diffraction peak is observed. The selectivity and sensitivity of the responsive system have been investigated by monitoring the static and dynamic reflectance spectra considering different solvents and a mixture of tert-butyl alcohol and water. An analytical model has been proposed and validated in order to assess the optical chromatic response, investigating the variation in frequency of the diffraction peak as well as the filling factor. The prepared structure and the interrogation approach can be seen as a suitable tool for the development of low cost portable and visual sensors for homologues and mixtures
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