1,045 research outputs found
Relative entropy in diffusive relaxation
We establish convergence in the diffusive limit from entropy weak solutions of
the equations of compressible gas dynamics with friction to the porous media equation away from vacuum.
The result is based on a Lyapunov type of functional provided by a calculation of the relative entropy.
The relative entropy method is also employed to establish convergence from entropic weak solutions
of viscoelasticity with memory to the system of viscoelasticity of the rate-type
Partial mixing and the formation of 13C pockets in AGB stars: effects on the s-process elements
The production of the elements heavier than iron via slow neutron captures
(the s process) is a main feature of the contribution of asymptotic giant
branch (AGB) stars of low mass (< 5 Msun) to the chemistry of the cosmos.
However, our understanding of the main neutron source, the 13C(alpha,n)16O
reaction, is still incomplete. It is commonly assumed that in AGB stars mixing
beyond convective borders drives the formation of 13C pockets. However, there
is no agreement on the nature of such mixing and free parameters are present.
By means of a parametric model we investigate the impact of different mixing
functions on the final s-process abundances in low-mass AGB models. Typically,
changing the shape of the mixing function or the mass extent of the region
affected by the mixing produce the same results. Variations in the relative
abundance distribution of the three s-process peaks (Sr, Ba, and Pb) are
generally within +/-0.2 dex, similar to the observational error bars. We
conclude that other stellar uncertainties - the effect of rotation and of
overshoot into the C-O core - play a more important role than the details of
the mixing function. The exception is at low metallicity, where the Pb
abundance is significantly affected. In relation to the composition observed in
stardust SiC grains from AGB stars, the models are relatively close to the data
only when assuming the most extreme variation in the mixing profile.Comment: 17 pages, 8 figures, 6 tables, accepted for publications on Monthly
Notices of the Royal Astronomical Societ
Magnetohydrodynamics of Cloud Collisions in a Multi-phase Interstellar Medium
We extend previous studies of the physics of interstellar cloud collisions by
beginning investigation of the role of magnetic fields through 2D
magnetohydrodynamic (MHD) numerical simulations. We study head-on collisions
between equal mass, mildly supersonic diffuse clouds. We include a moderate
magnetic field and two limiting field geometries, with the field lines parallel
(aligned) and perpendicular (transverse) to the colliding cloud motion. We
explore both adiabatic and radiative cases, as well as symmetric and asymmetric
ones. We also compute collisions between clouds evolved through prior motion in
the intercloud medium and compare with unevolved cases.
We find that: In the (i) aligned case, adiabatic collisions, like their HD
counterparts, are very disruptive, independent of the cloud symmetry. However,
when radiative processes are taken into account, partial coalescence takes
place even in the asymmetric case, unlike the HD calculations. In the (ii)
transverse case, collisions between initially adjacent unevolved clouds are
almost unaffected by magnetic fields. However, the interaction with the
magnetized intercloud gas during the pre-collision evolution produces a region
of very high magnetic energy in front of the cloud. In collisions between
evolved clouds with transverse field geometry, this region acts like a
``bumper'', preventing direct contact between the clouds, and eventually
reverses their motion. The ``elasticity'', defined as the ratio of the final to
the initial kinetic energy of each cloud, is about 0.5-0.6 in the cases we
considered. This behavior is found both in adiabatic and radiative cases.Comment: 40 pages in AAS LaTeX v4.0, 13 figures (in degraded jpeg format).
Full resolution images as well as mpeg animations are available at
http://www.msi.umn.edu:80/Projects/twj/mhd-cc/ . Accepted for publication in
The Astrophysical Journa
A Sr-Rich Star on the Main Sequence of Omega Centauri
Abundance ratios relative to iron for carbon, nitrogen, strontium and barium
are presented for a metal-rich main sequence star ([Fe/H]=--0.74) in the
globular cluster omega Centauri. This star, designated 2015448, shows depleted
carbon and solar nitrogen, but more interestingly, shows an enhanced abundance
ratio of strontium [Sr/Fe] ~ 1.6 dex, while the barium abundance ratio is
[Ba/Fe]<0.6 dex. At this metallicity one usually sees strontium and barium
abundance ratios that are roughly equal. Possible formation scenarios of this
peculiar object are considered.Comment: 13 pages, 3 figures. Accepted to ApJ
An extreme paucity of second population AGB stars in the normal globular cluster M4
Galactic Globular clusters (GCs) are now known to harbour multiple stellar
populations, which are chemically distinct in many light element abundances. It
is becoming increasingly clear that asymptotic giant branch (AGB) stars in GCs
show different abundance distributions in light elements compared to those in
the red giant branch (RGB) and other phases, skewing toward more primordial,
field-star-like abundances, which we refer to as subpopulation one (SP1). As
part of a larger program targeting giants in GCs, we obtained high-resolution
spectra for a sample of 106 RGB and 15 AGB stars in Messier 4 (NGC 6121) using
the 2dF+HERMES facility on the Anglo-Australian Telescope. In this Letter we
report an extreme paucity of AGB stars with [Na/O] > -0.17 in M4, which
contrasts with the RGB that has abundances up to [Na/O] =0.55. The AGB
abundance distribution is consistent with all AGB stars being from SP1. This
result appears to imply that all subpopulation two stars (SP2; Na-rich, O-poor)
avoid the AGB phase. This is an unexpected result given M4's horizontal branch
morphology -- it does not have an extended blue horizontal branch. This is the
first abundance study to be performed utilising the HERMES spectrograph.Comment: 5 pages, 2 figures, 4 tables (full Table 1 online). Accepted for
publication in MNRAS Letter
La construcción de la integración latinoamericana : el discurso político de Hugo Chávez : (estudio de caso)
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