1,047 research outputs found
Milky Way's Thick and Thin disk: Is there distinct thick disk?
This article is based on our discussion session on Milky Way models at the
592 WE-Heraeus Seminar, Reconstructing the Milky Way's History: Spectroscopic
Surveys, Asteroseismology and Chemodynamical models. The discussion focused on
the following question: "Are there distinct thick and thin disks?". The answer
to this question depends on the definition one adopts for thin and thick disks.
The participants of this discussion converged to the idea that there are at
least two different types of disks in the Milky Way. However, there are still
important open questions on how to best define these two types of disks
(chemically, kinematically, geometrically or by age?). The question of what is
the origin of the distinct disks remains open. The future Galactic surveys
which are highlighted in this conference should help us answering these
questions. The almost one-hour debate involving researchers in the field
representing different modelling approaches (Galactic models such as TRILEGAL,
Besancon and Galaxia, chemical evolution models, extended distribution
functions method, chemodynamics in the cosmological context, and
self-consistent cosmological simulations) illustrated how important is to have
all these parallel approaches. All approaches have their advantages and
shortcomings (also discussed), and different approaches are useful to address
specific points that might help us answering the more general question above.Comment: 7 pages, no figure. To appear in Astronomische Nachrichten, special
issue "Reconstruction the Milky Way's History: Spectroscopic surveys,
Asteroseismology and Chemo-dynamical models", Guest Editors C. Chiappini, J.
Montalban, and M. Steffe
Planetary nebulae in the inner Milky Way
New abundances of planetary nebulae located towards the bulge of the Galaxy
are derived based on observations made at LNA (Brazil). We present accurate
abundances of the elements He, N, S, O, Ar, and Ne for 56 PNe located towards
the galactic bulge. The data shows a good agreement with other results in the
literature, in the sense that the distribution of the abundances is similar to
those works. From the statistical analysis performed, we can suggest a
bulge-disk interface at 2.2 kpc for the intermediate mass population, marking
therefore the outer border of the bulge and inner border of the disk.Comment: 2 pages, 1 figure, uses iaus.cls, in press, IAU Symp. 265, Chemical
abundances in the Universe: Connecting the first Stars to Planets, Ed. K.
Cunha, M. Spite, B. Barbu
A new method for estimating the pattern speed of spiral structure in the Milky Way
In the last few decades many efforts have been made to understand the effect
of spiral arms on the gas and stellar dynamics in the Milky Way disc. One of
the fundamental parameters of the spiral structure is its angular velocity, or
pattern speed , which determines the location of resonances in the
disc and the spirals' radial extent. The most direct method for estimating the
pattern speed relies on backward integration techniques, trying to locate the
stellar birthplace of open clusters. Here we propose a new method based on the
interaction between the spiral arms and the stars in the disc. Using a sample
of around 500 open clusters from the {\it New Catalogue of Optically Visible
Open Clusters and Candidates}, and a sample of 500 giant stars observed by
APOGEE, we find km s kpc, for a local
standard of rest rotation ~km s and solar radius ~kpc.
