2,183 research outputs found
Physics of rotation in stellar models
In these lecture notes, we present the equations presently used in stellar
interior models in order to compute the effects of axial rotation. We discuss
the hypotheses made. We suggest that the effects of rotation might play a key
role at low metallicity.Comment: 32 pages, 7 figures, lectures, CNRS school, will be published by
Springe
A model of transport nonuniversality in thick-film resistors
We propose a model of transport in thick-film resistors which naturally
explains the observed nonuniversal values of the conductance exponent t
extracted in the vicinity of the percolation transition. Essential ingredients
of the model are the segregated microstructure typical of thick-film resistors
and tunneling between the conducting grains. Nonuniversality sets in as
consequence of wide distribution of interparticle tunneling distances.Comment: 3 pages, 1 figur
High Energy neutrino signals from the Epoch of Reionization
We perform a new estimate of the high energy neutrinos expected from GRBs
associated with the first generation of stars in light of new models and
constraints on the epoch of reionization and a more detailed evaluation of the
neutrino emission yields. We also compare the diffuse high energy neutrino
background from Population III stars with the one from "ordinary stars"
(Population II), as estimated consistently within the same cosmological and
astrophysical assumptions. In disagreement with previous literature, we find
that high energy neutrinos from Population III stars will not be observable
with current or near future neutrino telescopes, falling below both IceCube
sensitivity and atmospheric neutrino background under the most extreme
assumptions for the GRB rate. This rules them out as a viable diagnostic tool
for these still elusive metal-free stars.Comment: 9 pages, 5 figures
Massive star evolution in close binaries:conditions for homogeneous chemical evolution
We investigate the impact of tidal interactions, before any mass transfer, on
various properties of the stellar models. We study the conditions for obtaining
homogeneous evolution triggered by tidal interactions, and for avoiding any
Roche lobe overflow during the Main-Sequence phase. We consider the case of
rotating stars computed with a strong coupling mediated by an interior magnetic
field. In models without any tidal interaction (single stars and wide
binaries), homogeneous evolution in solid body rotating models is obtained when
two conditions are realized: the initial rotation must be high enough, the loss
of angular momentum by stellar winds should be modest. This last point favors
metal-poor fast rotating stars. In models with tidal interactions, homogeneous
evolution is obtained when rotation imposed by synchronization is high enough
(typically a time-averaged surface velocities during the Main-Sequence phase
above 250 km s), whatever the mass losses. In close binaries, mixing is
stronger at higher than at lower metallicities. Homogeneous evolution is thus
favored at higher metallicities. Roche lobe overflow avoidance is favored at
lower metallicities due to the fact that stars with less metals remain more
compact. We study also the impact of different processes for the angular
momentum transport on the surface abundances and velocities in single and close
binaries. In models where strong internal coupling is assumed, strong surface
enrichments are always associated to high surface velocities in binary or
single star models. In contrast, models computed with mild coupling may produce
strong surface enrichments associated to low surface velocities. Close binary
models may be of interest for explaining homogeneous massive stars, fast
rotating Wolf-Rayet stars, and progenitors of long soft gamma ray bursts, even
at high metallicities.Comment: 21 pages, 13 figures, 3 tables, accepted for publication in Astronomy
and Astrophysic
Diagnoses to unravel secular hydrodynamical processes in rotating main sequence stars
(Abridged) We present a detailed analysis of the main physical processes
responsible for the transport of angular momentum and chemical species in the
radiative regions of rotating stars. We focus on cases where meridional
circulation and shear-induced turbulence only are included in the simulations.
Our analysis is based on a 2-D representation of the secular hydrodynamics,
which is treated using expansions in spherical harmonics. We present a full
reconstruction of the meridional circulation and of the associated fluctuations
of temperature and mean molecular weight along with diagnosis for the transport
of angular momentum, heat and chemicals. In the present paper these tools are
used to validate the analysis of two main sequence stellar models of 1.5 and 20
Msun for which the hydrodynamics has been previously extensively studied in the
literature. We obtain a clear visualization and a precise estimation of the
different terms entering the angular momentum and heat transport equations in
radiative zones. This enables us to corroborate the main results obtained over
the past decade by Zahn, Maeder, and collaborators concerning the secular
hydrodynamics of such objects. We focus on the meridional circulation driven by
angular momentum losses and structural readjustements. We confirm
quantitatively for the first time through detailed computations and separation
of the various components that the advection of entropy by this circulation is
very well balanced by the barotropic effects and the thermal relaxation during
most of the main sequence evolution. This enables us to derive simplifications
for the thermal relaxation on this phase. The meridional currents in turn
advect heat and generate temperature fluctuations that induce differential
rotation through thermal wind thus closing the transport loop.Comment: 16 pages, 18 figures. Accepted for publication in A&
Abundances of Baade's Window Giants from Keck/HIRES Spectra: II. The Alpha- and Light Odd Elements
We report detailed chemical abundance analysis of 27 RGB stars towards the
Galactic bulge in Baade's Window for elements produced by massive stars: O, Na,
Mg, Al, Si, Ca and Ti. All of these elements are overabundant in the bulge
relative to the disk, especially Mg, indicating that the bulge is enhanced in
Type~II supernova ejecta and most likely formed more rapidly than the disk. We
attribute a rapid decline of [O/Fe] to metallicity-dependent yields of oxygen
in massive stars, perhaps connected to the Wolf-Reyet phenomenon. he explosive
nucleosynthesis alphas, Si, Ca and Ti, possess identical trends with [Fe/H],
consistent with their putative common origin. We note that different behaviors
of hydrostatic and explosive alpha elements can be seen in the stellar
abundances of stars in Local Group dwarf galaxies. We also attribute the
decline of Si,Ca and Ti relative to Mg, to metallicity- dependent yields for
the explosive alpha elements from Type~II supernovae. The starkly smaller
scatter of [/Fe] with [Fe/H] in the bulge, as compared to the halo, is
consistent with expected efficient mixing for the bulge. The metal-poor bulge
[/Fe] ratios are higher than ~80% of the halo. If the bulge formed from
halo gas, the event occured before ~80% of the present-day halo was formed. The
lack of overlap between the thick and thin disk composition with the bulge does
not support the idea that the bulge was built by a thickening of the disk
driven by the bar. The trend of [Al/Fe] is very sensitive to the chemical
evolution environment. A comparison of the bulge, disk and Sgr dSph galaxy
shows a range of ~0.7 dex in [Al/Fe] at a given [Fe/H], presumably due to a
range of Type~II/Type~Ia supernova ratios in these systems.Comment: 51 pages, 6 tables, 27 figures, submitte
Impact of rotation and disc lifetime on pre-main sequence lithium depletion of solar-type stars
Aims: We study the influence of rotation and disc lifetime on lithium
depletion of pre-main sequence (PMS) solar-type stars. Methods: The impact of
rotational mixing and of the hydrostatic effects of rotation on lithium
abundances are investigated by computing non-rotating and rotating PMS models
that include a comprehensive treatment of shellular rotation. The influence of
the disc lifetime is then studied by comparing the lithium content of PMS
rotating models experiencing different durations of the disc-locking phase
between 3 and 9 Myr. Results: The surface lithium abundance at the end of the
PMS is decreased when rotational effects are included. During the beginning of
the lithium depletion phase, only hydrostatic effects of rotation are at work.
This results in a decrease in the lithium depletion rate for rotating models
compared to non-rotating ones. When the convective envelope recedes from the
stellar centre, rotational mixing begins to play an important role due to
differential rotation near the bottom of the convective envelope. This mixing
results in a decrease in the surface lithium abundance with a limited
contribution from hydrostatic effects of rotation, which favours lithium
depletion during the second part of the PMS evolution. The impact of rotation
on PMS lithium depletion is also found to be sensitive to the duration of the
disc-locking phase. When the disc lifetime increases, the PMS lithium abundance
of a solar-type star decreases owing to the higher efficiency of rotational
mixing in the radiative zone. A relationship between the surface rotation and
lithium abundance at the end of the PMS is then obtained: slow rotators on the
zero-age main sequence are predicted to be more lithium-depleted than fast
rotators due to the increase in the disc lifetime.Comment: 8 pages, 11 figures, A&
B-type supergiants in the SMC: Rotational velocities and implications for evolutionary models
High-resolution spectra for 24 SMC and Galactic B-type supergiants have been
analysed to estimate the contributions of both macroturbulence and rotation to
the broadening of their metal lines. Two different methodologies are
considered, viz. goodness-of-fit comparisons between observed and theoretical
line profiles and identifying zeros in the Fourier transforms of the observed
profiles. The advantages and limitations of the two methods are briefly
discussed with the latter techniques being adopted for estimated projected
rotational velocities (\vsini) but the former being used to estimate
macroturbulent velocities. Only one SMC supergiant, SK 191, shows a significant
degree of rotational broadening (\vsini 90 \kms). For the remaining
targets, the distribution of projected rotational velocities are similar in
both our Galactic and SMC samples with larger values being found at earlier
spectral types. There is marginal evidence for the projected rotational
velocities in the SMC being higher than those in the Galactic targets but any
differences are only of the order of 5-10 \kms, whilst evolutionary models
predict differences in this effective temperature range of typically 20 to 70
\kms. The combined sample is consistent with a linear variation of projected
rotational velocity with effective temperature, which would imply rotational
velocities for supergiants of 70 \kms at an effective temperature of 28 000 K
(approximately B0 spectral type) decreasing to 32 \kms at 12 000 K (B8 spectral
type). For all targets, the macroturbulent broadening would appear to be
consistent with a Gaussian distribution (although other distributions cannot be
discounted) with an half-width varying from approximately 20 \kms
at B8 to 60 \kms at B0 spectral types.Comment: 4 figures, 8 pages, submitted to Astronomy and Astrophysic
Sensors and packages based on LTCC and thick-film technology for severe conditions
Reliable operation in harsh environments such as high temperatures, high pressures, aggressive media and space, poses special requirements for sensors and packages, which usually cannot be met using polymer-based technologies. Ceramic technologies, especially LTCC (Low-Temperature Cofired Ceramic), offer a reliable platform to build hermetic, highly stable and reliable sensors and packages. This is illustrated in the present work through several such devices. The examples are discussed in terms of performance, reliability, manufacturability and cost issue
Domain and lattice contributions to dielectric and piezoelectric properties of Pb(Zrx, Ti1âx)O3 thin films as a function of composition
In situ reactively sputter deposited, 300-nm-thick Pb(Zrx, Ti1âx)O3 thin films were investigated as a function of composition, texture, and different electrodes (Pt,RuO2).X-ray diffraction analysis, ferroelectric, dielectric, and piezoelectric measurements were carried out. While for dielectric properties bulklike contributions from lattice as well as from domains are observed, domain wall contributions to piezoelectric properties are very much reduced in the morphotropic phase boundary (MPB) region. Permittivity and d33 do not peak at the same composition; the MPB region is broadened up and generally shifted to the tetragonal sid
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