Exploring a range in and within the acceptable values, 200-240 km
s and 7.5-8.5 kpc, respectively, results only in a small change in our
estimate of , that is within the error. Our result is in close
agreement with a number of studies which suggest values in the range 20-25 km
s kpc. An advantage of our method is that we do not need
knowledge of the stellar age, unlike in the case of the birthplace method,
which allows us to use data from large Galactic surveys. The precision of our
method will be improved once larger samples of disk stars with spectroscopic
information will become available thanks to future surveys such as 4MOST.Comment: 10 pages, 6 figures, 4 tables, accepted for publication in MNRA
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Modern Lattice Boltzmann methods for multiphase micro-flows
This paper was presented at the 2nd Micro and Nano Flows Conference (MNF2009), which was held at Brunel University, West London, UK. The conference was organised by Brunel University and supported by the Institution of Mechanical Engineers, IPEM, the Italian Union of Thermofluid dynamics, the Process Intensification Network, HEXAG - the Heat Exchange Action Group and the Institute of Mathematics and its Applications.During the last decade, the Lattice Boltzmann (LB) method has captured an increasing attention as an efficient tool for the numerical simulation of complex
fluids, particularly multi-phase and multi-component flows. In this paper, we revisit the basic features of two modern variants of lattice Boltzmann models for non-ideal
fluids, which offer promising perspectives for the numerical simulation of complex micro-
flows.This study is funded from the European Project INFLUS, NMP3-CT-2006-031980
Identification of the plastic zone using digital image correlation
In this paper Digital Image Correlation (DIC) is used to study the evolution of the plastic zone close to a crack tip. A modified CT-specimen was used in order to fulfill the plane stress condition. The strain field around the crack tip was measured using two cameras and stereo DIC, so that out-of-plane movements are taken into account. Then, the Virtual Fields Method was used to identify the plastic zone, looking at the parts of the specimen which deviates from the linear elastic behavior. With such approach, it was possible to individuate the onset of plasticity close to the crack tip and follow its evolution. A comparison with FEM results is also provided
An estimate of the time variation of the O/H radial gradient from planetary nebulae
Radial abundance gradients are a common feature of spiral galaxies, and in
the case of the Galaxy both the magnitude of the gradients and their variations
are among the most important constraints of chemical evolution models.
Planetary nebulae (PN) are particularly interesting objects to study the
gradients and their variations. Owing to their bright emission spectra, they
can be observed even at large galactocentric distances, and the derived
abundances are relatively accurate, with uncertainties of about 0.1 to 0.2 dex,
particularly for the elements that are not synthesized in their progenitor
stars. On the other hand, as the offspring of intermediate mass stars, with
main sequence masses in the interval of 1 to 8 solar masses, they are
representative of objects with a reasonable age span. In this paper, we present
an estimate of the time variation of the O/H radial gradient in a sample
containing over 200 nebulae with accurate abundances. Our results are
consistent with a flattening of the O/H gradient roughly from -0.11 dex/kpc to
-0.06 dex/kpc during the last 9 Gyr, or from -0.08 dex/kpc to -0.06 dex/kpc
during the last 5 Gyr.Comment: 9 pages, 7 encapsulated postscript figures, LaTeX, uses Astronomy and
Astrophysics macro aa.cls, graphicx package, to be published in Astronomy and
Astrophysics (2002), Also available at: http://www.astro.iag.usp.br/~macie
High-resolution abundance analysis of red giants in the globular cluster NGC 6522
The [Sr/Ba] and [Y/Ba] scatter observed in some galactic halo stars that are
very metal-poor stars and in a few individual stars of the oldest known Milky
Way globular cluster NGC 6522,have been interpreted as evidence of early
enrichment by massive fast-rotating stars (spinstars). Because NGC 6522 is a
bulge globular cluster, the suggestion was that not only the very-metal poor
halo stars, but also bulge stars at [Fe/H]~-1 could be used as probes of the
stellar nucleosynthesis signatures from the earlier generations of massive
stars, but at much higher metallicity. For the bulge the suggestions were based
on early spectra available for stars in NGC 6522, with a medium resolution of
R~22,000 and a moderate signal-to-noise ratio. The main purpose of this study
is to re-analyse the NGC 6522 stars previously reported using new
high-resolution (R~45,000) and high signal-to-noise spectra (S/N>100). We aim
at re-deriving their stellar parameters and elemental ratios, in particular the
abundances of the neutron-capture s-process-dominated elements such as Sr, Y,
Zr, La, and Ba, and of the r-element Eu. High-resolution spectra of four giants
belonging to the bulge globular cluster NGC 6522 were obtained at the 8m VLT
UT2-Kueyen telescope with the UVES spectrograph in FLAMES-UVESconfiguration.
The spectroscopic parameters were derived based on the excitation and
ionization equilibrium of \ion{Fe}{I} and \ion{Fe}{II}. Our analysis confirms a
metallicity [Fe/H] = -0.95+-0.15 for NGC 6522, and the overabundance of the
studied stars in Eu (with +~0.2 < [Eu/Fe] < +~0.4) and alpha-elements O and Mg.
The neutron-capture s-element-dominated Sr, Y, Zr, Ba, La now show less
pronounced variations from star to star. Enhancements are in the range 0.0 <
[Sr/Fe] < +0.4, +0.23 < [Y/Fe] < +0.43, 0.0 < [Zr/Fe] < +0.4, 0.0 < [La/Fe] <
+0.35,and 0.05 < [Ba/Fe] < +0.55.Comment: date of acceptation: 31/07/2014, in press, 24 pages, 19
figures,Astronomy & Astrophysics, 201
Effects of thermohaline instability and rotation-induced mixing on the evolution of light elements in the Galaxy : D, 3He and 4He
Recent studies of low- and intermediate-mass stars show that the evolution of
the chemical elements in these stars is very different from that proposed by
standard stellar models. Rotation-induced mixing modifies the internal chemical
structure of main sequence stars, although its signatures are revealed only
later in the evolution when the first dredge-up occurs. Thermohaline mixing is
likely the dominating process that governs the photospheric composition of
low-mass red giant branch stars and has been shown to drastically reduce the
net 3He production in these stars. The predictions of these new stellar models
need to be tested against galaxy evolution. In particular, the resulting
evolution of the light elements D, 3He and 4He should be compared with their
primordial values inferred from the Wilkinson Microwave Anisotropy Probe data
and with the abundances derived from observations of different Galactic
regions. We study the effects of thermohaline mixing and rotation-induced
mixing on the evolution of the light elements in the Milky Way. We compute
Galactic evolutionary models including new yields from stellar models computed
with thermohaline instability and rotation-induced mixing. We discuss the
effects of these important physical processes acting in stars on the evolution
of the light elements D, 3He, and 4He in the Galaxy. Galactic chemical
evolution models computed with stellar yields including thermohaline mixing and
rotation fit better observations of 3He and 4He in the Galaxy than models
computed with standard stellar yields. The inclusion of thermohaline mixing in
stellar models provides a solution to the long-standing "3He problem" on a
Galactic scale. Stellar models including rotation-induced mixing and
thermohaline instability reproduce also the observations of D and 4He.Comment: 12 pages, 9 figures, accepted for publication in A&
Influence of thermodynamic behaviour on HLA system performances
Paper presented at the 5th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 1-4 July, 2007.In the recent years, a new interest on kinetic energy recovery (KER) of vehicles arose. In USA, car manufacturers (Ford, Sheep et al.) developed research programs on the use of hydraulic devices to achieve KER, also with economical support of government body (EPA). In 2002 Ford built a very interesting prototype (Ford Tonka) using such recovery system (HLA, Hydraulic Launch Assist), achieving improvements better than 20 % on fuel consumption and pollution, besides increasing of acceleration. At present time, also DIMEG is working about HLA systems, carrying on two work lines: one (based on the building of a simulation program) to verify operation limits and system performances and another experimental (to study particular thermodynamic aspects of cyclic operation of hydro-pneumatic accumulators).
In previous works the authors studied application of HLA system on small vehicles, emphasizing the influences of mechanical parameters and kind of mission profile. Here they mean to treat the influences of some thermodynamic characteristics on performances and operation field of such system. The nitrogen behaviour is described by BWR state equation. Particular attention will be devoted to analyze, from a global point of view, the penalties introduced by using a more simple state equation for describing nitrogen, the influences on global performances of the efficiency of expansion and compression transformations and of heat exchange between gas and outside. The analysis will be performed for a commercial vehicle with mass of about 2000 kg.cs201
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